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

[Magnus Danielson]

On 06/12/2010 11:29 PM, Bruce Griffiths wrote:
> 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.

True. The same technique is being used for LPLL phase noise 
measurements. The reference oscillator will still be a limit, but wither 
you can go below the reference oscillator noise or not is what makes the 
difference. Such a setup costs about twice of a single-channel TPLL. 
Usually there is two ADC channels available.

The cross-correlation processing isn't too hard to achieve and is 
efficiently performed using FFTs and a little support-processing. FFTW 
is a good tool to toss the FFT processing to. The remaining wrapping is 
in a few ten lines of codes or so. Going down the FFT path will give the 
frequency plot for free, getting it back into the time-domain cost extra.

>> 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.

Using time-stamping counters at high rate would be possible if being 
able to cope with the rate of samples. You want a frontend to do that if 
you want to run continously.

As for digital filtering. When doing measurements in the 0,1 - 1000 s 
range for the G.813 measurements, a 10 Hz low-pass filter is being required.

> 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).

Depends on the details. Some counters (SR620 for instance) can have 
biases for frequency data which their time-difference measures do not 
have. A TPLL does not suffer from that particular problem, as it cranks 
out its frequency estimation by a different method.

> 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.

Care to hand a reference or two for this statement?

Regardless, care must be taken to ensure high enough bandwidth compared 
to the tau for the measurements not to be affected.

Cheers,
Magnus