[hpsdr] ANAN200D RX1/RX2 Phase Coherent and Phase difference (2)

[Warren C. Pratt]

Hi All,

Comment added below.

73,

Warren  NR0V


On 1/28/2017 11:39 AM, Joe Martin wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Jan,
>
> I find the description of your implementation of diversity using the ANAN-200D/PowerSDR interesting.  It is, of course, distinctly different from using the Diversity button option in PowerSDR.  Perhaps a few comments regarding the fundamental differences and a few words about how the Diversity feature is implemented in firmware will aid in evaluating the various possibilities that exist for phase-coherent diversity operations and perhaps shed some light on why the relative phase shift you observe between Rx1 and Rx2 occurs upon changing bands.
>
> A major difference between your configuration and what is done in the PowerSDR Diversity option is that you are comparing phases and amplitudes of two demodulated/audio IQ streams from two DDC receiver modules  whereas the PowerSDR Diversity feature  combines the two raw  rf IQ data streams, one from each receiver, to form a single raw rf IQ stream that is then processed by the software DSP routines to form a single panadapter display and single audio output signal.  PowerSDR Diversity provides the user the ability to digitally shift the phase and amplitude of an rf IQ stream from one DDC receiver module relative to the rf IQ stream from a reference receiver module to form the single IQ stream.
>
> The significance of this is that in your configuration selecting VFOsync on the PowerSDR front panel simply forces the frequency of Rx1 and Rx2 to be the same value and those values are used to form phase words for each receiver that are then sent separately to the two receiver modules in the firmware.  This is generally fine to achieve “pseudo-coherent” operation of two receiver modules, as they will be operating on the same frequency, but because the receiver modules are running independently and are not updated with precisely the same phase word at the same instant a phase shift between the two can occur when new frequencies/bands are commanded by the user, due to the fact that the phase words are delivered to the firmware in a serial stream of Command & Control (C&C) data.  This situation is avoided in the PowerSDR Diversity feature by forcing Rx1 and Rx2 to use precisely the same phase word from the C&C stream to update the frequencies of the two receivers simultaneously.
>
> If the firmware updates the receiver frequencies by using phase words from different locations in the C&C stream there will be a time delay (phase shift) introduced between the two receivers, even if the phase words themselves are identical in value because the receivers will be updated at slightly different instants in time due the fact that the arrival times for the two values from the serial C&C stream are different, as is the case in your configuration.  In the PowerSDR Diversity option, however, both receivers are forced in firmware to use the same phase word in the C&C stream, thus avoiding any such time shifts.
>
> Although I haven’t examined this in detail in PowerSDR and firmware with respect to band changes, I suspect that changing bands causes a greater shift in arrival times of the C&C values from the software than does a minor frequency change within the same band.  Also, of course, when bands are changed the RX filters being used change and that fact in itself can certainly introduce phase and amplitude changes in the relative signals of Rx1 and Rx2.
>
> I think there is an option in PowerSDR to send raw IQ values from a receiver to a VAC output so that may be useful to you, and operate with the PowerSDR Diversity option selected.  I’m not familiar with that possibility actually, but it would seem to me that using such a scheme would allow you to maintain a fixed phase relationship between Rx1 and Rx2 even upon band changes. I think you’d need to use two VAC connections though, maybe not.  Maybe someone who has used that raw IQ output capability can add a useful comment or two for you.
Unfortunately, this will not work.  The diversity mixing of the two I-Q 
streams is done shortly after the data arrives to the software. This is 
well before the provisions to export data via VAC.  In diversity mode, 
you can export the mixed I-Q stream via VAC; however, not the two 
individual streams.
>
> Anyway, if I’ve missed something in my interpretation of your configuration I apologize.  I hope these comments will be helpful to you in any case.  Others may have better information for you.
>
> Good luck with your project!
>
> 73, Joe K5SO