Syllabus Sections:-

Receiver architecture

3I1  25 Understand the block diagram of superheterodyne and double superheterodyne receivers and the functions of each block.

So what is the meaning of the word the Superhetrodyne ??
Superhetrodyne is a system by which the receiver produces a tunable signal which is combined with the incoming signal to produce a predetermined intermediate frequency, on which most of the amplification is formed.

Superheterodyne Receiver

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The double Superheterodyne Receiver, also know as the communication receiver, as you would no doubt imagine is a far more complex piece of electronics than the "simple" Superheterodyne Receiver

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

In the diagram above the RF Amplifier can also incorporate a Pre-selector.

A pre-selector within the RF Amplifier is a band-pass filter that blocks  frequencies other than the wanted frequency from passing through from the antenna into the radio receiver's mixer stage. in other words it pre-selects the wanted frequency !!

The original signal in this case is 145.500MHz which mixes with the 1st Local Oscillator to give the Intermediate frequency of 10.7MHz and this frequency passes through all the other stages until the stages which extract the audio as AM, SSB, CW, FM and pass this recovered audio to the Noise limiter and onto the Audio Amplifier.

The AGC can also extend to the RF amplifier and can be the source of voltage for a "S" meter.

The double superheterodyne allows the choice of high frequency 1st IF, which is desirable to reduce problems with image frequency  response, it can also can be know as Image Channel  (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). The low 2nd IF allows good filtering and plenty of gain to be easily realised.

See also the Fig 9.3 page 58 in the RSGB Full Manual for anther picture to help you get a grasp of the design.

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

As two local 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.

Local Oscillator (LO)

• The LO is the VFO which generates an RF signal for use by the mixer. The circuit of the Local Oscillator has to give a clean and stable signal to the mixer circuit BUT it must be tunable .The local oscillator must be based on a tuned circuit for example a frequency synthesiser type or it can use a direct digital synthesis also known as DDS.

• The LO is the VFO which generates an RF signal for use by the mixer. The circuit of the Local Oscillator has to give a clean and stable signal to the mixer circuit BUT it must be tunable .The local oscillator must be based on a tuned circuit for example a frequency synthesiser type or it can use a direct digital synthesis also known as DDS.
  

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

Intermediate Frequency Amplifier (IF)

• The initial RF amplifier will have removed the image channel but there may well still be other frequencies adjacent to the wanted frequency. The IF, which has a high Q factor and fixed frequency must select and then amplify this wanted signal and it provides the main amplification and includes filters, which removes adjacent signals and wrong mixing products.

AGC (Automatic Gain Control)

• The AGC provides a constant input level of signal to the system so that the audio remains at about the same output level irrespective of the input 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.

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3I2   26   Understand the function of a mixer, the generation of the Intermediate Frequencies (IF) and other mixer products

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 with the resultant frequency being the IF or Intermediate Frequency.