User's Manual

Maintenance GMH Series VHF Radio
Page 5-8 BK RADIO
From the Edit Screen, press F6 (Menu). Press the down arrow key to select "Tune Radio" then
press Enter. The program reads the EEPROM map of the core microprocessor Ii (system board).
This takes about 5 seconds, during which the radio displays PROG.
5.5.4 SYNTHESIZER CALIBRATION
Select "Synthesizer Calibrations" from the "Tune GMH" menu then press Enter. The radio will
automatically perform any necessary adjustment. This takes about 15 seconds, during which the
radio displays PROG.
Synthesizer calibration affects the way the voltage from the pretune DIA converter applied to
the VCO STEER line changes with frequency. This adjustment may be necessary if VCO
components are replaced. Synthesizer calibration should be performed before any other
adjustments are made.
During the automatic calibration, the core microprocessor Ii (system board) will direct the
synthesizer to acquire lock at various frequencies in both receive and transmit mode. For each
test frequency the microprocessor will vary the voltage from the DIA converter network 1102
(system board) used to assist the synthesizer in obtaining frequency lock. The LOCK DETECT
signal is monitored to determine when lock has occurred. The DIA voltages that result in
proper operation will be recorded in the microprocessor non-volatile memory.
5.5.5 CRYSTAL COLOR
Select "Crystal Color" from the "Tune GMH" menu then press Enter. A list of crystal colors
appears. Select the correct color from the list and press Enter. The message "Write Crystal Color
to EEPROM (Y/)?" appears in the lower left corner of the computer screen. Press Y to store the
new color value, or N to cancel.
The reference oscillator crystal must be compensated for temperature variations. One of four
types, specified by a color dot on the crystal, is used in the GMH. If the reference oscillator
crystal is replaced with one of a different color code, the programmed compensation value
must be changed.
Different crystal temperature compensation curve parameters are stored in the radio core
microprocessor Ii (system board) for each color dot. In normal operating mode, the
microprocessor observes the temperature using RT1OO. This information, along with the
stored curve parameters, is then used to develop a crystal temperature compensation value.
This value is used to slightly modify the reference oscillator frequency using DIA network RN1
and varactors CR102 and CR103 (system board).
5.5.6 TRANSMITTER POWER
NOTE: Transmitter should not be keyed for extended periods while setting TX power.
Prolonged transmitting will cause the thermal protection circuitry of the radio to
decrease the TX power.