Thanks, I'm going to consider all diode types from all manufacturers so I'd be happy to use BAT parts if they work OK. I'd welcome any other suggestions as well. I do have an old Avago diode kit here with their whole Schottky diode range in it but this is all in SMD.
I did another test using a decent ESGD sig gen as the source. I used my (SMA) homebrew compensated tee with the far end terminated in a decent 18GHz SMA load. Normally, I would externally level the generator using a power meter and external ALC but in this case, I'm not after great precision (yet). This sig gen is usually well within +/- 0.1dB across this frequency range when using its own internal levelling as long as I pad the output and don't turn the level up too high.
If I test the channel A probe for flatness with my homebrew tee or a homebrew compensated termination I see a gradual upwards slope up to about 500MHz. However, this slope is only about +0.25dB in total. I don't think this is in the sig gen. Some of it will be in my tee and some of it in the probe itself. However, this agrees with my old HP 8405A when I do the same test on its channel A probe.
If I then plug the channel B probe PCB into the channel A probe on the newer HP 8405A, the performance degrades a lot in terms of flatness. It was so disappointing I didn't write down the results but it exceeded 1dB by about 150MHz and hit +2dB by 250MHz.?
So something is clearly wrong with this probe element. I have a fair bit of design experience designing Schottky detectors up to a few GHz and usually when something like this upwards slope happens it will be a function of the input capacitance and any stray package inductance in the overall detector design. This typically causes some peaking up towards UHF. Adding damping resistance can help but this can spoil the input Rp.
I think it could take me a while to find the right diodes to use, but I'm in no hurry as I have the other HP 8405A here and I rarely use it anyway. I want to get this newer HP 8405A working really well, hopefully as good as the older one in terms of probe performance :)
Note that even in its current state I think it would be fine to use up to about 100MHz even with the wrong diodes in the probe. I suspect that the majority of 8405A uses will typically use these meters below 150MHz.
