[Skimmertalk] red pitaya noisy
Pete Smith N4ZR
n4zr at contesting.com
Sun Jul 31 21:03:04 EDT 2016
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
> 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
>> 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.
>> 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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