Page 231 - Analog and Digital Filter Design

228 Analog and Digital Filter Design

produce lower levels of mixing (intermodulation). The amount of isolation
obtained would depend on two things: (1) the order of the filter, and (2) the fre-
quency of the signals in relation to its cutoff point. The further the signal is
from the cutoff frequency, the greater the isolation. Higher-order filters produce
a greater isolation. The simplest diplexer uses just a single capacitor and a single
inductor to produce a highpassllowpass network. The performance of this
circuit is very limited, but in many cases it is better than having no diplexer at
all.

Designing a Diplexer

This process can be carried out manually or by using a computer program.
The DIPLEXER program supplied at www.bh.com/companions/0750675470
will design 0.1 dB Chebyshev diplexers of third-, fifth-, or seventh-order. A
diplexer using Butterworth or Chebyshev types having 0.01 dB, 0.25 dB, 0.5 dB,
or 1 dB ripple could be designed with the FILTECH Professional program, or
manually using tables. In either case, several decisions must be made before the
design process can begin.

First, choose whether you would like a highpassllowpass or a bandpasslband-
stop combination. The choice depends on the frequency range of the wanted
signals. For example, if the wanted signals are in a narrow band of frequencies,
such as a 10.7MHz intermediate frequency (IF) stage in a radio, then a band-
passhandstop combination would be the ideal choice. If it is desired to sepa-
rate signals having frequencies below, say, 5MHz from signals above that
frequency, then a lowpasslhighpass combination is required.

Second, choose the frequency, or frequencies, where band splitting is required.
This may be a more difficult decision because the output ports need the correct
load impedance within the passband of each filter. The filter is required to be a
lowpass/highpass combination with 100 kHz cutoff frequency and load imped-
ance of 1OOi2. The lowpass filter’s load must have IOOQ impedance at all fre-
quencies up to and, ideally, slightly beyond l00kHz. The highpass load must
have 100Q impedance at all frequencies at and above 100kHz. In practice, a
diplexer with a cutoff frequency of 100 kHz would only be usable up to perhaps
100MHz. Above that frequency the filter components would become less ideal
(parasitic inductance and capacitance would begin to dominate) and a poor fre-
quency response would result.

Third, decide how much ripple in the passband is acceptable. Both the passband
ripple (if any) and the filter order will determine the amount of isolation pro-
vided at any specified frequency. You may remember from earlier chapters that
Chebyshev filters have ripple in their passband, which results in steeper skirt
attenuation. Also, in general, higher-order Bters have steeper skirt attenuation.