The MB510 should give more predictable results, having much more uniform frequency response than the UPB1506.
If tweaking bias and input frequency response tricks just don't cut it even with the MB506, and you must have no output (can't just ignore it) when there's no input signal, there is another option. This would entail having an RF signal level detector in the amplifier chain, and a comparator circuit that overrides the prescaler's bias so it can't respond unless the detector says there's enough input level to work right. The prescaler can then be run in stock mode, with maximum sensitivity, whenever it's enabled. This of course adds another set of considerations like what trip limits apply, and how accurately it all must be determined, and the added circuit complexity.
HP apparently got the original system to work adequately to avoid this nuisance response (right?), and meet some sort of specs - I'm not familiar with this model so don't know if these are true. Anyway, if they did, then you can too, but with the grief of having to use some different parts. I doubt that the original is extremely clean and flat and totally free of instabilities, but just enough for whatever the specs are.
Ed
