Output Amplitude Control - Agilent Technologies 33220A User Manual

Chapter 7 Tutorial

Output Amplitude Control

Output Amplitude Control
The Agilent 33220A uses a variable reference voltage to control the
signal amplitude over a 10 dB range. As shown in the simplified block
diagram below, the output of the waveform DAC goes through an anti-
aliasing filter. Switching circuitry selects either the waveform output or
the output of the separate square/pulse DAC. Two attenuators (-10 dB
and -20 dB) are used in various combinations to control the output
amplitude in 10-dB steps over a wide range of amplitude values
(10 mVpp to 10 Vpp)
.
Variable
Vref
Waveform Anti-Aliasing
DAC Filter
Switching
Attenuators
Switching
Main
Circuitry
Circuitry
Output
0 dB or
0 dB or
-10 dB -20 dB
Square/Pulse
DAC
Output
DC Offset
Amplifier
DAC
0 dB or
+20 dB
Note that the dc offset is summed with the ac signal in the output
amplifier. This allows relatively small ac signals to be offset by relatively
large dc voltages. For example, you can offset a 100 mVpp signal by
almost 5 Vdc (into a 50Ω load).
When changing ranges, the 33220A switches attenuators such that the
output voltage never exceeds the current amplitude setting. However,
momentary disruptions or "glitches" caused by switching can cause
problems in some applications. For this reason, the 33220A incorporates
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a range hold feature to "freeze" the attenuator and amplifier switches in
their current states. However, the amplitude and offset accuracy and
resolution (as well as waveform fidelity) may be adversely affected when
reducing the amplitude below the expected range change.
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