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Measurements >> RF Measurements >> spectrum analyzer resolution bandwidth
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Message started by aaron_do on May 2^nd, 2007, 12:13am

Title: spectrum analyzer resolution bandwidth
Post by aaron_do on May 2^nd, 2007, 12:13am

Hi all,

i was recently measuring the gain of my LNA/Mixer. I noticed that the gain was around 10 dB with the resolution bandwidth higher than 100 kHz, but it steadily reduced as the resolution bandwidth was reduced below 100 kHz. There was no modulation, but the LO signal was provided by an on chip oscillator. does anybody know the relation between the measured signal strength and the resolution bandwidth?

thanks,
Aaron

Title: Re: spectrum analyzer resolution bandwidth
Post by ACWWong on May 2^nd, 2007, 8:00am

This effect can result from a couple of reasons:

When reducing the RBW, did you reduce the span and or sweep time ? This is normally done automatically to give the signal enough time in the RBW filter to be detected properly... too little time results in a lowered signal amplitude because of inadequate detection.

another possible reason is jitter in your LO which causes a moving (albeit slightly) mixer output IF signal. using a wider RBW will capture a wider signal range (ie all your IF power), whereas narrowing the bandwidth will on register signal power in a narrower bandwidth... so you might get a wider peak at a lower amplitude (on very narrow RBW and slow sweeptime you can see if your IF is moving, if so using a wider RBW would result in a larger amplitude as you would capture all your IF power in one peak).

cheers
aw