Hi, Bruce,
Thank you for the comments and useful link. Probably you did not understand
the goal and positioning of this "project" and I did not tell the history of
how it was build :)
So, the solely goal of making this "test set" was to assist with the design
of the synthesizer unit for my HF transceiver. The synthesizer PN goals are
to archive PN better then -145..-150dBc/Hz@1kHz offset and better
then -150..-155dBc/Hz@5kHz and farther. So I do not need something perfect
to measure parts of the synthesizer or the complete unit.
Now some words how it was build. Several years ago I experimented with the
voltage regulators and needed to measure their noise. So I made an AD797 LNA
for my soundcard. Later I added the mixer which I used (along with the
signal generator) as a selective meter or primitive spectrum analyzer.
Several months ago I started to work at the synthesizer project, so I needed
PN "test set". I found a board with two TL071 in suitable configuration in
my "junk box" and after several minutes of soldering I had the PLL board :)
Usually I am not a fan of such construction methods, but that time it solved
problem quickly.
1) The chosen mixer isnt as low noise as the various Minicircuits phase
detectors.
I just used what I have. There are some very bad things here :( (it is way
out of the list theme), so buying parts (especially ones not widely used) is
not a simple task here. The Minicircuits parts are expensive and exotic
here.
2) The 50 ohm load after the filter merely serves to halve the phase
detector
gain. The IF port is terminated by a 15nF capacitor at RF and LO
frequencies
and their harmonics. This produces a frequency dependent gain, however it
will
likely be relatively flat over the sound card bandwidth.
I know it does not terminate mixer correctly, but it is simple and it works.
I tried the termination suggested in the NIST papers (with 50Ohm RF
termination and 1kOhm DC/AF one) with no success - the noise floor of the
"test set" was higher. As for the gain flatness, I checked it - you can see
the results of the quick test here
http://skydan.in.ua/PNTestSet/Screen(432)-e.png (it was 60MHz LO + signal
generator slowly tuned around 60MHz, the SA was set to peak and hold mode).
It completely satisfies my needs.
3) Saturating both mixer ports increases the phase detector gain
substantially
and has the lowest noise
In this case the simple and reliable calibration method I use will not work,
cause the mixer output will not be sinusoidal anymore. Another problem is
the signal levels - two good RF LNAs will be needed to amplify signals up to
the necessary levels to saturate mixer.
4) Cascading the PLL circuitry with the preamp causes interaction between
the
Preamp gain settings and the PLL bandwidth. Driving the PLL circuit in
parallel with the preamp input directly from the low pass filtered mixer
output
avoids this issue as well as your 0.1x amplifier in the PLL section.
I see no reason to use 20dB preamp gain for measurements (the sound card
noise will have too much influence with this setting), so it useful only for
calibration or the other LNA use (not in PN test set). On the other hand if
the PLL circuit connected to the LNA output we have minimal
components/wires/traces/connections in the most sensitive part of the test
set, so the chance to pick up some external noise is also minimized.
I can add that other good and simple/cheap additions will be the integrator
reset button, two buttons to move integrator in positive or negative
direction manually (to speed up the initial lock in some cases, or shift the
output voltage into the necessary EFC range), potentiometer for the manual
VCO/VCXO frequency control (for the calibration) with the switch to
close/open PLL.
An OCXO like the 10811A has an EFC gain of around 0.1Hz/volt.
The PLL bandwidth should ideally be less than 1/10 of the lowest offset
frequency for which the PN is to be measured.
If the system frequency response is measured then the PLL bandwidth can be
a
little higher albeit with a reduction is sensitivity and an increase in
system
PN at the low offset frequency end of the range.
AS is the PN noise of this test set is far too high to measure the PN of
state
of the art OCXOs or indeed most modern OXCOs.
Ohh... I am not a time nut (or maybe not a time nut YET ;). I did not try to
make something "state of the art" - my goals were/are different (see
earlier).
As for the PLL BW of cause one should be aware what the BW is. In my
measurements the PLL BW is less then 30Hz. I am not interested in PN closer
then 1kHz, so no need for any additional correction of the results.
Much more sophisticated system can be made - better ADC, better LNA, RF LNAs
to push mixer in saturation, better software, two channels with cross
correlation and etc. Or we can even use two high speed ADC and move more
things into digital domain. But it can not be done in one evening and for
the less then $40 ;).
Best wishes!
Oleg
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