EBME Forums Forums Maintenance Technical Assistance HF RMS voltmeter

Hi,

Can anyone recommend a high frequency true RMS voltmeter? I need one to calibrate an Ethicon Harmonic Scalpel. I've tried our Diathermy tester but it doesn't have the accuracy and has relatively low input impedance. Obviously being the NHS I'd have to go for an inexpensive (cheap!) one.

Cheers,

Bill

Have you tried making one? The way I measure RF power is to use an oscilloscope and for the load I use the thick film non-inductive power resistors you can get from Farnell Electronic Components. Of course using the oscilloscope will give you the peak power, but you’ll be able to work out the RMS from there. The only cost is the power resistors, from memory the 500W ones are about £50 to £60 each depending on the type.

Thanks for the suggestion Paul, unfortunately we needed a meter with an accuracy/ resolution of 4 digits. I looked at using a scope and calculator but I wouldn't be able to get the accuracy. I managed to borrow a meter and have calibrated both of our harmonic scalpels.

We bought one for the same thing, its an HP/Agilent 34401A. got if from farnell with a lot of discount

That HP meter only claims plus or minus 4% for AC voltage's over 100kHz so the "scope" suggestion wasn't so stupid ?

OK, HF ... but what frequency are we actually talking about here, I wonder? And are we measuring voltage, or power? What (where) are we adjusting, and why the need for such accuracy?

And (last question), what does the manufacturer (service manual, etc.) say?

The manufacturer states accuracy to 300KHz but the output is about 55KHz so I reckon 100KHz would suffice. We are only measuring a low voltage, >500mv. The calibration test is for the output current, the manufacturer has kindly fitted 2X 0.5ohm series resistors to measure the voltage across and as if by magic i=v/r so we just compare the reading on the display with that on the voltmeter, simples