Re: Procedure to measure rf watts

Jim Potter
 

Randy,

The technique is to use calibrated attenuators to bring the power level down to 10 mW or so. 10 mW is 1V peak or 2V Pk-pk. Ideally you should calibrate the scope, but to do that you need a calibrated signal generator and a 50 Ohm load. If you frequency is well within the bandwidth of the scope you can calibrate by other means or accept the scope calibration as is.

If you have 10 W you need about 30 dB of attenuation. Its not critical as long as you get the signal into the amplitude range of the scope. It's best to be at the few volts pk-pk not down in the mV. You also need to use the scope at 50 Ohms input impedance. If it doesn't have a 50 Ohm setting then a Tee and a terminator will do for frequencies below 20 or 30 MHz. Above that you need an accurate 50 Ohm in-line terminator.

You then measure the pk-pk volts on the scope. The power into the scope is Ps =1/4 Vpp^2/R where R is 50 Ohms. The power is P = 1/2 V^2/R where V is the peak voltage and Vpp = 2*V is the pk-pk voltage.

Now you need to correct for the attenuator. The attenuation factor is 10^(A/10) where A is the attenuator attenuation. The power from your amplifier is then Pa = Ps * 10^(A/10).

One thing to be careful of is harmonics. If harmonics are an issue you need an appropriate low pass filter. It needs to be calibrated at your operating frequency and it's attenuation needs to be added to the attenuator attenuation.

If you are not used to working with rf this may be too brief an explanation.

I use a Tek 2467B to measure peak power of rf pulses at 425 MHz all the time. This is outside the scope bandwidth, so I calibrate the scope with a CW signal and an average power meter. The 2467B reads about 0.7 V for a 1 V signal at 425 MHz. This lets me get accurate peak power readings with out buying a peak power meter. Used Gigatronics peak power meters and heads run about $4k total used. The 2467B is under a $1000. (That's a guess. It's been a few years since I've purchased one.)

de K9GXC, Jim

At 07:25 PM 5/22/2019, you wrote:
On Wed, May 22, 2019 at 04:00 PM, Randy.AB9GO wrote:


Hello everyone,

I have a need to check the calibration on some low power watt
meters (10 watts
or less, 21 megahertz or less) and was wondering if anybody had a favorite
procedure using their scope to measure peak-to-peak rf voltage
without letting
out the magic smoke. My thought is to use a t connector, hook one
side to the
transmitter, one side to a dummy load and then the center
connector straight
to the 10X oscilloscope probe. Any other precautions I should take?

Thank you,
Randy.
I will say above anything else, 10W is way too much power to be putting into most test equipment, especially spectrum analyzers. You *will* burn up something in the frontend and then you'll be sad. You want something like an RF sampling tee that will couple a much smaller version of your TX signal into your instrument.

Sean

James M. Potter, PhD, President
JP Accelerator Works, Inc.
2245 47th Street
Los Alamos, NM 87544

TEL: 505-690-8701

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