Page 227 - Analog and Digital Filter Design

224 Analog and Digital Filter Design

If the signal source needs splitting, but each path carries signals in a separate
frequency band, a diplexer can be used. The advantage of diplexers over power
splitters is that they have very little loss in each path. Diplexers use two filters,
connected together at the source but feeding into separate loads. Each ater must
have the same (odd) order. An odd-order filter is necessary to present high
impedance outside its passband, to both source and load.

Diplexers are used in RF design to split signals from different frequency bands
in either a highpassllowpass or a bandpasslbandstop combination, as shown in
Figure 8.1. The same -3 dB cutoff frequencies must be used for both flters. For
example, a 1 MHz highpass must be combined with a 1 MHz lowpass filter. If
different cutoff frequencies are used the overall response is difficult (or impos-
sible) to calculate.

(a)
Amplitude

Amplitude
HIGHPASS
LOAD 2

I
Frequency

- Amplitude
BANDPASS

BANDSTOP Amplitude

Figure 8.1
Diplexer Filter Combinations