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Chapter 7 Tutorial
Square Waveform Generation
7
Square Waveform Generation
To eliminate distortion due to aliasing at higher frequencies, the 33250A
uses a different waveform generation technique to create square waves.
For frequencies above 2 MHz, square waveforms are created by routing
a DDS-generated sine wave into a comparator. The digital output from
the comparator is then used as the basis for the square wave output.
The duty cycle of the waveform can be varied by changing the comparator’s
threshold. For frequencies below 2 MHz, different waveshapes are
loaded into waveform memory to minimize jitter.
Square Waveform Generation Circuitry
Pulse Waveform Generation
To eliminate distortion due to aliasing at higher frequencies, the 33250A
also uses a different waveform generation technique to create pulse waves
.
For pulse waveform generation, clock cycles are counted to derive both
the period and pulse width. To achieve fine period resolution, the clock
frequency is varied from 100 MHz to 200 MHz by a phase-locked loop
(see the following page). To achieve fine period width resolution, an
analog delay (0 to 10 ns) is applied to the trailing edge. The rising and
falling edge times are controlled by a circuit that varies the charging
currents in a capacitor. Period, pulse width, and edge time are controlled
independently, within certain limits.
Anti-Aliasing
Waveform
Comparator
Threshold
DAC
Filter
Voltage DAC
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