User's Manual

Maintenance GMH Series VHF Radio
Page 5-10 BK RADIO
5.5.8 DEVIATION ADJUST
Connect a suitable attenuator and service monitor to the antenna output of the radio. Connect the
modulation output of the service monitor to the Mic High input of the radio. Adjust the service
monitor modulation output to a level of 0.15 VRMS and a frequency of I KHz. Select "Deviation
Adjust" from the "Tune GMH" menu and press Enter. Insure that "Start Deviation Adjustment" is
highlighted and press Enter. "Ok to transmit?" appears in the lower left corner of the computer
screen. When you press Y the radio begins transmitting and the present tuning values appear.
Press the Tab or arrow keys to move the cursor to the 2-character hexadecimal control value of
the first transmit frequency. Type in a new value or press the Page Up or Page Down keys to set
TX FM deviation to 4.5 kHz. Repeat for each of the four frequencies, then select "Store Deviation
Adjustment" and press Enter. "Write Dev Comp values to radio?" appears in the lower left corner
of the computer screen. Press Y to store the new values in the radio, or N to cancel changes.
This adjustment controls the maximum FM deviation of the transmitter. Since the deviation
sensitivity changes with carrier frequency, the adjustment must be performed at several
locations in the band. If components in the VCO or transmit audio amplifier or filter are
changed this adjustment may be necessary to maintain an FM deviation below 5 kHz.
The level of the transmit 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.
5.5.9 REFERENCE MODULATION
NOTE: For radios using Digital Code Guard only.
"Start Freq Adjust" appears in the middle of the computer screen. When you press Enter, the
message "Ok to transmit?" appears in the lower left corner of the computer screen.
Connect a suitable attenuator and service monitor to the antenna output of the radio. Adjust the
service monitor modulation output to a level of 0 VRMS. In the "Tune GMH" menu, select
"Reference Mod" and press Enter "Start Ref Mod adjust" appears in the middle of the computer
screen. When you press Enter, the message "Ok to transmit?" appears in the lower left corner of
the computer screen. When you press Y the radio begins transmitting and displays "PROG." On
the computer screen, the hexadecimal tuning value is outlined.
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 change the value. Observe the Digital Code
Guard signal on the service monitor display. Adjust the control value up or down to obtain the
flattest waveform with as little overshoot as possible. When adjustment is complete, press the Esc
key on the computer.
Reference modulation is used to allow transmission of low frequency Digital Code Guard
signals. The reference modulation sensitivity must be adjusted to match the transmit audio
deviation. This adjustment should be made after any necessary "Transmit Audio Deviation",
"Reference Oscillator Crystal Parameters" or "Reference Oscillator Frequency" adjustments.