[Skimmertalk] red pitaya noisy

[Pete Smith N4ZR]

I believe that the schermatic shows that there is an amplifier with a 
gas protection tube right after the jumpers.

73, Pete N4ZR
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On 7/31/2016 5:10 PM, W3OA wrote:
>
> Hi Dave -
>
> This looks like great progress in using the Red Pitaya.  Thank you.
>
> But I'm concerned because of the following FAQ on 
> http://redpitaya.com/faq-page/
>
> 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
>>
>