PLL noise question[from interview ]

adolflau · Mar 24, 2012

Hi,All;
I was asked a PLL noise question on phone interview:
If there's a noise appear at the reference at 100Hz, what frequency position of this noise will appear at the output of PLL.

P.S: are there any books/papers regarding the PLL noise, VCO phase noise recommend that fit for a PLL beginner???

Many Many Many Thanks!!

sky_123 · Mar 25, 2012

That's a weird question, do they mean a reference frequency of (say) 1MHz, and a spur at 100Hz on the reference signal? That is
probably unlikely to occur, since the ref freq is usually xtal generated.
Otherwise, if the reference frequency is 100Hz and there is noise around that point of the spectrum, i.e. phase noise, then it's
effect is felt within the loop bandwidth of the PLL, and you'll see it around the output frequency. That's why it is important to have a
good reference oscillator.

adolflau · Mar 25, 2012

sky_123 said:
That's a weird question, do they mean a reference frequency of (say) 1MHz, and a spur at 100Hz on the reference signal? That is
probably unlikely to occur, since the ref freq is usually xtal generated.
Otherwise, if the reference frequency is 100Hz and there is noise around that point of the spectrum, i.e. phase noise, then it's
effect is felt within the loop bandwidth of the PLL, and you'll see it around the output frequency. That's why it is important to have a
good reference oscillator.

Hello, I think so too.... the noise will stand around the output frequency.....
Do you know how cycle to cycle jitter in VCO influence the PLL....
I found some book discuss about the VCO, but I can't find some book that is easier to read,do you have any recommend?

sky_123 · Mar 25, 2012

For the cycle to cycle jitter on the reference oscillator, this is the same as phase noise.
Jitter/phase noise can be considered interchangeable.
If there is jitter/phase noise on the VCO, then that will mainly be visible outside the loop bandwidth of the PLL, which is pretty neat.
Check this link out, which shows it graphically and probably better than most textbooks:
https://www.analog.com/library/analogDialogue/archives/33-05/phase_locked/index.html
Unfortunately I don't know of a good book that addresses the noise in detail, or even the best book on PLLs, but hopefully someone else can comment.

iVenky · Mar 25, 2012

Hello adolflau. Could you please tell me the name of the company? I am a little bit curious.

PLL noise question[from interview ]

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