Meaning
A performance specification quantifies the attenuation of radio frequency signals between the two frequency band ports of a shared antenna filter network. Adequate diplexer isolation prevents the high power transmit signal from overloading the sensitive receiver input in co-located or frequency division duplex architectures. This separation is measured in decibels and is critical for maintaining receiver sensitivity during active transmission.
Signal Protection
Insufficient attenuation leads to receiver desensitization as the leaked transmit power raises the active noise floor. Low noise amplifiers at the receiver input can be driven into saturation or even damaged by the coupled transmit energy. To prevent these failures, the design must maintain a high level of isolation across the entire bandwidth of both the transmit and receive channels.
Filter Topology
High isolation is achieved through the use of bandpass or bandstop filters fabricated with high quality factor resonators. Cavity filters and ceramic monoblock filters provide excellent attenuation characteristics at the cost of larger physical dimensions. Multi layer ceramic diplexers offer a compact alternative for mobile devices, although they generally provide lower isolation than their larger cavity counterparts.
Surface mount diplexers designed for cellular modules typically achieve between thirty and fifty decibels of isolation in a compact footprint.
Measurement Routine
Vector network analyzers measure this metric by tracking the transmission coefficient between the transmitter port and the receiver port. The measurement requires careful calibration of the test cables to exclude the attenuation of the test setup. A successful validation shows a clean sweep across both bands with the attenuation levels remaining below the specified minimum limit.
