Diplexer Filter
Understanding Diplexer Filters
A diplexer filter is constructed from two bandpass (or band-pass/band-reject) filters connected to a common junction. Each filter passes its assigned frequency band while rejecting the other. The common port sees a matched impedance at all frequencies because one of the two filters always presents a match.
Diplexer Design
- Constant-impedance diplexer: Complementary filter pair. Sum of filter responses covers all frequencies. Common port always matched.
- Non-contiguous diplexer: Guard band between the two pass bands. Easier design but does not cover gap frequencies.
- Waveguide diplexer: Uses cavity-based filters. Very low loss, high power handling. Standard for satellite feeds.
- Microstrip diplexer: Compact integration on PCB. Used in mobile devices and small cells.
Key Specifications
- Passband insertion loss: 0.1-1.0 dB per channel.
- Isolation: 20-60 dB between ports.
- Return loss: > 15-25 dB at all ports.
- Guard band: Frequency separation between the two bands.
Frequently Asked Questions
How does a diplexer filter work?
A diplexer uses two frequency-selective filters joined at a common port. Band 1 frequencies pass through Filter 1 to Port 1 while being rejected by Filter 2. Band 2 frequencies pass through Filter 2 to Port 2 while being rejected by Filter 1. Ideally no power is lost.
What is the advantage of a diplexer over a power divider?
A power divider splits all frequencies equally, losing 3 dB per port. A diplexer routes each frequency band to its designated port with virtually no splitting loss. The trade-off is that a diplexer only works with non-overlapping frequency bands.
Where are diplexer filters used?
Satellite feeds (separating uplink/downlink or two frequency bands), cellular base stations (combining bands on a shared antenna), cable TV (separating upstream/downstream), and multi-band radio systems.