One challenging area of radio receiver design is that if achieving a good strong signal and overload performance whilst also maintaining a high level of sensitivity.
The overload performance is important because the radio needs to be sufficiently sensitive to receive very weak signals, but it must also be able to manage the strong signals without overloading.
As the range of signals that any receiver needs to handle may extend over 100dB or more, the strong signal performance is important. Overloading can lead to effects that can mask the wanted signal and actually reduce the sensitivity.
There are several key aspects and parameters that detail the radio performance in the presence of strong signals, each of which is key to the performance in a particular area.
With the proliferation of cell phones, Bluetooth devices, Wi-Fi, NFC, as well as more traditional radio systems, it is necessary for all of these to work in the presence of the others which may be in close proximity and radiating signals on adjacent channels or nearby frequencies. Accordingly the receiver overload and strong signal handling capability is very important.
Key overload and strong signal performance parameters
The overall radio overload and strong signal handling performance is governed by a number of separate specifications and parameters:
- Receiver blocking: Radio receiver blocking occurs when a strong signal close to that being received reduces the sensitivity of the receiver. It is very important, especially in these days where there are many short range, and other longer range wireless cellular and other radio communications devices around.
- Intermodulation performance : Intermodulation distortion and intermodulation performance are important parameters for a receiver performance. The presence of intermodulation products gives rise to unwanted signals being generated which may mask the wanted signal. These intermodulation present themselves as signal and may appear as ‘real’ signals, sometimes making a band that may have little activity, and much busier and full of signals.
- Intercept point: The Intercept Point, or possibly more correctly, the third order intercept point gives an indication of the strong signal handling capacity of an amplifier, mixer or receiver. It is the level on a graph where the level of the third order products equal the wanted products, i.e. where the lines for the levels intercept.
- Cross modulation : As the name implies, cross modulation manifests itself by transferring the modulation on one very strong signal onto other signals being received.
RF amplifier & receiver overload
One of the key elements of any receiver that governs its overload or strong signal handling capability is the RF amplifier. Its performance is key to the performance of the whole receiver.
Under normal conditions the RF amplifier of a radio receiver should remain linear with the output remaining proportional to the input. However, even the best RF amplifiers have limits to their output capability. Beyond a certain point the output can no lnger rise in proportion to the input as the amplifier enters a region where compression starts. When this happens their output starts to limit and the output is less than expected.
Compression in itself is not a problem for receiver overload performance. The absolute values of a signal are of little value and in any case the automatic gain control, AGC, used in many radio receivers means that the gain is reduced when strong signals are being received.
It is the side effects of compression give rise to major problems. Effects like intermodulation distortion, cross modulation, blocking and others mean that the operation of the radio receiver can be seriously impaired. It is these aspects which are of great importance for the overload performance of in the radio receiver design.
In view of the importance of the various aspects of overloading, a number of specifications quantify the various problems caused. However to look at these it is necessary to look at the basic effects and how they arise.
RF mixer & receiver overload
Another element of the receiver that has a major bearing on the strong signal handling performance is the first mixer. The performance of the mixer has a major effect on the receiver overload and strong signal handling performance.
Note on RF Mixing / Multiplication:
RF mixing or multiplication is a key RF technique. Using a local oscillator, it enables signals to be translated in frequency, thereby enabling signals to be converted up and down in frequency.
Read more about RF mixing / multiplication
To obtain the best radio overload performance a high level mixer is used. A high level mixer has a high level local oscillator input level. This enables it to cope with the high level signals that may give rise to problems with overload.
Improving receiver overload performance
To improve the radio overload and strong signal handling performance prevent these problems occurring at reasonable signal levels, radio receivers have a number of methods of reducing the signals levels:
- High level RF amplifier : To ensure the optimum radio overload performance, the first RF amplifier should be able to cater for high level signals while still remaining linear. It should have a high third order intercept point.
- High level mixer: Likewise to ensure the mixer does not become overloaded by any strong signals that may be encountered, it should have as high a third order intercept as possible commensurate with other factors. For the best performance it should have a high level local oscillator input level.
- AGC automatic gain control: Many receivers have a scheme known as an automatic gain control built in. This takes a sample of the output signal level and reduces the gain of the early stages. This works well for most situations, but it may not be able to prevent the earliest stages of the radio receiver from overloading. This is particularly true when signals are off the received channel, because these signals will not affect the AGC, but will still be presented to first stages of the receiver before they are filtered out.
- Input RF attenuator: Some specialist radio receivers may incorporate an RF attenuator. This can be used to reduce the RF input level when strong signals are present.
Any radio receiver design will need to balance the requirements for strong signal handling and radio overload performance with those for other aspects of the design like sensitivity and other parameters. Achieving excellent strong signal handling and overload performance will be a balance of many factors including cost.
More Essential Radio Topics:
Radio Signals Modulation types & techniques Amplitude modulation Frequency modulation OFDM RF mixing Phase locked loops Frequency synthesizers Passive intermodulation RF attenuators RF filters Radio receiver types Superhet radio Radio receiver selectivity Radio receiver sensitivity Receiver strong signal handling
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