Bredhurst Receiving and Transmitting Society

Syllabus Sections:-

Receiver architecture

4k.1 Understand the block diagram of superhet and double superhet receivers and the functions of each block.

Superhet receiver

The diagram of the superheterodyne receiver that you have seen in the ILC is as follows.

If you want to check back click here. This is too simple for the advanced course and so let's introduce you to the "next" size up in Superhet.

All that has been added is an AGC control and RF amplifier. Note: Sometimes the AGC is also liked to the RF amplifier.

Most of the text below you have seen in the ILC.

RF Amplifier

• Amplifies the wanted weak signal from the antenna while largely ignoring signals outside of the required range by the use of tuned circuits

Mixer

• The mixer combines the wanted amplified RF signal with the VFO signal (or as it is sometimes stated, with the local oscillator signal) to produce sum and difference frequencies of the RF signal and the LO signal which in fact are modulated identically to the incoming signal.

Local Oscillator (LO)

• The LO is the VFO which generates an RF signal for use by the mixer.

Intermediate Frequency Amplifier (IF)

• Provides the main amplification and includes a filters, which removes adjacent signals and wrong mixing products

AGC (Automatic Gain Control)

• The AGC provides a constant level of signal in the system so that the audio remains a about the same level irrespective of the signal strength.

Detector (or Demodulator means the same is interchangeable)

• The demodulator recovers the modulating audio signal

Audio Amplifier (AF amp)

• Amplifies the signal to drive a loudspeaker.

Double superhet receiver or communication receiver.

The double superhet as you would no doubt imagine is a far more complex piece of electronics than the "simple" superhet.

Take a moment to look through the diagram below and before reading through the explanation try to work for your self what parts are doing what.

The AGC can also extend to the RF amplifier.

The double superhet allows the choice of high frequency 1st IF, which is desirable to reduce problems with image frequency response (an image frequency is another signal that is like a reflection in a mirror in that it is as far from the local oscillator as the wanted image - see section 4m.2 for more explanation). The low 2nd IF allows good filtering and plenty of gain to be easily realised.

By the use of good filters fitted to IF1 and IF2 will be the key to good selectivity.

As three oscillators are used, there is scope for unwanted frequencies within the receiver. Note the use of the detector/demodulators to accommodate the three modes. The noise limiter seeks to reduce the volume of a loud "spike" of interference without affecting the volume of normal signals.