HP 8566B RF interconnect cable

[Bill Woodbridge]

I’ve just taken the plunge and bought the mighty 8566B, complete with YTO Unlock problem (on 2 – 22GHz only) that I’m sort of looking forward to investigating.

But I have a simpler query please.  My unit came with what seems to be a third-party RF interconnect cable – the connector shells are black and flimsier than the battleship-like HP genuine article (the 50-pin digital bus interconnect however definitely is the genuine article).  More concerning than the cosmetics is that my cable has three subminiature coax connectors, whereas I know the genuine HP cable has four.  Despite this, (although I’ve barely scratched the surface) I can’t see anything obvious that isn’t working on the instrument.

Looking at the IF/display section troubleshooting manual, the missing connector corresponds to J1-5, labelled as ‘IF OUTPUT TO FREQUENCY COUNTER’, which is a signal going back into the RF section from the IF/display section.  But I can’t find any reference to this signal as an input to the RF section – for example, I can’t see it on the overall block diagram in the RF section troubleshooting manual. 

Is this signal perhaps only relevant to the 8568B RF unit (unfortunately I don’t have the troubleshooting manual for that, but I know its other documentation mentions an ‘internal frequency counter’ and that its architecture is completely different to the 8566B)?  Apologies as always if this information is somewhere on the forum - I did do a search but might have missed it.

Thanks for any help, Bill

[Bill Woodbridge]

I think I've now answered my own question.  Disconnecting the RF cable from the RF section (which I should have thought of before - when I bought the unit it was only disconnected at the IF section end) I see there are only three subminiature connectors present on the 8566B RF section.  Whereas the IF/display section has four.

So that seems pretty conclusive evidence that the 'IF OUTPUT TO FREQUENCY COUNTER' originating from the IF/display section isn't used by the 8566B - and presumably is only needed for the 8568B.

Thanks to those who read this.