Front-End Filter. Motorola GM950 403-470 MHz, GM950, GM950 136-174 MHz, GM950 66-88MHz

UHF (403-470MHz) Tuning Procedure

1.6

Table 2-2 Reference Oscillator Alignment.

RF-Band

All bands

Target

±

150 Hz

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 2-4.

5.

Set the RF test generator to the receive test frequency, and set the RF level to 10

µ

V modulated with a 1kHz tone at the normal test deviation shown in table 2-3.

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 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 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.

Table 2-3 Normal Test Deviation.

Channel Spacing

12.5 kHz

20 kHz

25 kHz

Deviation

1.5 kHz

2.4 kHz

3 kHz

Table 2-4 Start Value for Front-End Pre-selector Tuning.

RF-Band

UHF

Target

0.5%

Stepsize

-2

Start Value

Maximum

Radio Tuning Procedure 5A.2-5