Biomedical Engineering Reference
In-Depth Information
TABLE 6.1 Capacitor Values and Types for the Precision
Sine-Wave Generator of Figure 6.2
Frequency Range
C6
C7 (
µ
F)
Capacitor Type
0.002-0.02 Hz
1000
Polycarbonate
0.02-0.2 Hz
100
Polystyrene
0.2-2 Hz
10
Te
fl
on
2-20 Hz
1
Te
fl
on
20-200 Hz
0.1
NPO ceramic
200 Hz-2 kHz
0.01
NPO ceramic
2-20 kHz
—
—
astable multivibrator formed by IC1A and IC1B. An approximate frequency for this oscil-
lator is given by
f
s
R18)C1. Counting can happen only when IC2's enable
line (pin 5) is set low. When trigger selector switch SW1 is in position 1, a trigger from the
astable multivibrator formed by IC1C and IC1D clocks
1/1.39(R17
op IC4A, enabling IC2 to
count. Once IC2 reaches a count of hex FF, output S15 of IC3 goes high. Flip-
fl
ip-
fl
op IC4B
is clocked after the rising edge on output S15 propagates through Schmitt triggers IC1E
and IC1F. The output of the
fl
op goes high, which resets IC4A and thus inhibits the
counter. This action also pulls the counter's parallel load line (IC2 pin 1), which asyn-
chronously forces the counter to hex 00. This state lasts for half a cycle of the sampling
clock, when IC4B is reset by IC1A. It is possible to trigger the signal generator externally
by supplying a 12-V clock signal to J1 and placing SW1 in position 2. Alternatively, plac-
ing SW1 in position 3 lets IC2 count freely, assigning equal time to each of its 16 cycles.
The setting of R1 de
fl
ip-
fl
nes a baseline level for the stepped signal voltage. The baseline
level lasts as long as the counter is in the hex 00 state. As such, the stepped waveform
developed across R21 is composed of a sequence of 14 levels (set by R2-R15), each pre-
sented for one cycle of the sample clock (output of IC1B), followed by a baseline level (set
by R1) that lasts for one trigger clock cycle minus the time it takes the counter to clock
through the 14 signal levels.
Although the step levels de
fi
ned by the linear slider potentiometers are analog in nature,
the time domain for the waveform is discrete. Of course, few real-world signals are appro-
priately represented by a stepped waveform with coarse jumps. Exceptions such as video-
like streams from scanned sensor arrays do exist, but in most cases, the waveforms needed
to develop or test a medical instrument should be representative of the smooth physiolog-
ical signals that they are designed to process.
In Chapter 5 we discussed use of a low-pass
fi
filter to reconstruct a signal from samples
acquired at a rate close to the Nyquist frequency. The same considerations apply here.
After the signal across R21 is o
fi
ltered by R24 and
C13 to reconstruct a smooth waveform. Selecting the proper sampling rate and
ff
set to zero by IC5A, it is low-pass-
fi
ff
requires some consideration. Let's take, for example, the way in which you would set up
the waveform generator to simulate an ECG signal. Figure 6.5 shows how the P-QRS-T
complex could be represented by the position of the 14-step linear slider potentiometers
(let's disregard the U-wave), while the isoelectric baseline level between the end of the
T-wave and the start of the P-wave is set by R1. Each step would last 40 ms, which requires
a sampling clock frequency of 25 Hz. If C1
fi
filter cutoff
0.1
µ
F and R17
100 k
Ω
, R18 needs to be
set somewhere near 187 k
Ω
1
for the multivibrator to oscillate at 25 Hz.
1
The exact frequency of oscillation of a CMOS astable multivibrator also depends on the supply voltage and on
the logic threshold voltage of the specific 40106 chip being used. The logic threshold can vary from 33 to 67%
of the supply voltage from device to device.
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