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Mixer Noise Figure Measurement - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


pigger
Post subject: mixer noise figure measurement
Unread postPosted: Sat Jul 10, 2004 2:03 am

we are measuring mixer noise figure. supposedly, it should be a smooth curve, but we saw lots of very high spurs along that curve. Has anybody here met this situation before? can anybody help me to analyze what happened?
Thanks in advance


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Guest
Post subject:
Unread postPosted: Sat Jul 10, 2004 12:32 pm

How are you measuring the noise figure? Are you using an automated noise figure meter that supplies both the reference signal and the LO, or are you supplying an external LO and just measuring noise floor difference from input to output, or some other method?

- Kirt B. :smt024


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pigger
Post subject:
Unread postPosted: Sat Jul 10, 2004 3:07 pm

we use the automated noise figure meter (actually, a VNA with noise figure measurement function) with both the ref noise source and LO

Anonymous wrote:
How are you measuring the noise figure? Are you using an automated noise figure meter that supplies both the reference signal and the LO, or are you supplying an external LO and just measuring noise floor difference from input to output, or some other method?

- Kirt B. :smt024


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pigger
Post subject:
Unread postPosted: Sat Jul 10, 2004 3:37 pm

I think I didn't make it clear.
I use a VNA ( just like an automated noise meter, I guess, Y-factor method) to measure the mixer noise figure. I swept the RF and LO frequency and fix the IF. VNA supplies the noise (or signal) and LO signal.
The phenomenon is at certain freq, there would be very high spurs instead a smooth curve. And "noise figure overload" would also be shown on the VNA.


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Kirt Blattenberger
Post subject:
Unread postPosted: Sat Jul 10, 2004 8:25 pm
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Assuming that your tester is not injecting spurious signals into your DUT, it is posible that you are generating a mixer spur that falls inside the measurement band. That can definitely cause an erroneous NF reading.

You might want to try backing off the mixer input power. If possible, connect the mixer output to a spectrum analyzer and look to see if there are discrete spurs stepping across the output band as the input steps.


Kirt B. :smt024


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DW
Post subject:
Unread postPosted: Sun Jul 11, 2004 6:32 pm

Few things to check:

1.) are you filtering your output such that only your IF (assuming a downconversion) is getting through?

2.) Make sure there are no other pieces of equipment in the area that are on.

3.) Make sure the LO and RF 10 MHz signals synced up. Not having these synced up can induce some drift which can mess up some measurements.

4.) If its an active circuit or there are amplifiers around the mixing circuit, check for oscillations/spurs.

BR,
DW


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pigger
Post subject:
Unread postPosted: Wed Jul 14, 2004 2:32 am

I don't see any obvious spurs when there are no RF and LO signal coming in. but I will try backing off the input power.

Kirt Blattenberger wrote:
Assuming that your tester is not injecting spurious signals into your DUT, it is posible that you are generating a mixer spur that falls inside the measurement band. That can definitely cause an erroneous NF reading.

You might want to try backing off the mixer input power. If possible, connect the mixer output to a spectrum analyzer and look to see if there are discrete spurs stepping across the output band as the input steps.


Kirt B. :smt024


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pigger
Post subject:
Unread postPosted: Wed Jul 14, 2004 2:35 am

I am not filtering the mixer IF output, and I also guess maybe it's the LO leakage or RF leakage who disturbed the NF measurement. But generally is it normal to use a filter, or it is a must when measuring NF.

DW wrote:
Few things to check:

1.) are you filtering your output such that only your IF (assuming a downconversion) is getting through?

2.) Make sure there are no other pieces of equipment in the area that are on.

3.) Make sure the LO and RF 10 MHz signals synced up. Not having these synced up can induce some drift which can mess up some measurements.

4.) If its an active circuit or there are amplifiers around the mixing circuit, check for oscillations/spurs.

BR,
DW


Posted  11/12/2012

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