Reference Oscillator - Motorola GM950E Service Manual

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 Tx Alignment.
2. Select Reference Oscillator to open the reference oscillator tuning window.
3. Press 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 on the RSS screen to achieve a frequency as measured
on the frequency counter to be within the limits shown in table 3-4 of the target frequency
displayed on the RSS window.
6. Press 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 have 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 (GTF180B) meter selection switch to the ÒAudio PAÓ position and connect a
SINAD meter to the ÒMETERÓ port.
2. From the Service menu, select Rx 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 Þrst test frequency shown, and set the corresponding value to the start value shown
in Table 3-5.
5. Set the RF test generator to the receive test frequency, and set the RF level to 10µV modulated
with a 1kHz tone at the normal test deviation shown in table 3-6.
6. Measure the RSSI voltage at accessory connector pin 15 with a dc voltmeter capable of 1 mV
resolution. The RSSI output is available on A2, 4 channel, radios but it is unbuffered. Therefore
a high impedance (1 MW) voltmeter must be used.
7. Decrease/increase the softpot value and note the RSSI voltage. The target softpot value is
achieved when the voltage change between 2 softpot steps is lower than 0.75% of the RSSI
voltage for the Þrst time. Set test box (GTF180B) audio switch to the ÒSPKRÓ position. The 1kHz
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. Press Program to store the softpot values.
Maintenance
Table 3-4 Reference Oscillator Alignment.
RF-Band
UHF
VHF
Radio Tuning Procedure
Target
±150 Hz
±150 Hz
3-11