Using Red Pitaya with the Reverse Beacon Network

Just about everything about Red Pitaya
n4zr
Posts: 4
Joined: Sat Jul 02, 2016 1:41 pm

Using Red Pitaya with the Reverse Beacon Network

Post by n4zr » Sat Jul 02, 2016 1:54 pm

Most participants in he Reverse Beacon Network currently use Software Research Labs' QS1R receivers for multi-band simultaneous monitoring. However, both cost and availability issues now have us looking for another option. I am one of the RBN organizers, and also manage its gap-filler program, which aims to get RBN nodes going in under-served areas, particularly along the equator and in the developing world. So cost is important to us.

From experiments that some friends have done, Pavel Demin's software appears to work very well for interfacing Skimmer Server with the Red Pitatya, covering up to 6 x 192 KHz slices. However, I am interested in learning whether anyone has professionally measured the input impedance and sensitivity of the Red Pitaya when used as an SDR. One source reports that the input impedance varies widely from 3-30 MHz, while another suggests adding a 50-ohm load across the inpuit, and making up the loss with a 50-ohm preamp of some sort. We are already providing a preamp in our gap-filler configuration, so maybe this would not be a difficult thing to do, but simplicity is also nice.

Any advice will be much appreciated.

73, Pete Smith N4ZR

pavel
Posts: 478
Joined: Sat May 23, 2015 5:22 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by pavel » Sat Jul 02, 2016 3:12 pm

Hello Pete,

Some information about the Red Pitaya inputs can be found at the following links:
http://wiki.redpitaya.com/index.php?tit ... e_Overview
http://wiki.redpitaya.com/index.php?tit ... d_-_Inputs

According to the specifications from the first link, there is 10 pF of capacitance in parallel to the inputs. So, it's expected that the input impedance varies with frequency.

According to the plots from the second link, the input impedance is about 25 kOhm at 1 MHz, 2.5 kOhm at 10 MHz and 500 Ohm at 50 MHz.

When I measured the input impedance using my VNA application, I obtained about 700 kOhm at 1 MHz and 200 Ohm at 50 MHz.
One source reports that the input impedance varies widely from 3-30 MHz, while another suggests adding a 50-ohm load across the input, and making up the loss with a 50-ohm preamp of some sort.
As far as I know, the following setups are known to work well with Red Pitaya:
  • wideband 1:9 impedance transformer (for example MiniCircuits T9-1, https://www.minicircuits.com/pdfs/T9-1.pdf) between antenna and the Red Pitaya input
  • wideband preamplifier between antenna and the Red Pitaya input (with or without terminating/load resistors)
I'd be also very interested to see more measurements of the input impedance and sensitivity as well as a comparison of the various impedance matching setups. The results that I've seen so far are summarized in my yesterday post on the HamSCI mailing list.

Best regards,

Pavel

n4zr
Posts: 4
Joined: Sat Jul 02, 2016 1:41 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by n4zr » Sat Jul 02, 2016 7:06 pm

Thanks, Pavel -very useful.

Pete

dl1sdzr
Posts: 3
Joined: Thu Mar 17, 2016 2:33 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by dl1sdzr » Sat Jul 02, 2016 8:48 pm

Hello,

I have using the Red Pitaya (RP) with the CW-Skimmerserver for over two months.
You will find measurements her:
http://forum.cq-nrw.de/viewtopic.php?f=18&t=34

Comparing figures the sensitivity (14 MHz) of the RP is - 106 dBm. With an easy modification you get a gain of about 6-7 dB. That adds up to -113 dBm. When I add 3 dB (my impression) of the skimmer digging in the noise you will end with around 116 dBm.

But this is not my point.

I ran parallel a Kx3 (-123 dBm)with a Px3 on a poor EndFed antenna between two houses. Of course I wanted to catch the good guys and had a hard time with CW and QRP. The reported SNR from skimmer was comparable(equal) to the Signals on the Kx3. Reported signals between 1-5 SNR were only to find with CAT and waterfall and I had nearly no chance to work them.
Now I use an active broadband CPOL antenna indoors. The loss of about 3-6 dB is equivalent to real signals on the EndFed.

Beside that an impedance matching for receivers seems to me quite academical, I have the practical impression that the sensitivity of the RP is more than sufficent. More would make reports coming from RBN-monitoring stations even harder to compare or evaluate. (I was just thinking about stations with 6*6*yagis ;-))

Beside that an additional amp connected to a big antenna could drive the AD-Converter of the RP into saturation and clipping could be a problem.

73 de Hajo DL1SDZ

n4zr
Posts: 4
Joined: Sat Jul 02, 2016 1:41 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by n4zr » Sat Jul 02, 2016 9:55 pm

Hi Hajo and thanks for your info. You mentioned a sensitivity of -106 dBm at 14 MHz. Is this the noise floor or some other benchmark? Have you done any comparisons with your KX3 on lower frequencies?

Thanks again and 73,

Pete N4ZR

dl1sdzr
Posts: 3
Joined: Thu Mar 17, 2016 2:33 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by dl1sdzr » Sun Jul 03, 2016 2:46 pm

Hi Pete,

It was MDS Minimum discernible signal - 106 dBm. Have a look at the link http://forum.cq-nrw.de/viewtopic.php?f=18&t=34 (last post) I provided.

Yes I have done recepetions on all bands (except 160m, 60m and 6m) and compared them with the Kx3. All signals had comparable S-Meter peaks. Of course signals on 80 and 40 were more hidden in local and other noise, but when the RP/Skimmer reported a signal it was visible also in the waterfall. I found no other additional decodable signals on the Kx3.

I had more problems on the higher bands 10m, where sometimes reports came, which I could not seen on the Kx3.

73 de Hajo

n4zr
Posts: 4
Joined: Sat Jul 02, 2016 1:41 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by n4zr » Sun Jul 03, 2016 3:15 pm

Hi Hajo - and thanks for the quick reply. Am I correct in assuming that this was MDS with no antenna connected, or with a dummy load? If so, -106 dBm is not very good. I have a 20M *band noise* floor of -120 dBm, so a -106 dBm internal limit would cost me many weak signals that I could otherwise hear. Of course, we could add a preamplifier to bring band noise above the internal limit.

73, Pete N4ZR

dl1sdzr
Posts: 3
Joined: Thu Mar 17, 2016 2:33 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by dl1sdzr » Sun Jul 03, 2016 3:35 pm

Hi Pete,

DC9UE used a
- Transmitter (R&S SML03) + (AN/USM 413 Ballantines Laboratories)
- Noise without signal but defined level
- Noise + Signal with 3dB
- 1:16 transformator on RP Input
- Frequenz 14340 KHz, Bandwidth 500Hz

So it is -106 + 7 = 113 dBm.

Compared to your unit it would be a difference of 7 dB = 1 S-Meter point. But with Alexs Skimmer in the noise the difference would shrink to 3-4 dB 1/2 S-Meter.

As I said we are talking about signals near the noise level. At least for me no problem.

73 de Hajo dl1sdz

Gerby
Posts: 1
Joined: Wed Jan 18, 2017 3:01 pm

Re: Using Red Pitaya with the Reverse Beacon Network

Post by Gerby » Wed Jan 18, 2017 3:08 pm

Tell me please, a similar scheme can be used in conjunction with a proxy server (advanced.name in my case)? I understand that for some, this question may be obvious, but I'm not really good at it.

ok2zaw
Posts: 9
Joined: Tue May 30, 2017 11:32 am

Re: Using Red Pitaya with the Reverse Beacon Network

Post by ok2zaw » Tue May 30, 2017 11:46 am

Hi guys!

Please I do have some problem with my RP as a skimmer at OL7M. I have tried a few images - thanks Keith W4KAZ - but still the same.
I can run RP with Peter's CWSL as CW + RTTY skimmer and also Winrad. Also if I run just CW skimmer directly, the problem is still there.
What is going on: skimmer works great ( using 4 bands / 96kHz ) but after abt 20-30hours the skimmer SW shows decoders, but nothing decoded.
I looked into the stream (is still the same on LAN) and there is empty band only with some "pings"
There is the screen: http://ok2zaw.com/SDR/skimmer.png

I have tried 3 PC, different LAN switch, RP images - BUT still last SDR RX from RP repository...

Thank you for any ideas!

Vy 73 JAN OK2ZAW / OL9A / OL7M

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