what is the fastest setup I can get with this 7854? The 7A24 with its 50 Ohm 400MHz inputs?The 7854 is specified to have 400 MHz vertical bandwidth (BW) when using either a 7A19 or 7A29 plug-in amplifier.
The 7A24 amplifier is specified to have 400 MHz BW when used in a 7104 mainframe. The 7104 is specified as quite a bit faster (1 GHz with 7A29) than the 7854. It is therefore to be expected that the 7854 / 7A24 combination may have trouble reaching the 400 MHz mark if you ignore the fact that actual BW of mainframes and plug-ins usually is significantly higher than the specification.
And the 7B85 ?The 7854 has a specification of 500 ps/div. maximum horizontally. The 7B85 is specified at only 1 ns/div. max, when using the 10x magnifier and so would be the limiting factor.
Just trying to figure out the ideal plugins to look out for.To fully utilise the horizontal and vertical speeds that the 7854 is capable of, a 7A19 (or 7A29) amplifier and a 7B92(A) would be a very nice setup. Be aware that the 7A19 and 7A29 are single-channel and that the 7B92(A) is a dual time base with delay.
With a 7B92(A) you'd have one horizontal slot unused. A 7D15 counter/timer would be nice to have real-time frequency and timing info.
What happens if you try to run the 1GHz plugins in the 7854? Are they just limited to 400MHz or what?See above. It's not a matter of "trying to run": It just runs at at least 400 MHz BW without a problem, as described above.
If you like cheating:
The fastest setup by far would use a sampling set (7S12 or 7S11 with 7T11A). Top BW would be around 14 GHz, depending on sampler. That would only work for repetitive signals though.
Although a sampling setup only needs a very slow 'scope (a few MHz would do nicely), the storage and processing capabilities make for a very nice set and beautiful screen traces.