[hfsig] Sidebands on Raspberry Pi WSPR carrier

Ron Ott ronott at sbcglobal.net
Mon Sep 5 15:19:20 CDT 2016


I have just run the same experiment you did: RPi connected directly to my antenna, which is a vertical with ground radials that eventually connect to ground rods, about 100 feet from the shack via RG213.  There are no other ground connections to the RPi.
There are sidebands, but now they are 30dB down from the carrier as indicated by the WSJT-X signal strengths.  This level is acceptable to me and I hope the balun project produces equal or better results.
Ron/W6XY


      From: Orrin Winton <orrin.winton at gmail.com>
 To: TAPR HF Modes SIG Mailing List <hfsig at tapr.org>; Orrin Winton <orrin.winton at gmail.com> 
 Sent: Monday, September 5, 2016 12:29 PM
 Subject: Re: [hfsig] Sidebands on Raspberry Pi WSPR carrier
   
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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