Red Pitaya SDR update

Wed Nov 11 21:53:19 CST 2015

What I would really like to see is a scheme similar to the openHPSDR hardware.  IIRC, it uses a 122.88MHz VCXO oscillator that can be locked to an external reference, via a PLL in the FPGA.  That external reference is either an onboard 10MHz oscillator, or a 10MHz signal from a GPSDO, rubidium standard, or whatever.  I have a few of each here, so I can take my pick.  I need to go back and check exactly how the 122.88MHz oscillator is locked in the Verilog code.

I would prefer to actually replace the 125MHz oscillator with a 122.88MHz one.  I know that messes up using the RP for a lab instrument, but I’m not sure that is the best use of the RP anyway.  Especially without all the voltage dividers and other front-end junk required to make it a true lab-grade instrument.

I’m a little concerned about actually running a 120+Mhz signal through the expansion connector, and the traces on both boards.  I’ve had experience with trying to run 76.8MHz through similar connectors on the Charleston SDR, without much success.  If we actually replace the oscillator on the RP board itself, then we only need to worry about getting access to the controlling voltage.

I’m glad to see that both the ADC and DAC clocks are derived from the same resistor settings.

It still would be nice to see a full schematic of the RP front end.  With that, we might be able to do away with the input lowpass filter components, and do some subsampling as well.

I wish my funds were less limited, as I would immediately get a second RP to do some creative dissecting on!
73, Terry, N4TLF

From: Martin 
Sent: Wednesday, November 11, 2015 10:21 PM
To: Jeff Scaparra 
Cc: Terry N4TLF ; Tacos 
Subject: Re: Red Pitaya SDR update

There's more discussion on the clock starting here: http://forum.redpitaya.com/viewtopic.php?f=11&t=265&start=60#p3062 

I hope we can find an off the shelf metal case that we can stuff everything into.  I'm using their plastic case with a little 5V fan on the Zynq module.  The whine is driving me crazy.

On Wed, Nov 11, 2015 at 5:30 PM, Jeff Scaparra <jeff at scaparra.com> wrote:

  Looks like we can utilize an external clock but it wouldn't be 10MHz. 

  http://wiki.redpitaya.com/index.php?title=External_ADC_clock

  http://forum.redpitaya.com/viewtopic.php?f=9&t=17

  On Wed, Nov 11, 2015 at 7:20 PM, Jeff Scaparra <jeff at scaparra.com> wrote:

    Has anyone evaluated the clock on the RP or used it for JT65? Is it good enough? Is there a way to provide a 10MHz reference that is common on some other radios? I always wanted a good reason to build a rubidium clock source. :) I do believe the Red Pitaya will be a better radio. 14 bit ADCs and Direct Sampling should be able to have better dynamic range and less noise. Some shielding will be needed for the Red Pitaya as I get much better results with a metal cover over it (probably just picking up noise in my "shack"). 

    On Wed, Nov 11, 2015 at 2:50 PM, Terry N4TLF <n4tlf at wb4jfi.com> wrote:

      I am definitely interested Martin.  After I straighten out a few things, I can do both the LF and RP.  Right now, I have so many different versions of PowerSDR and Quisk on so many computers, it’s hard to keep track!

      I am glad that you are taking charge of this!  The Red Pitaya is definitely a wonderful SDR platform.  I wish the HPSDR guys would get more interested in it, as I think the RP platform would be better than the present Hermes Lite.  Of course, the RP costs a little more, but look at what the hardware can do!
      73, Terry, N4TLF

      From: Martin 
      Sent: Wednesday, November 11, 2015 1:35 PM
      To: Tacos 
      Subject: Red Pitaya SDR update

      Thanks to Pavel Demin, the Red Pitaya now has a couple of SD card images to support a wide array of interesting and useful software.   

      There's a dual receiver single transmitter configuration that acts like a HPSDR network interface with dual receivers and a transmitter.  This opens up the possibility of using PowerSDR, Quisk, an Android app or other software.  It seems like diversity reception works in PowerSDR.

      I'd like help design and build the rest of the stuff it takes to turn the RP into a robust, portable transceiver.  I've never done this before and hope there's others on this list who would be interested in joining me.  We have some great resources in the club although I know everyone is time constrained.  I'm willing to commit significant time and effort to the project.

      I realize there's time sensitive work going on in the AMRAD Low Frequency world. That said, I'd like to propose this as a AMRAD project to follow on the Charleston receiver as time permits.  As most of the digital heavy lifting has been done it's mostly a matter of building the analog transmit/receive chain and ancillary stuff.  

      Terry N4TLF has done testing on the RP LF performance and found it too noisy to be useful but perhaps there's a fix.  

      The Red Pitaya is now $238 so an affordable, complete HF SDR transceiver should be possible.  

      This could be a great project for the club that hopefully attracts interest and new members. 

      So - anyone else in?  What do you think?

      73 Martin W6MRR.

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