How To Draw Receiver Block Diagrams
This is a block diagram of a superhetrodyne receiver. Before the actual stages are discussed, consider the diagram itself. It is drawn to show the signal flow entirely from left to right, shown by the arrows.
It starts with the antenna (aerial) on the left. The signal flows through many stages, shown by arrows from left to right. It ends with the speaker (or phones) on the right.
The Superhet Receiver
The diagram shows a super-sonic heterodyne - or superhet - receiver, the standard pattern for receivers in general use today. The first thing to note is that three amplifiers are shown, the RF amplifier, the IF amplifier, and the AF amplifier. Let's look at each in turn.
The Radio Frequency Amplifier
This provides amplification for the signal as soon as it arrives from the antenna. The amplified signal is then passed to the mixer/oscillator. Mixers. The purpose of the mixer/oscillator is to translate the frequency of the incoming signal to the intermediate frequency, i.e. to the IF amplifier.
The mixer stage is usually acknowledged as being the noisiest stage in the receiver so an RF amplifier is positioned ahead of it to mask that noise with a higher signal level.
The RF amplifier stage should use a low-noise amplifying device - such as a low-noise transistor - to keep the internally-generated noise of the receiver to as low as possible. All the following amplifying stages will amplify this RF stage noise as well as the signal, so a low-noise device at the start of the receiving process is very important.
The Intermediate Frequency Amplifier
It is in the IF amplifier where most of the amplification in a receiver takes place. Sometimes there may be two or more stages of IF amplification. The IF frequency is carefully selected, but more about that below. The filter block prior to the amplifier shapes the passband of the receiver.
The filter pass-band should be tailored to fit the signal being received - in the interests of keeping out unwanted noise and unwanted signals. A 500 Hz pass-band for CW reception, a 3 kHz pass-band for SSB, and 6 kHz for AM, would be typical.
From the IF stages, the signal passes to a detector. Here demodulation of the radio-frequency signal takes place to produce an audio signal.
The diagram shows a product detector with a Beat Frequency Oscillator - or Carrier Insertion Oscillator (CIO) - for SSB and CW reception. Mixers
The Audio Frequency Amplifier
Finally the audio signal is amplified in the audio amplifier and passed on to a speaker or phones for the listener to enjoy.
Receiving A Signal
The superhet receiver is really in two parts:
From IF amplifier onwards, it is a fixed frequency receiver, a receiver pre-tuned and optimised for the reception of a signal on the IF frequency.
The RF amplifier and mixer/oscillator receive signals from the antenna and then convert them to the frequency of this optimum receiver - to the IF frequency. It is in the RF amplifier and mixer/oscillator sections of the receiver where the actual operator adjustment and tuning for the selection or choice of received signal takes place.
Tuning a Superhet Receiver
To change the frequency of the incoming signal to the IF frequency, the tuned circuits in the RF amplifier, the mixer input, and the local oscillator, must be adjustable from the front panel. A look inside a typical conventional superhet receiver cabinet may disclose a three-gang tuning capacitor. Each section of this component tunes part of the first stages of the receiver.
Note that it is the INPUT to the mixer which is tuned by a variable capacitor - the output is fixed-tuned at the IF frequency.
The choice of Intermediate Frequency
There are two conflicts with the choice of the IF Frequency:
A low intermediate frequency brings the advantage of higher stage gain and higher selectivity using high-Q tuned circuits. Sharp pass-bands are possible for narrow-band working for CW and SSB reception.
A high intermediate frequency brings the advantage of a lower image response.
The image frequency problem can be seen in this example.
Consider a receiver for 10 MHz using an IF frequency of 100 kHz. The local oscillator will be on either 10.1 MHz - i.e. 100 kHz higher than the required input signal - or on 9.9 MHz. We will consider the 10.1 MHz case - but the principles are the same for the case where the oscillator is LOWER in frequency than the wanted signal frequency. .
Because of the way that mixers work, a signal at 10.2 MHz will also be received. This is known as the IMAGE frequency.
The image rejection of a superhet receiver can be improved by having more tuned circuits set to the required input frequency, such as more tuned circuits in the RF amplifier ahead of the mixer. This brings practical construction difficulties.
Another solution is to choose a high IF frequency so that the required received frequency and the image frequency are well separated in frequency.
Choosing an IF of 2 MHz for the 10 MHz receiver would put the local oscillator at 12 MHz, the image frequency then being at 14 MHz.
When receiving a signal at 10 MHz, it is easier to reject a signal at 14 MHz (the image in the 2 MHz IF case) than at 10.2 MHz (the image in the 100kHz IF case).
Note that the Image Frequency is TWICE the IF Frequency removed from the WANTED signal frequency - on the same side of the wanted frequency as the oscillator.
The Double Conversion Eeceiver
The double-conversion superhet receiver brings the good points from both IF choices. A high frequency IF is first chosen to bring a satisfactory image response, followed by a low-frequency IF to bring high selectivity and gain.
Typical examples would be a 5 MHz first IF and a 100 kHz second IF - but many designs are possible. There may be front-panel-selectable quartz or mechanical filters used at either or both IF's to give added selectivity.
The only two disadvantages of the double-conversion receiver are the added complexity and the additional oscillators required. These oscillators, unless carefully shielded, can mix with each other and produce unwanted signals at spots throughout the spectrum.
Count up the number of oscillators involved - including the BFO / CIO.
The F M Receiver
A receiver for FM signals follows the same general principles as a receiver for CW and SSB reception.
The frequency coverage for an FM receiver is different to that of a SSB / CW receiver. FM is a distinct VHF-and-higher mode. So FM receivers are for VHF and higher reception. In hand-held transceivers, the receiver will be channelised for switch-channel reception.
The IF amplifier is much wider in bandwidth than that of a CW/SSB receiver. So the IF amplifier will be higher in frequency - (say) 10.7 MHz.
The demodulator will usually be a discriminator and may even be of a phase-lock-loop variety. There will be a limiter before the descriminator to remove noise peaks and amplitude-changes before detection of the FM signal. Signals Oscillators
A Simple Receiver
A simple receiver for the reception of amateur radio SSB and CW signals can be constructed by you at home. Yes! You could build it yourself!. It uses the direct conversion principle.
This receiver consists of a mixer stage and an audio amplifier.
The mixer converts the incoming signal frequency down to a lower frequency - this time right down to audio frequencies. It can be considered to be a superhet receiver with a 0 kHz (zero) intermediate frequency. The derived audio is passed through a simple audio filter to an audio amplifier to drive headphones or speaker.
This block diagram of the simple receiver shows the down-conversion process with a numerical example: