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GNS 400W Series Maintenance Manual
Page 4-39
190-00356-05
Rev. A
4.
Measure the AM modulation of the transmitted signal.
5.
Unkey the transmitter.
6.
Repeat steps 2-6 at carrier frequencies of 127.000 and 136.975 mHz.
7.
Verify that for a 10 watt com radio modulation must be between 80% and 95%. For a 16
watt com radio modulation must be between 80 and 100%.
Next test:
1.
Set the unit to 127.5 mHz.
2.
Set the signal generator for a standard microphone audio signal.
3.
Key the microphone.
4.
Measure the AM depth of modulation.
5.
Set the microphone audio for 2.75 V.
6.
Measure the AM depth of modulation.
7.
Verify difference in modulation is 0 dB (± 0.5dB).
Carrier Noise Level
The demodulated noise on the transmitter output without audio modulation must be at least 45 dB
below the demodulated audio level obtained when the transmitter is modulated with a standard
microphone input signal. Measure at channels 118.500, 127.500, and 136.500.
1.
Generate and apply a standard microphone test signal.
2.
Set the carrier frequency to 118.000 mHz.
3.
Key the transmitter.
4.
Measure the signal to noise ratio (SNR) of the transmitter output.
5.
Unkey the transmitter.
6.
Repeat steps 3-5 using RF carrier frequencies of 127.000 mHz and 136.975 mHz.
7.
Verify SNR
≥
45 dB.
Frequency Stability and Tolerance
The RF carrier frequency must be within 0.0005% (5 PPM) of the selected channel frequency
when the transmitter is modulated with a standard microphone input signal. Measure at channels
118.500, 127.500, and 136.500.
1.
Set the unit to 136.500 mHz.
