Crown Broadcast FM10X Manual

Crown Broadcast FM10X Manual
Ten-Watt FM Broadcast Transmitter
Model FM10X
Technical Manual
Copyright © 1991
by Michael P. Axman
Elkhart, Indiana, USA
All Rights Reserved
Printed in U.S.A.
K80456—5
Table of Contents
T.0=INTRODUGTION ......:cimisousomvsoasassvsnssasssssss 4
2.0 - SPEGIFIGATIONS : : 2 5 5 à à à à @ à à à à 4 1 6 à di à à @ 800 6 06 (as d'A Na 00 5
3.0 = DESCRIPTION [email protected]@ sn uses esties pie pau IN Ua MENÉS RES UC N 6
3.1 PRONT PAÑEL ooo v5 mem means sav ammeas dns esc dsss sass 6
3.1.1 AUDIO INPUT CISDIAY | 5: vw isi 6s wa 5 à à 6 8 6 as 8 0 8 5 50 6 io 6 ко а 6
3.1.2 INPUT GAIN си СЛОВ. - « 5: os wc wis sot vio ba % 6 ais re 8 оао аа 6
3.1.3 HIGH BAND and BROAD BAND display. ......... iver. 6
3.1.4 PROCESSING control : à @ à a: + à à 6 0 ба 0 68 6 o % 0 3 90 5 4 3 68 6 6% 5 G 6
3.1.5 Four-digit multimeter 5: « uv: v5 5: 6 56 6 8 tiv 4 € d'a 4 0 0 50 is 5506 5 56 8 @ 18) § 7
3.1.0 FAULT indicators. « =:» » 46037606 648 win 6 ob 8 a0 0 4 6 5 6 67 © 6 068 § 6 00 & 7
3.1.7 STEREO:-MONO switch vo «5s «ais ao a 5 06 oie 660 8 5 @ oo fai 8 65% & aa a 7
3.1.6 NORMAI-TEST switch 2.220054 600000 0 00 8 wis & 8560 8 o 8% 8 406 8 7
2.1.9 MODULATICON display 46 5 400 va 0 5 win 5 000 5 600 4 50 8 км вЫ бобов 7
3400 CARRIER SWR! is von sow 5 900 об ром & oie § 955 6 3 95 5 8 5° 8 8 § 6 0 b 7
31:11 РОМАН БМ 2 250 à srs on 5 00% 5 Slaps 5 рб М ко 97 6 8 160 в вот коня 8 7
3.2 REAR PANEL . . oii i ee et ee ee eee 8
3.2.1 AC Power receptacle module .......cooordeeresrarece na rare 8
3.2.2 EXTERNAL RF AMPLIFIER connector ........eoom)eeararoa ran. 8
3.2.3 AF MONITOR jacks, RIGHT 6 LEFT — aaa aaa e 8
22.4 SGA INPUT à à us à à ui 6 à 0 € 4 0 à 0 765 à à ré 6 A 07 6 8 AAN AE NA O e E 8
3.2.5 Audio INPUT, RIGHT & LEFT iii it eit ee eee a 8
3.2.0 ВР OUTPUT connector :.:.... ::. wire 6 160 6 a oo» 5% 260 40 50% at % 80 a = 8
4.0 - INSTALLATION ee ee a a aa 4 à 9
4.1 ACLINE VOLTAGE SETTING 2212 5 4% à a 6 à nu 307 8 0 6 6 68 9 sd 00 8 0 & 5 %i 5 à 9
4.2 FREQUENCY SETTING «саван сз sms suds ass amass son sa sess 9
4.2.1 RF Frequency Synthesizer . ........ 4044404 44 4 4 4 4 4 enn. 9
4.2.2 Ten-watt RF amplifier , «i « ss 06 a nev ams ao 0% vo min sw a smisneis 9
4.3 RF POWER AMPLIFIER INTERFACE : . à 2 à 40 6400 44 6 60 0 4 0 6 0 0 va à 10
4.4 AUDIO INPUT connections. $5 а + à + ‘4 à à à à 010 6 0e en dd CR SU CUT 6 10
4,5 ARFOUTPUT CONNEOCTIONS. ; 4 € à à à #6 407 à 40 @ 40e @ 8 4 9 € à i608 5 58 в а ав 10
S.U = OPERATION oc :oiva nes wos mes meso owes ions ed msessns 11
5.1 BETUP. i vss wanes moans ise me ami isn $6585 58585 wed ds 11
5.2 NORMAL OPERATION + ui sons vm omns 407 € 9 € mss ses ss mes soos 11
9.2:1 AUDIO INPUT "indicators 5 « soo 65 6 woo mas mu ETES REVUE 11
5.2.2 BROAD BAND indicator . . « io eco sia waa im som esse ens eos ins 12
5.23 HIGH BAND Indicator à 0: 6 à 00 am cam aamas ео сока ев ва ей 12
H.2.4 PROCESSING CcONTtrol а в: коз кока кв с ива жены EEE LANE 12
5.2.5. Digital Multimoeter os re WR WELLEN WEN ENG 12
5.2.6 FAULT indicators. . . . «vv i vi tt te ee te ee ee ee eee ee ee 13
5.2.7 STEREO-MONO switch. . ..... iii 4 4 4 4 4 4 4 4 4 4 4 a 4 0 14
5.2.8 NORMAL-TEST switch. 2220002044 444 4 4 4 4 4 4 4 4 4 4 4 4 4 4 40 14
5.2.9 MODULATION display. ..........e.oee 0. 4 A A 4 A 4 4 1 4 4 1 4 4 40 14
5.2.10 CARRIER switch. 222020404044 444 4 4 4 4 4 4 4 4 4 4 4 4 4 4 a 4 440 14
5.2.11 POWER switch 40240444 RER 4 4 naaa oa 14
6.0 - CIRCUIT DESCRIPTION .......1012010 0111 LL at 15
6.1 AUDIO PROCESSOR .....x......0.0sracau ran nava nene adaraa ea 15
6.2 STEREO GENERATOR ...........00drxvarednaodooaordrarvordo 16
6.3 FREQUENCY SYNTHESIZER ...........e0Ñ,docoeoedooederncoerev ran 17
6.4 ALC - METERING ....exr.se..reedra rea remraoon0dndom aun ena 18
6.5 POWER SUPPLY 1122200004 44 4 4 4 4 4 4 4 4 4 4 4 4 A 4 4 4 4 4 4 4 A 4 aa 0 20
6.6 DISPLAY vs ir moors wim 5 300m 5 000 #0 101 4 ca die) a ca RE CAD CON 6 SE M A TN 6 A 0 4 й чина 20
6.7 TWO-WATT RF AMPLIFIER 2420044 44 4 4 4 4 4 4 4 4 1 4 4 4 4 4 440 21
6.8 TEN-WATT RF AMPLIFIER 2020004 044 4 4 4 4 4 4 4 4 A A 4 A 4 eee as 22
6.9 DB15 INPUT FILTER 1.200000 04 0 4 4 4 4 4 A 4 4 A 4 4 A 4 4 A 4 A A 4 A 44 40 22
6.10 Auxiliary ALC 210000 114 4 4 4 4 4 4 4 4 4 4 A 4 4 4 4 4 4 4 A 4 A 4 a aa V4 à 22
7.0 - INTERNAL ADJUSTMENTS .......... i, 23
7.1 AUDIO PROCESSOR . oti ee ee ete ee eee ee eee, 23
7.1.1 PRE-EMPHASIS SELECTION U 4 4 4 a a a à 23
7.1.2 PRE-EMPHASIS ADJUST 1.122002 44 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 a aa 0 23
7.2 STEREO GENERATOR RR a 4 VA 23
7.2.1 SCA LEVEL ott 4 A 4 4 4 4 4 4 4 4 A 4 A 4 4 4 a aa à 23
7.2.2 L-RLEVEL ....eñc—rÑecreorecsooooreorervoao oa rareaonea da ee 24
7.2.3 19-kHz LEVEL .........o0Fenxrrdroaoa da dodo nadraredoeaa 24
7.2.4 19-kHz PHASE ..........0.090000drerecesredredeereraca 24
7.2.5 COMPOSITE LEVEL ........ e... 20dr0d0dAoasaddadvadretra.o 24
7.2.5.1 Using modulation monitor. .........oeecenecrecoercedr 24
7.2.5.2 Using Bessel nulls. . .......... 008 4 4 4 A 4 4e a 4 a à 25
7.3 FREQUENCY SYNTHESIZER .....024 20204 4 4 4 4 4 4 4 eee eee eens 26
7.3.1 FREQUENCY (CHANNEL) selection. ........202420202 44444 11440 26
7.3.2 MODULATION compensator . 0.000 1 4 a a 1 aa a à 27
7.3.3 MODULATION MEASUREMENT and adjustment. .............. 27
7,4 ALC - METERING 0220021444 4 A 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 a eee 28
7.4.1 POWER CALIBRATE 12044404 44 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 44 40 28
7.4.2 POWER SET 1.222001 4 144 2 44 4 4 4 4 4 4 4 4 4 4 4 4 dea dao 28
7.4.3 SWR CALIBRATE 1.222000 2 04 4 4 4 4 4 4 4 4 A AA A A 4 Ra AAA 28
7.4.4 PA CURRENT LIMIT .. 12020214 14 4 4 A 4 4 4 4 A 4 4 A 4 4 4 a A a Va 29
7.5 DISPLAY- Modulation calibrate . .......... vivir nnn 29
7.6 TEN-WATT RF AMPLIFIER 1202440444 41 4 4 4 4 4 a 4 ee ee eee 29
7.6.1 BIAS SET ........e.0eceereredam area 4 4 4 A 4 неее, 30
8.0 - PERFORMANCE VERIFICATION ........................ 31
8.0.1 Audio proof-of-performance measurements. . ............o.... 31
8.0.2 De-emphasis input network. . . «vv vv vt tt ee ee ee ee eee eas 32
8.1 CARRIER FREQUENCY . . . i titi tte et ee eee ee eee ee 32
8.2 OUTPUT POWER 1.122004 00 4 4 4 4 4 4 4 4 A 4 4 4 4 4 4 4 4 4 4 A a a 440 32
8.3 ВЕ ВАМО\/МОТН ап ВЕ НАВМОМЮС5 ............. 2.2... 2..2... 32
8.4 PILOT FREQUENCY .........oeeesvdracoararoanooa deere na 33
8.5 AUDIO FREQUENCY RESPONSE ..............s00.0d000ddresaa 33
8.6 AUDIO DISTORTION ....12020 100002 14 4 4 4 4 4 4 4 4 4 4 A 4 4 4 4 4 4 4 4 4 40 33
8.7 MODULATION PERCENTAGE er 214 114 44 4 4 4 4 a 4 A a a a 4 VA 34
8.8 FM and AM NOISE ........... 02022 202vao0adedoavaoordoara 34
8.9 STEREO SEPARATION ......... ..oes6erreadowarederdoerdrera 34
8.10 CROSS TALK ........n...srddreeramendiandondonara recada 34
8.10.1 MAIN-CHANNEL INTO SUB ........e.e.emesorameerernenro 34
8.10.2 SUB-CHANNEL INTO MAIN 22221111 te ee eee eee eee, 34
8.11 38-KHz SUBCARRIER SUPPRESSION ................0ds0crdia, 35
8.12 ADDITIONAL PERFORMANCE CHECKS ...........e0 remar 35
9.0 - DRAWINGS
9.0.1 FM10X FM TRANSMITTER
9.1 AUDIO PROCESSOR
9.2 STEREO GENERATOR
9.3 FREQUENCY SYNTHESIZER
9.4 ALC - METERING
9.5 POWER SUPPLY
9.6 DISPLAY
9.7 TWO-WATT RF AMPLIFIER
9.8 TEN-WATT RF AMPLIFIER
9.9 DB15 INPUT FILTER
9.10 TOP CHASSIS WIRING
9.11 BOTTOM CHASSIS WIRING
9.12 AUXILARY ALC
1.0 - INTRODUCTION
The FM10X is a ten-watt FM stereo broadcast transmitter. Development of the
FM10X grew out of a need to provide quality, easily transportable FM broadcast
stations, that could be set up and operated anywhere in the world. The result is a
unit that integrates audio processing, stereo generation, and RF generation into one
compact, rugged, and inexpensive unit.
The following design criteria were employed:
® Integrate audio processing, stereo generation, and RF generation without
compromising signal quality.
® Build in metering and diagnostic capabilities.
® Minimize internal adjustments. (Where trim pots are used, in most cases they
provide only fine adjustment.)
© Insure long-term stability and reliability in harsh environments.
® Provide for selection of operating frequency on the field.
e Setup and verification of operating parameters with the minimum of test
equipment.
® Provide technical standards commensurate with the state of the art and comply
with government standards worldwide.
In the interest of providing long-term stability and reliability, the FM10X employs
precision laser-trimmed analog multipliers, digitally-synthesized subcarriers, and
crystal-controlled switched-capacitor filters.
An audio processing formula that integrates short-term expansion with the
conventional short- and long-term gain reduction enhances program loudness
without degrading quality.
A special test point facilitates the use of an inexpensive short-wave receiver to
make precise measurements of deviation (using Bessel nulls) and operating
frequency.
The FM10X is small enough to carry in a suitcase. With the addition of an audio
source---mixer, tape deck, satellite receiver, etc.--- and an antenna, the FM10X is a
complete ten-watt FM stereo broadcast station, capable of serving a town or
village. Provision is made for the metering and control of an external RF amplifier
for wider coverage.
FREQUENCY RANGE:
FREQUENCY STABILITY:
RF OUTPUT:
RF SPURIOUS PRODUCTS:
AC POWER:
AUDIO RESPONSE:
With processor,
Without processor,
HARMONIC DISTORTION:
With processor,
Without processor,
STEREO SEPARATION:
FM & AM NOISE:
~ AUDIO INPUT:
SCA INPUT LEVEL:
SIZE
WEIGHT
2.0 - SPECIFICATIONS
87 - 108 MHz
Better than 1000 Hz from O to 50 degrees C.
10 watts (VSWR 1.5:1 or better.)
Better than -65 dB.
120 or 240 volts AC, 50-60 Hz
Conforms to 75 uS pre-emphasis curve as follows:
50Hz-10kHz, + 0.25, -1dB
50Hz-15kHz +0.25, -3dB
50Hz - 15kHz , +/- 0.25dB.
50Hz - 15kHz, THD plus noise
less than 0.5%
less than 0.2%.
Better than 40dB
Down better than 62dB
Nominal -10dbm to + 10dbm into 50k balanced
60-100KHz, 1V RMS for 7.5 KHz deviation.
3.5 in. high x 16.5 in. wide x 15 in. deep.
(Exclusive of rack adapters)
17 Ib, 6 oz.
3.0 - DESCRIPTION
The FM10X is a complete 10-watt FM broadcast transmitter contained in a 3.5" x
16.5" x 15" package. The unit has built-in audio processing, stereo generation, RF
frequency synthesizer, and digital multimeter.
Audio levels, processing, and other operating parameters are shown on front panel
dot- and bar-graph displays, along with a four-digit numeric display.
3.1 FRONT PANEL
3.1.1 AUDIO INPUT display
10-segment graphic bar-dot display of relative left and right audio processor
input level. (After the input gain-set attenuator.)
3.1.2 INPUT GAIN switches.
" + 6db" and " + 12db" slide switches set audio input sensitivity.
Switches Nominal Input
+6 DB + 12DB Sensitivity
DOWN DOWN +10 DBm
UP DOWN +4DBm
| UP UP -2dbm
| UP UP -8DBm
3.1.3 HIGH BAND and BROAD BAND display.
During audio processing, indicate amount of gain control for broadband and
pre-emphasized audio, respectively.
3.1.4 PROCESSING control
Sets depth of audio processing. See 5.2.4
(3.0 - DESCRIPTION - continued)
3.1.5 Four-digit multimeter
Pushbuttons select one of eight parameters to be displayed on the four-digit
multimeter. Green LED indicates parameter selected.
3.1.6 FAULT indicators
RF drive and PA supply voltage are reduced when a fault condition exists.
See 5.2.6 for a description of faults. [Note: Some units manufactured
before May 1992 did not have PA supply voltage control.] See 6.10
3.1.7 STEREO-MONO switch
STEREO activates stereo generator. In MONO, only left channel audio is
used to provide 100% modulation.
3.1.8 NORMAL-TEST switch
NORMAL, broadband level threshold 2db below high-band threshold.
TEST, used in proof-of-performance testing, sets the thresholds the same.
(Operating in TEST mode will degrade the high frequency audio response of
most program material.)
3.1.9 MODULATION display
Indicates modulation percentage. See 5.2.7.
3.1.10 CARRIER switch
Switches the 24-volt DC supply to the internal 10-watt RF amplifier and
provides 24-volt control signal to 15-pin connector on the rear panel to
control the power supply in an external RF power amplifier.
3.1.11 POWER switch
Main power switch for the unit.
(Section 3 - DESCRIPTION - continued)
3.2 REAR PANEL
3.2.1 AC Power receptacle module
Input for AC power. Module also has fuse and 120/240V switch. See 4.1.
3.2.2 EXTERNAL RF AMPLIFIER connector
Control and metering lines for external RF amplifier. See 4.2.
3.2.3 AF MONITOR jacks, RIGHT & LEFT
Processed, de-emphasized (75usec.) sample of stereo generator input
signals. Suitable for feeding a studio monitor, and for doing audio testing.
3.2.4 SCA INPUT
An external SCA generator may be connected to the SCA IN connector on
the rear panel. The input is intended for the 60- to 99-KHz range, but a
lower frequency may be used if the FM10X is operated in the MONO mode.
(The band of 23 to 53 KHz is used for stereo transmission.) The subcarrier
input should be a nominal 1 volt RMS. The input sensitivity may be set
using the SCA trim-pot on the STEREO GENERATOR circuit board. See
1:2. 1.
3.2.5 Audio INPUT, RIGHT & LEFT
See 4.4
3.2.6 RF OUTPUT connector
Ten watts or less RF output. See 4.5.
4.0 - INSTALLATION
CAUTION - BEFORE OPERATING THE FM10X FOR THE FIRST TIME:
CHECK A.C. LINE VOLTAGE SETTING (see 4.1)
CHECK FREQUENCY SETTING (see 7.3.1)
4.1 AC LINE VOLTAGE SETTING
The FM10X may be set to operate from either a 120-volt or 240-volt AC power
source. To set the input voltage, unplug the AC power cord from the rear of
the unit. On the power receptacle is a plastic slide cover; slide to expose the
cartridge fuse. A small printed-circuit board is plugged in just beneath the fuse.
The orientation of this board determines the line voltage setting. A "120" or
"240" will be visible under the fuse. If a change is called for, use a hook or
long-nose pliers to pull out the card, turn so that the appropriate voltage is
indicated, and reinsert card into the receptacle assembly.
4.2 FREQUENCY SETTING
4.2.1 RF Frequency Synthesizer
Thumbwheel switches for setting the operating frequency of the FM10X are
accessible by removing the top cover of the unit. See 7.3.1 for details.
The MODULATION trim-pot, just to the left of the thumbwheel
frequency-set switches, compensates for slight variations in deviation
sensitivity with frequency for the individual RF exciter boards. The trim-pot
dial should be set according to the Modulation Compensator chart. See
71:3.2.
4.2.2 Ten-watt RF amplifier
Tuning of the ten-watt RF amplifier stage is fairly broad. In many cases it
will not require retuning after a frequency change. If, however, the power
output has dropped off due to a frequency change, repeak the output with
the two trimmer capacitors located on the 10-watt RF amplifier circuit board
- accessible by removing the bottom cover. See 7.6.2 for more detail.
5.0 - OPERATION
CAUTION - BEFORE OPERATING THE FM10X FOR THE FIRST TIME:
CHECK A.C. LINE VOLTAGE SETTING (see 4.1)
CHECK FREQUENCY SETTING (see 7.3.1)
5.1 SETUP
1. CARRIER switch OFF.
2. POWER switch ON.
3. STEREO-MONO to desired mode.
Note: For MONO, feed audio only to the left channel. In MONO, audio is taken
from the left channel, but right channel audio can affect the audio
processing,
4. NORMAL-TEST switch to "NORMAL".
5. Apply program audio at a normal level.
6. Set PROCESSING control to "50."
7. Find experimentally, the combination of INPUT GAIN switch settings that will
bring the BROAD BAND gain-reduction indicator to mid scale for "normal"
level program. The audio processor will accommodate a fairly wide range of
input levels with no degradation of audio quality. See 3.1.2.
5.2 NORMAL OPERATION
5.2.1 AUDIO INPUT indicators
Under normal operating conditions the left and right AUDIO INPUT indicators
will both be active, indicating the relative audio input level. During pauses
in program the red LOW LED will come on.
11
(Section 5 - OPERATION - continued)
5.2.2 BROAD BAND indicator
The BROAD BAND indicator will show short-term "svllabic-rate" expansion
and gain reduction around a long-term (several seconds) average gain set.
The magnitude of the short-term expansion and compression (the rapid left
and right movement of the green light) is determined by the program
material and the setting of the PROCESSING control.
5.2.3 HIGH BAND indicator
Activity of the HIGH BAND indicator is determined by the high-frequency
content of the program. With 75usec. pre-emphasis, HIGH BAND
processing will begin at about 2000 khz and increase as the audio frequency
increases. Some programs, especially speech, will show no activity; for
some music there will be lots.
5.2.4 PROCESSING control
The PROCESSING setting will be determined by a combination of program
material and personal taste. In general, mid-range (40 to 70) should be
satisfactory. For the classical music purist 10 to 40 might be a good range.
The audio will be heavily processed at 70 to 100.
If the program source is already well processed, set the PROCESSING to "0"
or "10".
5.2.5 Digital Multimeter
The digital multimeter is a four-digit numeric display in the center of the
front panel. "Up" and "down" pushbuttons facilitate selection from a menu
of eight parameters. A green LED indicates the one selected.
The parameters are:
RF POWER Actually reads RF voltage-squared, so the accuracy can be
affected by VSWR (RF voltage-to-current ratio). See 7.4.1 and 7.4.2 for
calibration. Requires calibration with the RF reflectometer being used.
12
(Section 5 - OPERATION - continued)
SWR Direct reading of the antenna standing-wave ratio. (The ratio of the
desired load impedance (50 ohms) to actual load.)
ALC DC gain-control bias used to regulate PA supply voltage and RF drive.
[Note: Some units manufactured before May 1992 did not have PA
supply voltage control.] With full output, ALC will read about 5.0 volts.
When the RF output is being regulated by the RF power-control circuit
this voltage will be reduced - typically reading 1 to 4 volts. It will be
less than one volt during a LOCK fault. The ALC voltage will be reduced
during PA DC overcurrent, SWR, or LOCK fault conditions. This is a
useful parameter to monitor when making adjustments to the power
output stage.
PA DC VOLTS Supply voltage of the external RF power amplifier. (Voltage
at positive side of PA DC current shunt.)
PA DC AMPS Transistor drain (or collector) current for external RF power
amplifier.
EXCITER DC VOLTS Supply DC voltage for internal 10-watt RF amplifier.
(Nominally 18-24 volts.)
EXCITER DC AMPS Transistor drain current for 10-watt RF amplifier.
(Typically 0.5 to 1.0 amps.)
SUPPLY DC VOLTS Internal 12-volt supply.
5.2.6 FAULT indicators. Faults are indicated by a blinking red light as follows:
SWR: Load VSWR exceeds 1.5:1. ALC voltage is reduced to limit the
reflected RF power. (For this to work properly, the POWER LIMIT
control must be set at, or just above, the operating power level. See
7.4.1 - 7.4.3.)
PA DC: Power supply current for the external RF amplifier is at the preset
limit. ALC voltage has been reduced to hold supply current to the preset
limit. See 5.2.5.
13
(Section 5 - OPERATION - continued)
LOCK: Frequency synthesizer phase-lock loop unlocked. Drive inhibited by
reduced ALC voltage. (Under normal conditions it will take up to 20
seconds at power up to lock.)
FUSE: Not used.
5.2.7 STEREO-MONO switch.
Selects mode. See 5.1.
5.2.8 NORMAL-TEST switch.
Set to NORMAL during operation. See 3.1.8
5.2.9 MODULATION display.
Indicates modulation percentage. Peak-hold bar-graph display of modulation
percentage. "100" coincides with 75 khz deviation. The display holds for
about 0.1 seconds after the peak. Resolution is increased over a
conventional 10-segment bar-graph display through the use of dither
enhancement. (Dither modulates the brightness of the LED.) The "PILOT"
indicator lights when the unit is in the stereo mode.
5.2.10 CARRIER switch.
Switches power to RF amplifiers. See 3.1.9
5.2.11 POWER switch.
Main power switch.
14
6.0 - CIRCUIT DESCRIPTION
6.1 AUDIO PROCESSOR
The audio processor is a 4.5 x 6-inch circuit board located in the upper
compartment of the unit at left front. The board is accessible by removing the
unit top cover. (One circuit of the processor - the 8th-order elliptical filter - is
on the stereo generator board.) The processor provides the audio control
functions of compressor, limiter, and expander. The pre-emphasis networks are
on this board. Refer to 9.1. Reference numbers will be for the left channel.
Where there is a right-channel counterpart, references will be in parenthesis.
Audio input from the XLR connector on the rear panel of the unit goes to
differential-input amplifier UTA (U2A).
The gain of inverting amplifier U1B (U2B) is set by binary data on the " + 6DB"
and " + 12DB" lines. One of four feedback points is selected by analog switch
U3 in approximate 6DB steps.
The output of U1B (U2B) goes to an eighth-order elliptical switched-capacitor *
low-pass 15.2 kHz filter. The filter is located on the stereo generator board to
take advantage of the ground-plane and proximity to the 1.52-MHz clock.
U4B (U4A), along with R22/C8 (R58/C20), form third-order low-pass filtering,
attenuating audio products below 30 Hz.
The output level of analog multiplier U5 (UB) is the product of the audio signal
at pin 13 and the DC voltage difference between pins 7 and 9. At full gain (no
gain reduction) this difference will be 10 volts DC.
When either the positive or negative peaks of the output of U5 (UB) exceeds the
gain-reduction threshold, DC bias is generated by U13A, producing broad-band
gain reduction. Q5 is a precision-matched transistor pair. It, along with U13B,
form a log converter, so that a given voltage change will produce a given
number of DB of gain control of U5 (U6). The log conversion insures uniform
level-processing characteristics over - and well beyond - the 20-DB control
range. The log conversion has an additional benefit; it allows display of gain
control on a linear scale with even distribution of DB.
Q1 (Q2) is a recover/expansion gate with a threshold about 18 DB below the
normal program level. The amount of short-term expansion and gain reduction
is controlled by R650, located on front-panel display board. See 5.2.4.
15
(Section 6 - CIRCUIT DESCRIPTION - continued)
Pre-emphasis in microseconds is the product of the capacitance of C10 (C22),
multiplied by the gain of U8 (U9), times the value of R31 (R67). For
75-microsecond pre-emphasis the gain of U8 (U9) will be about 1.11. The
pre-emphasis curve - 75uS, 50uS, 25uS, or FLAT - is selected by jumpering the
appropriate pins on header JP1. Fine adjustment of the pre-emphasis is made
with trim pot R29 (R65). See 7.1.2.
For high-band processing the peak output of U10B is detected and
gain-reduction bias is generated, as with the broadband processor. The
high-band processing, however, modifies the pre-emphasis curve rather than
overall gain.
Peak audio voltages are compared to a plus and minus 5.00-volt reference, U17
and U18. This same reference voltage is used by the stereo generator,
metering, and display boards.
For explanation of on-board adjustments see 7.1.
6.2 STEREO GENERATOR
The stereo generator is the right-rear 4.5 x 4.5-inch board in the upper
compartment of the unit, accessible by removing the top cover. The
component-side of the board is mostly ground plane. The board has inputs for
left- and right-channel audio from which it generates a composite stereo signal.
(8th-order, 15.2-kHz elliptical low-pass filters (U201 and U202) are on this
board, but belong to the audio processor.)
U207A, with Y201, is a 7.6-MHz crystal oscillator from which the 19- and
38-kHz subcarriers are digitally synthesized. U207F is a buffer. The 7.6 MHz is
divided by 5 in U208A to provide 1.52 MHz at pin 6, used by filters U201 and
U202. 3.6 MHz, 1.9 MHz and 304 kHz are also derived from dividers in U208.
Exclusive-OR gates U210A and B provide a stepped approximation of a 38 kHz
sine wave - a scheme described in the CMOS Cookbook by Don Lancaster
(Howard W. Sams &. Co., Inc., Indianapolis, IN - 1978). With the resistor ratios
used, the synthesized sine-wave has very little harmonic energy below the 7th
harmonic. U210C and D generate the 19-kHz pilot subcarrier. U211 is a dual
switched-capacitor filter, configured as 2nd-order low-pass filters with Q,s of 5.
The 38 and 19 KHz outputs of pins 1 and 20, respectively, are fairly pure sine
waves. Harmonic distortion products are better than 66 dB down - THD of less
than 0.05%.
16
(Section 6 - CIRCUIT DESCRIPTION - continued)
U212 is a precision four-quadrant analog multiplier. The output of U212 is the
product of 38 kHz applied to the "Y" input and the difference of left and right
audio - the L-R signal - applied to the "X" input. The resulting output is a
double sideband suppressed carrier - the L-R subcarrier.
SCA subcarrier, left, right, left-minus-right subcarrier, and 19-kHz pilot
subcarrier are combined into the composite stereo signal by summing amplifier
U206A.
Analog switch U205, at the input of U206A, provides switching of left and right
audio for stereo and mono modes. In the mono mode the subcarriers are turned
off, right channel audio is disabled, and the left channel audio is increased from
45% modulation to 100%.
MON L and MON R outputs go to the AF MONITOR jacks on the rear panel.
R210 (R222) and C207 (C211) are a 75 usec de-emphasis network. Processed,
de-emphasized (75usec.) samples of the stereo generator input signals are used
for feeding a studio monitor and for doing audio testing.
VR201 and VR202 supply plus and minus 6 volts, respectively. Supply for the
subcarrier generators is 5.00-volt reference from the audio processor board.
For explanation of on-board adjustments see 7.2.
6.3 FREQUENCY SYNTHESIZER
The frequency synthesizer is a 4.5" x 7.0" board located in the upper
compartment of the unit. The entire component side of the board is ground
plane. Thumb-wheel switches along the front edge of the board set the
operating frequency. The VCO (voltage-controlled oscillator) circuitry is inside a
cast aluminum shield cover. There is an input connection for modulation.
About 35 mW. output is available into 50 ohms.
VCO, Q301 operates at the synthesizer output frequency of 87 MHz to 108
MHz. The frequency is controlled by voltage-variable capacitors D307 and
D308. Q304 and Q305 form an active filter to supply clean DC to the drain of
Q301. Q305 also serves as a common-base RF amplifier for Q301. U308 and
U309 are hybrid RF amplifiers to provide buffering and gain. The output level of
the synthesizer is controlled by Q302. With 5 volts DC at the ALC input, the
sythesizer will produce full output; as the ALC is reduced, so is the RF output.
17
(Section 6 - CIRCUIT DESCRIPTION - continued)
A sample of the RF from the VCO goes to the input of U302. U302 is a dual
modulus 10/11 divider that has been connected to divide by 10. The output of
U302 is 8.7 to 10.8 MHz. This signal, available at TP202, may be used with an
HF receiver for deviation and frequency measurements. See 7.2.5.2 and 7.3.3.
U304 is a phase-locked-loop frequency synthesizer IC. The 10.24 MHz from
the crystal oscillator is divided to 10KHz. Internal programmable dividers (along
with dual modulus divider U1) divide the 8.7 to 10.8 MHz RF to 10kHz also.
Differences between the two signals produce error signals at pins 7 and 8.
The U304 dividers are set with thumb-wheel switches. The binary output of
the 0.1-MHz switch programs the "A" counter directly. BCD data from the
100's, tens, and units thumbwheels is converted to binary data by EPROM
U310 to set the "N" counter. There is also provision on the circuit board for
using either DIP switches or jumpers to set the frequency, See 7.3.1.
U305B is a differential amplifier and filter for the error signal. Audio that is out
of phase with that appearing on the error voltage is introduced by U305.
U306A and B are a filter/integrator. It is configured to isolate the VCO from
op-amp noise. Q303 and D302 are VCO input voltage clamps.
Lock and unlock status signals are available at the outputs of U303E and
U303F, respectively. Modulation is introduced to the VCO through R351 and
R326 to R329. About 4.1 millivolts across R329 produces 75 kHz deviation.
6.4 ALC - METERING
The ALC and metering circuitry is on a 4.0" x 4.5" circuit board in the upper
compartment of the chassis. This board processes information for the RF and
DC metering, and produces ALC (RF level-control) bias. It also provides
reference and input voltages for the digital panel meter, and drive for the
front-panel fault indicators.
The 10-watt RF output amplifier has a 0.1-ohm current metering shunt in the
24-volt transistor drain supply lead. The shunt resistor is located on the
10-watt circuit board. The two sides of that resistor come to the "IPA
SHUNT +" and "IPA SHUNT-" inputs of the ALC-meter board. Op-amp U405
drives transistor Q404 to produce a voltage across emitter resistor R431 equal
to the voltage across the meter shunt. Assuming a transistor alpha of better
than .995, voltage drop across collector resistor R432 will very nearly equal
18
(Section 6 - CIRCUIT DESCRIPTION - continued)
that of R31, thus producing a ground-referenced voltage source for metering of
0.1 volts per amp.
A voltage divider from the "IPA SHUNT +" line feeds the line for DC voltage
metering.
The metering circuits for the drain (or collector) current and voltage of an
external RF amplifier are identical to that just described for the IPA. U403 and
Q403 form the shunt-voltage level translator circuit. The PA shunt lines come
from the 15-pin connector on the rear panel. It should be noted that, because
of the voltage rating of the U403 op- amp, this metering may not be used with
external amplifiers that have supplies greater than 24 volts.
U410A, U411A, and U411B - with their respective diodes - are diode linearity
correction circuits. Their DC inputs come from diode detectors in the RF
reflectometer (either the reflectometer on the 10-watt RF amplifier board, or an
external reflectometer via the 15-pin connector on the rear panel).
U401A, U401B, 0401, and 0402 are components of a DC squaring circuit.
Since the DC output voltage of U401B is proportional to RF voltage squared, it
is also proportional to RF power.
U402A, U404A, and U404B are level sensors for RF power, reflected RF power
and external PA current, respectively. When either of these parameters exceeds
the limits, the output of U402B will be forced low, reducing the ALC (RF level
control) voltage.
The DC voltage setpoint for U404A (reflected RF voltage) is one-fifth that of
U402A (forward RF voltage). This ratio corresponds to an SWR of 1.5:1. SWR
protection is available only if the POWER SET control has been set to either
control the RF power, or just above the point at which it will control power - as
may be the case when running an RF power amplifier at saturation.
The U406 invertors drive the front panel fault indicators.
To get a direct reading of SWR, the reference input of the digital panel meter is
fed from a voltage proportional to the forward-minus-reflected RF voltage, while
forward-plus-reflected is fed to the DPM input.
19
(Section 6 - CIRCUIT DESCRIPTION - continued)
U408 and U409 are used as data selectors for the digital panel meter input and
reference voltages. Binary select data for U408 and U409 comes from the
display board.
6.5 POWER SUPPLY
The power supply for the FM10X occupies the right-hand side of the chassis.
The power transformer and two bridge rectifiers are mounted on the chassis.
Three 10,000 MFD filter capacitors are mounted on the underside of the chassis
base plate. On the top of the chassis, opposite the capacitors, is a circuit
board. The board has the capacitors and rectifier bridge for the negative supply
and the three three-terminal regulators. The regulators are heat-sinked to the
chassis inside partition. The power supply provides + 24 volts at 1 amp., +12
volts at 1 amp., and -12 volts at .25 amp.
AC input for the power supply is through a receptacle/filter module on the rear
panel. It receives the AC line cord, holds the line fuse, and has facility for
switching between 120 and 240 VAC.See 4.1.
Each of the two 12VAC secondaries has a bridge rectifier. The DC outputs
(about 17VDC) of the bridges are connected in series, so there is available a
no-load voltage of about 17 and 34 volts DC.
The 34 volts goes to a 24-volt three-terminal regulator; the 17 volts to a 12-volt
regulator.
AC from the lower secondary is AC-coupled to a bridge to provide the negative
supply. The negative supply has a three-terminal -12V regulator.
6.6 DISPLAY
Front-panel indicator LEDs, numeric display, slide switches, and processing
control are mounted on the 3.5" x 14.5" display circuit board. The component
side of the board may be accessed by removing the front panel. (Remove 8
flat-head screws.) The board has the circuits for the digital panel meter,
modulation peak detector, and LED display drivers, as well as indicators and
switches mentioned above.
Left and right audio from input stages of the audio processor board (just after
the INPUT GAIN attenuator) go to the L VU and R VU input on the display
board. Peak rectifiers U601A and U601B drive the left and right AUDIO INPUT
20
(Section 6 - CIRCUIT DESCRIPTION - continued)
displays. The LED driver gives a 3 dB per step display. The lowest step of the
display driver is not used; but rather a red LOW indicator lights when audio is
below the level of the second step. Transistors Q601 and Q602 divert current
from the LOW LEDs when any other LED is lit.
Resolution in the linear displays - HHGH BAND, BROAD BAND, and
MODULATION - has been improved through utilization of dither enhancement.
With dither, the brightness of the LED is controlled by proximity of the input
voltage relative to its voltage threshold. The effect is a smooth transition from
step to step as input voltage is changed. U606A, U606B, and associated
components are the dither generator. Dither output is a triangular wave.
U604, U605, and UB08 are linear drivers for the HIGH BAND, BROAD BAND,
and MODULATION LED displays.
Composite stereo (or mono) is full-wave detected by diodes D605 and D606.
U607, U613, Q603, and Q604 are components of a peak sample-and-hold
circuit.
Oscillator U609F supplies a low frequency square wave to the FAULT
indicators, causing the fault indications to flash on and off.
Digital multimeter inputs are selected with pushbuttons located to the right of
the multimeter menu. Signals from the push buttons are conditioned by U609A
and U609B. U610 is an up/down counter. Binary input to U611 from U610
selects a green menu indicator light, and lights the appropriate decimal point on
the numeric read-out. The binary lines also go to analog data selectors on the
ALC/metering board.
PROCESSING control R650 is part of the audio processor. See 6.1.
The DPM IN and DPM REF lines are analog and reference inputs to digital
multimeter IC U612. They come from analog data selectors on the
ALC/metering.
6.7 TWO-WATT RF AMPLIFIER
The two-watt RF amplifier uses a CA2832 hybrid high-gain wideband amplifier
capable of 2 watts output at 100 MHz. The amplifier operates from the 24-volt
supply, and is turned on with the CARRIER switch. It amplifies the signal from
the RF frequency synthesizer, feeding the 10-watt amplifier.
21
(Section 6 - CIRCUIT DESCRIPTION - continued)
A resistor pad reduces input level. The circuit board has components for input
and output coupling, and for power supply filtering.
6.8 TEN-WATT RF AMPLIFIER
The ten-watt RF amplifier is located in the underside of the chassis on the left
side. All circuits for the amplifier are on a 7.5" x 2.25" printed circuit board.
The board includes the amplifier, a matching and filtering network, and a
reflectometer. The amplifier is turned on with the CARRIER switch.
The amplifier device is a MRF136 MOSFET with 24 volts on the drain.
Capacitors C806 and C808 facilitate optimization of load matching.
Forward and reflected power information (RF FWD and RF REFL) from the
reflectometer goes to the DB15 connector on the rear panel of the unit. The
reflectometer information will go to the metering board via the DB15 connector,
only if the FM10X is being used as a transmitter. (If the FM10X is being used
as an exciter, then a reflectometer at the output of the external amplifier should
be connected to the appropriate DB15 pins.)
6.9 DB15 INPUT FILTER
If the FM10X is to be used as an exciter, control and metering of an external RF
amplifier is made possible by a 15-pin DB15 connector on the rear panel. The
connector is mounted on a 1.5" x 3.0" circuit board. Resistor, capacitors, and
RF chokes provide RF filtering. If the unit is being used as a 10-watt transmitter,
it is necessary to jumper pin 2 to 9, 3 to 10, and 4 to 11. This will connect the
10-watt RF reflectometer to the ALC-meter board.
See 9.9.1.
6.10 AUXILIARY ALC
[Note: Some units manufactured before May 1992 did not have this circuit.]
The auxiliary ALC control circuit, located just to the right of the power
transformer, is a series linear regulator that controls the supply voltage to the
RF power amplifier. With 5 volts of ALC, full (24 volts) voltage is applied to the
RF PA. As ALC is decreased to about 2.5 volts, the supply voltage drops close
to zero. Below about 2.5 volts ALC the RF drive is reduced, preventing RF
feedthrough when the frequency synthesizer is out of lock.
22
7.0 - INTERNAL ADJUSTMENTS
7.1 AUDIO PROCESSOR
7.1.1 PRE-EMPHASIS SELECTION
The pre-emphasis curve - 75uS, 50uS, 25uS, or FLAT - is selected by
jumpering the appropriate pins on header JP1.
Note: If other than 75uS pre-emphasis is used and the AF MONITOR
outputs are used, the de-emphasis components on the stereo generator
circuit board should be modified accordingly. See 6.2.
7.1.2 PRE-EMPHASIS ADJUST
Fine adjustment of the pre-emphasis is made with trim pot R29 and R65 for
left and right channels, respectively. Pots are set to bring de-emphasized
gain at 10 kHz equal to that of 400 Hz. (At the proper setting, 15.0 kHz will
be down about 0.7 dB.)
When making these adjustments, it is important that signal levels be kept
below the processor gain-control threshold.
Measurements may be made using either the de-emphasized output of an
FM modulation monitor, or the AF MONITOR outputs on the rear panel of
the FM10X.
A preferred method is to use a precision de-emphasis network ahead of the
audio input, and then use the non-de-emphasized (flat) output from the FM
modulation monitor for measurements.
7.2 STEREO GENERATOR
7.2.1 SCA LEVEL
Using an SCA modulation monitor, set for 7.5 kHz deviation.
(Approximate setting of the SCA level can be made by looking at TP301 on
the RF synthesizer board with an oscilloscope. At 10% modulation, the
subcarrier will be close to 340 millivolts peak-to-peak. This is best observed
in the MONO mode.)
23
(Section 7 - INTERNAL ADJUSTMENTS - continued)
7.2.2 L-R LEVEL
Feed a 400 Hz sine wave into one channel. Observe the classic
single-channel composite stereo waveform at TP301 on the RF synthesizer
circuit board. Adjust the L-R LEVEL control for a straight center line.
Since proper adjustment of the control will coincide with best stereo
separation, an FM monitor should be used to make or confirm the
adjustment.
7.2.3 19-kHz LEVEL
Adjust the 19-kHz pilot for 9% modulation as indicated on an FM
modulation monitor.
(The COMPOSITE LEVEL should be set first, since it follows the 19 kHz
LEVEL control.)
7.2.4 19-kHz PHASE
Apply 400-Hz audio to the left channel.
Look at the composite stereo signal at TP301 on the synthesizer circuit
board with an oscilloscope, expanding the display to view the 19-kHz
component on the horizontal center line.
Switch the audio to the right channel input. When the 19-kHz PHASE is
properly adjusted, the amplitude of the 19-kHz will remain constant when
switching between left and right.
Recheck separation and adjustment of L-R LEVEL per 7.2.2. above.
7.2.5 COMPOSITE LEVEL
7.2.5.1 Using modulation monitor.
1. Set STEREO-MONO switch to MONO.
2. Check that the setting of the MODULATION compensation control,
R51 on the synthesizer circuit board, is according to the chart for
the frequency of operation. See 7.3.2.
24
(Section 7 - INTERNAL ADJUSTMENTS - continued)
3. Feed a sine-wave signal source of about 2.5 kHz into the left channel
at a level sufficient to put the BROADBAND gain-reduction indicator
somewhere in the middle of its range.
4. Set the COMPOSITE level control to produce 90% modulation as
indicated on an FM monitor.
5. Apply pink noise or program material to the audio inputs and confirm,
on both MONO and STEREO, that modulation peaks are between
95% and 100%.
7.2.5.2 Using Bessel nulls.
If done properly, the Bessel Null method to be described will give better
accuracy than a modulation monitor. The following procedure will set
the gain-control threshold for 90 percent modulation.
1. Set STEREO-MONO switch to MONO.
2. Check that the setting of the MODULATION compensation control
R351 on the synthesizer circuit board is according to the chart for
the frequency of operation. See 7.3.2.
3. Feed a 2807 Hz sine-wave signal source into the left channel at a
level sufficient to put the BROADBAND gain-reduction indicator
somewhere in the middle of its range. The modulation setting will
be only as precise as the frequency of the sine wave; a one-percent
error in frequency will result in a one-percent error in deviation.
4. Couple a shortwave receiver tuned to one-tenth the operating
frequency (8.70 MHz to 10.80 MHz) to TP302 on the frequency
synthesizer circuit board. (In most cases there need not be a direct
connection.)
The receiver must have either a narrow IF bandwidth setting or a BFO
(beat frequency oscillator).
5. Adjust the COMPOSITE level control to null the carrier.
a) If using'a BFO: Set the BFO for a low frequency audio tone.
Listen for this tone to disappear at the carrier null.
25
(Section 7 - INTERNAL ADJUSTMENTS - continued)
b) Without a BFO: Watch the signal-level indicator on the receiver or
determine by ear the point at which the carrier nulls. (This only
works if the bandwidth is narrow enough to discriminate against
the 2.8 kHz sidebands.)
6. Apply pink noise or program material to the audio inputs and confirm,
on both MONO and STEREO, that modulation peaks are between
95% and 100%.
7.3 FREQUENCY SYNTHESIZER
7.3.1 FREQUENCY (CHANNEL) selection.
Thumbwheel switches set the operating frequency of the FM10X. Any
multiple of 0.1 MHz is available in the FM broadcast band from 87 to 108
MHz. |
Examples of thumbwheel settings:
Lols[e]r)
L'Le17 1°]
for 88.1 MHz:
for 107.9 MHz:
Provision is made on the synthesizer circuit board for using either DIP
switches or jumpers in place of the EPROM and thumbwheel switches. If
the thumbwheel switches are not used, the EPROM (U310) and resistors
R345, R346, R347, and R348 must be removed. The operating frequency
will be the sum of the switches on. For example, 64 +16+8+ 0.1 for 88.1
MHz; or 64+32+8+2+1+0.8+0.1 for 107.9 MHz.
26
(Section 7 - INTERNAL ADJUSTMENTS - continued)
7.3.2 MODULATION compensator.
The MODULATION trim-pot, just to Frequency: Compensator
MHz: Setting:
the left of the thumbwheel switches, 88 70
compensates for slight variations in 90 65
deviation sensitivity with frequency. 92 60
Each mark (of ten) on the trim-pot 94 55
represents about 1.8% modulation 96-108 50
compensation. (For exact setting,
see 7.2.5 for measuring deviation.)
The compensator settings shown are approximate.
7.3.3 MODULATION MEASUREMENT and adjustment.
Next to the 10.24 MHz crystal on the circuit board is a 5.5-18 PF ceramic
trimmer capacitor (C307). Use C307 to set the frequency of the 10.24
MHz crystal while observing the output frequency of the synthesizer. Use
one of three methods for checking frequency:
1. Use an FM frequency monitor.
2. Couple a frequency counter of known accuracy to the output of the
synthesizer and observe the operating frequency. Or connect a
counter to TP302 and measure one-tenth operating frequency. (Do
not connect to the 10.24-MHz clock circuit.)
3. Using a shortwave receiver and test point TP301 will, in most cases,
give better accuracy than 1 or 2 above.
a) Temporarily set the synthesizer to 100.0 MHz.
b) Tune a shortwave receiver to a 10-MHz standard such as WWV
or JJY. Loosely couple the receiver to TP302 so that the 10
MHz signal strength from TP302 is about equal to that being
received from the standard station. Usually setting the receiver
(if it’s a portable, such as the Sony 2010) on top of the FM10X
with the receiver antenna a few inches from the TP302 test
point will be about right.
c) Use the C7 trimmer capacitor to "zero beat" the two signals.
27
(Section 7 - INTERNAL ADJUSTMENTS - continued)
7.4 ALC - METERING
7.4.1 POWER CALIBRATE
When changing between the internal reflectometer in the 10-watt RF
amplifier and an external reflectometer, the power metering will require
recalibration.
While looking at RF POWER on the digital panel meter set the POWER
CALIBRATE trimpot to agree with an external RF power meter.
7.4.2 POWER SET
Adjust for desired output power. (Operation of the POWER SET control is
not affected by the POWER CALIBRATE.)
There may be circumstances where the RF output amplifier will be running
at saturation (full output).
The POWER SET trim pot adjustment sets both the forward and reflected RF
power thresholds (see 6.4). Therefore, if it is desired to run the power
amplifier at saturation, the POWER SET control should be set to a point just
above where it would provide power regulation. This will insure limiting of
power in the event the SWR exceeds 1.5:1.
7.4.3 SWR CALIBRATE
Set the digital panel meter to read SWR.
With the FM10X operating at normal power output, ground pin 3 of U411
on the ALC/METER circuit board. This simulates zero reflected power - the
condition that exists when the VSWR is 1:1.
With the pin grounded, adjust the SWR CAL trim pot for a reading of
"1,01 ;"
28
(Section 7 - INTERNAL ADJUSTMENTS - continued)
7.4.4 PA CURRENT LIMIT
Since it may not be practical to increase the PA current for purposes of
setting the PA CURRENT LIMIT control, an indirect method will be
suggested.
Looking at the DC voltage at pin 6 of U4 on the ALC/METER circuit board,
adjust the PA CURRENT LIMIT control for a voltage equal to 0.1 x Current
Threshold, in amps. If, for example, the limit is to be 15 amps., set the
voltage to 1.5 volts.
7.5 DISPLAY- Modulation calibrate
CAUTION: Disconnect power before removing the front panel.
The POWER switch connects to the AC power line, so use care
when making the following adjustment.
The MODULATION CALIBRATE trim pot sets the sensitivity of the front panel
MODULATION bar-graph display. The control is accessible with the front panel
removed (8 flat-head screws).
This adjustment may be made only if the COMPOSITE LEVEL has been properly
set. See 7.2.5.
1. Set STEREO-MONO switch to MONO.
2. Feed a sine-wave signal source of about 2.5 kHz into the left channel at a
level sufficient to put the BROADBAND gain-reduction indicator somewhere
in the middle of its range.
4. Set the MODULATION CALIBRATE trim pot so that the "90" light on the
front panel MODULATION display just starts to light.
7.6 TEN-WATT RF AMPLIFIER
Tuning of the RF output network of the FM10X is done with two ceramic
trimmer capacitors located on the 10-watt RF amplifier printed circuit board.
The board is accessed by removing the bottom cover of the FM10X.
29
(Section 7 - INTERNAL ADJUSTMENTS - continued)
7.6.1 BIAS SET
The BIAS SET pot is found only on early units. If present, it should be set
full counter clockwise.
7.6.2 Output Network tuning
The RF output network for the ten-watt amplifier is set at the factory to
provides 10 watts output into a 50-ohm resistive load over the frequency
range of 88 to 108 MHz. This setting was achieved by tuning C806 and
C808 for maximum RF output at 104.1 MHz. This is a compromise setting
that, in most cases, will allow for changes in operating frequency without
the necessity of retuning the output network.
In most cases, however, the efficiency of the RF amplifier can be
improved--less heating of the RF output transistor--by retuning C806 and
C808 to optimize the amplifier for a particular operating frequency or to
accommodate an RF load mismatch. (Mismatches that show an SWR of
1.5:1 or more should be corrected by repairing or tuning the antenna.)
Retuning of the amplifier may also be called for when the RF power output
is low or when the EXCITER DC AMPS reads more than 1.2 amps (for 10
watts).
1. Turn the POWER SET trimpot on the meter board full clockwise.
2. While observing the RF output power with an RF wattmeter or on the
FM10X digital multimeter, alternately tune C806 and C808 until
maximum RF output power is obtained.
3. Note the EXCITER DC AMPS reading.
4. Turn C806 by a small amount in the direction that causes a decrease in
EXCITER DC AMPS. Turn far enough to cause a drop of a few percent
in RF output power. (If turned in the wrong direction, the EXCITER DC
AMPS will increase.)
5. Set the POWER SET control for 10 watts output.
(Power output can be increased at 108 MHz, if necessary, by spreading
the turns of L803.)
30
8.0 - PERFORMANCE VERIFICATION
8.0 PERFORMANCE VERIFICATION
Measurement of the following parameters will provide a comprehensive
characterization of the performance of the FM10X:
. Carrier frequency.
. RF output power.
. RF bandwidth and RF harmonics. See 8.3 below.
. Pilot Frequency.
. Audio frequency response.
. Audio distortion.
. Modulation percentage.
. FM and AM noise.
. Stereo separation between left and right.
10. Crosstalk between main channel and subcarrier.
11. 38-kHz subcarrier suppression.
oN OTA Wh =
o
In addition to the above tests which pertain to signal quality, a complete check of
the unit will include items listed in 8.12.
8.0.1 Audio proof-of-performance measurements.
References to "100 percent" modulation assumes nine percent pilot and 91
percent for the rest of the composite stereo signal.
Because the audio processing threshold is at 90 percent modulation, it is not
possible to make audio proof-of-performance measurements at 100 percent
modulation through the audio processor. Instead, audio data for 100 percent
modulation is taken from the input of the stereo generator, and then data
including the audio processor is taken at a level below the audio processing
threshold.
The 100-percent-modulation data for response, distortion, stereo separation,
crosstalk, and noise is taken with left and right audio fed into the ten-pin
connector that plugs into J2 on the audio processor circuit board.
A second set of data is taken at 80 percent modulation with the audio source
feeding the normal left and right audio inputs of the FM10X.
31
(Section 8 - PERFORMANCE VERIFICATION - continued)
8.0.2 De-emphasis input network.
A precision de-emphasis network connected between the oscillator and the
audio input of the FM10X can be very helpful when making the audio
measurements. Care must be taken that the network accuracy is not affected
by the input impedance of the FM10X or by the source impedance of the test
oscillator.
With the de-emphasis network, oscillator level adjustments need only
accommodate gain errors, instead of the whole pre-emphasis curve.
8.1 CARRIER FREQUENCY
Carrier frequency is measured at the output frequency with a frequency monitor
or suitable frequency counter.
For adjusting frequency, see 7.3.3 (FCC tolerance +/- 2000 Hz. FCC Part
73.1540 and 73.1545.)
8.2 OUTPUT POWER
The RF power reading on the digital multimeter should be verified with a
wattmeter in the RF output line. Output power should be in the range of 90%
to 105% of rating.
See 7.4.1 and 7.4.2 for power setting.
8.3 RF BANDWIDTH and RF HARMONICS
RF bandwidth and spurious emissions are observed with an RF spectrum
analyzer.
In the STEREO mode, feed 15.0 kHz audio into one channel to provide 85%
modulation as indicated on a monitor. This will produce 38% main, 38% stereo
subcarrier, and 9% pilot per FCC Part 2.989. An alternative is to use pink noise
into one channel.
32
(Section 8 - PERFORMANCE VERIFICATION - continued)
Using a spectrum analyzer, verify that all of the following are true: (FCC
73.317)
1. Emissions that are more than 600 kHz from the carrier are at least
43 +log,, (power in watts) dB down. That would be 53 dB for 10
watts; 63 dB for 100 watts. The scan should include to the tenth
harmonic.
2. Emissions that are between 240 kHz and 600 kHz from the carrier are
down at least 35 dB.
3. Emissions that are between 120 kHz and 240 kHz from the carrier are
down at least 25 dB.
8.4 PILOT FREQUENCY
The pilot frequency is to be within 2 Hz of 19,000 Hz. (FCC Part 73.322.)
Using a frequency counter, measure 1.9 MHz at pin 1 of U209 on the STEREO
GENERATOR board. A 200-Hz error here corresponds to a 2-Hz error at 19 kHz.
If the frequency is off by more than 50 Hz, the value of C213 may be changed.
(Changing C213 from 56PF to 68PF lowered the 1.9 MHz by about 35 Hz.)
8.5 AUDIO FREQUENCY RESPONSE
For the response tests, readings are taken from an FM modulation monitor.
Audio frequency response measurements for left and right channels are made at
frequencies of 50, 100, 400, 1k, 5k, 10k, and 15k Hertz.
See 8.0.1 and 8.0.2.
8.6 AUDIO DISTORTION
Distortion measurements are made from the de-emphasized output of an FM
modulation monitor.
Audio distortion measurements are made for left and right channels at
frequencies of 50, 100, 400, 1k, 5k, 10k, and 15k Hertz.
See 8.0.1 and 8.0.2.
33
(Section 8 - PERFORMANCE VERIFICATION - continued)
8.7 MODULATION PERCENTAGE
While feeding an audio signal into the left channel only, it should be confirmed
that the total modulation percentage remains constant when switching between
MONO and STEREO.
Modulation percentage is measured with an FM modulation monitor, or by using
an HF receiver and Bessel nulls. See 7.2.5., 7.3.2., and 7.5.
19 kHz pilot modulation should be 9 percent.
8.8 FM and AM NOISE
Noise readings are taken from a de-emphasized output of a modulation monitor.
8.9 STEREO SEPARATION
Left-into-right and right-into-left stereo separation measurements are made with
an FM modulation monitor for frequencies of 50, 100, 400, 1k, 5k, 10k, and
15k Hertz.
8.10 CROSS TALK
For the stereo crosstalk measurements, both left and right channels are fed at
the same time. For best results, there needs to be a means of correcting small
imbalances in levels. The balance is made at 400 Hz.
8.10.1 MAIN-CHANNEL INTO SUB
Left and right channels are fed in phase with audio (L+R) at 50, 100, 400,
1k, 5k, 10k, and 15k Hertz at 100 percent modulation, while the stereo
subcarrier (L-R) level is observed on an FM modulation monitor.
8.10.2 SUB-CHANNEL INTO MAIN
Audio is fed into the left and right channel as above, except the polarity of
the audio of one channel must be reversed ("L-R" input). Using the
frequencies of 8.10.1 above, observe the main channel (L +R) level with a
modulation monitor.
34
(Section 8 - PERFORMANCE VERIFICATION - continued)
With no modulation - but in the STEREO mode - the 38-kHz subcarrier, as
indicated on an FM modulation monitor, should be down at least 40 dB.
8.12 ADDITIONAL PERFORMANCE CHECKS
In addition to what has been covered, a complete check of the unit will include
the following:
1. Perform physical inspection. Look for damage. Check that chassis
hardware is secure. Check that circuit boards are secure.
2. Check functionality of switches and processing control.
3. Observe that all indicators function.
4, Check frequency synthesizer lock at 72 MHz and 110 MHz.
5. Measure AC line current with and without carrier on.
6. Perform functional test of SCA input, MONITOR outputs, and DB15
connector.
~J
. Check all metering functions.
8. Test ALC action with PA current overload, SWR, and PLL lock.
(NOTE: The FCC type acceptance procedures call for testing of the carrier
frequency over the temperature range of 0 degrees to 50 degrees centigrade, and at
line voltages from 85% to 115%of rating. See FCC Part 2.995.)
35
(CPN CBS16—4)
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FM AUDIO PROCESSOR
9.1
#1 BUG Y:
XLR connectors. NC Connector Stereo Monitor Jacks DB-15 connector.
LEFT & RIGHT AF Inputs. SCA Input Processed, de-emphas|zed| |External RF Amplifier
600-ohm or 23K Balanced, ` | Metering and control. |
| / |
L
BNC connector
RF Output
10-watts
e: /
u ED ©) 00 — Y
RF EXCITER
10-WATT RF AMPLIFIER
Bottom Compartment.
STEREO GENERATOR
Crystai-controliea, digital ly-
Besse! test point for modulation
synthes!zea 19KHz and 38KHz. p
and frequency callbration w!th
a short-wave recelver,
15.2kHz Bth-order elliptical
_ filter. / /
switchea-capacitor low-pass f1lter / / Thumbwheel switches set Frequency
/ 87 — 108 MHz, 0.1 MHz steps.
/
Doo
É
AUDIO PROCESSOR
/
a /
ession / Expansion / Limiting
120/240 val
50/60 HZ
ALC and METERING
Pre—emphas|s selectable for
75us, S0us., 2Z5us. or FLAT,
SWR and Overcurrent protection.
Л
/ Carrier Level Control,
Switches internal C10-watt),
ond external RF amplifiers,
\
Flashing red light,
RF drive reduced.
Z
Set for nominal input level |
of —B, —2, +4, or +10dBm. |
Short-term “syllabic rate”
expansion and gain-—reduction \
around long-term average
gain set. 10 - 40 Light Processihng
40 —- 70 Norma! setting
70 - 100 Heavy Processing
| High-band Overhead
| 0 dB on Test
— AUDIO INPUT HIGH BAND ABRAN \ ON
LEFT RIGHT Æ 19 O 110
O HIGH O 00000 Qe Eon O 100
О О EXPAND <«@ COMPRESS One O o CARRIER
O 0 O 900000000Q \ | | O PA DC VOLTS © 80
o о BROAD BAND \ | O PA DC AMPS О 70 pee
О -8 O =" 9 = “|| O EXCITER DC VOLTS E ON
o О \ O EXCITER DC AMPS O 50
O -20 +8DB +1208 \ O SUPPLY DC VOLTS STEREO NORMAL О 40
O Le O ; | = E a POWER
o LOW o \ | O PILOT
—l INPUT GAIN PROCESSING MONO pe OFF
5 7 \ \ Model FM10X FM EXCITER | | | O
À , em | /
Z / \ \ / |
[Processor input level. I RL cone of | [Digital Multimeter Select| _ | |
| Peak Cotch-end-Hold
Copyright © 1991 Michael P. Axman
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