User's Manual

GMH Series VHF Radio Maintenance
BK RADIO Page 5-11
Unlike normal voice and audible tones, the low frequency signals present in Digital Code
Guard cannot be applied as VCO modulation only. They must be presented to the reference
oscillator using the DIA network RN1 and varactors CR102 and CR103 (system ). The sub-
audible information passes through the core microprocessor Ii (system board) where the
variable attenuation is effected by software. The Digital Code Guard signals are generated by
the control processor Ii (control board) and sampled by microprocessor Ii 5 stem board) on
the LF AUDIO (pin 13) line using an AID converter. The final setting is stored in non-volatile
memory.
5.5.10 TUNE RECEIVER
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. Select "Tune Receiver" from the "Tune GMH"
menu and press Enter. Insure that "Start Rx Front End Alignment" is selected and
press Enter. The radio displays "PROG". Press the Tab key to select an
alignment frequency and perform the following procedure for each of the three
listed frequencies.
Set the RF signal generator carrier frequency to the alignment frequency. Adjust
the generator output level to obtain a SINAD of approximately 12 dB. Press the
Tab or arrow keys to move the cursor to the hexadecimal tuning value in the
"Common" column. Type in a new value or press Page Up or Page Down to
change the value and obtain the maximum SINAD reading. (During the procedure,
anytime the SINAD exceeds 18 dB, the generator output level should be
decreased to return to 12 dB SINAD.)
Press the Tab or arrow keys to move the cursor to the hexadecimal tuning value
in the "VARS" column. Type in a new value or press Page Up or Page Down to
change the value and obtain the maximum SINAD reading. Once the SINAD has
been maximized using the "VARS" value, repeat the procedure for VAR4, VAR3,
VAR2, then VAR1. Press the Tab or arrow keys to select "Store Front End
Alignment" and press Enter. The message "Write radio core EEPROM?" appears
in the lower left corner of the computer screen. Press Y to store the new settings
in the radio, or N to cancel changes.
The final settings of VAR1, VAR2, VAR3, VAR4, and VARS should be greater
than 04. If this is not the case, decrease the "Common" value and repeat the
adjustment of the VAR settings. When finished, press the Esc key to return to the
main screen.
Bandpass filters in the receiver front end are varied for different receive
frequencies to optimize radio performance. The tuning is customized using the
receiver front end tuning adjustment. If components in the preselectors, RF
amplifier, or mixer are replaced this adjustment may be necessary to maintain
specified RF sensitivity.