[hfsig] Sidebands on Raspberry Pi WSPR carrier

Zoltan Doczi zoltan.doczi at gmail.com
Mon Sep 5 12:14:40 EDT 2016

Hi Ron,

You might also give a try to run RPi from WiFi connection instead of
ethernet or simply remove ethernet connector for shake of testing momentary.


On Mon, Sep 5, 2016 at 6:08 PM, Ron Ott <ronott at sbcglobal.net> wrote:

> Last night I completed some revealing tests with the Raspberry Pi (=RPi)
> TX with TAPR 20m WSPR kit.  *The bottom line is that grounds are the
> cause of the observed spurious sidebands.*
> * I see no sidebands when the TX is connected to a 50-ohm load and my HP
> scope with a *floating ground *input.  I'm receiving on an SDR with an
> outside 20m vertical antenna, picking up stray RF.  In this setup the RPi
> TX is fully isolated from any grounds in the shack.
> * When I move the 50-ohm load to the input of an HP frequency counter with
> a grounded-shield (1Mohm) input, I see several sidebands on the carrier
> that are multiples of 60Hz. In this setup the shield side of the RF output
> coax is connected to the mains ground through the frequency counter.  I
> decoded four (4) WSPR signals simultaneously - the carrier and three of the
> sidebands.
> * I played with ground leads to the RPi USB connector shells and to the
> ground side of the RF output terminal strip, but there was no change in the
> sidebands.  Since the RPi is a small computer, the grounds are not that
> easy to discern and it may take some experimenting to discover what works.
> ** Potential solutions*: Perhaps running the antenna lead directly to the
> transmitter without a ground connection in the shack might help - an
> experiment for tomorrow.  An un/un transformer might be used to isolate the
> RPi grounds from the shack ground and avoid the ground loops.  I'll look
> into making an un-un myself or buying a commercially built one from Balun
> Designs.
> Ron, W6XY
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