[time-nuts] Advantages & Disadvantages of the TPLL Method

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jun 12 17:29:06 EDT 2010


WarrenS wrote:
> subject: Advantages&  Disadvantages of the TPLL Method.
>
> Here is a new and unique Idea that may be useful for many.
> Rather than focusing on what some members may or may not already know,
> or  how good or bad one specific working BB configuration is.
> How about focusing on what the TPLL method can and can not do well.
> If someone will make a place to post and compile a  couple of list,
> I can start it off with what I've learned so far:
>
>
> DISADVANTAGES of the TPLL method:
> -------------------------------------------------------
> #1) The TPLL method is limited by it's reference OSC.
>    
This isn't necessarily correct, one could use a pair of tight PLL loops 
and use correlation techniques to reduce the contribution of the 
reference oscillator noise.

> The ref osc (or the DUT)  needs to have an Analog&/or Digital EFC control input with a bandwidth that is wider than the desired Tau0
>
> #2)  It basically measures Freq and not Phase differences, and few understand how and why it works so well or it's many advantages.
>    
This is not true, there is no inherent SNR advantage in measuring 
frequency changes as opposed to measuring phase differences.
When the phase measurement system and the frequency measurement systems 
being compared have the same noise bandwidth then the measurement floors 
are comparable.
For example, the TSC5120A is a narrow band system based on measuring 
phase differences with a comparable or lower noise floor than your 
implementation of the tight PLL.

The common technique of using a time interval counter to measure the 
phase difference between 2 RF signals once ever second or so is a 
wideband technique with severe undersampling, consequently the system 
noise floor is much higher than for narrow bandwidth techniques. If the 
phase difference between the 2 signals were measured more frequently and 
digitally low pass filtered the noise will be much lower.

Since one has to calculate average frequency from the frequency samples 
by integration/averaging this is mathematically equivalent to 
reconstructing the phase change between the start and end of the 
averaging time (Tau0).

One effect of undersampling is to convert (in the sampled data) a 
proportion of any flicker phase noise (and other non white phase noise 
components) to white phase noise.
The effect of this is to change the ADEV vs Tau plots from their true shape.

With a single pole RC filter the required minimum sampling rate to 
ensure that such effects are acceptably small cannot be known unless the 
phase  noise spectra of the 2 oscillators being compared is known.

However the extra phase noise filtering due to the finite PLL bandwidth 
(including any EFC filtering built in to the reference oscillator) 
allows an estimate of the maximum sampling rate likely to be required to 
ensure that such phase noise whitening effects are acceptably small.

> #3) TBD
>
>
>
> ADVANTAGES of the TPLL method:
> ---------------------------------------------------
> 1 thru 30) same as I've posted several times before.
> I'm sure others will find many more if they try it or at least understand it better.
>
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>    
Bruce





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