Measuring High Power Waveforms - Agilent Technologies 83487A User Manual

Optical/electrical plug-in module

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Measuring High Power Waveforms

The Agilent 83487A is specified to accurately measure peak modulated signal

powers up to 400 µW (–4 dBm)

. If a signal has an average power of 200 µW

(–7 dBm) with an extinction ratio of 10 dB or higher, then the peak power may

be assumed to be roughly double the average power, or 400 µW. When signal

powers exceed this 400 µW level, the photodiode amplifier of the

Agilent 83487A may begin to saturate. This in turn can distort the shape of the

waveform and produce a false waveform image. A device that has a compliant

waveform may then actually fail a mask test.

This issue becomes more complex for devices which have a large overshoot in

the "0" to "1" transition. It is not unusual to have 100% overshoot when

working with high speed multimode transceivers. If the nominal '1' level is

400 µW, and the overshoot is 100%, the peak power seen by the

Agilent 83487A is 800 µW (with 100% overshoot present, peak power is

roughly four times average power). This power level is likely to cause amplifier

saturation and waveform distortion. If tests are made in the Agilent 83487A fil-

tered mode, the overshoot is suppressed by the filtering that takes place after

the amplification. Post-amplification filtering can hide the overshoot that may

cause distortion.

Steps to guarantee accurate results

Achieving accurate measurement results may require limiting the power going

into the Agilent 83487A optical port. For the 100% overshoot example above

(200 µW average power, 400 µW '1' level, 800 µW peak power), the signal

must be attenuated by a factor of two (3 dB). A basic rule of thumb for signals

with up to 100% overshoot is that the average power should not exceed

100 µW (–10 dBm).

1. While the Agilent 83487A module is specified to receive a continuous wave peak power of up

to 600

W (–2.2 dBm), high frequency ringing in a modulated signal can cause compression

at lower levels around 400

W peak power.

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