Front-End Filter - Motorola GM950E Service Manual

1.6

Front-End Filter

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. If the radio supports 20 or 25 kHz channel spacing
selection, use the parameters for 25 kHz channel spacing.
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 Rx Alignments.
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 the start value shown
in table 6.2-4.
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 6.2-3.
6. Measure the RSSI voltage at accessory connector pin 15 with a dc voltmeter capable of 1mV
resolution and at least 1Mohm input impedance.
7. Change the softpot value by the stepsize shown in table 6.2-4 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 6.2-4, 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 (GTF180) 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.
Radio Tuning Procedure
Table 6.2-2 Reference Oscillator Alignment.
RF-Band
All bands
Table 6.2-3 Normal Test Deviation.
Channel Spacing
12.5 kHz
20 kHz
25 kHz
Table 6.2-4 Start Value for Front-End Pre-selector Tuning.
RF-Band Target
216-246MHz 0.29%
216-246MHz Tuning Procedure
Target
±150 Hz
Deviation
1.5 kHz
2.4 kHz
3 kHz
Stepsize Start Value
+4 Minimum
6.2-5