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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 13 04:51:17 UTC 2010

WarrenS wrote:
> Thanks for the positive contrbution,  A good example of one of the 
> TPLL's obvious disadvantages.
> The simple cheap analog version of the TPLL is limited by it's need to 
> have a dedicated Ref OSC.
> One way I have got around that problem, which would not apply to all, 
> is to put the DUT unit as the controlled OSC, and use a special Tbolt 
> as the reference Oscillator.
> The other way around the problem is the Digital version of the TPLL 
> that uses DSS.
> BTW that limitation is not nearly as big as one would think. This is 
> because the long term accuracy is already limited by the reference 
> osc, so one would not generally use this kind of system out past 1000 
> sec or so anyway.  So If doing long term multichannel Osc, One would 
> likely be MUCH better off with a more basic  undersampled Phase system 
> for long term testing and just go thru and cheek each Osc one at time 
> for a short time with a low tau tester such as this type.
> Keep the advantages and disadvantages coming in, so the Time Nuts can 
> compare which methods work best for their application.
> Now if we just had some place to log the responses.
> Summery: If you have multi oscillators to test simultaneously that do 
> not have EFC input, and that you want to do continuous sampling on, 
> and do not have multiple TSC boxes,  the TPLL is not the right tool 
> for the job.
> Be better off with one simple lower resolution multiplexed time 
> stamped TI phase system and a single TPLL.
If one has a production requirement to test/compare several hundred 
oscillators simultaneously, the TSC5120A and its variants, being 2 
channel instruments, aren't really that useful even if one could afford 
several hundred of them. Such a requirement may be difficult to meet 
within a modest budget whilst still achieving the performance 
requirements (eg 1E-13/tau system noise).

Even with a much smaller number of oscillators (eg 8 -16) devising an 
affordable measurement system may be challenging.

> Bruce posted:
>> The poor cost scaling of the tight PLL system is another reason
>> why it has fallen out of favour for those who have more than
>> 2 frequency standards to compare simultaneously.
> Thanks for that opinion, but I don't think we should list the above as 
> a unique disadvantage.
> Maybe need a new column heading for that one, Any name suggestions?
> Does not sound all that valid or unique of a reason to me.
> It seems the same can be said about a TSC or any new high cost system.
> I would think a more important reason is that the simple TPLL is not a 
> universal do all system.
> Because the simple analog version is "Limited by it's reference Osc" 
> in many ways,
> This does give it some possible major disadvantages like not working 
> so good with a CS or Rb standard.
> If one has more time than money, there are ways around that.
> ws
> *******************
> Bruce Griffiths bruce.griffiths at xtra.co.nz
> Sun Jun 13 01:25:13 UTC 2010
> Another disadvantage of the Tight PLL system that only applies to
> multichannel systems is that a dedicated reference oscillator is
> required for each channel.
> i.e. for an N channel system N reference oscillators are required.
> If correlation techniques were to be employed then an N channel system
> requires 2N reference oscillators.
> N channel versions of Dual Mixer systems by contrast only need a single
> offset oscillator and a single reference oscillator.
> Similarly an N channel heterodyne system only requires a single offset
> oscillator.
> An N channel direct RF phase sampling system (like that employed by the
> 2 channel TSC5120A) only requires a single samplign clock source.
> An N channel time interval counter that periodically (eg at a 1Hz rate)
> measures phase differences between 2 RF signals only requires a single
> reference source.
> The above system can be regarded as an undersampled version of the
> direct RF phase sampling system.
> The poor cost scaling of the tight PLL system is another reason why it
> has fallen out of favour for those who have more than 2 frequency
> standards to compare simultaneously.
> Bruce
> WarrenS wrote:
>> Great start
>> Now if we just had a list that someone would add the advantages and
>> disadvantages to, so that any non relevant stuff could be easily seen
>> and removed or moved to a third list, It would all become much clearer.
>> ws
>> ****************
>> Magnus Danielson wrote:
>>> 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.
>> Yes the cost of the reference oscillator dominates the system cost, the
>> additional $10 (omitting the cost of the phase detector) to implement
>> the tight PLL is relatively insignificant.
>>> 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.
>> If one is calculating the FFT then it is possible to calculate ADEV
>> directly from the FFT (of the frequency samples) with little additional
>> effort, for the relevant formulae see:
>> http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
>> Note such processing doesn't increase the cost of the system as one
>> needs a PC to calculate frequency stability measures, unless one
>> wants/needs to do it in real time.
>> One disadvantage of a tight PLL system is that finite EFC range and EFC
>> non linearity may preclude its application to noisier sources.
>> Linearising the EFC transfer function will help but the reference
>> oscillator EFC range will ultimately provide an upper limit to the
>> measurable 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.
>>> 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.
>> Yes, but I thought that we were calculating the required averages from
>> the frequency (EFC) samples by approximating the required integrals.
>>>> 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?
>> References for the whitening effect of undersampling:
>> http://www.obs-besancon.fr/tf/publis/metrologia98a.pdf
>> http://www.obs-besancon.fr/tf/publis/metrologia98b.pdf
>> The change in shape of the ADEV vs Tau plot is a consequence of the
>> whitening of the phase noise.
>>> Regardless, care must be taken to ensure high enough bandwidth
>>> compared to the tau for the measurements not to be affected.
>>> Cheers,
>>> Magnus
>> Bruce
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