[Skimmertalk] red pitaya noisy

W3OA w3oa at roadrunner.com
Sun Jul 31 17:10:20 EDT 2016

Hi Dave -

This looks like great progress in using the Red Pitaya.  Thank you.

But I'm concerned because of the following FAQ on 

Do you think it would be advisable to provide some external protection 
circuit on the antenna input?

73 - Dick, W3OA

On 7/31/2016 11:28 AM, David Robbins wrote:
> Previously for those not included in my discussion with pete:  “Got it 
> running now with the hermes interface dll, but I am plugged in to the 
> router.    Running it on the splitter with a qs1r on the other port, 
> both running cw skimmer server 1.50.  the rp receive seems much less 
> sensitive.  On 20m the qs1r has 340 decoders running, the rp only has 
> 45 and is generating a lot less spots.  The ones that both of them 
> spot about the same time the snr is 2-6db lower on the rp.   More 
> later, going to cut some grass or something while the wx is nice.”
> Then when I dug into it some more:  “While I would like someone with 
> professional instrumentation to repeat this I think you can see a 
> clear difference in the narrow frequency displays of hdsdr below.  The 
> one on the left is the qs1r, the right is the rp.  It looks like the 
> rp is about 5db noisier.  I zoomed in on the signal peaks to compare 
> them closer and the amplitudes are almost identical as close as I can 
> tell, so the signal strength calibration is about the same.
> cid:image001.png at 01D1E020.4FD7C0D0
> There are of course many things that could contribute to this.
> 1. Power supplies are different, I am using the supplied rp supply for 
> it but an after market supply for the qs1r.
> 2. Front end or adc noise
> 3. The rp is not in an enclosure like the qs1r maybe some local noise 
> pickup
> 4. The sample rate conversion could contribute noise
> p.s. that was a xg2 at 1uv on 40m going into a dxengineering splitter 
> feeding each of the receivers.  The rp is using the hermes interface 
> dll to cwsl_tee to hdsdr and skimmer server.  the qs1r is the same 
> software setup except using the qs1r interface dll.
> Today:
> Hmmmm, nice rainy morning so played some more with the rp.  I think I 
> found the problem.  on the input lines there are two sets of jumpers… 
> I didn’t quickly find a final schematic, apparently they are keeping 
> those private, but I did find a ‘development’ schematic, excerpt shown 
> here:
> So jp1 sets the input to one of two attenuators, and jp2 sets the 
> output.  I thought I would try the HV setting, which attenuated my 1uv 
> to below the noise level.  But I noticed when I pulled the jp2 jumper 
> off the output noise went well below the display level I had set on 
> the hdsdr.  So after playing around injecting my signal to pin 5 of 
> jp2 through capacitors and jumpers and generally making a lot more 
> noise, I just put one of their jumpers from pin 2 to pin 5 which 
> bypasses the two input attenuators and voila, now the two cw skimmers 
> are generating almost identical spots.  In fact some of them from the 
> rp are 1db higher snr, and a few are a bit lower, but that is bound to 
> happen just from the skimmer software timing of when it generates the 
> snr value, one may sample before or after the other and get a noise 
> spike or not.
> So my guess is that somehow the input attenuator is coupling noise 
> from the board ground into the input signal going to the amp/filter 
> block.  But it would take someone who knows more about the low level 
> design of the amp to figure it out.  I guess I would have expected to 
> see the inputs isolated from the digital board ground and going into a 
> differential amp, but I can’t tell that from the schematic I found. 
>   it could also just be that the attenuation of the input before the 
> amp dropped the signal closer to the amp noise floor and all I’m doing 
> is boosting it a bit, but whatever the mechanism it seems to have 
> helped a lot.
> David Robbins K1TTT
> e-mail: mailto:k1ttt at arrl.net
> web: http://wiki.k1ttt.net <http://wiki.k1ttt.net/>
> AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373

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