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156 Agilent E1441A Function Generator Tutorial
Appendix C
Phase Truncation Harmonics ≤ –20 x log
10
(P) dBc
where “P” is the number of waveform points in RAM.
Output Amplitude Control
The Agilent E1441A uses a 12-bit digital-to-analog converter (DAC) to convert the
digital representation of a signal to an analog output voltage. The
DAC can create
waveforms represented by 4,096 (2
12
) discrete amplitude levels. All 4,096 amplitude
codes are used for the built-in waveforms. Output levels from full maximum to
minimum output are controlled by applying varying amounts of signal gain or
attenuation to the signal created by the
DAC as shown in the block diagram below.
The output waveform is always described by the full 12-bit vertical resolution. You
can download user-defined arbitrary waveforms using less than the full 12-bit
vertical resolution; however, it is recommended that you always use the full
DAC
amplitude resolution to minimize amplitude quantization errors as previously
discussed.
The Agilent E1441A has a fixed output source resistance of 50 ohms (see next
diagram). During calibration, output amplitudes are calibrated for both the
open-circuit voltage (no load) and the terminated output voltage (loaded). The
terminated output amplitude is calibrated for an exact 50 ohm load. Since the
function generator's output resistance and the load resistance form a voltage divider,
the measured output voltage of the function generator will vary with load resistance
value and accuracy. When the function generator's output is loaded with a 0.2%
accuracy termination, an additional (negligible) 0.2% amplitude error is created.
Using a 5% accuracy termination will add 5% additional error to specified output
amplitudes.
If the function generator's output is measured with no load connected, the output will
be approximately twice the displayed amplitude (V
gen instead of Vload). In some
applications, you might continually use the function generator in a “no-load”
conditions. In such applications, remembering to double the function generator's
displayed amplitude can cause many errors. The Agilent E1441A allows you to
specify the function generator's load condition using the OUTPUT:LOAD
command; thus enabling the function generator to display the correct output
amplitude.
Figure C-4.
Figure C-5.