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Design >> RF Design >> Synthesizer PEC re-quantization modulator with rapid changing input stream
https://designers-guide.org/forum/YaBB.pl?num=1281545817

Message started by neoflash on Aug 11^th, 2010, 9:56am

Title: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by neoflash on Aug 11^th, 2010, 9:56am

Hi,

Hope can have some synthesizer experts to comment on this
question.

Some reference and many recent approaches show that it is getting popular to do phase error correction in fractional synthesizers.

Furthermore, people are re-quantizing the PEC correction DAC so that there is no need for a very fine DAC.

The re-quantization of the PEC residue inside synthesizer is usually done by a sigma-delta modulator.

As shown in the figure, the requantization modulator is usually working at the same clock rate of the first modulator. As we know, the modulator is usually working on the input which changes much slower than the clock rate. However, in this setup, the 2nd modulator is working on the input signal which changes in significant step every clock cycle, is there any issue with it?

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by neoflash on Aug 12^th, 2010, 10:03am

I did an simulation and found that the quantization noise of the 1st order re-quantization modulator is white (not shaped) when its input is rapidly changing.

When the input is stable, the quantization noise is shaped with 20-dB slope.

Very confused...

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by vp1953 on Aug 12^th, 2010, 6:27pm

Hi Neoflash,

Is the accumulator block shown in your figure really a SDM?

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by neoflash on Aug 12^th, 2010, 7:12pm

Yes. Accumulator is actually a 1st order sigma-delta modulator.

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by vp1953 on Aug 13^th, 2010, 1:09pm

Hi Neoflash,

I am not an expert on SDM's but that both the SDM's together in the figure looks similar to a second order SDM with some portions missing. In a second order SDM the error output from the first modulator going to the second one changes at the reference clock frequency. Therefore I would have expected some quantization noise at the output of the second SDM provided that the input is changing at no more than the reference clock (if the input was changing much faster than the reference clock, then the noise might be unshaped).

How did you measure the quantization noise and what input signal did you use for this?

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by vp1953 on Aug 13^th, 2010, 4:20pm

I just did a quick simulation using a 1bt SDM; input is a sine wave and it is oversampled by 100. The error output of the first SDM was fed to another 1bit SDM (having the same clock as the first) and the quantization noise of the second has a 20 db slope.

Title: Re: Synthesizer PEC re-quantization modulator with rapid changing input stream
Post by neoflash on Aug 15^th, 2010, 3:19pm

I also re-do the simulation and found that the noise of the 2nd stage is indeed shaped.

I guess something went wrong in my previous setup.

Thanks for your trial.

I realized that although the input of the 2nd stage is changing rapidly, the quantization noise will get the same transfer function. The output noise spectrum will be shaped as long as the quantization itself is still white.

Thanks,
Neo