MM0704, Lesson 2
Receiver. The design of a filter for the use in a receiver power supply is a relatively simple matter. Current
requirements and output voltages are comparatively low, and high-inductance chokes (choke is the term used in filters
for inductors) and high-capacity filter capacitors cost little. The usual procedure is to select a cutoff frequency well
below the fundamental ripple frequency, and with a given capacitor, calculate the required inductance. In most cases, L
and C are much larger than is actually required for satisfactory filtering. Such a filter is called a "brute force" filter.
Transmitter. The design
problem is quite different
in filters for transmitter
power supplies. With high current drain,
the choke must be wound with large wire and the core must contain plenty of iron to avoid saturation. The high
voltages also demand additional insulation precautions. The filter capacitors must be rated at a much higher breakdown
voltage. Where a 30-h choke for a receiver supply may be quite expensive, the cost of a 30-h choke for a 20,000-V, 6-
amp supply would be practically prohibitive and enormous in size. Where an 8-fd electrolytic capacitor costs only a
few cents, a 1-fd filter capacitor, to be used in a 20,000-V power supply, may cost several thousands of dollars. Thus,
in the design of filters for transmitter power supplies, cost is a major factor.
The filter is designed to provide just enough attenuation of the ripple frequency to satisfy the requirements of the
RESISTOR-CAPACITOR FILTER CIRCUITS
When it is necessary to separate the DC and AC that may be flowing in a circuit, a capacitor may be used to provide a
path for the AC and a resistor to provide a path for the DC. Diverting the signal current through the capacitor will not
affect the voltage drop across the resistor.
Low-pass filters are commonly used after the rectifier networks in power supplies. In order to convert the pulsating DC
output of the power supply to a low ripple current, the low-frequency ripple must be reduced. Low pass filters are also
used to reject the higher of two frequencies and pass the lower.
This low-pass filter,
in its simplest form,
is an inductance
in series with the line and
a capacitor across the
line (figure 2-1). Also called a half-section filter, the L-section offers high resistance at high frequencies to the flow of
current toward the load, because the inductive reactance of the coil is large. Also, most of the high-frequency current
that gets through the coil passes through the capacitor, whose reactance to high frequencies is low; the high frequency
does not reach the outputs. For low-frequency or DC currents, the inductive reactance is small and the capacitive
reactance is large. Accordingly, these currents readily pass through the coil to the load.
Another analysis of a low-pass filter is done by comparing the voltages developed in the output at the low and high
frequencies. Remember, "low-pass filter" means that a high voltage output is developed at the low frequency to