[hpsdr] coherent dual mercury receivers

[Joe Martin K5SO]

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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