Understanding Superhet Radio Specifications

There are many different specifications associated with any receiver and often looking at the datasheet parameters may not always be enlightening.

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Selecting a receiver or a transceiver is not always easy as there are many different specifications associated with superhet radio receivers.

The parameters set out in a datasheet need to be understood to be able to compare one radio with another.

Specifications for selectivity, sensitivity, image rejection, blocking, cross modulation, frequency range, . . the list goes on.

Discover what the key superhet radio specifications and parameters mean so that it is possible to make sense of the different datasheets.

Sensitivity specification

One of the key specifications for many superhet radio, as for any form of radio is its sensitivity.

The receiver sensitivity is actually determined by the noise generated within the receiver and hence specifications like signal to noise ratio, SINAD and noise figure are used. Often HF receivers are specified in terms of signal to noise ratio, often FM receivers use SINAD, and VHF receivers use noise figure, although all the specifications and parameters can be applied to any receiver.

The sensitivity requirements for receivers vary according to their application, and also the frequencies to be used. At HF, for example, the main limitation is not the receiver noise, but that of the received background noise. At VHF and UHF, the receiver generated noise then becomes the main factor limiting the sensitivity specification.

Note on the Receiver Sensitivity:

The sensitivity of a radio receiver of any form is a key aspect of its performance. There are several methods used to define the sensitivity including signal to noise ratio, SINAD, noise figure and others.

Read more about Receiver Sensitivity.

Adjacent channel selectivity

Another key specification parameter for any superhet radio receiver is that of its adjacent channel selectivity.

The adjacent channel selectivity is ley because it is the parameter which specifies the way in which the superhet receiver is able to accept signals on the wanted channel or frequency, and reject those on adjacent channels or frequencies.

Note on the Receiver Adjacent Channel Selectivity:

The adjacent channel selectivity of a receiver is the ability of a set to accept signals on the wanted channel or frequency, and reject any on other adjacent channels or frequencies as these would interfere wit the wanted signal. In a superhet radio, this selectivity is primarily provided by the filters in the intermediate stages.

Read more about Receiver Adjacent Channel Selectivity.

Image rejection

The image rejection specification is key for any superhet radio. A poor image rejection performance will mean that unwanted signals enter the IF and appear at the output. They are not wanted signals and when tuning they can cause unwanted heterodynes as they track in a different direction to the wanted signal.

Removing the image signal in a superheterodyne receiver
RF tuned circuit removes image in a superheterodyne receiver

Note on the Receiver Image Rejection:

The receiver image is a feature of superhet radios. It occurs because there are two signals frequencies that can mix with the local oscillator to enter the intermediate frequency stage. Tuning prior to the mixer attenuates the unwanted signal to an acceptable level, but there will always be a small level that enters. The image rejection level is the amount by which the image signal is reduced.

Read more about Receiver Image Rejection.

IF breakthrough

The IF breakthrough parameter is of particular importance for receivers that use a wide bandwidth IF over which latter stages tune as in the case of the crystal controlled first conversion double superhet receiver.

Strong signal handling specification

Whilst the weak signal performance of a superhet radio is of importance, so too is the strong signal handling. There is no point in having a very good level of sensitivity, if spurious signals and other issues arise when strong signals are present.

Note on the Receiver Strong Signal Handling:

Although the sensitivity of a receiver is important, the way in which it handles strong signals can be equally important. If stages of the receiver become non-linear, then a number of effects may occur that can result in spurious signals being generated, low levels signals being masked, and a number of other issues arising.

Read more about Receiver Strong Signal Handling.

Dynamic range

The dynamic range parameter for a superhet radio effectively defines the range of signals for which the receiver is designed to operate. In effect, it combines the sensitivity and strong signal handling specifications for the superhet radio.

Ian Poole   Written by Ian Poole .
  Experienced electronics engineer and author.



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