View Index by Level





Radio Receiver - Superhet

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

To gain access to revision questions, please sign up and log in.



The Supersonic Heterodyne Receiver


The superhet receiver works much better than the simple receiver for the following reasons ...

Superhet disadvantages ...

Superhet Subsystems


The antenna will often be a half wave dipole. This is ideal for short wave radio between 1.6MHz and 30 MHz. At lower frequencies a ferrite rod antenna is often used.


A simple LC tuned circuit selects the desired radio frequency. The image frequency is generally so different from the wanted frequency that this single stage of tuning is sufficient.

Radio Frequency (RF) Amplifier

This amplifies the radio frequency. If the RF amplifier has a variable gain design, automatic gain control can be applied here.

Local Oscillator

In cheap AM radios, this oscillator is 455kHz higher in frequency than the wanted signal. It could be 455kHz lower but this might cause more unwanted image signal problems.

In FM radios, the local oscillator is 10.7 MHz above the wanted signal.


Mixer Example

Intermediate Frequency Amplifier

Intermediate Frequency (IF) Filtering


Sometimes called the Detector.

The audio (or digital data) is separated from the modulated carrier.

Automatic Gain Control

AF Amplifier

Superhet Signals

Superhet Receiver Signals

  1. A wide range of radio frequency signals including the unwanted image signal or second channel.
  2. A much smaller range of radio frequency signals.
    The unwanted image frequency is removed by the tuned circuit. This works well but might not be 100% removed.
    Other frequencies close to the wanted frequency are NOT removed.
  3. An amplified copy of the smaller range of radio frequency signals.
  4. A pure sine wave signal generated by the local oscillator. To tune in different stations, this frequency has to be altered.
  5. The sum and difference frequencies produced by mixing 3 with 4.
    The wanted input frequencies is mixed to become the intermediate frequency.
    The other input frequencies are mixed and fall above or below the intermediate frequency.
  6. The wanted intermediate frequency at a higher signal level. All the other unwanted signals have now been filtered out.
  7. A small audio signal.
  8. A much larger audio signal sufficient to drive the speaker.
  9. A DC voltage proportional to the radio frequency signal strength.
    The DC voltage is fed back and used to reduce the gain of the RF amplifier and IF amplifier.
    It the antenna signal is very strong, the receiver gain is reduced.
    If the antenna signal is weaker, the receiver gain is increased
    This is a closed loop control system. It uses negative feedback.
    It measures the IF output signal strength and alters the receiver gain to keep this signal strength constant.
    The audio level does not increase or decrease if the RF signal gets stronger or weaker.

Satellite TV LNB

Satellite TV is transmitted at a frequency that is so high that low cost co-axial cables can't be used to connect the dish to the receiver. The solution is to use superhet technology. The LNB (low noise block) mounted at the focal point of the dish antenna, uses a local oscillator and mixer to down-convert the satellite frequencies to a much lower frequency. These frequencies can be handled with low cost cables and a relatively simple satellite receiver design.

Audible Heterodynes

To receive Morse code and digital-text with amplitude, frequency or phase modulated signals, the radio frequency is mixed with another locally generated signal to produce an audible heterodyne at about 600 Hz. Without this the Morse would sound like a faint hiss being keyed on and off.

A common and unpleasant cause of interference is unwanted heterodyne signals. These sound like a continuous whistle or whine in the background of the wanted signal. They are caused by carrier waves on a frequency very similar to the wanted frequency. Top quality radio receivers include notch filters to remove the unwanted heterodyne signal.


IF Amplifier - Not in the AQA Exam

A Possible IF Amplifier circuit with AGC.





reviseOmatic V3     Contacts, ©, Cookies, Data Protection and Disclaimers Hosted at, London