Larcan FM-250 Technical Manual

FM-250 FM BROADCASTING EXCITER

FM-250

250W STEREO

TRANSMITTER/EXCITER

TECHNICAL MANUAL

LARCAN Denver Office (LARCAN-TTC)

1390 Overlook Drive #2, Lafayette, CO 80026 USA

Tel: 303-665-8000 Fax: 303-763-9900

E-Mail: [email protected] Web: www.larcan.com

Issue Date: 21 Jan 2003


NOTE

AMENDMENT

Please take note that any reference to 230V and 50Hz in this manual should be read as 110V and 60Hz respectively.



FM-250

250W STEREO TRANSMITTER/EXCITER


CONTENTS:

1.

2.

3.

4.

5.

6.

General Description

Technical Specifications

Operating Information

Installation Information

Technical Information

Maintenance information



Tel: 303-665-8000 Fax: 303-763-9900






INTRODUCTION

This manual contains the operating, technical and installation information for the NCI FM-250

FM Broadcasting Exciter. The Exciter forms part of a new range of Professional FM Broadcast

Exciters produced by Broadcast Solutions Electronics (Pty) Ltd and supplied by LARCAN

USA in the United States.

1. The FM-250 Exciter forms a compact, solid state FM Broadcast transmitter with a RF output in excess of 250W in the FM Broadcasting band (87.5MHz to 108MHz). The unit is housed in a 19-inch rack mount case occupying only a 2U space. The Exciter features a range of customer options including a built in, high quality Stereo coder.

2. The FM-250 Exciter offers a standard specification, better than the general requirements of the major broadcasters in the world. This level of performance is only found in the best equipment that the market has to offer.

3. The following basic versions are available: a) FM-250 (W) Wideband MPX Exciter (For use with composite input). b) FM-250 (S) Stereo Exciter (With built in Stereo coder and Audio Limiter). c) FM-250 (M) Mono Exciter (With built in Mono input filter and Audio Limiter).



FM-250



General Description



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GENERAL DESCRIPTION

1. STRUCTURE.

The Exciter comprises of the following modules. a) Synthesizer/Modulator module (980507). b) Control/Monitoring module (980508). c) 250W RF PA module (980510). d) 250W Power supply module (980515). e) Display module (980509). f) Stereo coder/Limiter (980512). (Optional). g) Fan Psu module (980518).

2. FEATURES.

The FM-250 Exciter has standard features including the following; a) Remote/Internal frequency selection (standard) with Thumbwheel switch option available. b) Remote telemetry with voltage free contacts (standard). c) +48V Battery operation (standard). d) Wideband input (MPX) with two auxiliary inputs (SST/SCA/RDS). e) Comprehensive metering including VU meters. (Built in Stereo decoder). f) ALC built in for absolute control of RF output power. g) Comprehensive protection built in.

3. BLOCK DIAGRAM DESCRIPTION.



Refer to the Front panel layout in figure 1, Rear panel controls and connectors in figure2 and the Block diagram of FM-250 in Figure 3



Figure 1: FM-250 Front Panel Controls



Figure 2: FM-250 Rear Panel Controls and Connectors





BACK PANEL

INPUTS

PL1

LEFT (XLR)

OV

PL2

RIGHT (XLR)

OV

PL3

PRE-EMPH

(SWITCH)

STEREO

CODER

980512

MPX OUT

SK1

PL4

OV

10 way ribbon cable

PILOT OUT

(BNC)

REAR PANEL

PL2

PL7

REMOTE

TELEMETRY

REAR PANEL

(D-25)

PL6

26 way ribbon cable

PL1

DISPLAY

PCB

980509

MPX (I/P)

AUX 1 (I/P)

AUX2 (I/P)

SK2

OV

SK3

OV

SK4

OV

PL2

SYNTHESISER

980507

PL3

DEVIATION

ADJ.

(OPTIONAL)

PL4

PL4

PSU

PL5

CONTROL PCB

980508

SYNTH

REMOTE

(D-15)

RF OUT

SK1

RF OUT

(N TYPE)

230V

50Hz

(IEC INPUT)

E OV

OV

N

L

MAIN PSU

RF P.A.

250W

980510

PL4

AUX PSU

SK1

PL1

PL2

PL3

THUMBWHEEL

(OPTION)

ALC

TEMP

CNTRL

FWD

REFL

PL3 PL1 PL2 SK1 SK2

8A

OV

RF MONITOR

FRONT PANEL

(BNC)

PL8

25A +

BRIDGE

~

~

RS485 (OPTIONAL)

REAR PANEL

20 LED MULTIMETER

STEREO VU METER

PILOT INDICATOR

FAULT INDICATORS

FUNCTIONAL INDICATORS

DEVIATION ADJ. (OPTIONAL)

RF POWER ADJ.

FRONT

PANEL

POWER

SWITCH

230V

50Hz

O/P

AUX.

VOLTS

500VA

TRANSFORMER

+24V

AC

+48V

AC

25A

BRIDGE

PL1

SK1

PL1 PL3

PSU PCB

980508

SK1/1

SK4 SK1/3 FAN

D.C.

INPUT

0V

+48V

10A

250W STEREO EXCITER BLOCK SCHEMATIC

Figure 3: Block diagram of FM -250


FM-250 FM BROADCASTING EXCITER a) SYNTHESIZER AND MODULATOR (980507)

The digital synthesizer derives the final operating frequency from a 10MHz reference (TCXO) using a PLL (Phase lock loop) and dividers. The frequency can be selected Locally (In 100kHz steps via rotary BCD switches) or Remotely (via a external parallel control). The direct carrier modulator uses special techniques to obtain very low distortion and high stereo channel separation (>60dB). This level of performance is not normally found on competitive equipment, especially at low audio frequencies.

The Synthesizer is equipped with four modulating inputs, three of which are external (BNC's) and one internal (SMB) for use with optional Stereo coder.

The synthesizer is protected from Out of Lock and Out of Band operation by removing the RF

Output when either of these conditions exists. b) RF PA MODULE (980510)

The RF output (Nominally +10dBm) from the synthesizer buffer is amplified by a three stage RF power amplifier to obtain the final output power of >250W. The final device is operated in push-pull mode to obtain high efficiency. All devices are operated well within their ratings.

Wideband techniques are used throughout the PA and a built in harmonic filter is used at the output to suppress all harmonics. Directional couplers are used to sample the output for control and monitoring purposes.

A temperature-sensing device is provided for thermal monitoring. This device is situated at the final RF Stage to provide accurate temperature monitoring of the RF PA Heatsink.

Current and voltage sensing is provided for control and monitoring purposes along with the

ALC circuit, which controls and monitors the RF output power.

A Series regulator is used to ensure protection and utilizes three parallel devices for maximum reliability. c) CONTROL MODULE (980508)

The control Pcb provides the standard telemetry functions and interconnections for the Exciter.

The unit receives two dc inputs, one from the external battery supply and one from the rectified a/c supply and has a ‘on board’ low power switching power supply. The various voltages are distributed to the other modules via the Control Pcb.

Forward and Reflected RF Inputs are processed and distributed for ALC control and monitoring purposes. RF PA current, Voltage and Temperature are also processed and their voltages distributed.

Various voltage free contacts are provided for telemetry purposes and are Link selectable for o/c or s/c condition. d) POWER SUPPLY MODULE (980515)



The power supply module contains the smoothing capacitors for the ‘off board’ 230V 50Hz a/c mains transformer/bridge rectifier combination. This +52V nominal smoothed supply is then distributed to a choke/capacitor filter at the module. This type of arrangement eliminates all ripple and therefor any AM products on the transmitter, even when the a/c input voltage is extremely low. The +48V (nominal) external battery supply is combined off the module with the smoothed supply via isolating diodes. The various voltages are then distributed to the other modules.

Auxiliary supplies are generated for use in the RF PA regulator stage and Modules. e) DISPLAY MODULE (980509)

The display module provides the Local user functions and alarm indications at the Exciter front panel. The display has three functional meters, which allow instant viewing of multiple functions.

1) Multimeter: Provides push button selectable functions for Forward power, Reflected power, PA current and Heatsink temperature.

2) ALC meter: Dedicated meter for instant viewing of ALC voltage and alarm.

3) MPX VU meter: Active peak detection meters for Left and Right channels, derived from built in Stereo decoder. This provides instant viewing of Transmitter deviation from Left and

Right channels. The Stereo pilot indication is also provided with the meter.

Alarm indications are provided by Front panel LED’s, enabling instant viewing of transmitter functions. Front panel adjustments are available for control of RF Output power and Deviation level.

Connection for the optional RS485 Module is provided on the Display, enabling remote viewing of Meter functions . f) STEREO CODER MODULE (980512) (optional)

This optional module allows the user to facilitate a very high quality stereo signal on board the

Exciter and provides all the necessary functions such as Pre emphasis switching, Mono- Stereo switching with the added advantage of a built in Audio Limiter. The module accepts Left and

Right audio inputs (600 ohms balanced) and after Limiting and filtering is converted into the

Stereo multiplex format via a new digital stereo multiplexer. A 100kHz Linear phase filter is used at the output with comprehensive phase adjustment to obtain absolute transparency.

A 19kHz Pilot signal output is provided for supplementary signal use. g) BACK PANEL CONNECTIONS

The 110V 60Hz a/c power input is connected via an IEC connector whilst the +24 dc input is connected via Binding terminals. Back panel Fuses are provided for maximum protection.

Standard telemetry output is provided via a 25 pin D connector, while the Remote frequency control has a 15 pin D connector.



There is provision for three (50-ohm BNC female) external modulating inputs, one for external

MPX (600 ohms unbal. or 1.2k ohms BAL.) And two for auxiliary use (20k ohms unbal.).

The RF Output is via an N Type (50-ohm) connector.

Optional connectors include two XLR (F) connectors (for Stereo coder), plus Pre- emph slide switch with a 9 pin D connector for the RS485 module. h) MONO FILTER (optional)

The Mono filter option provides the user with a XLR (F) 600 ohm balanced input and 15kHz low pass filter with an internal output connection to the Synthesizer and modulator. Preemphasis switching (50uS or 75uS) is provided on board and is switched via the Pre-emphasis switch on the back panel. An Audio Limiter is supplied as standard issue with internally adjustable threshold.

Note: The following modules are common to the range of Exciters produced by NCI

Electronics.

•

Synthesizer module

•

Control module

•

Stereo coder (option)

•

Mono filter (option)





Technical Specifications



Tel: 303-665-8000 Fax: 303-763-9900




TECHNICAL SPECIFICATIONS

GENERAL RF SPECIFICATIONS

FREQUENCY RANGE 87.5 MHz to 108MHz

FREQUENCY CONTROL PLL Synthesizer

FREQUENCY INCREMENTS

FREQUENCY STABILITY

RF OUTPUT POWER

RF HARMONIC SUPPRESSION

RF SPURIOUS SUPPRESSION

OUT OF LOCK ATTENUATION

OUT OF BAND ATTENUATION

AM NON SYNCHRONOUS NOISE

AM SYNCHRONOUS

MEAN CARRIER VARIATION

100kHz steps (Local and Remote control)

±

2ppm (10MHz TCXO)

≥

250W (Adjustable via front panel control)

≥

70dB

≥

75dB

≥

80dB

≥

80dB (87.5MHz

≤

F

≤

108MHz)

≤

-60dB (Filtered 20Hz to 20kHz)

≤

-60dB (Filtered 20Hz to 20kHz)

≤ ±

200Hz

WIDEBAND COMPOSITE OPERATION

BASEBAND RESPONSE

≤

0.2dB (30Hz to 100kHz)

STEREO SEPARATION

≥

60dB (30Hz to 15kHz)

STEREO DISTORTION

WIDEBAND S/N RATIO

≤

0.1% (30Hz to 15kHz)

≥

80dB (20Hz to 20kHz) 50uS de emph.

INPUT IMPEDANCE

DEVIATION SENSITIVITY

1k2 ohms balanced (other on request)

3.5V p-p for

±

75kHz deviation (adj.)

STEREO OPERATION (With built in Stereo coder/Limiter)

STEREO SEPARATION

STEREO DISTORTION

STEREO S/N RATIO

MONO/STEREO CROSSTALK

INPUT IMPEDANCE


AUDIO LIMITER SENSITIVITY

ATTACK TIME

DECAY TIME

LIMITER DISTORTION

LIMITER THRESHOLD

≥


≤

0.1% (30Hz to 15kHz)

≥

75dB (20Hz to 20kHz) 50uS de emph.

≥


600 ohms balanced (XLR)

+6dBm for

±


10dBm nom. for 62.5kHz deviation (adj.)

≤

2mS for increased 10dB I/P step

≥

80mS for a decreased 10dB I/P step

≤

0.35% (10dB into limiting)

Internally adjustable



MONO OPERATION (With built in Mono filter)

AF RESPONSE

MONO DISTORTION

S/N RATIO

≤ ±

0.3dB (30Hz to 15kHz)

≤

0.1% (30Hz to 15kHz)

≥

80dB (20Hz to 20kHz) 50uS de emph

INPUT IMPEDANCE


ATTACK TIME

DECAY TIME

LIMITER DISTORTION

600 ohms balanced (XLR)

+6dBm for

±


≤

2mS for increased 10dB I/P step

≥

80mS for a decreased 10dB I/P step

≤

0.35% (10dB into limiting)

Internally adjustable LIMITER THRESHOLD

ENVIRONMENTAL SPECIFICATIONS

A/C INPUT POWER

A/C FREQUENCY VARIATION

DC INPUT POWER

STORAGE TEMPERATURE

OPERATING TEMPERATURE

110V

±

10% (other on request)

60Hz

±5

% (other on request)

+24V nominal Battery

-40

°

C to +60

°

C

-10

°

C to +45

°

C

RELATIVE HUMIDITY

OPERATING ALTITUDE

COOLING SYSTEM

20% to 90% (non condensing)

≤

2500m above sea level

Forced air (4 x 24Vdc blowers)

PHYSICAL DIMENSIONS

PHYSICAL MASS

480mm x 485mm x 88.8mm

Approximately 16kg

STANDARD ALARM SETTINGS AND LOCAL INDICATIONS (adjustable)

RF FAULT INDICATION

VSWR FAULT INDICATION

VSWR FAULT TRIP INDICATION

TEMP FAULT INDICATION

TEMP TRIP INDICATION

≤

175W (Red led on front panel)

≥

20W (Red led on front panel)

≥

25W (Red led/front panel/10Sec re-cycle)

≥

55

°

C (Red led on front panel)

≥

65

°

C (Red led on front panel)

ALC FAULT INDICATION

PSU FAULT INDICATION

Out of range (Red led/front panel meter)

Internal Psu fault (Red led on front panel)

RF PSU FAULT INDICATION

OVERVOLTAGE FAULT

RF PA Psu fault (Red led on front panel)

≥

+52V at Control (Red led on front panel)

The unit has a number of operational customer link options, which should normally be configured before delivery. Consult Control module technical description section.

•

RF Power backed off with high temperature.



•

RF Power backed off with high VSWR.

•

RF Power backed off with high RF PA Current.





Operating Information



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OPERATING INFORMATION

FRONT PANEL

1. POWER ON/OFF SWITCH

The front panel On/Off switch connects the External A/C and D/C supplies to the equipment.

2. POWER ADJUST CONTROL

This single turn potentiometer is used to set the RF output power via the internal ALC

Circuit. Fully anti-clockwise gives minimum power (

≤

25W) while clockwise gives

Maximum power (

≥

250W).

3. DEVIATION ADJUST

This single turn potentiometer is used to fine trim the Transmitter final deviation level.

The standard variation is

±

1.5dB (other on request). This controls the total deviation

Level.

Note: When the stereo coder option is installed, the built in Limiter controls the maximum

Deviation level of the Left and Right program inputs and adjustment of this level is done

Internally, to customers requested level.

4. MULTIMETER

The Multimeter comprises a linear scale, 20 led (green) custom bar display. The meter has a push button selector with individual LED's (yellow) for indicating the four selectable

Functions.

•

Temperature: RF PA Heatsink temperature with 100

°

C f.s.d.

•

RF PA Current: RF final stage current with 10A f.s.d.

•

RF Forward power: RF Output power with 320W f.s.d.

•

RF Reflected power: RF Output Reflected power with 32W f.s.d.

5. ALC METER

This dedicated 10 led (9 green and 1 red) bar display provides instant information on the

ALC loop control function and voltage. The meter indicates the dc control voltage at the

Gate of the final RF Mosfet stage and as such gives the user instant feedback on the status of the ALC loop.

The meter scale is -5V to +4V (1volt per led) with the +4V led (red) indicating ALC ‘out of range’.

6. VU METERS

This dedicated stereo meter is used to indicate the dynamic peak deviation of the Left and Right channels. This provides the user with instant visual information on the status of the system modulation.

The meters operate to the DIN 45406 specification with precision Full wave peak


FM-250 FM BROADCASTING EXCITER detection.

The signals are derived from the built in stereo decoder and calibrated accordingly.

The VU meter scales are as follows:

LEFT CHANNEL

+3dBm

+2dbm

RIGHT CHANNEL

+3dBm

+2dbm

LED STATUS

Over deviation (red)


+1dBm

+0dBm

-1dBm

-3dBm

-5dBm

-7dBm

-10dBm

-20dBm

+1dBm

+0dBm

-1dBm

-3dBm

-5dBm

-7dBm

-10dBm

-20dBm


Peak program (yellow)

Normal program (green)






The stereo pilot is also monitored by the decoder and is indicated by a led (green) between the Left and Right VU meters. If the pilot is lost the led will extinguish and if desired the Right channel VU meter can be actively disabled in this circumstance

(Internal link option). This would normally happen for instance if Mono had been selected.

7. INDICATORS

FUNCTION

RF ENABLED

FRONT PANEL

INDICATION

Green led

STATUS

Indicates ext. command on.

PLL LOCK

PRE EMPHASIS

REMOTE PLL

RS485 (option)

RF FAULT

VSWR FAULT

AC SUPPLY

EXT. DC

OVERVOLTAGE

PSU FAULT

RF PSU FAULT

Green led

Yellow led

Yellow led

Yellow led

Red led

Red led

Green led

Green led

Red led

Red led

Red led

Indicates PLL is in lock.

Indicates pre emphasis is on.

Indicates PLL in remote.

Indicates RS485 module on.

Indicates low RF Forward.

Indicates high RF Reflected.

Indicates a/c supply is on.

Indicates dc supply is on.

Indicates high voltage.

Indicates internal Psu fault.

Indicates RF Psu fault.



BACK PANEL a) A/C INPUT

Standard IEC male connector for 110V 60Hz mains operation. b) A/C FUSE (110V a/c operation)

20mm (10 Amp) fuse connected in live line. c) DC INPUT

Terminals for +48V dc operation. d) DC FUSE (+48V dc Battery operation)

20mm (10 Amp) fuse in +48V line. e) WIDEBAND MPX INPUT

BNC (female) for use with external stereo multiplex input. f) AUXILIARY #1 AND #2 INPUTS

BNC (female), for use with supplementary signals, (RDS/SST/SCA). g) LEFT INPUT

Balanced 600 ohm XLR (female) input for Left channel (Stereo coder or Mono filter installed). h) RIGHT INPUT

Balanced 600-ohm XLR (female) input for Right channel (Stereo coder installed). i) PRE EMPHASIS SWITCH

Slide switch for pre emphasis switching (Stereo coder or Mono filter installed). j) PILOT OUTPUT

BNC (female) with pilot (19kHz) sine wave output 1V p-p (Stereo coder installed). k) RF OUTPUT

N Type (female) RF output connector. l) REMOTE PLL

This 15 pin D type (female) connector is used for remote frequency operation m) TELEMETRY OUTPUT

This 25 pin D type (female) connector is used for monitoring and control purposes.


FM-250 FM BROADCASTING EXCITER n) RS485 OPTION

This 9 pin D type (female) connector is used for remote monitoring of meter functions. o) TELEMETRY CONNECTIONS (PL7)

FUNCTION

RF FAULT

PL7 CONNECTOR PINS

Pins 1 and 2

CONTACT STATUS

Voltage free contacts s/c for fault

VSWR FAULT Pins 3 and 4

TEMPERATURE FAULT Pins 5 and 6



PSU FAULT

STEREO OPERATION

Pins 7 and 8

Pins 9 and 10


Voltage free contacts s/c for stereo

PLL LOCK

ENABLE COMMAND

MONO/STEREO

COMMAND

+12V / 20mA SOURCE

Pins 11 and 12

Pins 13

Pins 14

Pins 15

Voltage free contacts s/c for Lock o/c to enable ground to disable o/c for Stereo ground for Mono

Supply for Test jig Leds

GROUND Pins 17 and 18 Connected to chassis

NOTE: Alternative contact positions are available internally, (via Link selections on the

Control module), for the Telemetry outputs.



p) REMOTE FREQUENCY PROGRAMMING (PL2)

15 pin connector BCD Parallel Code Code information

PL2 pin 1

PL2 pin 2 x 0.1MHz x 0.2MHz

These four data lines contain the information from 0 MHz to

PL2 pin 3

PL2 pin 4

PL2 pin 5

PL2 pin 6

PL2 pin 7 x 0.4MHz x 0.8MHz x 1MHz x 2MHz x 4MHz

0.9MHz in BCD code.

These four data lines contain the information from 1MHz to

9MHz in BCD code.

PL2 pin 8

PL2 pin 9

PL2 pin 10

PL2 pin 11

PL2 pin 12

PL2 pin 13 x 8MHz x 10MHz x 80MHz x 100MHz

Data enabled

Lock indication

These three data lines contain the information, 80MHz, 90MHz and 100MHz.

+5V Logic '1'output.

Open collector for Lock.

Example

PL2 pin 14

PL2 pin 15

Remote enable

Ground

Connect to 0V for Enable.

0V

Freq. pin 11 pin 10 pin 9 pin 8 pin 7 pin 6 pin 5 pin 4 pin 3 pin 2 pin 1

107.6 1 0 0 0 1 1 1 0 1 1 0

98.9

88.5

0

0

1

1

1

0

1

1

0

0

0

0

0

0

1

0

0

1

0

0

1

1

The logic '0' condition only requires an open circuit, and logic '1' condition has to be connected to PL2 pin 12.







Installation Information



Tel: 303-665-8000 Fax: 303-763-9900




INSTALLATION

STAND ALONE CONFIGURATION (Stereo Exciter)

The following information sets out the general requirements for installation and operation of the above equipment in a "stand alone" configuration.

Note: The Audio program inputs (Left and Right), may vary in level from the standard factory sensitivity setting, and adjustment may have to be made internally (due to the Stereo coders built in Limiter). The Limiter threshold level is factory set to limit the deviation of the audio signal to

±

62.5kHz. (Adjustable)

The pilot deviation is factory set to

±

7kHz. The total deviation must not exceed

±

75kHz. A fine deviation level adjustment of 0 to -3dB is available on the front panel to allow for small variations.

The operating frequency will normally be set at factory configuration, but if a different operating frequency is necessary then the unit top cover has to be removed and a new frequency selected on the Synthesizer module (980507). This is simply done by setting the direct readout on the rotary BCD switches.

1. ELECTRICAL CONNECTIONS

Ensure that the power on/off switch on the front panel is switched off before proceeding. a) A/C MAINS SUPPLY (110V 60Hz).

Connect a/c power cable to IEC connector on back panel. b) DC SUPPLY (+48V dc).

Connect dc power input to the dc supply terminals on the back panel observing correct polarity. c) RF OUTPUT.

Connect the Antenna cable to the RF Output on the back panel using an N type connector or via suitable adapters. d) AUDIO INPUTS.

Connect the Left and Right audio feeds (XLR male) to the Left and Right (XLR male) inputs on the back panel. e) PRE EMPHASIS.

Set the pre emphasis switch to "ON" at the back panel.

Note: When the Telemetry connector is not in use the unit requires no external commands to enable the RF. The unit is also ready for Stereo operation. These functions can be changed via link commands on the Telemetry connector. See "9" Telemetry connections (PL7).



2. OPERATION

With the electrical connections completed above the unit is now ready for operation. a) SWITCH ON SEQUENCE

Adjust the RF power control on the front panel, fully acw. then set the power on/off switch to ON. b) FRONT PANEL LED's:

RF ENABLE (green led)

PLL LOCK (green led)

PRE EMPHASIS (yellow led)

ON

ON

ON

REMOTE PLL (yellow led)

RF FAULT (red led)

VSWR FAULT (red led)

TEMP FAULT (red led)

A/C SUPPLY (green led)

EXT. DC (green led)

OVER VOLTAGE (red led)

PSU FAULT (red led)

OFF

ON

OFF

OFF

ON

ON

OFF

OFF

RF PSU FAULT (red led) OFF c) ALC METER

All the Leds on the ALC meter should be OFF when PWR ADJ is set to minimum. d) MULTIMETER

Select the four functions in turn and check the meter readings:

•

TEMPERATURE Room ambient.

•

PA CURRENT 0 Amps (no Leds on).

•

FORWARD POWER 0 Watts (no Leds on).

• REFLECTED POWER 0 Watts (no Leds on).

e) DECODED MPX (VU Meters).

The Left and Right, VU meters will be indicating the incoming program levels and should be operating in the green zone of the meters with occasional peaks switching the 0dB yellow led on. The program peaks should not be operating in the red zone of the meters. If the meter is operating below the 0dB level (yellow), the internal sensitivity setting may have to be increased on the Stereo coder module to suit the incoming program levels. See calibration section for information.



The front panel deviation adjustment should not be used for the purpose of deviation adjustment in the Stereo operational mode. (This is for use with Wideband operation).

With the standard factory setting, the unit should not be over deviating due to the Limiter operation and protection. The Pilot led should be on, indicating that the stereo pilot signal is present assuming a Stereo signal is present. f)

SETTING THE RF POWER LEVEL

Select FWD POWER on the Multimeter and observe the reading.

Adjust the Front panel PWR ADJ potentiometer clockwise to set the RF output power to

250W. (This should be achieved with 16 LED’s on).

Now check the PA CURRENT and REFL POWER readings:

PA CURRENT should be approximately 7 Amps (14 Leds).

REFL POWER should be less than 25W (15 Leds).

Check all the fault Leds on the front panel and ensure that there are no red Leds on.

The unit is now operational and only the final temperature must be monitored after approximately 15 minutes. The temperature reading on the front panel Multimeter should settle down to approximately 12

°

C above Room temperature.

WIDEBAND OPERATION

The connection for this mode of operation is almost the same as for the Stereo mode, the difference being in the connection of the Program inputs.

The Wideband system does not use the Left and Right program inputs or the pre emphasis switch (not normally supplied). The Composite input or MPX is connected to the MPX input on the unit back panel (BNC female).

The Decoded MPX meter is still operational in this mode and displays the Left and Right signals as before with exactly the same operation.

The Deviation adjust control on the front panel is now used to set the level of the modulation on the meters. The pilot led is also still operational if the incoming MPX signal is in stereo and will be off if the pilot is not present.

MECHANICAL

The unit is designed to fit in a standard 19-inch rack, cabinet structure. There is a Slide rail kit

(option) available for this purpose which allows the unit to be installed in this manner.

Due to the weight of the unit it is not recommended to mount the unit without proper

support.

INTERCONNECTIONS

The external cables and connectors are not normally supplied as standard kit, and due to the various customer configurations the customer must request the cable and connector requirements.







Technical Information



Tel: 303-665-8000 Fax: 303-763-9900




TECHNICAL INFORMATION

1. SYNTHESIZER MODULE (980507) a) Power supply.

The single +21.5V dc input at PL4 pin 4 is routed via diode D25 to capacitor C68, which acts as a dc storage mechanism that allows the synthesizer module to remain temporarily operational during power dips in the final system. The dc voltage is then regulated via IC14

(+15V) and IC15 (+5V) before distribution to the other circuits.

Led D28 indicates +15V supply on.

A voltage divider is formed via TR6, (+7V.) which is used to provide the a/c ground inputs to the operational amplifiers. b) Synthesizer circuit. (refer to circuit diagram)

The reference for the synthesized operating frequency is obtained from a 10MHz TCXO,

(IC3).

The frequency output is then divided down by 6400 via a fixed divider (IC2), which provides the comparison frequency for the PLL (Phase lock loop) of 1562.5Hz.

The frequency programming is obtained by another divider (IC1), which derives the comparison frequency (1562.5Hz) by a division ratio, set by Rotary BCD switches SW1 to

SW4. A pre scalar (IC12) is used to divide the operating frequency by 64, which is used as the input for IC1.

The synthesizer can be switched in steps of 100kHz from 87.5MHz to 108MHz.

Provision is made for external operation of the switching process via PL2, which is activated by connecting pin 14 and 15 on PL2 which disables the rotary BCD switches via diode

D12. This in turn switches on transistor TR1 that provides the logic 1 signal for the external data on pin 12 of PL2. Led D38 provides indication of remote operation.

Provision is also made for a thumb wheel switch option and connector PL1 is available for this purpose.

IC8/c and IC8/d are used to provide correct startup of dividers at switch on. c) PLL (Phase lock loop)

Comparison of the above 1562.5Hz signals is done by IC9, which provides an tri-state output on pin 13, depending on the phase relationship between the two signals. This output is then

integrated by IC10/a and the dc output is then filtered by a Low pass filter IC10/b,

IC17/a and IC17/b. The filter cuts off above 7Hz and provides

≥

40dB attenuation above

25Hz. The loop delay of the filter is

≤

50mS, which allows for fast correction of VCO disturbances.

The filtered dc output is now used to control the VCO (Voltage controlled oscillator) which is separately screened on the module. The filtered dc input above is buffered by TR3 (which is a low noise transistor) and the output is used to drive the varactor diodes D20, D21,

D31 and D22. This arrangement provides very high linearity and deviation flatness. The


FM-250 FM BROADCASTING EXCITER input sensitivity of the VCO is approximately 2.2MHz/Volt, with a tuning voltage range between 3 and 12 volts

The Fet oscillator TR4 is tuned to the operating frequency via L2 and the above varactor diodes.

The output from the oscillator is split and attenuated to the separately screened Pre- scalar

(IC12) and a RF output buffer IC11, which provides the final RF output to SK1 (Typically

+10dBm). d) Out of Band detection

The synthesizer has a feature to prevent accidental or deliberate operation outside the standard FM Broadcasting band. The circuit operates by continuous monitoring of the frequency programming lines at the synthesizer programmable divider IC1.

IC4/d, IC5/b, IC5/c, IC5/a, IC7/b, IC7/c, IC6/d and IC5/d, provide the data required detecting the condition of

≤

87.5MHz.

IC4/a, IC6/a, IC6/b, IC6/c, IC4/c, IC7/a, IC4/b and IC8/a, provide the data required detecting the condition of

≥

108MHz.

The signals are finally gated by diodes D16 and D15 which are input to IC8/b pin 5, which will force the Lock led, (D18) off This will also inhibit the RF output at SK1 via TR5, which removes the dc supply to IC11 (RF output buffer). The RF output is attenuated by

≥

40dB. e) Out of Lock detection

In the case of an Out of Lock condition, the Phase detector (IC9 pin 1) will provide an input to IC8/b pin 4 that will switch off the Lock led (D18) and output buffer (IC11) as described above. f) Signal inputs and Phase correction

The MPX input at SK2 (BNC) can be operated in a Balanced or Unbalanced mode by insertion of LK1 (unbalanced). The input impedance is 600 ohms (Unbalanced) or 1.2k ohms (Balanced).

The output from IC13/a is routed via R80 (MPX Adjust) potentiometer, which is used to set the required level for operation.

The summing amplifier IC13/b is used to combine all the inputs including the three unbalanced

Auxiliary inputs on SK3, SK4 and SK5 (BNC's). SK3 and SK4 have trimming potentiometers (R82 and R84) to set their required levels, while SK5 is input directly

(normally from Stereo coder option).

MPX phase correction is now required: Low frequency phase correction (30Hz) is done at

IC16/a via R93 while High frequency phase correction (15kHz) is done at IC16/b via

R116 (All pass filter).

A MPX output is provided for metering purposes at PL4 pin 3 while the direct output is routed to the VCO via R57.

Input is provided for external level control at PL3, which is normally used to adjust the final deviation in the Wideband mode of operation.



The adjustment to control the deviation flatness is provided by R53. This is used to provide constant deviation by adjusting the dc conditions to Varactor diodes, D21 and D22, when the synthesizer frequency is changed.

2. CONTROL MODULE (980508) a) PSU and Internal Monitoring

The Control module has two dc power inputs. The dc (+48V battery) input is connected to

PL8 pin 3, while the rectified and smoothed (a/c supply) input is connected to PL8 pin 1.

The nominal voltage in this case is +28V.

Each of these inputs is monitored for indication purposes via R88 and R89, which drive external Leds on the Exciter front panel. Led D42 indicates voltage applied.

The two, dc voltages are combined via D36 and D37 (providing isolation), to fuse F1

(protection). The voltage is now distributed to various points as follows:

•

IC7 (Switch mode Psu). This provides +21.5V output and has Undervoltage startup protection via TR9 and overvoltage protection (nominally +52V) via IC6/a and TR8.

Overvoltage trip level set via R102.

•

IC9 (Switch mode Psu). This provides -12V output from the +12V supply.

•

IC8 (Linear regulator). This provides +12V output from the 21.5V supply.

•

PL4 (pins 5 and 6). This provides the dc output to the Stereo coder module (option).

•

PL5 (pin 4). This provides the +21.5V dc output to the Synthesizer module.

•

PL6 (pins 24 and 25). This provides the +21.5V dc output to the Display module.

•

IC6/b (-12V Psu sensing). This provides Psu fault indication via TR10/RL6. b) RF Psu Monitoring

PL2, (pins 3 and 4) provide a differential current monitor for the RF PA power supply via

IC4/a. This configuration gives good common mode rejection and has an adjustment (R69) for zeroing the common mode signal. The output is now amplified by IC4/b to give a voltage level proportional to the RF PA Current. (I.e. 1 volt = 1 amp). Level adjustment is via R72

(I Cal). The voltage is then output to PL6 pin 16.

PL2 (pin 1) provides the voltage sensing for the RF PA power supply, and IC5/a detects for low voltage against the +12V reference. The sense output is then distributed to PL6 pin 26. c) Forward RF detection and Alarm

The Forward RF monitoring input is via SK1 (SMB), which provides a RF signal (nominally

0dBm), proportional to the RF forward power of the Exciter. A dc output proportional to the input power is achieved via D1, D2 and IC1/a (The diodes provide temperature compensation). Adjustments are provided for Level (R8) and Offset (R6). The Offset voltage is adjusted for 0.8V nominal with no input signal at SK1. This is done to allow the use of linear metering of the RF Forward power in the Display module. The nominal adjusted dc level is then +5V/OdBm.



IC2/a provides the detection for a Forward RF Alarm, with R10 (FWD Alarm cal) setting the required threshold when the level is low. The output is used to give an Alarm signal via

TR1/RL1. d) Reflected RF detection and Alarm

The Reflected RF monitoring input is via SK2 (SMB), which provides a RF signal (nominally

0dBm), proportional to the RF reflected power of the Exciter. A dc output proportional to the input power is achieved via D5, D6 and IC1/b (The diodes provide temperature compensation). Adjustments are provided for Level (R21) and Offset (R19). The Offset voltage is adjusted for 0.8V nominal with no input signal at SK2. This is done to allow the use of linear metering of the RF Reflected power in the Display module. The nominal adjusted dc level is then +5V/OdBm.

IC2/b provides the detection for a Reflected RF Alarm, with R24 (REFL Alarm cal) setting the required threshold when the level is high. The output is used to give an Alarm signal via

TR2/RL2.

The detected dc output from IC1/b is routed via IC2/c and R32 (REFL Alarm trip), which sets a threshold to activate the Timing circuit at IC2/d. This provides a 10 second re-cycle time in the Exciter if the High reflected power persists. When the output of IC2/d is low the

Reflected Alarm is held on via D7, which latches the Alarm indication during re-cycling via

IC2/b. e) Temperature detection and Alarms

The temperature sensing Thermistor, which is mounted in the RF PA module is input at PL1

(pins 1 and 2). The nominal resistance of the Thermistor is 4700 ohms at 25

°

C. R41 (Temp

cal), is adjusted to obtain a voltage output at IC3/a of 2.5V for 25

°°C. The circuit sensitivity is 10

°°C per Volt.

The Temperature detected output voltage is now routed via IC3/b, TR3 and RL3c. to give an

Alarm indication, and R45 (TEMP Alarm cal) is provided to set the alarm level (normally

50

°

C). The voltage is also routed via IC3/c to provide a fixed trip level at 65

°°C. This output switches off the RF PA Power supply.

f) ALC (Automatic level control)

The ALC output at PL3 pin 2 is derived from IC3/d, which has the following inputs.

•

ALC CONT. This dc input is derived from the Display module via PL6 pin 19.

•

FWD-V. This dc input is derived from IC1/a (Forward power detected dc).

•

REFL-V. This dc input is derived from IC1/b (Reflected power detected dc).

•

TEMP-V. This dc input is derived from IC3/a (Temperature detected dc).

•

CURR-V. This dc input is derived from IC4/b (RF PA current detected dc)

The circuit operates by comparing the set voltage from the ALC Control to the FWD-V voltage derived from the RF Forward power. This is done to correct any change in the


FM-250 FM BROADCASTING EXCITER output power in the Exciter with respect to the Control voltage (The ALC Output is used to control the Gain of the RF PA via the Mosfet gate voltage). This arrangement controls the power output of the RF PA and the other inputs to the circuit are only there for protection purposes, by reducing the output power during fault conditions.

•

LK1 is used to insert protection from High reflected power. (option)

•

LK2 is used to change the standard Temperature back off level from 55

°°C to 60°°C.

•

LK3 is used to insert protection from High RF PA Current. (option) g) Telemetry connection (see circuit diagram)

The Telemetry outputs at PL7 provide voltage free contacts for six functions ,

•

RF Fault. This provides indication of low Forward power.

•

VSWR Fault. This provides indication of High reflected power.

•

TEMP Fault. This provides indication of High temperature at RF PA.

•

PSU Fault. This provides indication of internal PSU fault.

•

STEREO Indication. This provides indication of Stereo Pilot (19kHz)

•

PLL Lock. This provides indication of Synthesizer in Lock.

The Pilot detection is routed from the Display module via PL6 pin 6 and TR5/RL4/c.

The PLL Lock detection is derived from the Synthesizer module via PL5 pin 1 and

TR6/RL5/c.

The Enable input, on PL7 pin 13 requires a ground connection to disable the RF PA. This is done via D29, D23, and TR4 to PL2 pin2.

The Mono/Stereo input, on PL7 pin 14 requires a ground connection for Mono. This is only used when the Stereo coder option is installed.

The +12VS voltage output at PL7 pin 15, is a 12V/20mA current source used for test purposes and external control use.

3. DISPLAY MODULE (980509) a) Psu and Input requirements

The power supply input is connected at PL1 pin 24 and 25. This dc input is nominally +21.5V with the 0V connection at PL1 pin 1 and 2. The -12V dc input is connected to PL1 pin 21.

IC3 provides the internal +12V supply and IC12 provides the +5V.

PL1

PL1 FUNCTION TABLE

Function Comment

PL1 pin 1

PL1 pin 2

PL1 pin 3

0V

0V

MPX Input

Ground connection

Ground connection

MPX metering input

PL1 pin 4

PL1 pin 5

PL1 pin 6

PL1 pin 7

PL1 pin 8

Lock (indication)

Enable (indication)

Pilot (Decoder output)

Pre emphasis (indication)

RF Fault (indication)

Provides data for Led D8


+9V nom. for detection





PL1 pin 9

PL1 pin 10

PL1 pin 11

PL1 pin 12

PL1 pin 13

PL1 pin 14

PL1 pin 15

PL1 pin 16

PL1 pin 17

PL1 pin 18

PL1 pin 19

PL1 pin 20

PL1 pin 21

PL1 pin 22

Reflected Fault (indication)

Temp Fault (indication)

Psu Fault (indication)

O/V Fault (indication)

Ext. dc (indication)

Reflected Voltage (input)

Forward Voltage (input)

Current Voltage (input)

Temp Voltage (input)

ALC Voltage (input)

ALC Control (Set power)

ALC Fault

-12V dc

Remote PLL (indication)





Provides data for Led D11 dc for Refl power meter dc for Fwd power meter dc for Current meter dc for Temp meter dc for ALC meter dc output from R27 dc input for fault detection

-12V dc input


PL1 pin 23

PL1 pin 24

PL1 pin 25

PL1 pin 26

A/C Internal (indication)

+Vs

+Vs

RF Psu Fault (indication)


+21.5V dc input

+21.5V dc input

Provides data for Led D9 b) ALC Meter and alarm

The ALC Voltage at PL1 pin 18 is routed via R17 (ALC meter cal), which is used to calibrate the scale for the ALC Front panel meter. The dc input signal, which is normally between -5V and +3V is offset via R19, which is connected to the +5V supply. This allows the meter to operate on a positive voltage only, at a scale factor of 1V/Led. (9 Leds). The input signal at

IC1 pin 5 (Linear scale bargraph driver) is used to drive the Leds (D13 to D21) of the

ALC Meter. The Bargraph drivers are operated in dot mode but configured to run in bar mode by connecting the Leds in series, this reduces the current consumption by a factor of 9 in this case.

The ALC Fault alarm Led (D22) derives its signal from PL1 pin 20, via IC7/d which operates as a comparator, turning the Fault Led on at a level set by R23. c) Multimeter operation

The Multimeter consists of a 20 Led custom bar display using linear scale Bargraph Drivers,

IC10 and IC11, which operate the Leds D47 to D66 inclusive. D47 is the least significant.

The sensitivity of the driver input is 1.2V dc, for Full-scale deflection. The Bargraph drivers are operated in dot mode but configured to run in bar mode by connecting the Leds in series, this reduces the current consumption by a factor of 10.

A Four-way selector switch using a Push button (SW1) is formed via IC8 and IC9. These operate on an increment function that allows a One in Four selection. IC8 is a counter that counts up from 0 to 3 and resets on the count of 4. IC9 is a dual “One in Four” switch whose outputs are controlled by the data from IC8 (Two lines). IC9 pins 1,5,2 and 4


FM-250 FM BROADCASTING EXCITER operate Leds D46, D45, D44 and D43, which indicate which switch selection is in operation.

The four inputs to the switch are as follows:

•

Temperature. R47 is used to calibrate this input function.

•

Current. R52 is used to calibrate this input while R51 sets the offset null (0 Amps).

•

Forward power. R55 is used to calibrate this input which is converted from a Square law scale to a Linear scale via IC7/b and temperature compensated diode arrangement D78 and D79.

•

Reflected power. R62 is used to calibrate this input which is converted from a Square law scale to a Linear scale via IC7/c and temperature compensated diode arrangement

D80 and D81. d) Stereo decoder

The Stereo decoder input is derived from the MPX input on PL1 pin 3, which is decoded via

IC14 (Stereo decoder chip). The IC provides both Left and Right decoded outputs on pins

11 and 10 respectively. The IC operates from a 456kHz ceramic resonator, which provides very good stability while requiring no adjustments. Stereo separation of 40 dB (Typical) is obtained.

The 19kHz Pilot detection is obtained via pin 18 of the Stereo decoder chip, and is a open collector output (open when pilot detected). This output is inverted by TR3 to activate Led

D67 (Pilot) and is also routed to the MPX VU Meter via TR4/LK1. The detected pilot signal is also routed via D83 to PL1 pin 6.

The Left and Right outputs are then filtered by Fourth order Low pass networks IC13/a,

IC13/b, IC13/c and IC13/d, which remove the 19kHz and 38kHz products produced in the Decoding process and prevent errors in the VU meter operation. e) VU Meters and peak detection circuits.

The Left and Right outputs from the Stereo decoder are routed via R28 (Left meter level) and R37 (Right meter level) which are used to calibrate the VU meters.

Two identical circuits are used to convert the incoming audio signals to a peak dc level. This is done by IC4/a, IC4/b, IC4/c and IC4/d. Taking one of the sections only for description purposes the circuit operates as a precision full wave peak detector which has a very fast attack time (1.7mS) and decay time (650mS), conforming to the DIN 45406 Specification.

This calls for a response 1dB down from steady state for a 10mS tone burst and 4dB down for a 3mS-tone burst. The decay time is 20dB in 1.5 sec’s.

The VU meters operate on a standard VU scale and the bargraph driver used to do this is calibrated internally to achieve this. IC5 and IC6 form the Left and Right bargraph drivers, which are operated in dot mode but configured in bar mode by connecting the Leds in series.

This is done to reduce current consumption. The sensitivity of the bargraph driver is 1.2V.

Leds D23 to D32 form the display for the Left channel with Leds D33 to D42 for the Right channel.

The Right VU meter bargraph driver has a inhibit function at the input. This is provided such that when the unit is operated in Mono the customer can choose to have the Left meter



operational only (LK1 in position B) or when operated in stereo, if the pilot signal fails then the Right meter could be inhibited automatically (LK1 in position A). This provides the customer with excellent visual status of the real time program. f) RS 485 Option

The display module has provision via PL2 to connect a RS485 module. The purpose of this is to provide remote viewing of the display meter functions . The +21.5V internal dc is routed via a fuse (F1) and is connected to pin 20 of PL2. Provision is made for an indication of the RS485 module connection via Led D85.

The monitored parameters are:

Parameter PL2 Connection Comment

PA Current

Reflected power

Forward power pin 1 pin 3 pin 5

0 to 1.2Vdc voltage output



Left VU meter

Right VU meter

Temperature pin 7 pin 9 pin 11




ALC meter

Pilot status pin 13 pin 15

-5 to +3Vdc voltage output

+0.7V for pilot/0V no pilot

These outputs are routed to the Analog to Digital converters in the RS485 module and the meter scales and alarms are calibrated and set in the software.

4. RF PA MODULE (980510) a) Psu and Regulator

The RF PA requires two dc-input supplies:

•

The +VR supply is the main supply for the PA and is typically between +50V and +60V.

This supply is connected to the main regulator power devices (which are capable of supplying up to 10 Amps current) and connected to E29 (circuit reference). The output of the regulator is at E33. The incoming dc has a ripple content of approximately 100mV p-p.

•

The Aux. supply (which is approximately 2.5V higher than the main supply) is connected to TR3 (via PL4) which operates as a 7mA constant current source, whose current output is supplied to the main regulator devices at E30, E31 and E32. Zener diode D7 controls the maximum voltage at the regulator output to 52V. Filter capacitor C31 ensures that no a/c ripple appears at the regulator output.



The main regulator can be disabled via TR4 / D10 which effectively shunts the constant current source to ground thus removing the output voltage. TR4 is switched via an external input from the control module.

Resistor R35 is used to monitor the current in the RF PA section and provision is made for external monitoring via R37, R38 and R39. Led D13 provides a visual indication of the presence of the regulated supply.

The regulator has the ability to operate at low dc voltages, as the a/c ripple content is always very low in level. This gives a large advantage to the PA operation as there is no AM produced even with a very low a/c mains supply. b) RF Amplifier stages

The RF amplifier comprises a three-stage amplification block. The first stage (Bipolar) amplifies the RF input (E1, E2) from 10dBm to 20dBm via IC1 (Wideband amplifier). The

second stage (Mosfet) amplifies the RF signal to approximately 5 watts via TR1

(Wideband amplifier). TR1 has a potentiometer R6 to adjust the device gate voltage for optimum operation which requires a idling current of about 100mA (No RF input).

The final stage (TR2) is operated in push-pull mode via a 3:1 wideband-input transformer

T1 and matching trimmer capacitor C8. Resistor R11 is used to stabilize the dynamic impedance of the Mosfet input. The output wideband transformer (1:2) T2 has a centre tap in which the dc input is applied. This provides optimum balance in the push-pull mode of operation and ac de-coupling is provided via L2 and associated components. R25 is provided to set the gate voltage for the output mosfet, which is operated in Class C. (See manufacturers data sheet for individual gate voltage settings). D.C. blocking capacitors C41 and C42 are provided to remove the dc component. c) RF Harmonic filter

The output from the final RF stage is routed via a 50 ohm 1/8 wavelength Balun to the RF filter stage. The filter used is a Hybrid ninth order type utilizing two notches. The first notch is designed to eliminate the third harmonic source current (by providing high impedance) from the final mosfet and is centered at 185MHz. The filter ripple is less than 0.1dB (0 to

120MHz) and reduces all harmonics to

≤≤ -70dBc. d) RF Couplers

Three microstripline couplers are available to provide Forward, Reflected and Monitoring

RF signals, two of which are used for control and protection within the Exciter. The 150p capacitors at the coupler outputs are required to ensure a flat RF response. A TAB capacitor is provided on the Reflected coupler to optimise the Directivity. The RF output at

E21 and E22 is connected externally to a 50 ohm N Type connector. e) Temperature sensor.

A Thermistor is provided (R27) at the base of the Final output device TR2, which is used to monitor the Heatsink temperature and whose output is connected externally.



5. PSU MODULE (980515) a) A/C Supply

The a/c input to the power supply is provided by a external 48V a/c Transformer and Full wave

Bridge rectifier which is connected to SK1 pin 1 (+) and SK4 pin 1 (0V). This raw dc input which is indicated by Led D2 is smoothed by capacitors C1, C2, C3 and C4. (4 x 4700uF) which is then routed via SK2 pin 1 through an external Inductor (15mH) and back to the

Psu Pcb at SK2 pin 3. This smoothed dc supply is then filtered by C5 (4700uF) to eliminate all ripple components. This constitutes the main dc supply from the a/c mains and supplies approximately 8A. b) +48V dc supply (battery)

The +48 dc battery supply input is connected to SK1 pin 3 (+) and is indicated by Led D1.

The 0V-battery return is connected to SK4 pin 3. c) RF PA Main supply

The above supplies in a) and b) are combined via SK3 pins 1 and 3 to an external Diode bridge to provide isolation between the supplies. This output is used to supply the main RF

PA. Fuse F1 (10A) provides protection on the a/c supply. d) RF PA Aux. supply

The battery supply and raw dc supply are combined via D4 and D5 (for isolation) to provide the auxiliary supply to the RF PA via R5 and PL4 pin 1. This supply is smoothed by C6 to reduce the a/c ripple. e) Control supply

The battery supply and raw dc supply are connected to PL1 pin 3 and pin 1 respectively via diodes D6, D7, D8 via F2/D3 to power the Control, Display, Synthesizer and Stereo coder modules. The dc return 0V is connected to PL1 pin 2. f) Fan supply

The combined supply at PL2 pin 1 is connected to power the built in fans at the RF PA assembly. The dc return 0V is connected to PL2 pin 2. The current requirement of the fans is approximately 600mA.

6. FAN PSU MODULE (980518) a) Power supply

The combined dc supply from the PSU Module is input at SK1 and is regulated by a switch mode power supply IC1 which supplies approximately +23V to the Fans via connectors

SK2 (+) and SK3 (0V).

Four fans are used to cool the RF PA assembly and are industry standard, +24V dc type.

80x80mm.



7. STEREO CODER MODULE (980512) a) Psu and distribution

The dc input is supplied via PL4 pins 5 and 6, and is typically between 24V to 40V dc. The

ground is connected to PL4 pins 1 and 2.

The incoming dc is routed via diode D7 that is provided for protection against negative voltages.

The dc is then applied to IC26, which is a switch mode regulator providing a dc output of

16V and indicated by Led D10. The regulator is prevented from switching on, (until the dc input reaches approximately 16V), by TR2 and D8.

The +16V supply is applied to IC27 to obtain a +12V stabilized output which is the connected to IC28 via D11 to obtain a -12V output. (This is also a switch mode supply). L3 is incorporated to remove any switching spikes from the -12V switch mode supply before distribution to other circuits.

The

±

12V dc supplies are distributed to Linear regulators IC14 (+5V) and IC15 (-5V) and are indicated by Leds D2 and D3 respectively. These supplies are required by the HCMOS digital circuitry. b) Audio Input circuits

(The Left and Right input circuits are identical and only one (Left) needs to be

described).

The audio input at PL1 can be balanced or unbalanced with an input impedance, which is nominally 600 ohms . Capacitors C1 and C2 are provided to remove any RF signals. IC1/a is configured as a differential amplifier, which is required to remove any common mode signal when the input is operated in balanced mode. The input circuitry has the capacity to operate up to 20V p-p (+19dBm).

Trimmers R9 (Left) and R37 (Right) are provided to allow for level adjustment to customer requirements which generally vary from -6dBm to +9dBm.

Note: This is the main adjustment in the final Exciter.

IC1/b is a buffer amplifier with a gain of +6dB. c) Audio Limiter

The buffer outputs above are connected to op-amps IC3/a and IC3/b whose gains are controlled via IC4 (Limiter gain cell). The cell operates by monitoring the outputs of IC3/a and IC3/b and compares them to a preset level set by R21 (Limiter thre shold adjust) at

IC5. Both Left and Right signals are monitored and if either or both signals exceed the preset threshold (which is equally positive and negative), the comparator IC5 switches on

(open collector output). This activates TR1 and charges C16, increasing the dc level into

pins 1 and 16 of IC4. This in turn reduces the gain of IC3/a and IC3/b via their negative feedback loops. The output from the comparator IC5 is a narrow pulse train and if the

Limiter circuit is totally overloaded or faulty, the pulses become much wider and start to illuminate Led D1. The nominal input to the Limiter is 3.1V p-p.

The attack time of the Limiter is approximately 1mS and decay time, 100mS. This is achieved by TR1/C16, which is essentially a peak detection circuit, which is discharged slowly by the high impedance into IC4 pins 1 and 16.


FM-250 FM BROADCASTING EXCITER d) High pass filter

The output from the Limiter circuit is connected to IC20/a and IC23/a, which form the High pass filters, removing any low frequencies below 10Hz. This is done to remove any dc switching transients contained in the incoming audio program. e) Pre emphasis

The outputs of the High pass filters are routed to the pre emphasis circuits IC20/b and IC23/b which have a time constant of 50uS (LK2 and LK3 In) or 75uS (Links out). The pre emphasis is selected by operating Relay RL2, which requires an external s/c between PL3

pins 1 and 2. R104 and R129 are provided for setting the required input level to the

Stereo coder section (see stereo coding section). f) 15kHz Low pass filters

The Left and Right 15kHz filters are identical and only the Left will be discussed.

The filter has three distinct sections and was designed for minimum ripple and group delay.

•

The first section, IC21/a and IC21/d, is configured as a notch filter centered at 23.8kHz.

The notch frequency can be adjusted by R108.

•

The second section, IC21/b and IC21/c, is configured as a notch filter centered at 19kHz.

The notch frequency can be adjusted by R113.

•

The third section, IC22/a and IC22/b is configured as a fifth order 19kHz low pass filter.

Adjustment is provided by C101.

The filter is adjusted by monitoring TP4, which is the output of a differential amplifier IC29, with very high common mode rejection. This unique feature enables very simple set up of the

15kHz filters by allowing the Left or Right filter to be measured individually and also allowing simple differential measurement with both Left and Right together. This is very important when setting up the Mono into Stereo calibration in the final Exciter.

The adjustment is made simple by setting the notches as described and adjusting the trimming capacitor for a flat response at 15kHz (with respect to 1kHz). The response of the filter should be as follows when adjusted:

1kHz

0dBm

8kHz

+0.15dBm

12kHz

+0.1dBm

15kHz

0dBm

19kHz

-40dBm

≥

23kHz

-40dBm

The amplitude variance of this filter is very small and should not present any problems in set up, unlike other filters found in competitive equipment. g) Stereo coding and switching

IC6/a and crystal XL1 form the reference oscillator. Fine frequency adjustment is via C38. The oscillator operates at 4.256MHz and is buffered by IC6/b.

The square wave output is then divided by 8 (via IC7/a) to give a frequency of 532kHz.

The 532kHz clock signal is applied to IC8/a and IC8/b (dual 4 bit shift register) which forms a

14 step multiplexer via IC9 and IC10/a, allowing a count of 7 (532khz/7 = 76kHz). Each


FM-250 FM BROADCASTING EXCITER output has a pulse repetition frequency of 76kHz with a 1.879uS pulse. The outputs of IC9 are routed to a Quad bilateral switch (IC11) which with precision weighting resistors (R53

to R60), a 14 step replica of a 38kHz sinusoid is produced. This has a distinct advantage over direct switching at 38kHz, as any spurious products produced appear at frequencies

532kHz and above and are easy to filter.

The output of IC8 pin 11 is routed via IC13/a, which produces a 38kHz square wave at its outputs. These outputs are applied to another Quad bilateral switch IC12 which alternately switches the Left and Right audio signals at the inputs of this switch at 38kHz These signals are applied to the weighting resistors above.

The output of IC13/a pin 5 is routed via resistors R51 and R52. (R51 is used to vary the phase). This 38kHz signal is divided by 2 (via IC7/b), whose output is at 19kHz. This signal is then routed to the 19kHz bandpass filters.

The stereo coded signal produced has virtually no commutation products due to the use of High speed CMOS switching and the final output from the weighting resistors is routed to IC17 via R73. h) 19kHz Bandpass filters

The 19kHz square wave above is routed to IC16/a and IC16/b, which are configured as bandpass filters centered at 19kHz. These filters convert the 19kHz square wave to a sinusoid with

≤

0.1% distortion. R63 and R67 adjust the filters for maximum amplitude at

19kHz and the output from IC16/b is routed to a SMB connector SK1. This is for external use and isolation is provided via R69.

The signal is also routed to IC17 to provide the 19kHz pilot signal in the final Composite MPX output. The pilot level is adjusted via R70. i) MPX Filter

The output from IC17 buffer is routed to a fifth order Linear phase filter (IC18/a and

IC18/b) which provides

≥≥ 50dB attenuation above 532kHz. The required filter Passband

(53kHz) has a flatness characteristic of

≤≤ 0.02dB with a phase variance of ≤≤ 0.1°°. Allpass filter IC19/a provides a Fine phase adjustment of the stereo multiplex signal at 15kHz. The total combination exhibits a group delay variance of

≤≤ 10nS. j) MPX output buffer

R91 (MPX Level) and R96 (Mono Level) provide adjustment for their respective functions and RL1 provides the switching mechanism for Mono or Stereo. The final signal is routed via buffer IC19/b to the output at SK2 (SMB connector).





Maintenance Information



Tel: 303-665-8000 Fax: 303-763-9900

















































































Larcan FM-250 Technical Manual

FM-250

250W STEREO

TRANSMITTER/EXCITER

TECHNICAL MANUAL

Please take note that any reference to 230V and 50Hz in this manual should be read as 110V and 60Hz respectively.

CONTENTS:

General Description

Technical Specifications

Operating Information

Installation Information

Technical Information

Maintenance information

General Description

Technical Specifications

Operating Information

Installation Information

Technical Information

Maintenance Information

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