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

WarrenS warrensjmail-one at yahoo.com
Tue Jun 15 14:49:08 EDT 2010


Magnus posted a bunch of good stuff,

>Neither. It's a characteristic, it needs to be analyzed. If the DUT is
>very sensitive, then additional care may be taken or maybe it just isn't
>a very good solution.

We have little disagreements for the most part. Except maybe for if it 
should be on 'the list'.
The characteristics are what makes the advantages and disadvantages, so they 
belong on the list.

The high natural rejection that the TPLL has to injection locking makes it 
much less sensitive than any other system, so it lets one keep it simple. 
TO me that is an advantage.

I did test for the effect by connecting the REF osc to the DUT osc with a 
variable attenuator, and could not make the TPLL system start to fail until 
I the coupling got so low that it started to short out the two signals, and 
still it was working, just got a bit noisier
I called the results of that test good enough and moved on to other ways of 
testing.
All of which it also passed with flying colors

Another  subtle side effect of that advantages is that the cables do not to 
be shielded so well.
example:
If I bring a third osc's that is connect to a RG58 BNC shielded cable and is 
offset in freq by 1 to 10 Hz
The effect of coupling (and or Injection locking)  between to BNC shielded 
cables can be seem at the Tester output.

What I have found is if I want to get the full accuracy of the TPLL, I can 
not have an offset osc cable on the same table, unless it is fully enclosed 
in a RF proof box.

ws

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

[time-nuts] Advantages & Disadvantages of the TPLL Method
Magnus Danielson magnus at rubidium.dyndns.org
Tue Jun 15 17:53:49 UTC 2010


On 06/15/2010 03:28 PM, WarrenS wrote:
> Bruce posted
>
>> But Adler's equation indicates that an oscillator is much more
>> susceptible to injection effects when the injected signal frequency is
>> very close to the oscillator frequency.
> No argument,
> BUT
> The thing that you (and maybe Adler?) are missing is that effect goes
> away when the two frequencies ARE exactly the same.
> I'm not talking close, I'm talking the exact same freq with phase held
> in quadrature within single digit femtoseconds.
> BIG difference, Once that is understood, then that sort of answers your
> other comments.

Actually, as we have this in a PLL, it doesn't work out quite like that.
Wolaver shows how the PLL is modified with an additional proportional
path (his analysis is on active PI loop, but the additional proportional
path also applies to active lag loops).

For the active PI loop, the injection locking will modulate the PLL
bandwidth. I would need to analyse the details for an active lag filter,
but since the gain increases in the loop, so will the loop frequency.
The implication for the EFC is that not all the gain goes through the
amplifier, so the EFC deviations will become less. However, a tight PLL
has high gain for starters, so unless there is very high injection gain
the effect will be very modest. If the gain is calibrated during closed
loop conditions and the injection is relatively stable, then the effect
is essentially cancelled anyway. I would assume that the gain-change of
typical injection is a small fraction of loop gain, so then it is a
small effect that should not affect the results to much. It may be good
to measure the injection gain and compare it to the loop gain.

For a loose PLL, as being used for phase noise measurements, injection
locking is a much bigger concern.

But, the effect is there even when the frequency is the same. It is just
that the application in tight PLL makes it very small.

>> For each and every oscillator pair someone may try?????
> Can't say for sure, I've only tried the ones I've tried, but even the
> ones that are highly susceptible were OK.
>
>
>> At best you've only shown this to be true for the particular
>> oscillator pair being compared.
> Yep, maybe I'm just real lucking again and it only applies to all the
> ones I've tested.
>
>
>> That's descending into the murky realms of pseudoscience.
> OR as I see it, it is using just a little common sense.
> When is the last time you heard of a problem with an oscillator
> injection locking to it's self?

I think these can be answered by much better arguments than
pseudoscience or pure luck. The loop gain dominates over the injection
gain and thus makes it a minor effect. and that by applying analysis.

>> Not only must the effect of injection locking be insignificant for the
>> reference, it has to be insignificant for the test oscillator as well.
> NO argument, If you are testing a DUT what this does effect, then one
> should buffer it or take other precaution.
> If you want to make sure the tester is not effecting the osc there is
> another choice besides the Buffer.
> DON'T connected it. The tester will work at better than -60 dB signal
> levels and if one just gets a couple of small wires close that is enough
> signal coupling that one can made 1 sec and slower tau readings.
> OR you do both the buffer and antennas you can test the OSC from across
> the room.
> Which side of the list do you think that should go on, advantages or
> disadvantages?

Neither. It's a characteristic, it needs to be analyzed. If the DUT is
very sensitive, then additional care may be taken or maybe it just isn't
a very good solution.

>> If injection locking is an issue the efc gain with the loop
>> open will differ from the efc gain with the loop closed.
>> It will change the loop parameters in particular the efc gain.
>> Its just a matter of how much it affects the efc gain.
> NO argument, The PLL loop is never opened. THAT will screw up everything
> and cause the injection, delta gain, etc.
>
> Sounds like it is time for someone to find or write another one of the
> fancy math papers that covers this case.

Page 102-104 in Dan Wolavers book covers this with sufficiently clarity
that I could answer this. I have been trying to hint to this before.
Anyway, page 103 is a very instructive conversion from different
representations of a PI-loop with injection into a simpler form. The
math follows up and remaining analysis becomes very easy to do.

Cheers,
Magnus 





More information about the time-nuts mailing list