How to safely look at transmitter output with a spectrum analyzer

I presume you are interested in the relationship between harmonics and fundamental i.e. relative harmonic level. The most usual way - if this is a standalone SpecAn and not inbuilt in a comms test set - is a (typically) 30db termination/attenuator. The attenuation level is dictated by the need (a) to protect the specan front-end from dangerous power levels, and (b) avoid generating unwanted products by overdriving RF amps or mixers.

There are also "sniffers" designed to be inserted into a line between RF source and any termination (resistive or antenna etc). Bird make such a product which has a reasonable bandwidth.

For specans within comms test sets, these are capable of monitoring inputs that the test set itself can withstand.

The typical SFDR of a spectrum analyzer is about 60dB. So you need a notch or highpass filter to attenuate the fundamental for another

20..30dB to make a valid measurement. You will also need the 20dB/50Wt attenuator to reduce the power to the reasonable level.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

"Jim Stewart"

** An "accurate measurement " of what - exactly ?

If you want to know if a given Tx is radiating harmonic energy that could cause interference to a receiver operating on that harmonic frequency - then you had better set up the Tx * with its intended antenna system* and then search for harmonics via ( calibrated) receiving antenna fitted to the analyser.

If you just want to see how the Tx behaves when driving a dummy load, then you will need a copy of the maker's service and alignment notes - and follow the procedures set out inside.

...... Phil

A directional coupler, and some attenuators ?

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
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A Heathkit cantenna dummy load and a low power resistive splitter off the cantenna input to get the input level you want.

Jim

-- "If you think you can, or think you can't, you're right." --Henry Ford

A wideband resistive divider of the measurement quality is not a very trivial thing. Especially if the goal is to measure the harmonics of the UHF band.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

and you are telling me this exactly why?