Superhet Radio RF Amplifier & Tuning

The RF amplifier circuit and tuning for a superhet or superheterodyne radio has some distinct requirements so that the optimum performance is obtained for the whole radio.

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Superhet Radio Circuit Blocks:
Block diagram / overall receiver RF amplifier & tuning RF mixer IF amplifier & filter Automatic gain control, AGC

Superhet Radio Includes:
Superhet radio Superhet theory Image response Block diagram / overall receiver Design evolution Double & multi-conversion superhet Specifications

The RF amplifier is one of the key areas of any superheterodyne radio receiver and its performance can govern many aspects of the performance of the receiver as a whole.

The RF amplifier circuit and tuning is the first area of the radio receiver that the signal encounters when it enters the radio.

As the title suggests, there are two main functions for this stage of the receiver circuitry, although there are also other requirements as well.

Superhet RF tuning

In terms of the operation of the superhet receiver, the front end RF tuning circuit is required to remove the image signal.

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

There are various methods that can be used to provide the required RF tuning.

Typically an RF variable LC tuned circuit is used. The tuning of this tracks that of the local oscillator, so that as the local oscillator moves to change the received frequency, so does the RF tuning.

In the days when manual tuning was used, two or even occasionally three section variable capacitors were used. These items were large and expensive, but were the only way in which the RF tuning could track the changing received frequency.

Today the tuning is normally carried out using varactor diodes that are driven by a voltage that is programmed from the microprocessor that controls the operation of the radio. This controls the frequency synthesizer used as the local oscillator and it is also able to calculate the required voltage for any given received frequency.

Normally a straightforward approach, possibly using a single tuned circuit may be sued for more basic receivers, but where high performance is required, more thought needs to be placed into the design of the overall receiver and in this case the RF filters. High performance filters will introduce loss and in turn this will degrade the noise figure, but if the filter is not sufficiently selective the image performance will be degraded.

IF rejection

Where an intermediate frequency may have signals that might be directly transferred from the antenna over some of the RF stages and into the IF, band reject filters may be used to reject the IF band at the antenna.

This reduces the level of signals at the IF band appearing before the first stages and reduces any IF breakthrough.

A broadband, band reject filter is often required in receivers where a fixed frequency first conversion is used along with a wideband IF. This occurred in amateur radio receivers of the 1960s to 1980s before the introduction of frequency synthesizers. Typically using an IF of between 5.0 and 5.5 MHz, there was the possibility of IF breakthrough, where signals were picked up directly into the IF. Introducing a band reject filter for this band reduced the level of this problem, although it was difficult to eliminate the problem.

Superhet RF amplifier

This stage of the receiver also provides some RF amplification. The amount needs to be carefully determined to enable sufficient pre-mixer gain to be obtained, without encountering overload.

The provision of gain at this stage helps overcome the noise generated by the mixer as mixers typically are more noisy than amplifiers and it may be necessary to overcome this by providing some pre-mixer gain.

Whilst this is true in many cases, some modern active mixers are able to provide very good performance and there is a growing trend in many high performance receivers to have no RF amplification and for the signal to enter the receiver via the input filtering / tuning.

Superhet RF amplifier noise performance

The noise performance of the RF amplifier circuit in a superhet radio, or any radio in fact is of great importance. Being the first amplifier in the receiver, its noise performance governs that of the whole radio. Noise generated in the RF amplifier is amplifier by successive stages and cannot be removed. As such noise generated here is the dominant noise in the receiver.

RF amplifier AGC control

Most superhet radios have an automatic gain control, AGC system. This typically takes a signal from the demodulator stage and applies this to preceding stages so that the gain is reduced. Not only does this control the audio volume for some types of modulation like AM and SSB, but it also serves to prevent overload in the RF and IF stages of the receiver.

Although the AGC is often applied to the IF stages, in high performance radios, it is also applied to the RF amplifier circuit to reduce the likelihood of the 1st mixer or even the RF stage itself from overloading and becoming non-linear for very strong signals.