What is the correct audio level for BPSK modulation? #hamradio

In my whispering airwaves article I mentioned the problem of over-modulation of the SSB transmitter when you feed it a PSK audio signal. In this article I will demonstrate the correct audio level and the corresponding ALC readings to prevent over-modulation. I’m speaking about the ALC meter of the Yaesu FT-991, be aware that the behavior of ALC meters on other transceivers may be different. This ALC meter has a blue zone on the left, when you transmit SSB it should stay in this zone, when your microphone gain is too high it overshoots to the right into a red zone where the white bar over the ALC blue zone indicator becomes red. In the image below you can see the correct maximum ALC value when you speak into the microphone, in my case a microphone gain of 80 percent prevents that the ALC meter overshoots the blue zone.

IMG_2892
The ALC meter on the Yaesu FT-991 while we transmit USB.

In order to measure what PSK modulation does I mounted a 50 Ohm dummy load via a PL-259 T-connector to the FT-991 transceiver. The signal inspection output of the T connector is a voltage over the 50 Ohm resistor that mimics the antenna load. You need to attenuate this voltage by 10dB or more to get it down to a voltage that your oscilloscope likes. The figure below shows my 10 dB attenuator, its input impedance is 11k Ohm and on the output it is 1 k Ohm, the scope can stay at a 1 MegaOhm or greater impedance setting.  In the image below the right BNC receives the input from the T connection and left BNC goes via a coax cable to the scope, I used 1 Watt resistors. Limit the transmitter to a power level that the dummy load can sustain, and tell your PSK program to set it at receive acknowledge mode at a high PSK datarate (like PSK-200). This generates a continuous PSK signal, next play with the trigger of your scope until the envelope remains fixed on the screen.

IMG_2882
My home made 10 dB attenuator

The following image shows a distorted PSK signal because the audio level was far too high. In this case the ALC meter was at the end of the blue zone which would have been an acceptable level for SSB, but it is not acceptable for PSK because the peaks of the sine waves are shaved off. They should be like sine waves.

IMG_2886
Here the audio level was too high, the ALC meter went up to the end of the blue zone, this is ok for USB but it is too much when you transmit PSK.

Next I turned the audio level of the PSK signal back to roughly 2/3 into the ALC blue zone, and this also shows that the sine waves are shaved off, it will lead to distortion in the radio spectrum, albeit less than that we had in the first measurement.

IMG_2885
Here the audio level was still too high, the peaks of the envelope are still clipped off, but less than in the previous figure, the ALC meter was at 2/3 into the blue zone.

Next I turned the audio level down up to the point where well shaped sine waves appeared on the scope, and this happens when you reduce the audio level back so that the ALC meters goes 1/3 into the blue zone.

IMG_2884
This is what you want, the ALC meter now reaches 1/3 into the blue zone. This is apparently the correct audio level to prevent unnecessary distortions in the spectrum

Required for the experiment is: a scope that has enough bandwidth, 30 MHz could do the job but a 400 MHz was used for these measurements, a 50 Ohm dummy load, a T-corrector and a 10 dB attenuator, and all required cables to convert from PL to BNC and N connector (my dummy load had an N connector).

Last update: 30-April-2016

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