[time-nuts] Re: PLL subharmonic spurs

[mail at vresearch.pt]

Hi Bob,

Thank you for clarifying. On Roland Best's book he states that for some phase detectors there are also subharmonics that will be generated (~ 1/20 reference frequency) but in fact the spurs I'm really interested to estimate are multiple of the reference frequency (i.e. at the input of phase detector). There are a few equations to estimate the reference spurs but I'm chasing the source material.

Regards,
Vasco Soares

-----Original Message-----
From: Bob kb8tq <kb8tq at n1k.org> 
Sent: 6 de fevereiro de 2022 19:08
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: [time-nuts] Re: PLL subharmonic spurs

Hi

Just to clarify: 

“Sub Harmonics” is a term often used to describe spurs at carrier / N where N is pretty small. If N = 4 you would be looking for spurs at 0.25, 0.5, 0.75, 1.25, 1.5 and 1.75 times the carrier frequency. This would be a common way to talk about them in the context of “sub harmonics are 120 db down”. 

Indeed if your PLL has a reference at 1/4 the output carrier the same term might be used. That would be an unusual case, but possible. 

In a more conventional PLL, the reference is a (very) small fraction of the carrier. The term “reference spurs” is typically used to describe them.
They still are at some fraction of the carrier, but the “N” value is quite large. It is more convienient to look at them as ref x 1, ref x 2 , ref x 3 rather than carrier / 3896. 

Typically reference spurs are part of a PLL and sub-harmonics are part of a multiplier chain. No, it does not have to be that way, but it is how the terms usually show up.

Again, only trying to clarify what sort of spurs you are chasing. 

Bob

> On Feb 6, 2022, at 1:21 PM, mail at vresearch.pt wrote:
> 
> Hi All,
> 
> I'm searching for a good reference to calculate the PSD of subharmonic spurs on an integer PLL. Roland Best uses a reference on his PLL book: "Volgers,  B.: "Phase-Locked Loop  Circuits: 74HC/HCT4046A &  74HC/HCTR7046A HCMOS Designer’s  Guide", Signetics/Phillips  Components. (Ordering Code:  98-2908350)". Does anyone can help?
> 
> Regards,
> Vasco Soares
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