[G0HZU_JMR]

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.