User's Manual

Maintenance GMH Series VHF Radio
Page 5-12 BK RADIO
Four varactors are used in the receiver front end bandpass filter. The control
voltages for these varactors change to accommodate different receiver
frequencies. The proper values are determined by microprocessor Ii (system
board) using data stored in non-volatile memory. All four varactors share a
common tuning voltage generated by DIA network I3 and RNA (RX/TX
board). Each varactor also receives an individual tuning voltage from one of
the D/A converters of 12 (RX[TX board) that corrects for minor differences in
the varactors and other components of each filter. During the tuning process,
all the individual varactors are set to a fixed value while the common voltage
from 13 and RN1 is adjusted for best receiver sensitivity. The individual
voltages from 12 are then adjusted to optimize the tuning of each varactor.
5.5.11 DISCRIMINATOR ADJUST
Connect an RF signal generator to the radio antenna input. Set the generator FM
modulation to a frequency of 1 kHz and a deviation of 3 kHz. Connect an AC
voltmeter to the Discriminator Audio output from the Accessory Connector.
In the "Tune GMH" menu select "Discriminator Adjust" and press Enter. The radio
unmutes and displays "PROG". "Start Discr Adjust" appears in the middle of the
computer screen. When you press Enter to begin, the frequency and hexadecimal
tuning value are outlined. Set the RF signal generator carrier frequency to the
alignment frequency. With the cursor under the 2-character hexadecimal value in
the right column of the menu, type in a different value, or press Page Up or Page
Down to adjust the control value to obtain a voltage reading of 0.707 V RMS on
the voltmeter. When finished, select "Store Discr value" and press Enter.
This setting controls the amplitude of the receiver audio signal from the FM
discriminator. If components in the discriminator are replaced, this adjustment
may be necessary.
The level of the discriminator audio is controlled using a digital potentiometer
19 (system board). The microprocessor Ii (system board) sets values for the
attenuation as the adjustment is performed. The final setting is stored in non-
volatile memory.
5.5.12 SQUELCH ADJUST
Connect an RF signal generator to the radio antenna input. Set the generator FM modulation to a
frequency of 1 kHz and a deviation of 3 kHz. Connect a SINAD meter to the radio audio output.
In the "Tune GMH" menu select "Squelch Adjust" and press Enter. The radio unmutes and displays
"PROG". "Start squelch adjust appears in the middle of the computer screen. When you press
Enter, the radio is automatically tuned to the indicated frequency (which can be changed before
the test starts). Adjust the RF level to obtain 12dB Sinad. With "Store squelch values" highlighted,
press Enter to save the changes in the radio.