Radio Tuning Procedure
3-10 Maintenance
7.5 Reference Oscillator
Adjustment of the reference oscillator is critical for proper radio operation. Improper adjustment will
not only result in poor operation, but also a misaligned radio that will interfere with other users
operating on adjacent channels. For this reason, the reference oscillator should be checked every
time the radio is serviced. The frequency counter used for this procedure must have a stability of 0.1
ppm (or better).
1.
From the Service menu, select Transmitter Alignment.
2.
Select Reference Oscillator to open the reference oscillator tuning window. The tuning window
will indicate the target transmit frequency.
3.
Click Toggle PTT to key the radio. The status bar will indicate that the radio is transmitting.
4.
Measure the transmit frequency on your frequency counter.
5.
Adjust the reference oscillator softpot in the tuning window to achieve a transmit frequency
within the limits shown in table 3-4.
6.
Click Toggle PTT again to dekey the radio and then press Program to store the softpot value.
7.6 Front-End Pre-Selector
Alignment of the front-end pre-selector is normally not required on these radios. Only if the radio has
poor receiver sensitivity or the pre-selector parts has been replaced the following procedure should
be performed. The softpot value sets the control voltage of the pre-selector. Its value needs to be set
at 7 frequencies across the frequency range.
1.
Set the test box (GTF180) meter selection switch to the "Audio PA" position and connect a
SINAD meter to the "METER" port.
2.
From the Service menu, select Receiver Alignment.
3.
Select Front End Filter to open the pre-selector tuning window. The window will indicate the
receive test frequencies to be used.
4.
Select the first test frequency shown, and set the corresponding value to maximum.
5.
Set the RF test generator to the receive test frequency, and set the RF level to 10µV modulated
with a 1 kHz tone at the normal test deviation shown in table 3-5.
6.
Measure the RSSI voltage at accessory connector pin 15 with a dc voltmeter capable of 1 mV
resolution.
7.
Change the softpot value by the stepsize shown in table 3-6 and note the RSSI voltage.The
target softpot value is achieved when the measured RSSI voltage change between step 6 and
step 7 is lower than the tuning target for the first time. The tuning target, shown in table 3-6, is
expressed as the percentage of the measured RSSI voltage and must be recalculated for every
tuning step. If the measured RSSI voltage decreases before the target value has been
achieved, approximation should be stopped and the current softpot value should be used as
target value. Set test box (GTF180B) audio switch to the “SPKR” position. The 1 kHz tone must
be audible at the target value to make sure the radio is receiving.
8.
Repeat steps 4 - 7 for all test frequencies shown in the window.
9.
Click the Program button to store the softpot values.
Table 3-4
Reference Oscillator Alignment
RF-Band Target
All bands ±150 Hz
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