[time-nuts] Re: PLL subharmonic spurs

[Bob kb8tq]

Hi

Ok, this is a non-trivially difficult problem. You get into a number of issues
that are *very* specific to your exact circuit. Often simulation is the most 
practical way to work out “what’s what”.

Bob

> On Feb 6, 2022, at 5:27 PM, mail at vresearch.pt wrote:
> 
> 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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