BNC 630 - Manual (Page 57)

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7.2 Introduction to the Arbitrary Waveform System
7.2.1 Description of the Arbitrary Waveform Generator
The Arbitrary Mode lets the user design custom waveforms on a PC and download them to the 630 for generation.
Included with the Arbitrary Waveform system is a fully featured Function Generator. The function generator offers a set of pre-
stored waveforms which are generated using the Arbitrary Waveform hardware. The user may select from many stored waveforms,
and may also specify a repetition rate to 2 MHz. All functions may be generated on a continuous or triggered basis. A high-going
pulse is given on the SYNC Out connector at the start of each waveform.
Also included with the Arbitrary Waveform system is a variable duty cycle Pulse Generator. The Pulse Generator allows the user to
generate pulse waveforms with varying amplitude, offset, frequency and duty cycle. The pulses may be generated on a continuous or
triggered basis.
Introduction
An Arbitrary Waveform is generated by sending values to a Digital to Analog converter (DAC) from a ram memory. A high speed
counter generates sequential addresses which indexes successive ram values for the DAC. The clock for the counter is derived from
the same DDS system used to generate the DC-21.5 MHz output of the 630. Because the Arbitrary waveform system uses the fully
synthesized DDS system for its clock, the arbitrary waveform generator has a highly accurate and stable clock source adjustable from
DC to 40 MHz in .01 Hz steps.
DDS
Clock
Generator
Freq.
Doubler
DC-20 MHz
Counter
32K x 16
RAM
12 Bit
DAC
Low Pass
Filter
Gain
Control
TTL/CMOS
Buffer
SYNC Out
(Front Panel)
SIG Out
(Front Panel)
DC-40 MHz clock
Counts 0,1,...N,0,1,...
Extra Data bit
15 13
9th Order Bessel
Fc = 10 MHz
Addr Data
Figure 7.2.1-1: Arbitrary Waveform Generator block diagram
A logic level output is provided on the SYNC Out connector which allows the user to assert a digital pulse on any data point. This
can be useful to generate logic pulses which are synchronous with the analog data or to generate arbitrary digital waveforms.
The 630 receives arbitrary waveform data through the serial port on the rear of the 630. An incoming waveform is stored in
successive ram locations beginning at address 0. After the last data point is received, the system processor adds a special “end of
data” bit to the last data point, instructing the counter hardware to reset to the waveform start address after the last data point is sent
to the DAC. Waveforms may be conveniently saved to nonvolatile memory.
These waveform values can be sent to the 630 in a variety of formats. ASCII formats include floating point, time & value floating
point, decimal, hexadecimal, and integer. Binary format is also supported.
Waveforms may be generated in Continuous or Triggered modes. In continuous mode, the waveform is immediately restarted after
the last point in the waveform is generated. In Triggered mode, the waveform halts after the last data point has been generated. The
system then awaits another Trigger condition before generating the waveform again.