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Is this a good approach to measure phase noise of a single ADF4351? #spectrum_analyzer

 

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 43nd 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.



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 using these settings:



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 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 as can be seen below



This looks more like the loop filter has a 60kHz bandwidth but according to ADIsimPLL the 10kHz noise should be 10dB lower with a 60kHz loop filter
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?
 

Hi Erik, this is a valid method for measuring phase noise. As you see, at these levels everything is sensitive. You could be seeing regulator noise as you suggest. What is the reference clock used by the ADF4351? This will also affect performance.

Let me examine the ADF4351 data sheet and ADISIM and come back to you. I have very high confidence in the performance shown in the data sheet. What board are you using for the ADF 4351?

Best regards,
Allen



On June 2, 2019, at 8:33 AM, erik@... wrote:


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 43nd 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.



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 using these settings:



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 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 as can be seen below



This looks more like the loop filter has a 60kHz bandwidth but according to ADIsimPLL the 10kHz noise should be 10dB lower with a 60kHz loop filter
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?
 

I'm using a 10MHz reference clock from the onboard xco. All settings can be seen in the screen grab of th ADF4351 tool in my first post. I tried with the doubler but that did not make a big difference. I also used the OCXO as reference as it has much better close in phase noise (below 1kHz) but no difference in noise above 10kHz
The ADF4351 module is the "green" ebay module. I also tested the black ebay module with 25 MHz onboard XCO reference, it has more spurs but comparable phase noise.
All modules have an ultra low ESR capacitor on the 3.3 regulator output
 

Erik, pls see attached application note for phase noise measurements. Page 31-32 describe the method you are trying to perform. Are you adjusting either signal to insure quadrature? The ADF4351 has the capability for adjusting the output phase, not sure if enough range is available to insure quadrature?

www.keysight.com/upload/cmc_upload/All/PhaseNoise_webcast19Jul12.pdf

Thanks!



On June 2, 2019, at 4:37 PM, erik@... wrote:


I'm using a 10MHz reference clock from the onboard xco. All settings can be seen in the screen grab of th ADF4351 tool in my first post. I tried with the doubler but that did not make a big difference. I also used the OCXO as reference as it has much better close in phase noise (below 1kHz) but no difference in noise above 10kHz
The ADF4351 module is the "green" ebay module. I also tested the black ebay module with 25 MHz onboard XCO reference, it has more spurs but comparable phase noise.
All modules have an ultra low ESR capacitor on the 3.3 regulator output
 

Allen, thanks for the excellent document!

Yes, I know the output phase is OK as the OCXO and the ADF4351 are slightly out of tune (200Hz) as can bee seen in the ARTA picture. This allows me to see both the signal and the phase noise but of course creates dynamic range problems.
My concern is that I also did measure the phase noise in-situ in the spectrum analyzer build with two of the ADF4351 modules and that resulted in a somewhat different picture (apart form the fact this measurement is done at 2.6GHz so DIV1
In-situ phase noise measurement
Another concern is that compared to the data sheet I seem to have a noise level at least 10dB higher from what is stated? in the data sheet.
 

Hi Erik, I'll try and duplicate your results. I'll have to purchase a USB? audio card for my PC, but I think I have all the other components on hand or can borrow. I have one of the `green` ADF4351 boards I use for a sig gen. I don't have one of the black boards.

Give me a week or so to pull it together and I'll come back to you.

Thanks!



On June 3, 2019, at 8:44 AM, erik@... wrote:


Allen, thanks for the excellent document!

Yes, I know the output phase is OK as the OCXO and the ADF4351 are slightly out of tune (200Hz) as can bee seen in the ARTA picture. This allows me to see both the signal and the phase noise but of course creates dynamic range problems.
My concern is that I also did measure the phase noise in-situ in the spectrum analyzer build with two of the ADF4351 modules and that resulted in a somewhat different picture (apart form the fact this measurement is done at 2.6GHz so DIV1
In-situ phase noise measurement
Another concern is that compared to the data sheet I seem to have a noise level at least 10dB higher from what is stated? in the data sheet.
 

Allen,

It may not bee needed to have an external USB audio. I also tried with 16bit 192kHz regular PC audio line and it also worked.
 

Hi Erik, my laptop doesn't have an audio in jack, thus the need for the audio card.

But.... I think there are bigger issues with the measurement technique. You alluded to this in your emails. For this method to work the reference signal (the standalone oscillator) must be frequency locked to the test signal (ADF4351). This then allows the phase of the 2 signals to be adjusted for quadrature prior to the mixer. This process eliminates the carriers and allows only the phase noise of test signal to be measured (as the phase noise of the reference signal is expected to be negligible.) Please see the details in the document I referenced in the prior email.

As you have it set up presently, you have an extremely large signal that happens to be a mixer product of.the 2 input signals. I'll experiment once I have the audio card in place. But I suspect what you are measuring is an artifact of audio card (large signal present and trying to measure a very small signal)?

Thoughts?

Thanks!



On June 4, 2019, at 8:54 AM, erik@... wrote:


Allen,

It may not bee needed to have an external USB audio. I also tried with 16bit 192kHz regular PC audio line and it also worked.
 

Could be as the actual performance of the SA using two of these modules is better than what I measure with the audio method

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