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Old 23rd Jul 2022, 5:35 pm   #129
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: 6-gang FM stereo tuner heads

It's also worth exploring the datasheet for that Rigol spectrum analyser as this model doesn't have good distortion performance when compared to an old school analyser from HP. The Rigol datasheet says that the noise floor with 0dB attenuation is typically -135dBm with a 1Hz BW. So that means the noise figure is -135 - (-174) = 39dB. An old school HP analyser typically manages a noise figure of about 25dB.

The input IP3 for the Rigol is quoted as +10dBm with 10dB attenuation so the equivalent IP3 for the Rigol mixer would be 0dBm with 0dB attenuation. These aren't good numbers at all... A typical HP analyser might have a mixer IP3 of about +10dBm but some newer analysers can get close to +20dBm.

If I put these numbers from the Rigol into a basic spreadsheet it is possible to predict the levels of any internal IMD3 terms produced by the Rigol analyser. Obviously this depends a lot on the accuracy of the datasheet but it should give some idea of what to expect.

There's a screenshot of the Rigol below, taken from the blog website showing input tones at -11dBm to the analyser with 25dB internal attenuation. My excel spreadsheet is also shown below and if I boost the Rigol mixer IP3 up by just 1dB to 1dBm then the IMD3 distortion numbers do seem to agree quite closely.

The spreadsheet predicts IMD3 terms at -85dBm and a delta of 74dB. This is very close to the screenshot performance. The analyser has been set to a 30Hz RBW to try and squeeze out this level of performance. You can see the sweep time is a whopping 277 seconds so the operator was desperately trying to get the most out of the analyser here by selecting a very narrow resolution bandwidth of just 30Hz. It looks like it took over 4 minutes for the analyser to complete each sweep so these measurements required a lot of patience...

The distortion performance of a typical spectrum analyser also degrades when used below about 50MHz so the performance of the Rigol may be worse than this when used down at 10.7MHz for example. At low frequencies like this the first IF and the second mixer of the analyser will also have to cope with any leaked LO energy from the first mixer and it also has to handle the nearby image term so the analyser datasheet will often omit to publish distortion data below (say) 50MHz. This isn't because the manufacturer is being sneaky, it's just difficult to spec how much the performance degrades at lower frequencies. This has a lot to do with the mixer port isolation and the IF1 filter stopband performance and these parameters tend to vary even for the same model of analyser. All the top manufacturers tend to avoid quoting the distortion performance of a microwave spectrum analyser below several tens of MHz.
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