Transmit Modulation Balance (Compensation) - Motorola GM950E Service Manual

Radio Tuning Procedure
7.10

Transmit Modulation Balance (Compensation)

Compensation alignment balances the modulation sensitivity of the VCO and reference modulation
(synthesizer low frequency port) lines. Compensation algorithm is critical to the operation of
signalling schemes that have very low frequency components (e.g. PL) and could result in distorted
waveforms if improperly adjusted. The compensation value needs to be set at 7 frequencies across
the frequency range. For 20/25kHz radios, the radio stores separate tuning data for 20kHz and
25kHz channel spacing. Therefore, both sets of tuning data should be set independently.
1. From the Service menu, select Tx Alignment.
2. Select ÔModulation BalanceÕ to open the deviation balance tuning window. This window is used
to set the values for 12.5kHz radios and the 25kHz data for 20/25kHz radios. The window will
indicate the transmit test frequencies to be used.
3. Set the Test Box (GTF180B) meter selector switch to the ÒGENÓ position, and inject a 2kHz (two
kilohertz) tone at 800 mVrms (eight-hundred millivolts) into the ÒAudio InÓ port.
4. Connect an AC meter to the meter port to insure the proper input signal level.
5. Select the Þrst test frequency shown in the window.
6. Press Toggle PTT to key the radio. The status bar will indicate that the radio is transmitting.
7. Measure the transmitter deviation.
8. Adjust the transmitter deviation using the appropriate softpot to the value shown in Table 3-8.
9. Press Toggle PTT to dekey the radio.
10. Repeat steps 5- 9 for the remaining test frequencies.
11. Press Program to store the softpot values.
12. If the radio is a 20/25kHz channel spacing model, repeat steps 1 - 11 for 20kHz channel
spacing using the ÔModulation Balance (20kHz)Õ window.
7.11 Transmit Control Voltage Limit
The transmit control voltage limit softpot sets the maximum power control voltage. All 7 voltage limit
softpots are tuned and programmed automatically when the Program button is pressed.
1. From the Service menu, select Tx Alignment.
2. Select Control Voltage Limit to open the control voltage limit tuning window.
3. Set the Power Factor to 1.3.
4. Press Program to store the softpot values.
3-14
Table 3-8 Transmitter Deviation.
Channel Spacing Deviation
12.5 kHz 2.1-2.2 kHz
20 kHz 3.4-3.5 kHz
25 kHz 4.3-4.5 kHz
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