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

[Magnus Danielson]

Warren,

On 06/13/2010 05:45 AM, 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.

I think one should best break the TPLL into three different cases, as 
they have different characteristics.

> 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.

The TPLL is a mid-tau stability test, since it's sweet-spot is in the 
0,1 - 1000 s range. Short-term is better handled in LPLL phase-noise 
measurements.

> 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.

Could be arranged. Best way would naturally be to whip up a draft 
article providing a survey. It may be useful to break up detailed 
analysis in separate articles. There is many details in this.

> 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.

Agreed... for large N.

> Be better off with one simple lower resolution multiplexed time stamped
> TI phase system and a single TPLL.

Actually that might not be what you want, the articles Bruce referred to 
points out some interesting problems which also needs to be understood 
as one progresses down that path.

For me, finding those articles was a good side-consequence of this 
discussion.

> 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?

It's a conditioned disadvantage, but still a disadvantage. Doesn't apply 
for 1 or 2 channel systems.

> 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.

Indeed. This is the N channel condition. The DTMT system proved much 
cheaper for many-channel setups. Infact, the many-channel setup 
situation has very few solutions even today when looking at commercial 
boxes.

> 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.

Sure is. The TPLL method does not fit all needs. It's fine, we just need 
to quantify properly what needs it fits. The N-channel case is a 
limitation which may or may not apply to a particular case.

For most hobbyists and many commercial usages, the N-channel case is not 
a serious limit.

It could also be argued that for small N (say 8) the cost would still 
not be prohibiting. Personally I lack mixers and such to setup an 
8-channel system, as I already have more than 8 10811 and an 8-channel 
ADC system suitable for the task. Oh, I still count myself as a hobbyist.

Cheers,
Magnus