110
Chapter 5 Theory of Operation
System ADC
5
System ADC
See “A1 Modulation Schematic” on page 190.
Modulation by an external source, both AM and FM, is performed digitally
by sampling the external input at the modulation BNC, adjusting the
data to match the current modulation specification (deviation in FM,
depth in AM). The sampled data are sent to the DSP which applies the
resulting data to the waveform generation hardware. The sample rate is
468.75 kHz. All modulation processes in the 33250A are DC-coupled, and
can follow DC changes in the external modulation signal.
Modulation by an internal source is similar, except that the modulation
waveform is stored in the DSP memory and “played” back at the
frequency specified by the current instrument setup. The sample rate for
internal modulation is the same as for external modulation, 468.75 kHz.
In Amplitude Modulation, the modulation data are applied to the
amplitude control DACs, which control the amplitude multiplier and
thereby, the amplitude of the carrier waveform. When amplitude
modulation is enabled in the 33250A, the carrier waveform amplitude is
reduced to 50% of its unmodulated value, and the modulation signal then
may vary its amplitude up to the full nominal setting (or above,
if greater than 100% modulation is specified).
In Frequency Modulation, the modulation data are applied to the Phase
Increment Register (PIR) of the main DDS waveform synthesizer to vary
the frequency of the carrier waveform. The amount of frequency
deviation depends upon the amplitude of the modulating signal and the
current instrument setting for deviation.
Modulation circuitry consists of an input multiplexer, U603, an anti-
alias filter, U604, an analog-to-digital converter (ADC), U606, and a
digital-to-analog converter (ADC), U607. There is also a voltage
reference, U601, that is used by the ADC and other circuits in the
system. In addition to modulation, the ADC is used for calibration and
self-test.
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