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_INFor 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.
PNA.pdf
Description: Adobe PDF document
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