40m CW/SSB
by George!
Contents
Introduction
1-1 Description of Receiver ....................................................................................................................
1
1-2 Specifications ...................................................................................................................................
2
Circuit Description
2-1 Bandpass Filter ................................................................................................................................
3
2-2 Mixer - Post Mixer Amplifier ...........................................................................................................
3
2-3 Crystal Filter - IF Amplifier - AGC Module .....................................................................................
4
2-4 400Hz Noise Filter/ Product Detector Module ..................................................................................
5
2-5 Audio Processing Board - AF Power Amplifier ...............................................................................
5
2-6 VFO .................................................................................................................................................
6
2-7 LO - Passband Generator Module .....................................................................................................
6
2-8 BFO Generator ..................................................................................................................................
6
2-9 Display Module ................................................................................................................................
6
2-10 Power Supply ...................................................................................................................................
7
Alignment and Calibration
3-1 Required Test Equipment ..................................................................................................................
9
3-2 VFO ..................................................................................................................................................
9
3-3 LO - Passband Generator Module ......................................................................................................
9
3-4 BFO Generator ................................................................................................................................
10
3-5 Bandpass Filter ...............................................................................................................................
10
3-6 Mixer - Post Mixer Amplifier ..........................................................................................................
10
3-7 Crystal Filter - IF Amplifier - AGC Module ....................................................................................
11
3-8 Audio Processing Board .................................................................................................................
12
3-9 Display Module ...............................................................................................................................
13
Illustrations
Figure 1-1 The K9GDT 40m Receiver .....................................................................................................
1
Figure 2-1 Receiver Block Diagram ........................................................................................................
3
Figure 2-2 Front Panel Controls ..............................................................................................................
7
Figure 2-3 Rear Panel Controls and Connections .....................................................................................
7
Figure 2-4 Interior Views - Module Locations ........................................................................................
8
Figure 3-1 LO - Passband Generator Module ...........................................................................................
9
Figure 3-2 BFO Generator .....................................................................................................................
10
Figure 3-3 Crystal Filter - IF Amplifier - AGC Module .........................................................................
11
Figure 3-4 Audio Processing Board .......................................................................................................
12
Figure 3-5 Display Module ....................................................................................................................
13 ii
Introduction
Figure 1-1 K9GDT 40m Receiver
1-1 Description
The K9GDT 40m receiver is a solid state receiver that offers a very high level of performance for the 40m
Amateur Radio Band. It has excellent sensitivity, selectivity, and dynamic range. In addition, the receiver features a digital frequency readout and interference reduction circuitry.
The PASSBAND tuning control is an operating aid useful in reducing or eliminating interfering signals.
It electronically positions crystal filter’s response curve around the displayed frequency. This enables the operator to shift that response such that a nearby interfering signal is positioned down on the filter’s skirt.
The front panel layout is optimized for ease-of-use.
Large controls are ergonomically placed in functional groupings, with frequently used controls positioned close to the large, weighted main tuning knob.
A doubly balanced switching-mode mixer is used as the front-end mixer in a single conversion scheme.
This technique ensures optimum front-end overload and IMD performance.
The NOTCH TUNE control varies the center frequency of the AF notch filter. This feature is useful for eliminating interference from CW carriers and is especially useful in the upper portion of the band, which is shared with foreign broadcast services.
The receiver features a “tuned” audio section capable of delivering 3.5 watts to an eight ohm speaker.
Frequency response is tailored by active, passive, and switched capacitor filtering. The front panel SELECTIVITY switch allows the operator to select either a 2.1KHz or 400Hz IF bandwidth. Each bandwidth is provided by inserting the appropriate eight-pole crystal filter between the post-mixer amplifier and the IF amplifier. When the
400Hz position is selected, a second 400Hz filter is inserted before the product detector to remove broadband noise produced by the IF amplifier.
Other features include external mute control, a transmit monitor mode, selectable AGC time constants, an AGC defeat mode, a CW keyer sidetone input, and a convenient tilt bail located on the receiver’s bottom panel.
Page 1
Introduction
1-2 Specifications
Frequency Coverage
Sensitivity
Selectivity
AGC
Intercept Point
40m Amateur Radio Band
Spurious Response
2.1KHz - Less than 0.4 µV
for 10 dB S+N
N
Frequency Readout
Accuracy
400Hz - Less than 0.15 µV
for 10 dB S+N
N
2.1KHz @
−
6 dB
3.9KHz @
−
60 dB *
400Hz @
−
6 dB
1.0KHz @
−
60 dB *
Frequency Stability
< 65 dB
10ppm ±100Hz
< 1KHz during first 60
minutes after power-up
< 100Hz /hr thereafter
Less than 6 dB output variation for a 100 dB input signal change, referenced to AGC threshold. (< 1µV )
Dynamic Characteristics:
Attack time - 1 ms.
Decay times - 20 ms.
500 ms.
5 seconds
Notch
+7 dBm **
Notch Depth > 40 dB
400Hz -3KHz
Passband Tuning Range
The receiver’s display frequency ±4KHz
Internally Generated
Spurs
None
IF Frequency
3.395MHz
Audio Output
Power Requirements
3.5 Watts RMS @ less than 5% THD into 8
Ω
102-132 VAC, 50-60Hz
30 Watts max
.
IF Rejection
> 100 dB
Dimensions
5¼ in. high
17 in. wide
10¼ in. deep
Image Rejection
> 60 dB
Weight
** Suitable test equipment was unavailable to measure this parameter. So Hayward’s rule-of-thumb was used to
“guesstimate” it’s value.
Page 2
Introduction
Circuit Description
K9GDT 40m CW/SSB Receiver
7-7.3 MHz
First Mixer
SBL-1
-6 db
Low Noise
Post Mixer
Amplifier
3.395 MHz
8 pole
Crystal Filters
500Hz
Filter
Gain EQ
I.F.
Amplifier
2.1 kHz
BPF SK3218 40673
MC1350 x2
400Hz
10.395-10.695 MHz
+7dbm
+15V
AGC
5.2v no signal
5.7v -10 dbm
LO Pre-mix
SBL-1
6-6.3 MHz S-meter
AGC
AC
4395 ±4 KHz
Passband Tuner
SBL-1
BFO Generator
3395 ±4 KHz
AC
1 MHz
L.O.
Display Mixer - 40673
Post I.F.
400Hz Filter
-6db
Pad
400Hz
Product
Detector
MC1496
BFO
3395 ±4 KHz
7258.0
BFO
7-7.3 MHz Frequency
Counter
7216C
Tuneable AF
Notch
400-3000 Hz
+5V
+12v
+15v
+5v
+22v
Switched
Capacitor
L. P. Filter
LPF
3.0 kHz
A.F.
Amplifier
3.5W 8
LM383
+22V
Power Supply by George!
7812
7815
7805
LM388AK
Version 3.1
1/94
Figure 2-1 Receiver Block Diagram
2-1 Bandpass Filter Module
The Bandpass Filter module has an input and output impedance of 50
Ω
and consists of two major sections:
a dual resonator parallel tuned bandpass filter and a seven element low pass filter for additional attenuation of image frequencies from 13.79 through
14.09MHz. The module’s insertion loss is under 1dB.
2-2 Mixer - Post Mixer Amplifier
The mixer stage is a doubly-balanced diode-ring device made by Mini-Circuits Laboratories, model
SBL-1. The 10.395-10.695MHz local oscillator drive level is +7dBm.
IMD performance and eliminating the requirement for a diplexer. Because a very low noise figure is normally not necessary around 7MHz, the pad’s 6 dB insertion loss is tolerable.
A 6 dB pad is placed between the mixer output and the post-mixer amplifier, which is a low noise, high dynamic range broadband bipolar amplifier with a
50
Ω
input impedance. This arrangement assures a proper non-reactive termination at the mixer’s IF port for all frequencies present, thereby preserving it’s
The post-mixer amplifier has a gain of 20 dB using an
NTE278 transistor, which has an f
T
of 1.2 GHz and a noise figure of about 4 dB. This device is heat sunk and biased to a standing current of 70 ma. The amplifier’s output is matched to the 2 k
Ω
input impedance of the crystal filter which follows.
Page 3
Circuit Description
2-3 Crystal Filter - IF Amplifier - AGC Module
Crystal Filters
The received signal is fed to this module and routed to one of two eight-pole crystal filters by steering diodes.
The bandwidths are selected by the front panel
SELECTIVITY control. For an IF bandwidth of
2.1KHz, this control places +15 VDC on the 2.1KHz
filter select input and GND on the 400Hz filter select input lines. To choose the 400Hz filter, the
SELECTIVITY control reverses this connection.
These filter selection signals are also supplied to the product detector for inserting a second 400Hz filter and to the IF amplifier for filter gain equalization.
when a strong signal appears while preventing most pops and noises from “hanging” the AGC system.
The AGC decay time is determined by the parallel combination of a timing capacitor and a 22
ΜΩ resistor at the JFET input of the AGC amplifier. The timing capacitor is one of three selected by the front panel AGC control. This arrangement provides AGC decay times of about 20ms, 500ms, and 5 seconds, thus allowing the operator to match the receiver’s
AGC characteristics to the received signal’s rate of fade. In addition, the front panel AGC control switch has an “OFF” position, which shorts the JFET’s input to ground, thereby defeating the AGC system.
IF Amplifier
The signal leaving the crystal filter is connected to a tuned amplifier (3.395MHz) which uses an RCA
40673 dual-gate MOSFET. The purpose of this amplifier is to establish the noise figure for the IF section and to provide gain equalization for the crystal filters. This low noise device provides ample gain that is easily controlled by a voltage at it’s second gate.
When the 400Hz filter is selected, +15 volts is applied to the GAIN EQ input, resulting in a 3.3 volt level on gate #2 for maximum gain. When the 2.1KHz filter is selected, the GAIN EQ input is grounded and the
MOSFET’s gate #2 voltage is then controlled by a
0-3.3 volt signal from the 2.1KHz FILTER GAIN EQ trim pot.
The AGC amplifier’s output is connected through a diode to the IF amplifier’s gain control line. Two other signals are similarly connected such that the most positive signal determines the receiver’s gain.
The front panel manual RF GAIN control raises the voltage on the control line to limit the maximum receiver gain with the AGC enabled. It will also provide total control of the receiver’s gain when the
AGC is defeated. The second signal mutes the receiver.
The signal is then amplified by a pair of cascaded
Motorola MC1350 amplifiers. These devices furnish most of the IF amplifier’s gain and over 120 dB of gain control. During no-signal conditions, the IF amplifier’s gain control line is set to 5.38 volts. Gain is reduced by increasing the gain control voltage.
Mute Control
The external mute control system is compatible with that used by the R. L. Drake Co. The receiver is muted when a companion transmitter removes the short to ground (normally supplied when not transmitting) from the EXT MUTE connector. This causes +12 volts to be placed on the IF amplifier’s gain control line, effectively silencing the receiver.
The EXT MUTE DEFEAT switch on the rear panel provides a ground to the EXT MUTE connector so the receiver can be operated without a transmitter.
AGC
The output of the IF amplifier is coupled to the AGC detector diode. This diode charges a timing capacitor through a resistor which establishes the AGC attack time of approximately 1 millisecond. This value is sufficiently fast to maintain a constant AF output
The monitor function also defeats the external mute feature by grounding the EXT MUTE input when the front panel function switch is turned to MON. This permits the operator to temporarily override the external mute for monitoring the transmitted signal or setting the transmit frequency.
Page 4
Circuit Description
2-4 400Hz Noise Filter/Product Detector
400Hz Noise Filter
A sample of the IF amplifier’s output is fed to the
400Hz noise filter/product detector for conversion to audio. When the SELECTIVITY control is set to
400Hz, the filter selection signal energizes two small relays that insert a 400Hz eight pole crystal filter before the product detector. This filter restricts the effective noise bandwidth to that of the first 400Hz filter. Without it, broadband noise generated by the IF amplifier will reach the product detector and produce an audio output.
When the SELECTIVITY control is set to the 2.1KHz
position, the 400Hz filter is removed and a 6dB pad is switched in the signal path. The purpose of this pad is to match the insertion loss of the 400Hz noise filter, thus insuring identical signal levels at the mixer input at both selectivity settings.
Product Detector
The received signal is then routed to the modulating signal input port of a Motorola MC1496-P balanced modulator used as a product detector. The BFO signal of 3395±4KHz is terminated in 50
Ω
, attenuated to
850 mV p-p, and applied to the balanced modulator’s carrier port.
The audio signal from the output port is connected to a low pass filter which removes any RF energy from the product detector’s AF output. The frequency response of this filter, as well as subsequent stages, is limited to 3000Hz. The result of this effort is clean sounding audio that is free from hiss and high pitched background garbage caused by filter “blow-by” from
very strong adjacent signals.
The audio is then routed to the audio processing board.
The requirement for a noise filter is much less stringent for a bandwidth of 2.1KHz, so one is not included. Almost all of the broadband IF noise at this selectivity setting is removed after detection by limiting the bandwidth of the audio section.
2-5 Audio Processing Board - AF Power Amplifier
Audio Processing Board
The signal is sent through an input level set potentiometer to a LF356 low noise op amp, which functions as a 3KHz low pass filter. The amplifier’s output is routed through a passive 200Hz highpass filter to another LF356 serving as a tunable notch filter. a unity gain buffer to supply the required low-Z drive to the filter input stage. The other is used to provide a
“stiff” source of V
CC
÷
2 bias. The clock frequency is adjusted to 300KHz, which sets the filter’s cutoff frequency to 3KHz.
The notch filter provides an adjustable response null from 400Hz to 3KHz by summing normal audio with the output of a phase inverting bandpass filter. The notch filter is disabled by removing the audio signal to the input of the bandpass filter.
The signal is routed through another 200Hz highpass filter network to the switched capacitor lowpass filter section.
Finally, the signal is routed to the board’s output connector through another highpass/lowpass network
.
AF Power Amplifier
The signal from the Audio Processing board is sent through the front panel AF GAIN control to the AF
Power Amplifier board. The amplifier uses a National
Semiconductor LM383, which is powered by a 20 volt regulated supply and is capable of delivering 3.5 watts into an 8
Ω
load.
The switched capacitor filter uses a National
Semiconductor MF6-100 6th order Butterworth
Lowpass filter. This chip also contains two op amps and an on-board clock. One of the op amps is used as
The SIDETONE input connector on the rear panel allows the receiver’s power amplifier and speaker to be used by an accessory CW electronic keyer.
Page 5
Circuit Description
2-6 VFO
The VFO functions as the main tuning element by supplying a 6-6.3Mhz signal to the LO - Passband
Generator module. The signal is produced by a temperature compensated series-tuned Colpitts JFET oscillator with 6.3 volt zener diode regulation. Tuning is accomplished by a high quality double ball bearing variable capacitor driven by a zero-backlash flywheel weighted 50:1 gear reduction drive.
The oscillator stage is followed by a two transistor buffer amplifier that produces a 5 volt p-p signal.
2-7 LO - Passband Generator Module
Passband Tuner
The Passband Tuner is an oscillator similar to the
VFO described above. However, the 4395KHz output frequency can be shifted ±6KHz by a tuning voltage from the front panel PASSBAND control. This DC voltage is bias for varactor diodes which are part of the oscillator’s series tuned circuit.
There is a buffered Passband Tuner signal available at the PBT connector for use by the BFO Generator module.
The effect of the ±6KHz offset is to shift the BFO and
LO frequencies. The shifted LO repositions both the received signal and nearby interference around the response curve of the crystal IF filter. This permits the interfering signal to be placed down on the filter’s skirt where it’s effect will be less severe. Because the
BFO frequency is also shifted by the same amount and in the same direction, there is no change in the received frequency.
LO Generator
The 4395KHz output of the Passband Tuner is mixed with the 6-6.3MHz VFO output to produce the local oscillator signal of 10.395-10.695MHz. A Mini-
Circuits SBL-1 is used as the mixer and is followed a two transistor amplifier section with suitable bandpass filtering. The 50
Ω
, +7 dBm LO output is very clean, with mixed products and harmonic energy greater than 65dB below the LO carrier level.
A second, less spectrally pure, LO signal is available at the DISP connector for use by the Display Module.
2-8 BFO Generator
The BFO Generator furnishes a 3395±4KHz signal at
+7dBm to the Product Detector, which converts the IF signal to audio. The BFO signal is produced by mixing the 4395±4KHz PBT signal with the output of a 1MHz crystal-controlled offset oscillator. The difference frequency of 3395±4KHz is amplified by a two stage tuned amplifier.
A second BFO output is used by the Display Module for frequency readout.
2-9 Display Module
The display module contains circuitry for the signal strength meter and digital readout. The S-meter is connected to a conventional balanced bridge circuit with one side driven by the AGC amplifier. The traditional ZERO and FULL SCALE adjustments are present for meter calibration. will indicate approximately S3, with a 50µV input indicating S9. A 30mV input will bring the S-meter to full scale and signals of 100mV and greater will peg the meter.
When the receiver is properly adjusted, the S-meter will indicate S1 when the AGC is off and the RF
GAIN control is at it’s clockwise end of rotation.
When the AGC is enabled, noise output from the IF amplifier will “tickle” the AGC and cause the meter reading to nudge upward slightly. An input of 1µV
The digital frequency readout is a straightforward frequency counter which counts a 7-7.3MHz signal that is produced by mixing the 10.395-10.695MHz
LO and 3395KHz BFO signals. The counter is based on an Intersil 7216C counter chip. Extensive shielding and decoupling are used in this and other modules to minimize leakage from the Display
Module to the receiver front-end circuitry.
Page 6
Circuit Description
2-10 Power Supply
The power supply consists of four regulated outputs of
+5, +12, +15, and +20 volts. The +5 volt output is used to power the TTL and CMOS chips used in the frequency display circuitry. The +15 volt section is used to supply power for the crystal filter selection diodes, the local oscillator output amplifier section, and the post-mixer amplifier. The AF power amplifier is operated from the +20 volt section, which is a National
Semiconductor LM338AK 4 amp voltage regulator.
The remaining circuitry is operated from the +12V section. LM78XX series 1 amp regulators are used for the +12, +15, and +5 volt outputs.
Digital Frequency Readout
Signal Strength Meter
Main Tuning
Notch Tuning
Passband Tuning
AC Power - AF Gain Control
AGC Control - Off/F/M/S
RF Gain Control
Figure 2-2 Front Panel Layout
Selectivity Control
2.1 kHz - 400Hz
Mode Switch
Monitor - Normal - Notch Enable
8 Ohm Speaker Outputs
1/4" Headphone jack or
RCA Connector.
Antenna Input
External Mute Defeat Switch
External Mute Input
Sidetone Input
Figure 2-3 Rear Panel Connections and Controls
AC Line Cord
Fuse 3/8 amp Slo-Blo
Page 7
AF Power Amplifier
Power Supply
Display Module
Circuit Description
Figure 2-4 Interior Views - Module Locations
VFO
Bandpass Filter
Crystal Filters
AF Processor
Board
IF Amplifier/ AGC
Mixer - Post Mixer
Amplifier Module
Top View
LO-Passband Generator Module
Audio Processing Board
BFO Generator
VFO
Bandpass Filter
Power Supply
Bottom View
400 Hz Noise Filter/ Product Detector
Page 8
Circuit Description
Alignment and Calibration
3-1 Required Test Equipment
• A DC voltmeter whose measurement uncertainty is better than 0.5% of reading.
• An oscilloscope with a bandwidth of at least 30Mhz.
• A frequency counter with 30MHz capability, 10Hz resolution, and a measurement uncertainty of 10 ppm.
• A spectrum analyzer with at least 70 dB dynamic range to 100MHz.
• A tracking generator for the spectrum analyzer.
• An RF generator with calibrated output from –130 dBm to +10dBm to at least 15MHz.
• A communications receiver with 10MHz coverage of WWV, NIST’s standard time and frequency station.
3-2 VFO
There is only one adjustment on the VFO module.
Tune the VFO for an output frequency of 6.1MHz.
This corresponds to a reading of 7100.0KHz on the display. Use an oscilloscope to monitor the VFO output at the spare RCA jack. Adjust the tuning slug located below the jacks near the bottom of the side panel for maximum output. Verify that the VFO’s output is about 5 volts peak to peak from 6.0 through
6.3MHz. NOTE: This is a very broad adjustment, which should not be required unless repairs have been made to the VFO.
3-3 LO - Passband Generator Module
Passband Center Frequency Adjustment
Center the front panel PASSBAND control. Connect a frequency counter to the PBT output and adjust L1 for a reading of 4395.0KHz. If a counter is not available, an alternate procedure would be to disconnect the antenna, select the 2.1KHz filter, center the
PASSBAND control, and adjust L1 for the lowest pitched noise.
PBT Buffer Amplifier Adjustment
Center the front panel PASSBAND control. Connect the LO output to a spectrum analyzer and observe the signal. Adjust L2, PBT buffer amplifier tune, for maximum output. Keep the spectrum analyzer connected. It will be needed for the next adjustment.
LO Bandpass Filter Adjustment
v
Adjust the receiver’s frequency to 7150.0KHz and adjust L3 for maximum output. v
Tune the receiver to 7300.0KHz and adjust C1 for maximum output.
v
Tune the receiver to 7000.0KHz and adjust C2 for maximum output.
v
Tune the receiver to 7200.0KHz and adjust C3 for maximum output.
v
Verify that the LO output is at least +7dBm from band edge to band edge. If this condition is not satisfied, repeat the previous steps. v
Verify that all harmonics and mixed products are at least 65dB below the LO carrier level.
L2- PBT Buffer Amplifier Tune
Passband Tuning Voltage
C1
L3
C2
LO Bandpass Filter Adjustments
C3
L1 - Passband
Center Frequency
Adjustment
+12v
4395 kHz PBT signal to BFO Generator
+15v
5v p-p from VFO
Figure 3-1 LO - Passband Generator
LO to Mixer
LO to Display
Page 9
Circuit Description
3-4 BFO Generator
1MHz Offset Oscillator Adjustment
Connect an oscilloscope to the 1MHz oscillator test point and adjust L1 for maximum output.
BFO Amplifier Adjustment
Center the front panel PASSBAND control. Use an oscilloscope to observe the signal at the RCA jack labeled DISP. Adjust C1 and C2 for maximum output.
L1 - 1 MHz Offset Oscillator Adjust
C1
3395 kHz BFO Output
BFO Output to
Display Module
1 MHz Offset Oscillator
Test Point
C2
4395 kHz PBT input
Figure 3-2 BFO Generator
3-5 Bandpass Filter
The two adjustments for the parallel resonators are accessible with the receiver’s top cover removed.
These adjustments are best made with a spectrum analyzer equipped with a tracking generator. Connect the tracking generator output to the filter input and connect the filter output to the spectrum analyzer’s input. Set the spectrum analyzer to sweep from 6 to
8MHz. Adjust the two capacitors for the flattest response from 7.0 to 7.3MHz. Verify that the insertion loss is less than 1dB.
Readjust the spectrum analyzer’s center frequency to
14MHz and verify that the filter’s response is at least
60 dB down.
3-6 Mixer - Post Mixer Amplifier
The only adjustment for this module tunes the resonator used to match the 50
Ω
output impedance of the post-mixer amplifier to the 2k
Ω
input impedance of the eight pole crystal filters. This adjustment is accessible with the receiver’s top cover removed.
Connect a signal generator to the antenna terminal and adjust the generator for a 7.05MHz, 3µV signal.
Verify that the receiver’s AGC is on FAST and the
RF GAIN control is fully clockwise. Select the 400Hz filter and tune in the generator’s signal. Adjust the trimmer capacitor for maximum indication on the signal strength meter.
Page 10
Alignment and Calibration
3-7 Crystal Filter - IF Amplifier - AGC Module
AGC Bias Adjustment
Rotate the front panel AGC switch to “F” and turn the front panel RF GAIN control fully clockwise. Adjust
R1 ten turns CCW and measure the voltage at the
AGC test point. Verify the voltage is 5.38±.01 volts.
This voltage is determined by a divider circuit formed by some fixed resistors and the RF GAIN potentiometer. Slowly turn R1 clockwise until the voltmeter reading increases by 10 millivolts to 5.39
volts. This ensures that noise output from the IF amplifier “tickles” the AGC system.
IF Amplifier Alignment
Connect the signal generator’s output to the receiver input. Adjust the generator for an output of 1µV at
7.05MHz. Set the receiver’s AGC to fast and select the 400Hz IF filter.
Tune in the generator’s signal and adjust C1, C2, &
L1 for maximum deflection on the signal strength meter. The meter should now indicate about S-3.
2.1KHz Filter Gain Equalization
Increase the generator output to 50µV, select the
400Hz filter, and adjust the PASSBAND control for a peak indication on the S-meter. The meter should be indicating approximately mid-scale. Make a note of the exact meter reading, then select the 2.1KHz filter.
Readjust the PASSBAND control for a new peak reading. Adjust R2 to obtain a meter reading that matches the peak value noted above for the 400Hz filter.
From AGC Switch
R1 - AGC BIas Adjust
RF Gain Control
+12 Volts
IF Amplifier
Output to
Product Detector
AGC Test Point
S-meter Signal to Display Module
Mute Control
Filter Module input
Connection
2.1 kHz Filter
Select Line
R2 - 2.1 kHz Filter
Gain EQ adjust
C2
C1
L1
400 Hz Filter Select Line
Figure 3-3 Crystal Filter - IF Amplifier - AGC
Page 11
Alignment and Calibration
3-8 Audio Processing Board
Notch Depth Adjustment
Connect the signal generator the receiver and adjust it’s output for a 500µV signal at 7.05MHz. Tune the receiver to 7050.6KHz, select the 400Hz filter and adjust the PASSBAND control for maximum S-meter reading. This will result in an continuous tone audio output of 600Hz.
Enable the notch and adjust the front panel NOTCH
TUNE control for minimum audible 600Hz tone.
Adjust R1 on the Audio Processing board for minimum audible tone. Note: it may be necessary to increase the AF GAIN control setting to hear the tone.
Because there is interaction between R1 and the
NOTCH TUNE control, it may be necessary to repeat the steps described in this paragraph. Do not change any settings for the next step.
SCF Cutoff Frequency
Connect an oscilloscope to the top arm of the AF
GAIN control. Tune the receiver to 7053.0KHz and adjust the PASSBAND control for maximum S-meter reading. This will result in an audio tone of 3KHz.
Turn R3 fully CCW and adjust the oscilloscope for a six division display. Slowly rotate R3 clockwise until the display is reduced to between 4 and 4.2 divisions.
Input Attenuator Adjustment
Verify that the notch filter is enabled and the 600Hz tone has been removed. Adjust R2 fully CCW and note the presence of substantial harmonic distortion.
Slowly rotate R2 clockwise to reduce the input level until all distortion products are inaudible. Rotate R2 for two additional turns. Turn off the notch filter and verify the presence of the 600Hz tone. Adjust the AF
GAIN control for a comfortable listening level, enable the notch, and adjust the NOTCH TUNE control until the 600Hz tone disappears. Verify there is no audible output from the receiver. Turn off the notch filter. Do not change any generator settings.
R1 - Notch Depth Adjust
Notch Enable FET Switch.
Applies signal to inverting
BPF for notch turn on.
R2 - Variable Input
Attenuator Pot.
AF Input Connector
Figure 3-4 Audio Processing Board
National Semiconductor
MF6CN-100 Switched
Capacitor Filter
R3 - SCF Cutoff Frequency
Adjustment.
AF Out to AF GAIN Control
Page 12
Alignment and Calibration
3-9 Display Module
Counter Timebase Calibration
Place the communications receiver in the AM mode and tune to WWV at 10MHz. Use a ten foot piece of wire as an antenna and position it near the display module. Adjust the antenna’s position such that the strength of the time base signal is comparable to
WWV’s. Adjust C1 such that the time base signal is zero-beat with WWV’s carrier frequency.
Note: It may be necessary to reposition the test receiver’s antenna very close to L3 in order to pick up a good signal.
Tune L1, L2, and L3 for maximum signal strength as indicated on the test receiver’s S-meter. Verify that the frequency readout on the DISPLAY MODULE is stable from band edge to band edge.
Counter Amplifier Adjustment
The counter amplifier tuning is not critical and adjustments should not be required unless repairs have been made to the DISPLAY MODULE.
Readjustment is required when the frequency readout will not display reliably from band edge to band edge.
S-meter Adjustment
Tune the receiver to 7050.0KHz, turn the RF GAIN fully clockwise, the AGC to AGC OFF and select the
400Hz filter. Remove the signal generator and adjust
R2 for a reading of S-1.
Tune the K9GDT receiver for a display reading of
7125.0KHz. Next, tune the test communications receiver to 7125KHz and pick up the signal generated by the DISPLAY MODULE.
Reconnect the signal generator and adjust it’s output to 30 mV at 7.05MHz. Turn the AGC switch to “F” and adjust the PASSBAND control for maximum reading on the S-meter. Adjust R1 for a full scale meter reading of +60 dB.
L3 - 7 MHz Amp Tune
L2 - 7 MHz Amp Tune
+12 Volts
LO Input (Top)
PBT Input (Bottom)
L1 - LO Buffer Amp Tune
R1 - Signal Strength Meter Full Scale Adj.
R2 - Signal Strength Meter Zero Adj.
M1 - 100 Microamperes F.S.
Figure 3-5 Display Module
C1 - Counter Time
Base Adjust
+5 Volts
Page 13
