| Radiocommunications Agency
|Adjacent channel rejection and Bandpass filtering|
What this technique is used for
Radio receivers are generally tuned to a single frequency, or “channel”. Each channel has a certain bandwidth, which describes the total frequency range that it inhabits. The receiver should accept signals (which may be very weak) within this channel without responding to signals on other channels, which may be much stronger. This means that the various circuits in the receiver must reject (attenuate) signals at other frequencies to a degree determined by the overall performance requirements. This is achieved by a combination of filtering and linearity in the receiver amplifiers.
Poor adjacent channel rejection results in an increased susceptibility of the receiver to large signals on nearby frequencies.
How this technique is used
A good receiver design will ensure that its filters have the steepest possible slopes on either side of the peak response so that signals outside the centre frequency are well attenuated. The further away from the centre, the greater attenuation is possible.
Since filter performance is a function of the centre frequency, it is hard to get good performance with narrow channel separations and high frequencies, so most receiver designs use intermediate frequencies (IFs) at which to perform the critical filtering.
The block diagram of such a “superheterodyne” receiver is shown below.
This in turn means that receivers must be designed to convert the incoming frequency to one or more IFs, but the circuitry needed to do this is before the main filtering and is exposed to the incoming unwanted signals. The unwanted signals must not be allowed to overload the input circuits as this would lead to effects such as cross-modulation, where the unwanted signal modulates and corrupts the wanted signal, or blocking, where the unwanted signal desensitizes the receiver and attenuates or even eliminates the wanted signal.
Key issues in employing this technique
Good filters are available but they naturally cost more than a simple, low-performance filter; although in certain high volume applications, such as mobile phones, the technology has evolved particular application-specific designs which have excellent performance and low cost. Also, linear and high dynamic range receiver circuits are generally expensive.
As well as cost, use of multiple IFs and other techniques to improve the filtering and linearity performance increases the complexity of the receiver, which reduces the design’s attractiveness for high volume, size and cost sensitive applications, unless the functions can be easily integrated.