VCO Phase noise: consequence for mixing...

Hi, Mr Woodgate, Thanks for your answer, It's true, my question was not so clear enought, when I've talked about phase noise injected in a mixer. Let's consider the problem again: In a PLL, the input signal (with phase noise, due to the elements before and emission) is mixed with the VCO's - filtered input controled- signal, like in this scheme

input signal =A6 Mixer----->----- filter --.-----> output signal ^___ VCO________=A6

Due to the loop, and of course non-linearities, I think - but I'm not sure- that the output signal would have a "white noise" component. I'll try it with a Matlab sim... .=2E. what do you think about it?

Thanks again for your help

Mike

OK, so I might wait a little :) No problem, thanks !!

Mike

I read in sci.electronics.design that Chromatik wrote (in ) about 'VCO Phase noise: consequence for mixing...', on Thu, 1 Sep 2005:

Phase noise is just a type of frequency modulation [delta(f) = delta(phi)/delta(t)]. Considered as frequency modulation, the spectrum has an additional gradient, proportional to delta(f).

I expect you can se that mixing doesn't change the shape of the spectrum of a frequency-modulated signal, so it doesn't change the spectrum of phase noise.

The effect on mixer sensitivity depends on how the signal is demodulated. An AM detector may be quite insensitive to small amounts of phase noise, whereas an FM detector (or similar) cannot distinguish it from the wanted signal.

I read in sci.electronics.design that Chromatik wrote (in ) about 'VCO Phase noise: consequence for mixing...', on Thu, 1 Sep 2005:

I'm afraid I'm not into the finer points of PLLs. But several people here are.

downconverting improves phase noise that's why offset PLLs are so popular...

Hi, maxfoo !

For you, what are the difference with a "classic" PLL and an Offset PLL? I mean in a "classic" PLL, the input signal is mixed with the looped VCO signal (like in my little scheme, 4 messages up), so phases are compared... If you have a link or docs on these Offset PLL, I'll be very kind !

Thanks a lot in advance! Regards,

Mike

Don't have any specific links, but IIRC Microwave Journal magazine had an article on offset plls back in sept 2004. think you can get the article from their website if you register with them. Offset PLLs are widely used in GSM.