SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 261

cient memory is available in an arb to generate a waveform, additional
memory can be used for a second waveform channel. Since both channels are generated
using a single clock, the two output waveforms are precisely synchronized in time. This
capability is essential for testing instruments that derive their measurements from the
phase relationship between two signals. Additionally, purely digital lines or marker chan-
nels are sometimes o

If more than su

ff ered to provide synchronization and position markers which are

coincident with speci ed points of the arb waveform. These can be very useful for trig-

gering oscilloscopes or other external instruments at speci

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fi c times within the arb wave-

form cycle.
Beyond generating synchronized signals, however, the greatest advantage of having an
additional channel is the possibility of summing both channels. In this way, two synchro-
nized arbitrary components of a single waveform can be controlled independently, making
it possible to test the e

ect of each component on the system. For example, in order to
study the immunity of a circuit to an unwanted phenomenon, channel 1 could be loaded
with the waveform that is normally seen by the system under test. Channel 2, however,
could be loaded with the anomaly at the desired time within the normal waveform. Then,
by varying the gain of channel 2, the amplitude of the anomaly can be adjusted without
changing the amplitude of the normal signal.
Summing of two arb channels can also be used to extend the dynamic range of the com-
bined signal beyond the maximum dynamic of each independent channel. Setting the gain
of the summed channels to di

ff

erent values makes it possible to generate large signals that
have very small features. Here, the macroscopic changes would occupy the full dynamic
range of one of the channels. The smaller “details” of the waveform would then be pro-
grammed to occupy the full dynamic range of the other channel. By ratioing the gains
between channels correctly, the summed signal can have a theoretical maximum resolution
equal to the sum of the independent channels' resolutions.

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PC-Programmable Arb

The instrument shown in Figure 6.10 is a simple arb built using standard SRAMs, a few
counter ICs, some glue logic, and DAC ICs. In the arb project presented here, three 32k

8 bit RAMs are used to store two 12-bit waveforms. An additional RAM IC provides seven
marker channels, and the additional bit is used to encode the last valid data sample of a
waveform sequence. As shown in Figure 6.11, the 15-bit address generator of the arb is
formed by a chain of 74LS191 synchronous counters (IC1-IC4). The output of the counter
chain is presented to 50-ns access-time SRAMs (IC5-IC8). From Figure 6.12 it can be
appreciated that as long as IC1 is enabled, each clock pulse supplied in parallel to all
counter ICs causes the address to advance. This process continues until the address points
to a data element (D31) on IC8, in which bit 7 is low. This causes the asynchronous reset
of the counter chain.
Notice how the arb's circuitry ensures that each sample of the waveform sequence has
equal length. Data contents presented on the RAM data bus (D0-D30) are latched on the
opposite edges of the clock than those which cause address transitions. In this way, since
the data at the output of latches IC9-IC12 lag the data at the inputs of these latches by half
of a clock cycle, the reset signal issued when the counters reach END_ADDRESS

1 (the
location in which bit 7 of IC8 is low) causes the address to be reset to zero without upset-
ting the data corresponding to END_ADDRESS. The next falling edge on the clock line
causes the data contents of the

first RAM address to be presented at the output of the
latches. Obviously, while the amount of time for which the

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first address is available is
shorter than that of any other address, the data corresponding to it is presented at the out-
put of the latches for exactly the same amount of time as for any other address.

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