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1080 ⏐⏐⏐ DECIBELS, FILTERS, AND BODE PLOTS
i
i t
Input
5 V
Regulator
(a)
5 V
V o
0 t
Feedback
High-frequency
noise
Output
i
problem for the equipment to which the dc voltage is applied: A highfrequency
noise signal has been developed. One way to subdue this
reaction and, in fact, slow the system response down so that very short
interval spikes have less impact is to add a capacitor across the output
as shown in Fig. 23.92(b). Since the regulator is providing a fixed dc
level, a large capacitor of 1 mF can be used to short-circuit a wide range
of high-frequency disturbances. However, you don’t want to make the
capacitor too large or you’ll get too much damping, and large overshoots
and undershoots can develop. To maximize the input of the
added capacitor, you must place it physically closer to the regulator to
ensure that noise is not picked up between the regulator and capacitor
and to avoid developing any delay time between output signal and
capacitive reaction.
In general, as you examine the schematic of working systems and
see elements that don’t appear to be part of any standard design procedure,
you can assume that they are either protective devices or due to
noise on the line that is affecting the operation of the system. Noting
their type, value, and location will often reveal their purpose and modus
operandi.
23.17 COMPUTER ANALYSIS
PSpice
dB
i 0 t
Input
5 V
Regulator
Output
High-frequency
1 µF
noise stabilizer–
bypass to ground
(Open-circuit for
5 V dc level)
High-Pass Filter The computer analysis will begin with an investigation
of the high-pass filter of Fig. 23.93. The cutoff frequency is
determined by f c � 1/2pRC � 1.592 kHz, with the voltage across the
resistor approaching 1 V at high frequencies at a phase angle of 0°.
For this analysis, the ac voltage source VAC was used. Within the
Property Editor, the quantities defined appear next to the source in
Fig. 23.93. Otherwise, building the circuit is quite straightforward.
Our interest lies in the effect of frequency on the magnitude of the
output voltage across the resistor and the resulting phase angle. Following
the selection of AC Sweep under the Analysis type heading, the
Start Frequency should be set at 10 Hz so that we have some data
points at the very low end and an End Frequency of 100 kHz so that
we extend well into the high frequencies. Here is the obvious advantage
5 V
Filter
FIG. 23.92
Regulator: (a) effect of spike in current on the input side; (b) noise reduction.
V o
(b)