I've got an HP 8640B Opt 2, and am quite satisfied with it's performance but
this one doesn't go below 500KHz since it doesn't have Opt 1.

But I don't think I'd have phase lock available which is nice but probably
not mandatory. I'm not sure how the Opt 1 works with the attenuator.

Hank-

I'm not that familiar with the specs, but it looks like the lowest band on the
8640B is 250 to 500 KHz, regardless of option.

The variable audio oscillator option has its own output level adjustment, but I
don't think it is calibrated like the main RF output. It certainly doesn't go
through the main attenuator except as modulation on the RF! I'm not sure, but
the variable audio oscillator's output may be 600 Ohms rather than 50.

When I've tried adjusting the frequency on the variable audio oscillator, I've
found it touchy. You definitely do NOT have phase lock on that oscillator. It
would be fine for use with AM and FM modulation, but not the greatest as an LF
RF signal where accuracy is a bit more important.

I can see using it with sufficient output to drive the counter's external
input, but with an external attenuator to set the impedance and level for your
application. It might work, but I wouldn't be happy with such an arrangement!

73, Fred, K4DII