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

[Bob kb8tq]

Hi

One “cute trick” that can be done on the lowpass filter:

Ideally you would like to terminate the mixer properly at the DUT and DUT x 2 
frequencies. Open circuit (or short circuit) is ok at audio. The quick and easy way
to do this is to put an appropriate resistor ( 50 ohm, 500 ohm, whatever ) in series
with C2 on your schematic. It is common to put an RF choke across it to get it 
“out of the way” as you head to audio. 

Spice is your friend in this case. It’s way easier to pump it into LTSpice or something 
similar and tinker then trying to come up with some sort of math solution. 

Stuffing in a resistor at the output of the lowpass is also a common thing. Again the 
value is a “that depends” sort of thing. 500 ohms or 5K are the likely candidates. The
main value is to take the port to zero when the mixer is not being driven. 

No, none of this is a big deal. 

Since the ref out of the OCXO is likely a pretty noisy item, I would not get it anywhere 
near the low noise audio part of the circuit. The mixer output is typically grounded and
the ref out is ignored. The EFC is driven off of the lock section of the circuit to keep 
things in quadrature. 

If things are “locking up” without a lock circuit then indeed the earlier post about injection
locking applies.

Bob

> On Jul 5, 2022, at 3:15 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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.<PNA.pdf>_______________________________________________
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