Hi All,

I am developing a carrier tracking module for 16QAM receiver based onCostas loop on an fpga platform. Tracking works well on either side of zerofrequency and over zero if crossing rate is several seconds or minutesapart. 
However it loses lock and relocks if zero crossing rate is few secondsapart.
Is this acceptable to an average RF expectations or do you think I canimprove on it.

Regards

kadhiem	   
					
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Posted through http://www.FPGARelated.com

To further explain my problem:

I use an NCO to generate stimulus of cos/sin pair, multiply with the QAMi/q symbols...then track the carrier as I change the NCO frequency insteps.

When crossing zero frequency, tracking manages first few crosses then mayfail but relocks. Looking at the waveforms at zero crossing point, I get into some doubts about my understanding in principle. I know we can getnegative frequency from positive frequency by inverting the sine wave or byreading the LUT in reverse time. These two ways are not identical at thecrossing point. My tracking is based on reversing LUT for -f and so is myNCO.
With reverse LUT reading, I notice that phase can suffer variation at thecrossing; for example if I hit the mid-poit of a symmetrical section ofwaves then read back to get -f then the continuity looks best. At otherextreme I could be before the peak of cosine only to reverse back leadingto anything between 0 ~ 360 phase variation of negative pair with respectto positive pair.
The question is how can I expect tracking not to fail with such phasevariation. Or in other words; how does a real RF upconverter/downconverterlink final frequency cross the zero and how to model it best?

Regards

kadhiem
	   
					
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Posted through http://www.FPGARelated.com

I believe I finally sorted out the issue thanks
to Matlab.

A LUT based approach for frquency synthesis is ok for positive or negativefrequency synthesis. However it is very glitchy at zero cross over. Theright model is to generate 2 frequencies(one drifting down, the other up sothat they criss-cross). The two frequency sources are then multipliedtogether to produce proper vector for zero crossing.

Regards

kadhiem	   
					
---------------------------------------		
Posted through http://www.FPGARelated.com

kadhiem_ayob
<kadhiem_ayob@n_o_s_p_a_m.n_o_s_p_a_m.yahoo.co.uk> wrote:
> I believe I finally sorted out the issue thanks to Matlab.
 
> A LUT based approach for frquency synthesis is ok for positive or negative
> frequency synthesis. However it is very glitchy at zero cross over. The
> right model is to generate 2 frequencies(one drifting down, the other up so
> that they criss-cross). The two frequency sources are then multiplied
> together to produce proper vector for zero crossing.

Reminds me of an HP signal generator, I believe 3325B, which uses
a combination of frequency generators and mixers to generate the
range of frequencies from 1uHz to 20.999999999 MHz.

I used to know more of the details about how it works, but part
rembering and part looking on the web, it generates 30MHz more than
the desired frequency, and then mixes down for the resulting output.

-- glen