The AM Forum

 I should know this, but here goes anyway.  I would like to use my older Fluke 1900A counter to set some local oscillator frequencies, etc.  What kind of probe is needed to keep any HV off the counters input?

Thanks,

Ed

Depending on the RF voltage and high voltage where you want to measure, you can use a scope probe. 

You can use a scope probe, but a close field probe, easy to make does not require contact to the device. Just take a piece of coax, wrap the center conductor back onto its shield. A tiny circular loop can be created based on its orientation and operate as an E field probe or an H field probe. In any case rotate the small loop to maximize pickup. I run mine into a small 1 GHz MMIC mounted at the of the probe and wrap the whole affair in shrink tubing.

Alan

P.S. Here is one link on the subject, 73.

http://www.edn.com/design/test-and-measurement/4380475/Near-field-probes-sniff-circuits

A lot of scopes have an output port that provides a sample of the vertical signal.  My standard bench scopes are a pair of Tektronix 7854 400 Mhz. scopes which allow you to direct the output from any of the vertical inputs to a rear BNC connector and I use this to connect to a counter.  This lets you use of the scope probe to reduce loading and protects the counter from excessive voltage.  Using a 50 ohm counter input the scope provides a 25mv per division output.

In some situations there is enough RF in the shack to just use a 2 meter rubber Duckie plugged into the counter.

Jim
WD5JKO

Per the manual, this counter can take 250v at the lower frequencies and less as frequency goes up.

With some care, I should be ok for my purpose.

Thanks,

Ed

If you will be checking that point from time to time it could be worth it to add in a test point of some kind, made so it won't change the load on the circuit much when the probe is placed, yet provides some standard-like voltage that counters like, with no DC.