[time-nuts] Re: PLL subharmonic spurs

[Bob kb8tq]

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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