HP 8656B "Reverse Power Condition"

[FenTiger]

Hiya,

I bought a HP 8656B signal generator from Ebay. When I try to use it, sometimes the amplitude section of the display starts to flash.

The manual says (on page 3-19), "Whenever a reverse power condition is detected, all segments and all LED annunciators associated with the AMPLITUDE Display flash until the source of reverse power is removed and the AMPTD key is pressed."

This is exactly what's happening to me.

What does it mean by a "reverse power condition"? My first thought was that the VSWR is too high, but it will happily run without anything connected to the output at all, so I don't think that can be the case.

It's connected to a purely passive circuit (a filter that I'm trying to tune) so it's not that I'm somehow trying to "transmit" back into its output.

Any suggestions?

[0culus]

My 8657A has done this when connected to a counter before. It's supposedly good to 50W 50Vdc reverse power; no way a counter is providing that much reverse power, perhaps a logic fault?

[RFDUK]

This is perhaps only when the output level is turned up near max??

I think you may be on the right track considering high vswr.

As you tune the filter the sig gen output will be presented with a high vswr at a wide spread of phase angles, some perhaps inducing a trip point.

If this is the explanation, connecting an external 10dB pad on the generator output before your filter under test should make the 8656B happy.

Good luck.

[FenTiger]

It was set to -10dBm at the time, and turning it down to -30dBm did seem to make it a lot more reliable.

But if it was a VSWR problem, wouldn't it behave like this with the output disconnected? That's as badly mismatched as it can get, but I can turn it up to +17dBm without triggering this problem, suggesting that it's quite happy with its own reflected power. (Maybe I should try it into a short, to see if the phase makes a difference?)

I had another thought, which was ESD. If I managed to accumulate a static charge, and discharged myself into the filter, this could have excited the first resonator and sent a brief pulse of RF back up the cable towards the generator's output. This wouldn't explain why turning the power down seemed to help, though, so I'm still puzzled.

Of course, being a second-hand unit from eBay, it might just have a minor fault.