By digital storage, I mean that the brain has full control over what's displayed, versus older types that have to present in real time, and can't store the information. Without storage, the repetition rate and activities of the SA have to be fast enough and transparent enough to present a usable display. With storage, the SA can do whatever it has to do, and take a fair amount of time doing it, but then only present the correct data on-screen. I assume the 492 and 492AP can do this.
So in my hypothetical SA process, let's say the 1IF and 2LO can be switched on the fly between 2072/2182 and 829/719 MHz. Let's also say there's a PIN-switched notch filter (stubs) in the input structure that can greatly reject either 2072 or 829 MHz, also switched on the fly.
Now suppose you want a full 0-4.2 GHz sweep. The first LO would start at 2072 MHz, and sweep to 2072 plus something less than 2072, say 1800 MHz to use round numbers. So, 1LO goes from 2072 to 3872 MHz, while 1IF is 2072, and 2LO is 2182, and the 2072 MHz notch is active. This would gather the correct amplitude info for 0-1800 MHz, while anything around 2072 MHz present in the signal would be rejected.
Then let's say the SA changes 1IF and the notch filter to 829 MHz, and 2LO to 719 MHz, then sweeps 1LO from 2529 to 5029 MHz. This would gather the correct amplitude info for 1700-4200 MHz, while rejecting signal content around 829 MHz.
Then let's say the SA takes the info from these two overlapping band results, and presents 0-1750 MHz from one, and 1751-4200 MHz from the other, in a continuous 0-4200 MHz display on screen, without telling you all the stuff it had to do.
Of course, without the benefits of LPFs or preselectors, there can still be various extra spurs, but at least the IF problem can be greatly diminished.
This may all appear confusing at first, but if you sketch it out it should make some sense. I made up these numbers on the fly while writing - hope I didn't make any logic or math mistakes that could confuse it more.
Ed
