[hpsdr] HPSDR and Beamforming

[pvharman at arach.net.au]

Murray,

This is  something we have discussed in relation to the HPSDR project. The 
Janus sound card Beta has the capability to be phase locked to an external 
reference. You could also use multiple Janus cards on the Atlas bus all phase 
locked together. 

Alternatively you could use one ADC running at say 192kHz and multiplex two 
96kHz I/Q streams over the USB. It may be easier to make a special purpose 
board (after all that is what the HPSDR project is all about) that has n ADCs 
all passed over the one USB interface. 

As a starting point you could take say two SoftRocks and feed them both from 
the same local oscillator and feed the I/Q outputs to inputs 1 & 2 and 3 & 4 
of a D44 sound card. Since the inputs all run off the same clock you should be 
able to trim any static phase error out in software. 

A modified version of PowerSDR would be required but to a hardware guy like me 
that is  just  "simply a matter of software" :)


73's Phil....VK6APH 





Quoting Murray Lang <murray.lang at metoceanengineers.com>:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hi Jeff
> 
> I don't think you're clicking "Reply All" because hpsdr at hpsdr.org is not 
> appearing in your headers. I suspect I'm the only one receiving your posts.
> More comments below.
> 
> At 04:10 AM 14/09/2006, jeff millar wrote:
> >>Now I'm wondering about synchronisation of sound cards if phase 
> >>adjustments are performed by PC software and there is a separate sound 
> >>card for each Tx/Rx. I suspect that they would have jitter relative to 
> >>each other due to task switching in the device drivers and to their own 
> >>clocks being unsynchronised. I suppose you would have to go for a 
> >>professional sound card with more inputs/outputs (at least 4 stereo 
> >>channels in each direction?). Maybe this is a candidate for an HPSDR
> board.
> >>
> >Hmmm...As the frequency drops from RF to IF to Audio, the amount of jitter 
> >tolerance goes up (as a percentage of the frequency).
> 
> But this contradicts what has been said earlier about the phase shift at 
> I/Q scaling proportionately to RF.
> 
> >Assume each audio A/D channel can DMA a continuous stream into into 
> >memory. Now we have to assume that each audio A/D runs from the same clock 
> >reference. I don't know how the HPSDR project arranges that across audio
> cards.
> 
> This basically proscribes using multiple sound cards unless they have 
> external clock inputs (which I'm not aware that any do). Even if they could 
> be synchronised, I'm not convinced that the data sent to them would be 
> synchronised unless maybe a single device driver drove all of them. Sounds 
> way too messy.
> 
> Perhaps it would be better if all the audio channels were delivered by USB. 
> I think this is relevant to the HPSDR project because any audio data packet 
> format will need to be designed to allow for more than two channels.
> 
> Murray
> VK6HL
> 
> 
> 
> 
> 
> 
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