WarrenS wrote:
ws said
if what is said does not agree with my experimental results, I'll
comment.
Bruce said:
0) If one follows that diagram blindly ...
Then that one should not be BUILDING the TESTER from scratch.
1a) The PLL BW... has to be adjusted to be close to that of the
oscillator under test.
The PLL Close loop BW is NOWHERE near the Osc freq. I agree that would
cause severe problems.
The PLL BW has to be high compared to the 1sec LP filter value they
are using.
What I have shown in the markup, is a 1K to 10 KHz PLL bandwidth which
makes a good typical value.
IT IS NOT CRITICAL, just has to be high compared to the low pass filter.
1b) so that the phase detector operates in its linear region.
The Phase detector is ALWAYS operating in its linear region, and NEVER
off by even a mv at its output.
The high bandwidth 10,000 gain amps between it and the ref osc will
see to that.
1c) A second order PLL may be a better choice.
A first order works fine at this bandwidth, with NO freq control RCs
in the loop.
When done right, there is only the natural pole of freq to phase
2 & 3) The mixer LP filter is far from optimum
Which is why I use a C R C R C for mine.
My R's are 49 ohm, Nothing magic about the value, just that with 100
ohms, they added more Johnson noise.
All such details are important, even the type of resistor and at high
frequencies the actual part no and manufacturer.
Component parasitics (capacitor inductance, capacitor esr, resistor
inductance, resistor shunt capacitance, etc)can be very important in RF
circuits.
Not all resistors (or capacitors) of the same style/size from different
manufacturers have the same parasitics.
4) One cannot substitute either a DVM or an oversampling ADC
If all that is wanted is a chart recorder output, You can use any DVM
as shown in the block.
As long as the oversamping ADC is fast compared to the Low pass 1 sec
filter used in their block
Then the system FREQ noise spectral response as recorded in the PC Log
file is just about totally determined by the Low pass filter
and NOT the freq response or type of ADC or VtoF converter used or its
update rate.
I think we're making progress, I didn't see any mention of the
nonexistent Phase recovering integrator this time.
It should always be present either explicitly or implicitly, to believe
otherwise is to misunderstand the relationship between average frequency
and phase differences.
Without an integrator or its equivalent (implicit or explicit) the phase
noise transfer function will differ from that of the NBS implementation.
One way to approximate a box car integrator is to average the output
samples in blocks of N samples where N is the number of samples in the
minimum vale of Tau.
The minimum value of Tau should be around half the RC low pass filter
cutoff frequency period (you need to read the Stein paper for details).
However the sampling rate has to be sufficiently high (well above the
Nyquist limit) to allow this without using WSK interpolation to estimate
the signal value between the samples.
thanks, it's always fun to read your comments
ws
*************
Bruce
----- Original Message ----- From: "Bruce Griffiths"
<bruce.griffi...@xtra.co.nz>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Thursday, February 11, 2010 4:02 PM
Subject: Re: [time-nuts] Tight PLL Tester
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" <t...@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
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