Output Amplitude Control - Agilent Technologies 33250A User Manual

Waveform Anti-Aliasing
DAC
Amplitude
DAC
Flatness/AM
DAC
7
Chapter 7 Tutorial

Output Amplitude Control

Output Amplitude Control
The 33250A uses an analog multiplier to control the signal amplitude
over a 10 dB range. As shown below, one of the multiplier's inputs is
routed from the output of an anti-aliasing filter. The other input is
routed from a dc control voltage which is the sum of the outputs from two
DACs. One of the DACs is set to a nominal voltage corresponding to the
desired output amplitude. The second DAC provides a voltage to correct
for frequency response variations in the function generator. The calibration
procedures for the 33250A provide all of the information necessary to
calculate the proper DAC values (see the Agilent 33250A Service Guide).
Two attenuators (-10 dB and -20 dB) and an amplifier (+20 dB) are used
in various combinations to control the output amplitude in 10 dB steps
over a wide range of amplitude values (1 mVpp to 10 Vpp).
Filter Multiplier
Note that the dc offset is summed with the ac signal after the attenuator
stages but before 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 33250A always 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 33250A
incorporates 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.
304
Attenuators
0 dB or 0 dB or
-10 dB -20 dB
DC Offset
DAC
Output
Amplifier
0 dB or
Main
+20 dB
Output