[time-nuts] DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Mike Monett]

To Erik:

Hi. The PNA block diagram I posted earlier had two errors. I
corrected them in PNA V2.0, as shown in PMA2.PNG

The first error was a missing connection to the Phase-Frequency
Detector (PFD). This was a trivial error and was due to issues with
the image viewer dropping lines. Do not mess with the Irfanview
viewer. I switched to the XNview instead, which gave good
performance. So if you find problems with missing lines in the PNG
files, try a different viewer.

The second error was a missing 90 degree coupler to the Double
Balanced Mixer (DBM), which you generate with discrete components.
If you are interested, Minicircuits makes 90 degree hybrids for many
different frequency ranges.

For example, the https://www.minicircuits.com/pdfs/JSPQ-65W+.pdf is
useful over the 5 MHz to 65 MHz range, and appears to be dead on 90
degrees at 10 MHz. 

When you get up to 100 MHz, the
https://www.minicircuits.com/pdfs/PSCQ-2-120+.pdf gives about 0.5
degree error at 100 MHz.

The advantage of a 90 degree hybrid is very low insertion loss and a
50 ohm environment. This eliminates the noise generated by resistors
and op amps.

Speaking of op amp noise, the lowest noise op amp I know of is the
LT1028. The datasheet is shown at  

https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf

It has 0.85nV/vHz typ at 1kHz. However, if you look at the Voltage
Noise vs Frequency curve, you find the noise increases drastically
below 10 Hz.

One way to get around this problem is a zero drift chopamp. An
example is the ADA4523, shown at

https://www.analog.com/media/en/technical-documentation/data-sheets/ada4523-1.pdf

As shown in Fig. 2, the noise is below 5nV/(sqrtHz) from 0.02 Hz to
10 KHz. This might be useful in some designs.

Another item that might be of interest is the PFD. The Hittite
HMC984LP4E has -231 dBc/root(Hz) of noise, which is quite low. The
datasheet is at
https://www.analog.com/media/en/technical-documentation/data-sheets/hmc984.pdf

I noticed in your phase noise plots numerous spikes starting at 50
Hz. These clearly show you are living in Europe. What a wonderful
place. I lived in Metz, France for a number of years while I was
with NATO. Thanks for the memories.

However, the noise spikes show another problem. They may be due to
noise on the VCC supply to the oscillators. This can be a serious
problem, as John Ackermann showed in his page at
https://www.febo.com/pages/oscillators/wenzel_uln/supply.html

You can reduce this with a simple ripple filter, shown in
2N3906S.PNG. This reduces the noise by 80 dB at 10 Hz, and 120 dB at
100 Hz, as shown in 2N3906G.PNG.

If you are interested in modifying the design, the schematic entry
for LTspice is shown in 54E696BA.ASC. The corresponding waveform
plot file is 54E696BA.PLT

The advantage of this configuration is easy to filter negative
voltages by switching to a NPN transistor, such as a 2N3904.

The disadvantage is this filter is only capable of supplying limited
current. It cannot supply the current needed by the OCXO heaters, so
if this is a problem, you might consider switching to VCXO
oscillators.

I am searching for ways to increase the maximum current, but it is
very difficult to keep the attenuation numbers. I have found a
MOSFET that gives good attenuation, but it can only handle 1.4 Amps.
It is the Onsemi FDC2512, and the datasheet is at
https://www.onsemi.com/pdf/datasheet/fdc2512-d.pdf

It might be necessary to use two separate filters, but this might be
an advantage by keeping crosstalk low between the oscillators.

If you would like to eliminate the problem of quadrature lock, the
Hittite HMC984LP4E PFD might be of interest. The -231 dBc/Hz of
noise is very low and might be hard to reach with a DBM.

If you are interested in following up on phase-frequency detectors
to eliminate the narrow lock range of double-balanced mixers, I can
supply you with a wealth of information on the design, implementation, 
and testing. Just let me know if this would help.

Mike
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