[time-nuts] Tight PLL Tester

[Bruce Griffiths]

If one follows that diagram blindly one will encounter a few problems 
with a 10MHz mixer/phase detector input frequency.

1) The PLL is a first order loop and the frequency of the OCXO being 
servoed to the oscillator under test has to be carefully adjusted to be 
close to that of the oscillator under test so that the phase detector 
operates in its linear region. A second order PLL may be a better choice.

2) The mixer IF port termination is far from optimum (see later NIST 
papers).
The phase detector sensitivity is much lower than with a better IF 
termination network.
A simple simulation (or test on an actual mixer/phase detector) will 
show this.

3) An off the shelf 750uH inductor will typically exhibit several series 
and parallel resonances in the 100kHz to 20MHz region.
Thus there may still be significant RF at the input of the dc amplifier 
with 80dB gain.
There will be a significant sum frequency (20MHz) component at the input 
to the LC filter.
The dc amplifier following the filter will rectify any RF at its input.
Amplifiers with FET input stages are less sensitive to RF.
An inductor with no resonances below 20MHz is preferred.
100uH inductors with a first SRF greater than 20MHz are available but 
from Germany.
It is usually advisable to use an RC filter between the LC filter output 
and the amplifier input to reduce the RF amplitude seen by the dc amplifier.
Another option is to use a cascaded set of passive RC filters instead of 
the LC filter, but this inevitably increases the noise.

4) One cannot substitute either a DVM or an oversampling ADC for the V 
to F converter and counter and produce a set of output samples that will 
necessarily allow one to calculate accurate values for ADEV without 
correcting for the fact that the system phase noise spectral response 
will differ from that when a VFC is used.

If the shape of the phase noise transfer functions differ from that when 
a VFC is used, the computed frequency stability measures obtained will 
not be ADEV, MDEV etc.

Bruce

WarrenS wrote:
>
> Thanks to the persistence and comments of others,
> I have marked up an old  NBS diagram to show, anyone that wants to 
> learn, how the Tight Phase lock method works to do its 'Magic'.
> Although it can be very simple and cheap to build, It does take a 
> certain amount of low noise design skill to be able to throw a bunch 
> of parts bin things together and make it work as well as it is capable 
> of.
> I do believe this information is enough for a well qualified person to 
> duplicate or even better my results.
> I'm happy to try and answer any specific questions.
>
> also see word discription from:
> Page 170 of 'NBS special publication 140' at:
> http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS140.pdf    81 meg, 
> 473+ pages  (Takes a while to download)
>
> For another block diagram and short description also see Figure 1.7 at:
> http://tf.nist.gov/phase/Properties/one.htm#oneone
>
> Have Fun
> ws
>
> ****** edited **********
>> Tom
>>
>> Things will turn out much better to do it the other way around.
>> When I find out who is going to build/test it,
>> I'll  make something specific for them that will allow them to be 
>> able to use there own parts and tool box.
>>
>> ws
>>
>> **************
>> ----- Original Message ----- From: "Tom Van Baak" <tvb at LeapSecond.com>
>>
>>>> If there are any Nuts out there interested in helping to make 
>>>> available to other Freq-Nuts a SIMPLE tester that I have found to 
>>>> be a VERY useful low cost tool,
>>>
>>> Warren,
>>>
>>> Yes, I think it's a good idea for a couple of people to try to
>>> duplicate your results; either to validate the resolution and
>>> features that you're claiming, or to locate or quantify the
>>> limitations in your implementation. Either way it will be a
>>> learning experience for you, and for the group.
>>>
>>> To that end, would you be able this week to write a quick
>>> word document or readme or web page with photo(s) of
>>> your setup, schematic, parts list, specific make/model of
>>> the equipment that you're using, etc. Since you say it is
>>> a simple setup, I suspect a number of us would then be
>>> able to dig in our parts bin and mimic your prototype
>>> as close as possible and then objectively measure how
>>> it works compared to other phase noise measurement
>>> systems.
>>>
>>> /tvb
>>>
>>
>