[hfsig] Sidebands on Raspberry Pi WSPR carrier

Orrin Winton orrin.winton at gmail.com
Mon Sep 5 14:29:27 CDT 2016


Hmm, my 20m antenna is only for the Pi. It is not connected to any ground.

The 2A power supply for the Pi is 2-prong. The 3rd prong (ground) is not
used.

Power out is 90-100mW.

And i get the sidebands. I'm not overly concerned. Maybe i should be; i
don't know. The sidebands are -15dB from my main signal, and the reception
reports i get (on wspr.aprsinfo.com) are -20 to -25, with no duplicates
that i can see, so i doubt anyone is hearing the sidebands.

Orrin wn1z

On Sep 5, 2016 09:11, "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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>
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