Biomedical Engineering Reference
In-Depth Information
To illustrate the magnitude of the problem, imagine a medical instrument's main circuit
board, consisting of a CPU, some glue logic, and memory ICs, that has been housed in an
unshielded plastic case. Let's assume that at any given time, a number of these ICs are tog-
gling states synchronously, at a frequency of 100 MHz, for instance. Furthermore, assume
that the total power switched at any given instant during a synchronous transition is
approximately 10 W. Now, in a real circuit, e
ciency is not 100%, and a small fraction of
these 10 W will not do either useful work or be dissipated as heat by the ICs and wiring,
but rather, will be radiated into space. Assuming a reasonable fraction value of 10
6
of the
total switched power at the fundamental frequency, the power radiated is 10
W.
Now, let's assume that an FM radio is placed at a distance of 5 m from the device. The
µ
fi
field strength
E
produced by the 10
µ
W at this distance may be approximated by the
formula
3
30 radiated power (W)
distance (m)
0
1
0
5
6
1
0
3.46
m
m
V
70.79
dB
m
V
E
Considering that the minimum
field strength required for good reception quality by a typ-
ical FM receiver is approximately 50 dB
fi
V/m, the radiated computer clock would cause
considerable interference to the reception of a radio station in the same frequency. In fact,
interference caused by the computer of this example may extend up to 50 m or more away!
From the past discussion, it is easy to conclude that a
µ
first method for reducing radiated
emissions is to maintain clock speeds low as well as to make rise and fall times as slow as
possible for the speci
fi
c application. At the same time, it is desirable to maintain the total
power per transition to the bare minimum. Transition times and powers depend primarily
on the technology used. As shown in Table 4.1, the ac parameters of each technology
strongly in
fi
uence the equivalent radiation bandwidth. In addition, the voltage swing, in
combination with the source impedance and load characteristics of each technology, deter-
mines the amount of power used and thus the power of radiated emissions on each transi-
tion. Figure 4.2 shows how the selection of technology plays a crucial role in establishing
the bandwidth and power levels of radiated emissions that will require control throughout
the design e
fl
ort.
Another problematic circuit often found in medical devices is the switching power sup-
ply. Here, high-power switching at frequencies of 100 kHz and above produce signi
ff
fi
cant
harmonics up to and above 30 MHz, requiring careful circuit layout and
fi
filtering. Fully
TABLE 4.1 The Most Popular Logic Families Have Very Different Timing and Driving Parameters, Resulting in Radiated
Emissions Spectra with Different Characteristics
Minimum
Minimum
Typical Bit
Equivalent
Single-Load
Output Source
Voltage
Transition
Pulse
Bandwidth
Input
Impedance
Technology
Swing (V)
Time
t
(ns)
Width τ (ns)
(MHz)
Capacitance (pF)
(Low/High) (Ω)
5-V CMOS
5
70
500
4.5
5
300/300
12-V CMOS
12
25
250
12
5
300/300
HCMOS
5
3.5
50
92
4
160/160
TTL
3
8
50
4
5
30/150
TTL-S
3
2.5
30
125
4
15/50
TTL-LS
3
5
50
65
5.5
30/160
TTL-FAST
3
2.5
25
125
4.5
15/40
ECL
0.8
2
20
160
3
7/7
GaAs
1
0.1
2
3200
1
N/A
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