[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[Erik Kaashoek]

Here a small schematic of the simple Phase Noise Analyzer
Hope this answers some of the questions.
The +5 V reference from the OCXO is buffered by U1.
The 3 ports of the used ADE-1 mixer are galvanically  isolated greatly 
helping to reduce ground loops.
The output of the mixer is low passed using C2,L1 and C3 and used as 
input to the tuning of the OCXO
R4/R4 are actually two potmeters linked with summing resistors for 
coarse/fine frequency adjustment.
Inside the OCXO is a R/C low pass filter with a corner frequency of 
about 0.5 Hz.
The potmeter setting do influence the loop gain but in practice this is 
not a problem.
The mixer output is also amplified by U2 using automatic bias done with 
a large C1 and send into the audio input of a PC running the FFT program.
The 10MHz output from the DUT goes into RF_GND and RF_IN
For simplicity the supply decoupling capacitors and the output DC 
blocking capacitor are not drawn.
The opamps use single +12 V, just like the OCXO.
The OCXO must have better (or just equal) phase noise performance 
compared to the oscillator being measured (the DUT)
The +12 V supply comes from a bench supply with floating ground

By shorting C3 one can check for unwanted behavior like injection 
locking and measure the  internally generated noise.
R5 is added to measure noise levels when no DUT is connected.

The performance is surprisingly good, although one has to use a 
frequency counter to bring the DUT and OCXO close enough for lock.
To check for 90 degrees lock the R3/R4 potmeters are tuned to maximum 
noise level while still having lock.
Listening to the audio out is like listening to a DSB receiver. One can 
hear any disturbance or stray 10MHz. like the 10MHz house clock 
distribution cable being too close (not connected) to the Phase Noise 
Analyzer.
Shielding is important to keep the noise down.
By ensuring the FFT has a bandwidth of 1Hz a calibrated noise source can 
be use to establish a power level reference, much needed because of the 
undefined gains in the PC audio path.

I know this design is far away from what many people on this list are 
used to, but it was good enough for me to quickly check the performance 
of some oscillators.
Erik.
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