Re: {SPAM?} Re: [linrad] RE: Detecting phase modulation with undersampling

["Dr. David Kirkby" <drkirkby@xxxxxxxxxxxxxxxxx>]

Krzysztof Kamieniecki wrote:

> This seems like a fully digital RF lock-in amplifier, 

Yes, basically it is.

> of which I have not seen an off-the-self model. 

They do exist - I bought one at the around 1995 when I started my PhD. 
However, the only high frequency versions I am aware of use a mixer to 
downconvert, then do the processing at lower frequencies.

That implements an IQ demodulator on the DSP - an approach I think might 
be the right one.

>
> Disclaimer: I'm a Software Developer and amateur EE. 

Don't worry. I develop software too - I tend to do a bit of everything - 
RF, optics, software, now starting on DSP.

> How about this?
>
> R = real reference signal sampled at 65MHz
> S = real signal of interest sampled at 65MHz
>
> Down-convert R and S separately to two low bandwidth/sample rate 
> complex signals, possibly with a CIC[1] filter but there is probably a 
> better method, since CIC is well suited for FPGAs and ASICs. 

I hope to do all the processing on the board I have. I certainly want to 
avoid FPGS's - I believe they are quite sepecialled, and need someone 
who is really an expert on them. The company selling the board I have 
has some FPGA based models, but warned me that you really need an 'FPGA 
engineer' to use them - they are far from easy to use.

> At this point you could divide S by R (since the sample rate will be 
> relatively low on the order of 5KS/s the c6701 may be able to handle it). 

I hope the C6701 can do that!! It is clocked at 150 MHz I think. I'm not 
sure a division is the way to go though. However, I am not convinced a 
division is the way to go. What makes you suggest that??

-- 
Dr. David Kirkby PhD CEng MIEE
Senior Research Fellow
Department of Medical Physics
University College London
11-20 Capper St
London WC1E 6JA
Tel: 020 7679 6406 (direct line)
Tel: 020 7679 6262 (office)



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