Electronics > RF, Microwave, Ham Radio

How to measure Noise figure of KU frequency up converter using spectrum analyzer


Hello Guy I would like to ask help if I can use a HP/Agilent Spectrum analyzer in measuring noise figure of KU band frequency upconveter. If it is possible could you please provide step by step procedure in making so. Thank you guys

You would measured the carrier to noise ratio on the input IF frequency, and then measure the carrier to noise ratio on the KU band output.

Lets say you see a 75 db C/n on the input and then 70 db C/N on the output.  You have lost 5 db and that's the noise figure.

You have to watch out for errors in the measurement, like select a proper resolution bandwidth on the analyzer and use the same setting on the input and output.  You also need to confirm the analyzer's noise floor does not change when you change frequencies on the analyzer - and that's a problem - as most have different paths internally for the low IF at 70 or 140 Mhz compared to 14 Ghz.  The noise floor usually rises the higher you go in frequency, in a spectrum analyzer.  Some analyzers have a "full" bandwidth setting and that should allow you to see where the analyzers noise floor is going at the input and output frequencies.

Usually a upconverter with two internal conversions is going to have a minimum of 12 db noise figure loss.  1st conversion up to say 1 Ghz, and then the next conversion up to the final frequency.  That 6 db is "ideal" for a balanced mixer and real world is usually 7 db and its not unusual for microwave mixers to have a lot more noise figure losses like 8-9 db.

After I made this post, I thought of a few items.  Most satellite upconvertors I worked with had 75 ohm input impedance and 50 ohm outputs.  All of them had input attenuators.

The last amplifier in the upconverter is going to set the noise floor on the output.

I know these are old, but HP published a wholes series on using spectrum analyzers.  The theory still holds today and they are worth a read.  https://www.hpmemoryproject.org/technics/bench/8568/bench_spectrum_docs.htm

When I said to choose the proper resolution bandwidth on the analyzer, its going to have to be below the noise floor of the output.  prudence is minimum 6 db, more than 10 db is excellent.  With that said, the input carrier will also have to be clean, you will have to work the gain back thru the converter and make sure the test signal is cleaner than the input amplifiers output.

The point is, this is not a trivial measurement.  It will also require a clean low phase noise signal generator.  It may be easier to use a data modulator/modem, were you can kill the modulation.  We use to do this by pulling our Forward Error Correction TX board and then the modem went CW on the output. 

I do not have access to some records I have but somewhere in my head, I thonk the C/N at EIRP had to be clean like better than 65 db.  The idea was not to have any intermodulation being thrown to the satellite. 


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