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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Jun 14 18:25:11 EDT 2010


On 06/14/2010 06:13 PM, WarrenS wrote:
> 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.

The main point I am trying to make is that it may not be useful to read 
the number of the data-sheet, but calibration methods should be 
performed. Should not be particularly hard to do, but if you do not make 
provisions for it, it will become a scaling error issue.

Hand-trimming the coarse offset should be done if far away.

> 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.

Yes, but since we derive our measurements from that EFC our sampled data 
will change so it will creep into the sample-series. Hand-calibrating 
towards zero EFC and let it stabilize should work well enough. 
Identifying potential problems is the first step to finding ways to 
avoid or compensate for them.

>>> 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.

The point I was trying to make that the total cost was more around 200 
USD including ref oscillator and 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 reference oscillator plays a different role in that system.

> 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.

Indeed. Cheap audio-boards could be hacked up.

> 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.

You may get very low on some of these, but what I was aiming for was a 
more average price for the average builder. You are usually not all that 
lucky when you want to. Also, recall that packaging and transport adds 
on top of that, including import taxes and VAT... which I did not included.

Your milage may vary a lot. I just 200 USD is a more realistic value. It 
doesn't make it less valuable as a tool, I am not trying to say that at all.

Cheers,
Magnus




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