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
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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.
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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
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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
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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
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