9.16.7 Why Must Radio Receivers Reject Unwanted Signals?
- What Is Receiver Selectivity?
- Why Is Selectivity Important?
- Isn't Tuning Enough?
- What Is a Filter?
- Why Are Band-Pass Filters So Important?
- What Is Adjacent-Channel Interference?
- What Is Co-Channel Interference?
- What Is Image Frequency?
- How Are Image Frequencies Rejected?
- What Is Receiver Blocking?
- What Is Desensitization?
- What Is Intermodulation?
- Why Is the Receiver Front End So Important?
- What Is Dynamic Range?
- How Does Digital Signal Processing Improve Selectivity?
- What Is Adaptive Filtering?
- Why Is Narrower Bandwidth Often Better?
- Where Is Excellent Selectivity Most Important?
- Why Are Modern Receivers Better Than Earlier Designs?
Description
Explore how receivers distinguish the desired signal from nearby transmissions. Learn about selectivity, image frequencies, adjacent-channel interference, intermodulation, filtering, and the importance of receiver front-end design.
Introduction
Imagine trying to hold a conversation with a friend in the middle of a crowded stadium. Hundreds of other people are talking simultaneously, music is playing, public announcements are being made, and background noise surrounds you. Yet your brain can usually concentrate on one particular voice while ignoring the others.
A radio receiver faces a remarkably similar challenge.
At any moment, the antenna may receive signals from broadcasting stations, mobile phone base stations, satellites, aircraft, ships, emergency services, Wi-Fi networks, Bluetooth devices, radar systems, and countless other transmitters. Some of these signals may be millions or even billions of times stronger than the transmission the receiver actually wishes to receive.
The receiver's task is not simply to detect weak signals—it must also reject all the unwanted ones. This capability is known as selectivity, and it is one of the defining characteristics of a high-quality receiver.
Achieving good selectivity requires much more than simply tuning to the desired frequency. Engineers must carefully design filters, mixers, amplifiers, oscillators, and digital signal-processing algorithms to suppress interference while preserving the wanted signal. As communication systems have become increasingly crowded, receiver selectivity has become more important than ever.
What Is Receiver Selectivity?
Receiver selectivity is the ability of a receiver to accept the desired signal while rejecting unwanted signals at nearby frequencies.
A receiver with good selectivity responds strongly to the desired transmission but greatly attenuates signals outside the required channel.
Selectivity is one of the principal measures of receiver performance.
Why Is Selectivity Important?
Modern radio spectrum is heavily occupied.
A receiver may simultaneously encounter:
- neighbouring communication channels;
- broadcasting stations;
- mobile telephone transmitters;
- satellite signals; and
- numerous other radio services.
Without adequate selectivity, these unwanted signals would interfere with reception of the desired transmission.
Isn't Tuning Enough?
No.
Tuning simply centres the receiver on the desired frequency. The receiver must still suppress signals occupying nearby frequencies. This requires carefully designed filtering throughout the receiver.
Without adequate filters, unwanted signals may still pass through the receiver even though they are not centred on the tuned frequency.
What Is a Filter?
A filter is an electronic circuit that passes some frequencies while attenuating others.
Receivers commonly employ several different types of filters, including:
- low-pass filters;
- high-pass filters;
- band-pass filters; and
- band-stop filters.
Each performs a different role within the receiver.
Why Are Band-Pass Filters So Important?
Most receivers employ band-pass filters because they pass only a relatively narrow range of frequencies centred on the desired channel. Signals outside this range are progressively attenuated.
Band-pass filters therefore improve:
- selectivity;
- interference rejection;
- receiver stability; and
- overall communication quality.
What Is Adjacent-Channel Interference?
Adjacent-channel interference occurs when signals occupying neighbouring frequency channels enter the receiver.
If receiver selectivity is inadequate:
- neighbouring transmitters may be heard;
- digital error rates increase;
- audio quality deteriorates; and
- communication reliability decreases.
As channel spacing becomes narrower, adjacent-channel rejection becomes increasingly important.
What Is Co-Channel Interference?
Co-channel interference occurs when two transmitters operate on the same frequency.
Examples include:
- distant broadcast stations;
- cellular frequency reuse;
- satellite frequency reuse; and
- microwave communication systems.
Unlike adjacent-channel interference, co-channel interference cannot usually be removed by filtering because both signals occupy the same frequency. Instead, engineers rely on techniques such as:
- directional antennas;
- power control;
- beamforming; and
- frequency planning.
What Is Image Frequency?
One important source of interference in superheterodyne receivers is the image frequency.
Because the mixer responds to both the sum and difference of frequencies, an unwanted signal at a different frequency may produce exactly the same intermediate frequency as the desired signal.
Unless removed before mixing, the image signal may also be received.
How Are Image Frequencies Rejected?
Receivers suppress image frequencies using RF front-end filters.
These filters:
- pass the desired signal;
- attenuate the image frequency;
- reduce interference; and
- improve receiver performance.
Selecting an appropriate intermediate frequency also assists image rejection.
What Is Receiver Blocking?
A receiver may be tuned correctly yet still experience interference from a very strong nearby transmitter. This effect is known as blocking. Strong unwanted signals may overload the receiver even though they occupy different frequencies. The result may be:
- reduced sensitivity;
- distorted reception;
- temporary loss of communication; or
- inability to detect weak signals.
Good receiver design minimises blocking.
What Is Desensitization?
Desensitization occurs when strong unwanted signals reduce receiver sensitivity.
Although the desired signal itself may remain unchanged, the overloaded receiver becomes less capable of detecting it. The effect resembles temporarily reducing the receiver's gain.
Desensitization is particularly important in crowded radio environments.
What Is Intermodulation?
When several strong signals pass through nonlinear receiver circuits, new unwanted frequencies may be generated.
These are known as intermodulation products. Some of these new frequencies may fall directly within the desired channel. The result can include:
- false signals;
- distorted reception;
- increased background noise; and
- reduced communication reliability.
Receivers therefore employ highly linear front-end amplifiers and mixers.
Why Is the Receiver Front End So Important?
The front end comprises the components immediately following the antenna.
Typically these include:
- RF filters;
- the low-noise amplifier;
- frequency-conversion circuitry; and
- associated matching networks.
The front end determines much of the receiver's:
- sensitivity;
- selectivity;
- blocking performance; and
- dynamic range.
A well-designed front end is essential for high-performance receivers.
What Is Dynamic Range?
Signals arriving at the antenna may differ enormously in strength.
The receiver must detect extremely weak signals while tolerating very strong nearby transmissions. This capability is known as dynamic range. A receiver with a large dynamic range:
- resists overload;
- maintains sensitivity;
- minimizes distortion; and
- performs well in congested radio environments.
How Does Digital Signal Processing Improve Selectivity?
Modern receivers increasingly perform filtering digitally.
Digital filters provide:
- extremely precise frequency responses;
- stable characteristics;
- programmable bandwidths;
- adaptive interference suppression; and
- consistent performance.
Unlike analogue filters, digital filters do not drift with age or temperature.
What Is Adaptive Filtering?
Adaptive filtering automatically adjusts filter characteristics according to current operating conditions.
For example, the receiver may:
- narrow the bandwidth when signals are weak;
- suppress specific interfering signals;
- compensate for changing propagation conditions; and
- optimize communication quality.
Adaptive filters are widely used in software-defined radios.
Why Is Narrower Bandwidth Often Better?
Reducing receiver bandwidth generally:
- admits less thermal noise;
- improves signal-to-noise ratio;
- increases selectivity; and
- suppresses nearby interference.
However, bandwidth must remain wide enough to accommodate the transmitted information.
Choosing the optimum bandwidth therefore involves balancing several competing requirements.
Where Is Excellent Selectivity Most Important?
High selectivity is particularly important in:
- cellular base stations;
- satellite earth stations;
- air-traffic-control receivers;
- military communication systems;
- amateur radio equipment;
- emergency-service radios; and
- spectrum-monitoring receivers.
In these applications, strong nearby transmissions are common.
Why Are Modern Receivers Better Than Earlier Designs?
Modern receivers combine advances in:
- analogue filter design;
- digital signal processing;
- software-defined radio;
- low-noise electronics; and
- sophisticated interference suppression.
Together these technologies allow receivers to separate signals that differ in frequency by only a very small amount while operating in extremely congested radio environments.
Why Is Selectivity Important?
Receiver selectivity is essential because the radio spectrum is shared by countless communication systems operating simultaneously. Without effective filtering and interference rejection, even the most sensitive receiver would struggle to recover the desired signal.
By combining carefully designed analogue front ends with sophisticated digital processing, modern receivers achieve remarkable selectivity while maintaining excellent sensitivity. This ability to distinguish one signal from many others is one of the key reasons modern wireless communication systems operate so successfully despite increasing demands on the available spectrum.
Summary
Receiver selectivity is the ability to isolate the desired signal while rejecting unwanted transmissions occupying nearby frequencies. Achieving good selectivity requires careful filter design, image rejection, highly linear receiver front ends, and increasingly sophisticated digital signal processing.
As the radio spectrum becomes more crowded, selectivity has become just as important as sensitivity. Together, these two characteristics largely determine the performance of every modern radio receiver, from inexpensive consumer equipment to advanced satellite, military, and scientific communication systems.
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