[hpsdr] coherent dual mercury receivers

[Phil Harman]

I do have a requirement to time stamp samples from Mercury.

This is for the Griffin project:

http://openhpsdr.org/wiki/index.php?title=GRIFFIN

I'm developing a Chirp beacon that can be used as a beacon (with an ERP
much higher than its actual power output) and a radar for measuring real
time propagation on the HF bands and in particular 6m.

The Chirp beacon linearly sweeps a carrier over 1kHz in one second (as an
example). The start of the sweep is triggered by the 1PPS signal from a
GPS receiver.

On receive we can feed 1PPS from a GPS receiver to the Atlas bus and use
this to time stamp the sample that coincides with the start of the Chirp.

If we time stamp at the output of the ADC then there is a high probability
that the sample will be removed during decimation.

If we time stamp after decimation and filtering then we can currently only
indicate which USB frame contains the sample that coincides with the 1PPS.
That assumes that we just set a bit in the C&C data. In which case we
could be out by a 62 samples.

Anyone see an elegant solution?

73 Phil...VK6APH


> ***** High Performance Software Defined Radio Discussion List *****
>
> Georg,
>
> Having coherent dual mercury receivers is exactly the reason that I
> developed the diversity modifications to PowerSDR.  Indeed, the
> underlying motivation for doing the mods was to operate with
> polarization diversity on 432 MHz EME and for 436 MHz pulsar radio
> astronomy.  PowerSDSR v1.19.3.3.diversity19 works just fine for both
> of those applications, as it currently exists!  That is true as long
> as the clocks on both boards are phase locked to a single reference
> clock such as the 10 MHz clock from Excalibur.
>
> The present configuration of the diversity program provides reasonably
> coherent signals from the two Mercury boards.  If this were not the
> case, you would NOT see the deep nulling capability that has been
> demonstrated very impressively by K2SDR and K1RQG already.   In truth,
> the present HPSDR diversity program permits full polarization
> diversity operations already when used with dual orthogonal rf feeds
> such as the V and H monopoles of a feedhorn or "cross-pol" yagi
> antennas.  Of course, it would be interesting to see the results of a
> measurement to determine how coherent the signals actually are, of
> course.  Perhaps you'd like to undertake such a measurement?  I could
> work with you on that if you desire.
>
> With regard to time delays, the diversity control of the program
> provides complete adjustment and compensation for up to +/- 180
> degrees of phase difference between the two IQ data streams to correct
> for differences in time delays associated with the two rf paths.  For
> true coherency, of course, one must make sure that the two rf paths
> from the antennas are within a single wavelength of each other.   You
> may, or not, have noticed that in the latest version of the diversity
> code I have put a user selectable choice for which IQ data stream is
> used as the reference channel for such phasing.
>
> In my opinion, the present diversity arrangement is entirely adequate
> to supply the coherent, orthogonal inputs that Linrad requires, so I
> personally don't think it is necessary to do any more on the hardware/
> firmware side of the issue.  What is missing for you is a
> communications interface between PowerSDR and Linrad.  It is not
> necessary, in my view, to incorporate any time stamping in the data
> streams to achieve you goal.
>
> As a matter of fact, in the past, I considered adding an interface
> routine to provide communications between PowerSDR and Linrad to
> enable MAP65 users to work with HPSDR rigs with the orthogonal IQ
> streams presently available with the diversity program.  I know there
> are plenty of people out there who would be pleased to see such a
> coupling.  Unfortunately, however, developing such an interface has
> not been a high priority for me.  I had hoped that someone else with
> more of a direct interest in using HPSDR in such a manner would take
> up the challenge to develop an interface to Linrad.
>
> If you don't attract the necessary software programming assistance
> with your project perhaps I can offer some help there and work with
> you to get the job done, even though my programming skills are not of
> a professional level.  I suspect that if we (you, me, and anyone else
> interested in joining in!)  focused on the issue we could be
> successful in coupling the diversity program to the Linrad program.
>
> I'd be interested to see if others have an interest in this.
>
> Joe K5SO
>
>
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