Transmit Deviation Limit; Transmit Modulation Balance (Compensation) - Motorola GM950E Service Manual

1.9

Transmit Deviation Limit

The transmit deviation limit softpot sets the maximum deviation of the carrier. The deviation limit
needs to be set at 7 frequencies across the frequency range. Unlike other radios, the deviation limit
for 216-246MHz is set using low frequency (PL) rather than the usual 1 kHz tone. No audio signal
must be injected, the radio generates a 82.5 Hz tone while the deviation limit alignment window is
open. This tone is used to set the maximum deviation. If the radio supports 20 or 25 kHz channel
spacing selection, the radio stores separate tuning data for 20 kHz and 25 kHz channel spacing.
Therefore, both sets of tuning data should be tuned independently
1. From the Service menu, select Tx Alignments.
2. Select 'Deviation Limit' to open the deviation limit tuning window. This window is used to set the
values for 12.5 kHz radios and the 25 kHz data for 20/25 kHz radios. The window will indicate
the transmit test frequencies to be used.
3. Select the first test frequency shown in the window.
4. Click the Toggle PTT button to key the radio. The status bar will indicate that the radio is
transmitting.
5. Adjust the transmitter deviation to the value shown in table 6.2-5.
6. Click the Toggle PTT button to dekey the radio.
7. Repeat steps 3-6 for the remaining test frequencies.
8. Click the Program button to store the softpot values.
9. If the radio supports 20 or 25 kHz channel spacing selection repeat steps 1- 8 for 20 kHz
channel spacing using the 'Deviation Limit (20 kHz)' window.
.
1.10

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 and for every channel spacing supported by the radio.
1. From the Service menu, select Tx Alignments.
2. Select Modulation Balance to open the deviation balance tuning window. This window is used
to set the values for 12.5 kHz radios and the 25 kHz data for 20/25 kHz radios. The window will
indicate the transmit test frequencies to be used.
3. Set the Test Box (GTF180) meter selector switch to the „GEN" position, and inject a 2 kHz (two
kilohertz) tone at 800 mVrms (eight-hundred millivolts) into the "Audio In" port.
4. Connect an AC meter to the meter port to ensure the proper input signal level.
5. Select the first test frequency shown in the window.
Radio Tuning Procedure
Table 6.2-5 Transmitter Deviation Limit Alignment Target.
Channel Spacing
12.5 kHz
20 kHz
25 kHz
216-246MHz Tuning Procedure
Deviation
375 kHz
600 kHz
750 kHz
6.2-7