Measuring method for phase noise of single ADF4351 module

[erikka]

Till now I always measured phase noise by using multiple ADF4351 modules. To understand the impact of loop current and loop filter changes it would be better if it would be possible to measure the phase noise of a single  ADF4351 module. The maximum XCO I have is at 10MHz. Because of the output divider in the ADF4351 the lowest multiple at 50MHz (DIV64) has too low phase noise to measure as can be seen in the datasheet
Instead I went for a setup at 430 MHz using a divider of 8 (DIV8) resulting in 20dB more phase noise

The 43th harmonic of the 10MHz signal is filtered out and amplified and send as LO signal to as passive DBD mixer. The output of the mixer goes to a PC using a good microphone input.
Without the LO signal connected to the mixer, ARTA show the noise floor of the microphone input at -120dB. Good enough for this measurement.
With the LO connected the noise floor goes up which will limit the noise measurement possibilities
With the ADF4351 connected and set to 430MHz I get the following measurement in ARTA using a 65536 bin FFT with FS at 192kHz (3 Hz per bin) so the measurement is 5dB above a /Hz measurement. Output signal of the ADF4351 is at +5dB but the noise is measured relative to the main output signal.
The 0dB peak at 200Hz (due to frequency error of the onboard XCO) can be moved around (even to 0 Hz) by tuning the OCXO. Phase noise at 10kHz seems to be at -100dB (compensating -5dB for the 3Hz FFT bins) which is about consistent with a simulation using ADIsimPLL using a loop filter of 15kHz

The phase noise however extends to much higher frequencies before it starts to decrease. Decreasing the charge pump current substantially leads to a noise increase at 10kHz which suggests a loop filter bandwidth of 10kHz

So now my questions before I start to exchange loop filter 0603 SMD components or low noise 3.3V regulators:
- Is this an appropriate method for measuring the phase noise of a single ADF4351 using only simple equipment?
- How to improve the measurement?
- Is the loop filter bandwidth 60kHz or is it 10kHz and is something else pushing the 60kHz noise up such as a noisy regulator?

[1design]

The best way that I know of to measure low phase noise on a budged is described in section 4 of the following document:
https://www.npl.co.uk/special-pages/guides/gpg68_noise

All you need is a phase shifter. The rest is trivial.

I was able to measure sources with phase noise in the 20fs range integrated from 10Hz to 10MHz using this method. Also added jitter below single digit fs very repetitively.

[erikka]

I tried a long coax cable as phase shifter (10 meters) and was able to tune the frequency of the ADF4351 to match the length of the cable to get DC output to zero but did not see any phase noise.
What length of coax would work between 2 and 4 GHz?

[erikka]

This is what I measure using two modules in a spectrum analyzer application.
Its indeed different as it does quicker

[Kleinstein]

With the phase shift though a long coax one would only get the phase noise for a given delay, so like the Allan deviation for that relatively small delay which corresponds to the rather high frequency phase noise.

As the modules are relatively simple, the best bet is likely using 2 such modules and a mixer.

Alternatively a good reference source could be used for one - the 10 MHz osc. and using the 43th harmonics may not be that low noise. Besides the normal increase in phase noise from going up in frequency, there can be additional noise from variations in how effective the harmonic generation is.

[Theboel]

Did You need to phase lock the out of DBD to OCXO ?

[erikka]

No, the ADF4351 and OCXO run at different frequencies to get the 200Hz mixer output. If one wants to get a zero beat you will need to phase lock