Radiocommunications Agency  EMC Awareness

Adjacent Channel rejection and Bandpass filtering

What this technique is used for

When transmitters and receivers at different but closely-spaced frequencies are used on the same installation – such as in a vehicle, ship, or aircraft, or on a single multi-user radio mast – then the signal fed from one antenna to another one nearby may be so high that you can’t gain enough adjacent channel rejection from a standard receiver design. In these cases you have to interpose an external bandpass filter between the antenna and the receiver.

How this technique is used

Like other EMC filters, bandpass filters are passive devices, that allow only certain frequencies through with little attenuation, and stop all others. They can use discrete inductors and capacitors for frequencies up to the low VHF range, or resonant cavity or transmission line structures for higher frequencies.

Unlike other filters, which are usually low pass, a bandpass filter will stop frequencies both below and above its “passband”. With good design, the passband can be made very narrow, possibly less than 0.1% of the centre frequency. The “stopband” may have an attenuation of greater than 60dB and with a high performance filter the transition from passband to stopband – that is, the steepness of the filter’s “skirts” – can be very rapid. So the receiver is protected from adjacent channel signals that can be very strong at the antenna but are well attenuated at the receiver input.

Key issues in employing this technique

Good screening and segregation of input and output are necessary, as is the case with all filters, although usually the connections are by well-screened coaxial cable. In extreme cases the receiver itself must also be screened, to prevent direct pickup of the strong signal from bypassing the filter.