Diplexer
Understanding Diplexers
Diplexers solve the problem of routing two different frequency bands to or from a single port (typically an antenna). Unlike a simple switch that selects one band at a time, a diplexer handles both bands simultaneously.
How It Works
A diplexer splits the frequency spectrum at a crossover frequency. Signals below the crossover pass through the lowpass path to one port, while signals above the crossover pass through the highpass path to the other port. At the common port (antenna), signals from both bands are combined.
Applications
- Satellite earth stations: Separate transmit (uplink) and receive (downlink) bands on a single feedhorn.
- Cellular base stations: Combine multiple frequency bands on a single antenna.
- Test equipment: Combine low-frequency and high-frequency paths to cover a wide measurement range.
Diplexer vs Duplexer
A diplexer separates two different frequency bands. A duplexer separates transmit and receive signals that may be at the same frequency (using time-domain or spatial separation). Circulators are sometimes used as duplexers.
Lowpass arm: passes DC to f_cross
Highpass arm: passes f_cross to infinity
Key specs:
Insertion loss: 0.1-1.0 dB per arm
Band-to-band isolation: 40-80 dB
Return loss: >20 dB in each band
Frequently Asked Questions
What does a diplexer do?
A diplexer separates or combines two non-overlapping frequency bands on a single transmission line. It allows two systems operating at different frequencies to share one antenna or cable without interfering with each other.
What is the difference between a diplexer and a duplexer?
A diplexer separates two different frequency bands (e.g., C-band and Ku-band). A duplexer separates transmit and receive paths that may be at the same or different frequencies. In FDD cellular systems, the duplexer is actually a diplexer because TX and RX use different bands.
Can a diplexer replace a filter?
A diplexer is two filters combined with a common port. It can replace individual filters when two frequency bands need to be routed to/from a single antenna or cable. However, a simple bandpass filter is preferable when only one band needs to be selected.