[TangerineSDR] Setting up DE for selected bands and sample rate

Engelke, Bill bill.engelke at ua.edu
Fri Apr 17 11:52:07 EDT 2020

In yesterday's HamSCI research telco<https://scranton.zoom.us/rec/play/tMd4JO39-j43H4WQ5gSDC_AtW9S_ffmsgHcd-KJZy0uzUiIAZlPwMuQSYbeIYiPAWTSPSeb1nkzcpCCR?startTime=1587063159000&_x_zm_rtaid=NMAJy4E0RWahMdU1UXtvsg.1587138515458.823452708c432bc18700b9c14edfeb54&_x_zm_rhtaid=286>, we discussed several ideas on how to accomplish several goals with the TangerineSDR; a summary is below for your review and comment.

  1.  We currently think that the Data Engine (DE) will have the capacity to simultaneously collect up to 8 channels of data for observing Doppler shift in carrier frequencies, i.e., WWV at 2.5, 5, 10, 15, 20 MHz plus CHU at 3.33, 7.335 and 14.670 MHz.  We think we can do this by observing excursions of up to +/- 1 Hz from the carrier frequency with a precision of 0.01 Hz, with measurements taken every second (see next point).
  2.  In earlier discussions on this board, it has been stated that this kind of precision (10 milliHz) would require a 102.4 second sample time; this assumes the processing is linear. CWRU is using a non-linear method in the low-cost approach to extract a frequency estimate, which uses fldigi to achieve the 10 milliHz resolution once per second, and it was discussed that TangerineSDR could be set up to do this as well. (Reference here: http://citeseerx.ist.psu.edu/viewdoc/download?doi= )
  3.  I won't try to explain the method mentioned above, as it is at the very limits of my understanding; however, a couple of observations...
     *   The CWRU low-cost system uses fldigi to implement the frequency estimator, and in looking at fldigi I see that it requires sound card input. In the case of the TangerineSDR, we will have buffers of IQ data. I don't think I want to use some sort of kludge to take the buffers and somehow emulate a sound card (do I?) - another option might be to implement the frequency estimator algorithm in the Small Board Computer (SBC) of the TangerineSDR. If we could/should do this, I would ask for a volunteer to code it; I fear that if I try to do it, it would take far too long for me to get up the learning curve. The code (C++) for the algorithm as used in fldigi is online (included with hundreds of other routines; see http://www.w1hkj.com/files/fldigi/ ). A third option might be to persuade the fldigi team to support buffer or file input.
     *   As I understand, the frequency estimator outputs a single value: the strongest signal in the band. A downside of this is that there seems to be a lot of other possible information in the data (Steve Cerwin has shown data he has collected showing Doppler effects but with multiple structures including various modes in both time and frequency domain). We need some guidance from the science community: what data do we really need to do the desired science?? Maybe we need to implement/support both raw data collection & frequency estimation at the same time(?)
     *   In yesterday's call, we discussed that we should be able to specify an 8 ksps sample rate and a center frequency, and get the desired 10 milliHz resolution - frankly, I am still fuzzy on the details of this, so I wish someone (David Kazdan? Kristina?) would chime in and explain, for the record, how this works.
  4.  Channel configuration settings for the DE:
     *   The DE will be able to tell the SBC what data rates it supports. We are working with 2-byte command codes; in general, a command ending in "?" is a query, i.e., send the DE the command "S?" and it answers "AK" , a simple health check.  Send the DE the command "R?" - it will respond with a table such as the following:
              1  2000    2  4000    3  8000    4  16000     etc. - where each entry is 2 integers: an index and a sample rate (Hz). To put a channel into data rate of 8000 sps, we put 3 into the channel setting.

Please comment!                   -73- Bill, AB4EJ
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