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Old Today, 6:10 pm   #321
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,025
Default Re: 6-gang FM stereo tuner heads

The 400MHz Marconi 2382 has a 3Hz RBW filter. I'm not sure if this is achieved using crystals or not. My old Advantest TR4172 analyser has the 7Hz RBW option fitted, and this is achieved with crystals. The HP8566/8 analysers use crystals for the narrower RBW filters and the narrowest has a RBW of 10Hz. I've not seen a 1Hz RBW filter made using crystals but maybe someone has managed it.

In practice, all the narrowest (crystal based) RBW filters (100Hz, 30Hz, 10Hz, 7Hz) are a bit ropey in terms of amplitude and frequency response so the best analysers will have an internal automated cal routine that is able to measure and correct for the errors in these filters. The operator usually has to connect a cable from a cal port to the RF input of the analyser and then run the automated routine. It normally takes a minute or two to complete. The accuracy and repeatability of the analyser should be much improved after this, especially when swapping between RBW settings. Usually, the analyser stores the cal corrections in NVRAM, so it isn't necessary to perform the auto cal routine every time the analyser is used.

Analysers with a digital IF don't need to do this anymore. However, my E4440A PSA analyser does have a suite of analogue preselector filters just ahead of the ADC. These are there to minimise/optimise the 'staring' bandwidth of the ADC. If they weren't there, the ADC would always stare at a 10MHz BW and this could make it easy to overload.

The E4440A has numerous LC and crystal preselector BPFs ahead of the ADC. These still need to be calibrated/checked for their response and the analyser does this at bootup and also every so often it pauses and does it during normal use. This can be quite annoying, but it does preserve the excellent performance of this analyser.
Regards, Jeremy G0HZU
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Old Today, 6:50 pm   #322
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 20,569
Default Re: 6-gang FM stereo tuner heads

There's a 1Hz RBW in the 8590A, and that uses crystals in its 100kHz IF. "RF" range is only 0-50kH, though.

The 859x analyser family have narrow (below 1kHz) RBWs done with a mix-down to a lower IF. 1kHz and wider is crystal, then LC at 21.4 MHz. Up-down keys will get you to 3MHz. But you can type in 5MHz and get it. Shape is not specified above 3MHz, but it's useful.

The higher performance 856x family can only go to 1MHz. They too switch in an extra conversion for the narrowest bandwidths, but later on, they were changed so the very narrowest IFs put the signal directly into what was otherwise the ADC after the log detector, and did the narrowest bandwidths as well as logging in DSP. This went on into the ESA family. I used the ESA as the basis of the noise figure analyser, but it has a completely different IF hardware and all RBW is DSP, aimed at 4MHz bandwidth for standard Y-factor measurements, but subsequent DSP can take it down to 100kHz if needed, but noise factor gets a bit slow to average down there. It was intended to be able to measure multiple 100kHz buckets at once and speed up measurement, but this didn't make it to the final product chiefly because of software development time for having to handle separate cal factors for each bucket. It was super-critical that the different IF gain stages had steps which combined arithmetically without combinatorial errors which were a limitation in the old 8970. The NFA has an extra mix-down from 21.4 to 6.25MHz. Partly, this helps with the ADC/DSP, but it also breaks up a lot of switched gain all at 20MHz in the 8970, and it's leakage around this chain which spoils the arithmetic combination of the gain steps.

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