[hfsig] Sidebands on Raspberry Pi WSPR carrier

Ron Ott ronott at sbcglobal.net
Mon Sep 5 14:39:21 CDT 2016


Hi Orrin. That's true if your signal is always 20-25dB below the peak noise level (as WSPR reports signal levels), but more local receivers will hear and be able decode multiple concurrent signals as I did last night.  Technically, these sidebands are not FCC acceptable, although it's difficult to get worked up over a 100mW transmitter with spurious sidebands.  
I appreciate your comment about the apparent isolation of your RPi from grounds, including the antenna. I'll run that experiment today using my own station; I can confidently say that I am sideband free when I run the RPi into a 50-ohm dummy load that is isolated from any grounds and I'm hoping an isolation balun will be the cure.
73, 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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