[time-nuts] EIP545A 18GHz counter query

[Charles P. Steinmetz]

Chris wrote:

>Band 2 should work from -20dbm (22mV RMS) right across its 10 MHz to 1
>GHz range according to the manual. Even with 190 MHz into it it takes
>at least 30mV to start triggering, sometimes up to 40 mV.  40 mV will
>reliably fire it across its full range once it warms up a bit.
>
>Band 1 specs are 22 mV from 10Hz to 1 GHz, and that band triggers with
>just 15 mV at 10 MHz, but needs 25 mV at 100 MHz.
>
>Band 3 specs are  12 mV at 1 GHz to 1.2 GHz (needs 15 mV @ 1GHz)

Ahh, I had missed that you were feeding the 10 MHz from the divider 
to the *measurement* inputs.  It sounds like a triggering issue.

Is the trigger coupling switchable from DC to AC, and is it 
adjustable for trigger level?

It is not unusual for the trigger point to drift a bit as a counter 
warms up, but at any temperature you should be able to find settings 
that will allow it to trigger on inputs at least close to the minimum 
the manufacturer specifies.  The figures you are seeing are not way 
out of bounds, especially if the trigger coupling and level have not 
been adjusted for optimum triggering.

Again, note that the divider output is unipolar (positive-only, does 
not cross 0V).  With a TTL input, you should get good, stable 
triggering with the trigger coupling set to DC and the trigger level 
set to ~+0.5V on the positive-going edge.  With sine-wave inputs, you 
may get best results with AC coupling and a trigger level at or near 
0V.  Look under "triggering" in the manual to see what the 
manufacturer recommends.

It's just like the triggering on an oscilloscope, if that helps -- 
what you would do to get a stable scope display is also what you need 
to do to get a stable frequency reading.

Best regards,

Charles