[amsat-bb] Using PowerSDR to copy AO-85 DUV telemetry

Alan wa4sca at gmail.com
Wed Nov 25 12:00:28 UTC 2015


There are several users on this list who use Flex rigs, pre-6k series, for satellite operations. There
is a problem with the Flex PowerSDR FM implementation because it lacks a way to bypass the audio
filtering. (Fixed in SmartSDR.) The low frequency attenuation is good enough to prevent using the
normal audio to decode DUV, but poor enough you can hear the DUV rumble, or the PL tones on a normal
repeater. However, there is a way, suggested by Dave, W0DHB to make this work for DUV telemetry.
Fortunately the PowerSDR I/Q output bypasses the filtering, and FoxTelem can accept I/Q inputs.
(According to the Flex people, the convention for I/Q is reversed in PowerSDR, but I have had no
problem interfacing it to FoxTelem.)

If your only interest is telemetry, there are alternate approaches which are probably better. I wanted
to be able to copy telemetry, but also be able to communicate at the same time. I will assume users
have already installed Virtual Audio Cables, know how to hook software programs together, and are
familiar with FoxTelem. All have good documentation. Additionally, different choices for sampling and
buffers may well work, as well as different procedures for tuning, but these are what I am using

In PowerSDR, go to VAC 1 and select 48k sampling, 1024 buffers. (Under Primary Audio, I use my normal
192k sampling, 1024 buffers.) Check both boxes in the Direct I/Q box. The VAC audio level control does
not have any effect on I/Q signal levels. Enable VAC 1.

In FoxTelem, select the correct cable to get the audio from PowerSDR, 48k sampling, and tick I/Q and
low speed.

I use SatPC32 to drive the Flex VU-5K. Tweak the downlink to center up the display, as you do for
ordinary voice. Start FoxTelem. You will see the characteristic AO-85 display in the lower FFT pane.
Do NOT use the Find Signal or Track Doppler, yet. Instead, click on the FFT display to tune FoxTelem.
You should be rewarded with a nice eye pattern, and SNR of 5-10. If you have a strong signal, select
the Track Doppler. It will hunt briefly, and then settle down to something close to optimum.

Generally leaving Doppler Track checked is the most flexible since it will adapt to the slight changes
in the downlink frequency. You may miss a few packets around AOS however. If you uncheck it, the
"sweet spot" will not change since SatPC32 will keep it tracking, so long as you adjust for the
current downlink frequency. I normally leave my checked.

This technique gives nearly 100% copy without compromising voice operations. In principle it should
work for high speed telemetry, but to date I have not been successful. Probably a sample rate/buffer




Keep Calm and Carry On

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