[hpsdr] Infinite precision, phased arrays,and us

Thu Sep 7 02:13:25 PDT 2006

 I am having difficulty in knowing whether you are not understanding what I 
do or whether I do not understand your problem.
I apologise if I am stating the obvious.
It is easier to understand the Tx path but the same reasoning applies on Rx

On the grounds that a diagram is worth a 1000 words I have attached a crude 
diagram (as a .gif)
In Fig 1 I have two antenna elements fed in phase - these add broadside on 
and produce a figure of eight polar diag
Fig 2 I feed them out of phase ie phase in one path180 degrees - they add 
end on now and cancel broadside.
By adjusting the phase through 0 to 180 degrees we have swung the beam 90 
degrees
Fig 3 shows the situation for an intermediate position. We can add as many 
elements as we like as long as we continue to add more and more phase shift 
in successive ones. we can have any distance between them as long as we take 
it into account in the phase.

We should note that the aperture of the antenna has gone down and thus its 
gain.

The question is "How do we achieve the phase change?"

Assuming we produce baseband digital information at some low clock rate we 
would naturally up the sampling rate with a CIC filter.
ie we would have a number of stages of differentiator, the a sampling rate 
changer and then a number of stages of integrator. This naturally does 
interpolation.
I have not examinined this in depth but I presume we simply put a series of 
identical samples into the integrator and the mere fact that it is an 
integrator produces the interpolated samples. So we have a baseband output 
at whatever sampling rate we want. I presume we would go up to the sampling 
rate of the Rx A/D say 100 MHz

How do we get the required phase change? - the easiest way conceptually is 
to now frequency shift with a mixer (digital)- so let us split the path into 
two mixers and  adjust the phase of the LO samples as desired. 0 to +- 180 
degrees for +- 90 beam swing. That demonstrates the accuracy required. This 
will produce the right phase at whatever RF we produce as phase is conserved 
through a mixing process.

We could have started the two channels at baseband but we would have to 
shift the phase across the whole audio band. We are all familiar with the 
complex networks used to do this in the past but would now be mucking about 
with Hilbert filters or the such like.
A few moments with a piece of paper multiplying  a+jb by 1+j0 or 0+j1 or 
sqrt(2)+j sqrt(2) etc will demonstrate that phase shifting need only a 
multiple ( for a single frequency)

The two outputs would feed separate Tx modules. While this is quite 
appropriate to Radar phased arrays with distributed PAs it is less so to 
amateur radio.

In a Radar array the problem of fill time arises. As a RF pulse hits the 
array it will reach one element of the array first. At that stage we have a 
very broad beam (Isotropic) as the other elements receive RF the beam it 
more and more forms. The same will happen at the back end of the pulse. It 
could prevent us using very narrow pulse in a big phased.

I had not seen the idea of beam forming as the way to go for HF radio but as 
we are always trying to resolve signals in QRM the inclusion of one or more 
null steerers in the Rx path would be very valuable - we take a lot of care 
to make our filtering good in the frequency domain but ignore the spacial 
domain that is just as valuable to us.

I hope this helps a bit

Nyall G8IBR

----- Original Message ----- 
From: "Murray Lang" <murray.lang at metoceanengineers.com>
To: <hpsdr at hpsdr.org>
Sent: Thursday, September 07, 2006 6:57 AM
Subject: Re: [hpsdr] Infinite precision, phased arrays,and us

> ***** High Performance Software Defined Radio Discussion List *****
>
> Thanks again Bob,
>
> Just to take a step backwards, this all came about from discussion in
> another thread a while back where the topic of beamforming arose. At one
> point I flippantly discounted controlling the RF phase shifts via I/Q but
> was pulled up and reminded that with I/Q you can do anything. OK, you can
> do phase modulation so why not a steady phase shift? I was uncomfortable
> with it but couldn't put my finger on exactly why so I let it go. Now I'm
> grabbing it back again.
>
> Yes, I knew that their would be more than one QSD/E with synchronised
> (English) clocks. I also understood that the HPSDR dudes were already
> catering for synchronised clocks - it's not new. I'm just trying to
> understand why the concept of achieving phase shifts via cunning
> manipulation of IQ is not sitting quietly in my head.
>
> It's one thing to shift the phase to accurately represent a modulating
> audio signal, but to have such fine control as to be accurate to within a
> fraction of a wavelength of the RF carrier seems to me a lot to ask of a
> sound card. Consider a phase modulated signal applied to such a phase
> shifted carrier. I/Q would have both shifts encoded into it: a component
> accurate at AF and a component accurate at RF.
>
> Murray
> VK6HL
>
>
>
>
>
> At 12:33 PM 7/09/2006, Robert McGwier wrote:
>>***** High Performance Software Defined Radio Discussion List *****
>>
>>Baaa.  You are doing it justice.  Believe me.  There are 100 hundred
>>people reading this that wanted to ask the questions and didn't.
>>
>>As I said in the previous,  for a phased array in the HPSDR system,
>>we are not likely to use only one QSE for transmit or QSD for receive.
>>  We are not likely to use a single sound card.   You will need one QSD
>>and one sound card pair for EACH ELEMENT in the phased array and the
>>QSD oscillator<<S>>  must all be the same source just as the sampling
>>clock on the A/D's must all be derived from the same clock.   But they
>>are separate devices, one per element in our narrow band phased array.
>>   If we chose to use only one QSD and sound card, then the combing of
>>the elements must be done before it and there must be a phase shifter
>>in each antenna element.
>>
>>I think you are getting there.  You are asking the right questions to
>>understand the system.
>>
>>Bob
>>N4HY
>>
>>
>>
>>At 09:41 AM 7/09/2006, Murray Lang wrote:
>> > ***** High Performance Software Defined Radio Discussion List *****
>> >
>> > What about the situation where QSE is the only "mixing" stage and I/Q
>> is from a sound card? Where does frequency response come into it? You've
>> only talked about word size. You seem to be saying that phase is a DC
>> component of I/Q.
>>
>>Sorry Bob, I realise that I haven't done justice to what you've
>>posted. I can see that the commutative nature of mixing helps but I'm
>>having trouble relating that to the situation I've identified above.
>>The only oscillator is the 4xoutput carrier and it's being modulated
>>by a sound card.
>>
>>Murray
>>VK6HL
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