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

[WarrenS]

Long explanations, cause I try to explain, the best I can, when I say 
something is "WRONG or misleading"

Magnus Posted:
> EFC linearity will remain an issue for analog oscillators.
> The oscillator gain will differ depending on offset voltage and 
> temperature.

TRUE it is an issue, but somewhat misleading because it need NOT be a 
problem or limitation (mostly)
EFC Linearity can be an issue because the TPLL is limited by the 
"performance" of the reference oscillator in lots of ways.
BUT
Oscillator EFC gain or linearity are not likely to be of much concern or a 
limitation for high end performance.

The gain nonlinearity I've measured can vary two to one over the full range 
of a good Oscillator but it is more like 10% over the normally used range, 
if one stays well away from the end points.
NOT so good but livable if you are not making something real accurate.
BUT
For all my accurate stuff, when using a HP 10811, I limit the full-scale 
change to 1e-9 or 1e-8 at most.
This uses such a small part of the total EFC range, that the nonlinearity 
effects are generally below the noise level and of little concern at all.

The fact that Oscillator gain does differ with the EFC voltage (offset 
voltage), means if you want to get max accuracy out of the TPLL, it will 
need to be calibrated at the EFC offset voltage it is being used at.  One 
simple solution, if the OSC also has a independent manual Freq adjustment 
like the single oven 10811, is to use it always set the EFC voltage to be 
near zero volts.
BTW calibration need not be much of a problem, because it can be a static 
calibration. What I use for a finial calibration & check is the 2G turn 
over, which I measure very accurately by other means before hand and then 
use that as a known freq offset to check operation and calibration. Of 
course there are any number of other ways.

As far as temperature having ANY effect on EFC gain, that is a total NON 
issue.
If temperature had any effect on EFC Gain then Temperature would also effect 
Osc Frequency at a fixed EFC voltage,
which would then effect the OSC freq drift and stability,
that would then effect anything that the Osc was used for, NOT just the 
TPLL.
The TPLL actually has a slight advantage over other methods,
because the PLL will adjust the freq to be correct, even if the EFC effect 
should change.



>> I think it is reasonable to assume that a TPLL weighs in at about
>> 200 USD with all support mixers, amplifiers, ADCs etc.  if you don't have 
>> the parts
> It is still a fairly cheap solution.

Yes I think that is ONE reasonable number to use and a fair conclusion.
BUT there are others.
The EBAY cost of the TPLL can be easy under $10, not including the reference 
Osc and the ADC.

Do note, NONE of items above are plural, Only one is needed per system 
unlike some other methods.
Because the cost of the Ref Osc is so variable and depends so much on what 
one is doing, I have noticed that its cost is generally not included in the 
base price. I think even on the $20K+  TSC 5120A that the reference Osc is 
an extra cost option.

The ADC is another BIG variable, depending on your needs and skill level and 
junk box, almost no limit in cost at the high end,
and can be as low as $0.00 dollars if you are a student doing a science 
project.
It can also be as low as $1.00 if one is good at programming PICS or other 
micros with built in ADC's.

The only other major part in the TPLL with any cost over $1 is the Phase 
detector.
The one I use most is a micro-circuits $15 single price device, but I've 
used all sorts of dual balanced mixers,
and if one is real cheap and good at design, I have found that a PD based on 
a 50 cent XOR gate works fine.

ws

*****************************

> On 14 June 2010 10:46, Magnus Danielson Posted:

> Steve
>> Still puts it in the mid-tau range as a method. The useful range and
>> precision of a particular implementation of the method will vary.
>
> By putting a GPSDO in the usual place of the DUT and putting the 10811
> in place of the reference oscillator it could work well beyond the
> 1000s point. CAVEAT: this only works for a DUT that has an EFC that is
> reasonably linear.

EFC linearity will remain an issue for analog oscillators. The
oscillator gain will differ depending on offset voltage and temperature.

>>> So if you are just thinking about the TPLL for taking ADEV data from 0.1
>>> to 1000 sec, then you're are missing 90% of the other useful stuff it
>>> can do as good or better than most anything thing else out there, and
>>> all for the same $10 (my cost).
>>
>> The typical price-tag of a 10811 is in 100-150 USD. I think it is 
>> reasonable
>> to assume that a TPLL weighs in at about 200 USD with all support mixers,
>> amplifiers, ADCs etc. It's not bad, but if you don't have the parts 
>> that's
>> about what you need to spend at least.
>
> If you really wanted to be a scrooge, you could open the case on one
> of those plethera of HP intruments and temporarily borrow that 10811
> that is just sitting there. As for something like a ADC, you could
> find a DAQ on fleeBay which could do duty here and also be a useful
> tool for general purpose use. Heck, you can use your sound card to
> digitize the EFC, provided it is DC coupled.

If you don't have the parts, then it will set you back with at least
that number. If you have the parts, you have already invested in them
and payed for them that way. You may be lucky to be given the
oscillator, but honestly you can't rely on that. I just want the cost
numbers to be more reasonably given. It is still a fairly cheap solution.

>>> BTW, with a couple of minor configuration changes, the TPLL BreadBoard
>>> can be transformed into a LPLL,
>>> so the usefulness of the basic Universal TPLL BB circuit has even more
>>> possibilities.
>>> In fact one could make yet another list of all the additional things it
>>> can do with no added cost,
>>> just by changing a few jumpers and values.
>>> But those things are for later discussions, one windmill at a time.
>>
>> Measurement of phase noise is what the LPLL is good at, especially when 
>> done
>> in cross-correlation mode. Interferometric setups use the mixers better.
>> Both these techniques could be used for LPLL and TPLL measures.
>
> Do you have some pointers to these setups please Magnus?

Look at Enrico Rubiolas site where his publications and presentations
should be inspirational. His focus is on LPLLs. but the methodology of
cross-correlation and interferometric setup should be as viable in the
TPLL world.

Cheers,
Magnus