HP 83487A, Руководство пользователя, Страница: 74 / 98

5-6
Reference
Measuring High Power Waveforms
Measuring High Power Waveforms
The HP 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 HP 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 HP 83487A is
800
µ
W (with 100% overshoot present, peak power is roughly four times aver-
age power). This power level is likely to cause amplifier saturation and wave-
form distortion. If tests are made in the HP 83487A filtered mode, the
overshoot is suppressed by the filtering that takes place
after the amplifica-
tion. Post-amplification filtering can hide the overshoot that may cause distor-
tion.
Steps to guarantee accurate results
Achieving accurate measurement results may require limiting the power going
into the HP 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 HP 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.