Agilent Technologies 83487A, Руководство пользователя

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5-6
Reference
Measuring High Power Waveforms
Measuring High Power Waveforms
The Agilent 83487A is specified to accurately measure peak modulated signal
powers up to 400
µ
W (–4 dBm)
1
. If a signal has an average power of 200
µ
W
(–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
µ
W. When signal
powers exceed this 400
µ
W 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
µ
W, and the overshoot is 100%, the peak power seen by the
Agilent 83487A is 800
µ
W (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
µ
W average power, 400
µ
W ‘1’ level, 800
µ
W 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
µ
W (–10 dBm).
1. While the Agilent 83487A module is specified to receive a continuous wave peak power of up
to 600
µ
W (–2.2 dBm), high frequency ringing in a modulated signal can cause compression
at lower levels around 400
µ
W peak power.