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PN measurement of freq div

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Alex_IC

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Hi!

How to measure phase noise of a static frequency divider if I have E5052B but don't have a generator with pn better than -120 dBc/Hz ?

Could somebody suggest a paper or book to know how to do that..

Thanks for help!
 

Use your generator, take a power splitter to divide the mw signal, feed TWO identical dividers in parallel, lowpass filter the output, and use the two divided outputs to go into a phase detector (you might need to add a phase shifter in one arm to make the mixer be in quadrature). When the pn measurement is over, take 3 dB off the measurement results.
 

Thanks for reply. But what should be the phase shift? Should it be exactly 90 degrees?
 

IF all the lines were exactly balanced in phase length, then yes, the phase shifter would be exactly set at 90 degrees. In practice, though, you find that the line lengths are not balanced. For instance, maybe you need more power on one port of the mixer/phase detector than the other input port. So you add an amplifier to that port. That, of course, screws up your phase balance.

So you temporarily hook a DC voltmeter to the phase detector/mixer output, and adjust the phase trim until you read 0 volts DC. Then you remove the voltmeter (so it does not add noise) and proceed to make the phase noise measurement. A mixer acts as a phase detector when it is tuned up in "quadrature", which means its IF port output is at 0 VDC, with small ac variations (that contain the phase information). If you have an automated phase noise measurement system, it will do these adjustments for you as part of its narrowband phaselocking system.

The reason this all works (being able to use a relatively poor microwave signal source for a high dynamic range phase noise measurement) is that you are measuring the phase noise variations of the dividers themselves, but the phase noise of the microwave source is "correlated" at both divider outputs, so its inital phase noise "cancels out".

IF your measurement system does not set the phase dectector's IF output to 0VDC, then you will have less phase detector gain AND you will also be measuring AM and PM noise combined.
Rich
 
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biff44 said:
IF all the lines were exactly balanced in phase length, then yes, the phase shifter would be exactly set at 90 degrees. In practice, though, you find that the line lengths are not balanced. For instance, maybe you need more power on one port of the mixer/phase detector than the other input port. So you add an amplifier to that port. That, of course, screws up your phase balance.

So you temporarily hook a DC voltmeter to the phase detector/mixer output, and adjust the phase trim until you read 0 volts DC. Then you remove the voltmeter (so it does not add noise) and proceed to make the phase noise measurement. A mixer acts as a phase detector when it is tuned up in "quadrature", which means its IF port output is at 0 VDC, with small ac variations (that contain the phase information). If you have an automated phase noise measurement system, it will do these adjustments for you as part of its narrowband phaselocking system.

The reason this all works (being able to use a relatively poor microwave signal source for a high dynamic range phase noise measurement) is that you are measuring the phase noise variations of the dividers themselves, but the phase noise of the microwave source is "correlated" at both divider outputs, so its inital phase noise "cancels out".

IF your measurement system does not set the phase dectector's IF output to 0VDC, then you will have less phase detector gain AND you will also be measuring AM and PM noise combined.
Rich
Click to expand...

Can someone give explanation from math on this method?
I am a little confused. Assuem one arm signal is cos(wt), the other arm after 90deg phase shift is sin(wt). Then the mixer output is cos(wt)*sin(wt)=sin(2wt)/2, where is the IF component for test phase noise at low freq?

And another question is: use this method, if we measure the PN at IF is -XdBc/Hz, so the RF signal PN should be (-X+3)dbc/Hz or (-X-3)dBc/Hz?

Thanks for your reply.
 
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    kayaoo

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Let me try:
Signal1= cos(wt+pn_1(t)+pn_source(t))*(1+noise_am(t)),
signal2= sin(wt+pn_2(t)+pn_source(t)+theta)*(1+noise_am(t)).
mixer out= (1+noise_am(t)*2)*sin(pn_1(t)-pn_2(t)-theta) /2
as E|pn_1(t)|^2= E|pn_2(t)|^2, and all items ~=0
=(1+2*noise_am(t)) *( pn_1(t)*sqrt(2) +theta)/2
if noise_am =0 and theta =0,
mixer out= pn_1(t)*sqrt(2)/2;
So the PN_actual= PN_measured－3dB.
 
kayaoo said:
mixer out= (1+noise_am(t)*2)*sin(pn_1(t)-pn_2(t)-theta) /2
as E|pn_1(t)|^2= E|pn_2(t)|^2, and all items ~=0
=(1+2*noise_am(t)) *( pn_1(t)*sqrt(2) +theta)/2
if noise_am =0 and theta =0,
mixer out= pn_1(t)*sqrt(2)/2;
So the PN_actual= PN_measured－3dB.
Click to expand...

1. So what is the freq for measured? Is it locked? What is the range of the measured freq? Sevral Hz? or KHz? Or MHz?

2. I noticed that the source phase noise was cancelled, so what should I do if I want to test source phase noise with this method? I remembered biff44 recommeded this method for 100G source phase noise test.
 
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    kayaoo

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tony_lth said:
1. So what is the freq for measured? Is it locked? What is the range of the measured freq? Sevral Hz? or KHz? Or MHz?

2. I noticed that the source phase noise was cancelled, so what should I do if I want to test source phase noise with this method? I remembered biff44 recommeded this method for 100G source phase noise test.
Click to expand...
It is measured in the zero frequency. Locked is unnecessary. I agree that a much more careful analysis is needed to determine the accuracy and the offset frequency range. An RF delay line may be used when measuing the source PN( with single source). Frankly, I have no idea for freqency above 2GHz.
 
Alex_IC said:
Hi!

How to measure phase noise of a static frequency divider if I have E5052B but don't have a generator with pn better than -120 dBc/Hz ?

Thanks for help!
Click to expand...

But E5052B signal source analysis test range start from10M, so how can test zero freq with E5052B?
 

tony_lth said:
But E5052B signal source analysis test range start from10M, so how can test zero freq with E5052B?
Click to expand...

You are right there. You will need to use a spectrum analyzer with a lower frequency input (and possibly a low noise video amp in front of the spectrum analyzer) to measure like described above.

Not familiar with the e5052b, and do not feel like reading the manual. But some of these newer analyzers do have cross-correlation techiniques or an IF input connector that might be used as above.

One quick way to do the measurement is to go out and get a really low phase noise 100 MHz crystal oscillator and drive one divider chain and measure with the e5052b. To some extent, the PN measurement does not vay much with input frequency.
 
you have the perfect instrument to do your measurement, following biff44 suggestions

E5052B performs internally a downconversion to DC and feeds this signal to a very low noise DC to 10MHz (or 40MHz if you have the right option) amplifier.

The input of the amplifier is available through front panel (baseband in connector).

Mazz
 
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