is the type receiver most familiar to you. You probably see one daily in
your home in the form of an AM and/or fm radio. We will discuss the
basic workings of both AM and fm types and their differences.
Amplitude Modulation Receiver
The picture below shows a block diagram with waveforms of a typical AM superheterodyne receiver developed to overcome the disadvantages of earlier type receivers. Let’s assume you are tuning the receiver. When doing this you are actually changing the frequency to which the rf amplifier is tuned. The rf carrier comes in from the antenna and is applied to the rf amplifier.
The output of the amplifier is an amplified carrier and is sent to the mixer. The mixer also receives an input from the local oscillator. These two signals are beat together to obtain the IF through the process of heterodyning.
At this time you should note the dotted lines connecting the local oscillator, rf amplifier, and the mixer. This is used on block diagrams and schematics to indicate GANGED TUNING. Ganged tuning is the process used to tune two or more circuits with a single control.
In our example, when you change the frequency of the receiver all three stages change by the same amount. There is a fixed difference in frequency between the local oscillator and the rf amplifier at all times. This difference in frequency is the IF. This fixed difference and ganged tuning ensures a constant IF over the frequency range of the receiver.
AM superheterodyne receiver and waveforms.
The IF carrier is applied to the IF amplifier. The amplified IF carrier is then sent to the detector. The output of the detector is the audio component of the input signal. This audio component is then passed through an audio frequency amplifier. The amplified audio component is sent to a speaker for reproduction. This allows you to hear the signal.
You should note that a superheterodyne receiver may have more than one frequency-converting stage and as many amplifiers as needed to obtain the desired power output. (Additional amplifiers are not shown.)
As you know the intermediate frequency is developed by a process called heterodyning. This action takes place in the mixer stage (sometimes called a converter or first detector). Heterodyning is the combining of the incoming signal with the local oscillator signal.
When heterodyning the incoming signal and the local oscillator signal in the mixer stage, four frequencies are produced. They are the two basic input frequencies and the sum and the difference of those two frequencies. The amplifier that follows (IF amplifier) will be tuned to the difference frequency. This difference frequency is known as the intermediate frequency (IF).
A typical value of IF for an AM communications receiver is 455 kilohertz. The difference frequency is a lower frequency than either the rf input or oscillator frequencies. This lower frequency gives slightly better gain but does increase the chances of image frequency interference. Image frequencies will be discussed later.
Once the IF stages have amplified the intermediate frequency to a sufficient level, it is fed to the detector. When the mixer is referred to as the first detector, this stage would be called the second detector. The detector extracts the modulating audio signal.
The detector stage consists of a rectifying device and filter, which respond only to the amplitude variations of the IF signal. This develops an output voltage varying at an audio-frequency rate. The output from the detector is further amplified in the audio amplifier and is used to drive a speaker or earphones.
Frequency Modulated Receiver
The function of a frequency-modulated receiver is the same as that of an AM superheterodyne receiver. You will find some important differences in component construction and circuit design caused by differences in the modulating technique.
The picture below is a block diagram showing waveforms of a typical fm superheterodyne receiver. Comparison of block diagrams in the picture above and the one below shows that in both AM and fm receivers, the amplitude of the incoming signal is increased in the rf stages.
The mixer combines the incoming rf with the local oscillator signal to produce the intermediate frequency, which is then amplified by one or more IF amplifier stages. You should note that the fm receiver has a wide-band IF amplifier. The bandwidth for any type of modulation must be wide enough to receive and pass all the side-frequency components of the modulated signal without distortion. The IF amplifier in an fm receiver must have a broader bandpass than an AM receiver.
Block diagram of an fm receiver and waveforms.
Sidebands created by fm differ from the AM system. You should recall that the AM system consists of a single set of side frequencies for each radio-frequency signal modulated. An fm signal inherently occupies a wider bandwidth than AM because the number of extra sidebands that occur in an fm transmission is directly related to the amplitude and frequency of the audio signal.
You should observe that only two fundamental sections of the fm receiver are electrically different from the AM receiver. These are the discriminator (detector) and the limiter.
Beyond the IF stage, the two receivers have a marked difference. AM demodulation involves the detection of variations in the amplitude of the signal; fm demodulation is the process of detecting variations in the frequency of the signal. In fm receivers a DISCRIMINATOR is a circuit designed to respond to frequency shift variations.
A discriminator is preceded by a LIMITER circuit, which limits all signals to the same amplitude level to minimize noise interference. The audio frequency component is then extracted by the discriminator, amplified in the af amplifier, and used to drive the speaker.
In normal reception, fm signals are almost totally absent of static while AM signals are subject to cracking noises and whistles. Fm followed AM in development and has the advantage of operating at a higher frequency where a greater amount of frequencies are available. Fm signals provide much more realistic sound reproduction because of an increase in the number of sidebands. This increase in the number of sidebands allows more of the original audio signal to be transmitted and, therefore, a greater range of frequencies for you to hear.
As you can see, fm requires a wide bandpass to transmit signals. Each transmitting station must be assigned a wide band in the fm frequency spectrum. During fm transmissions, the number of significant sidebands that must be transmitted to obtain the desired fidelity is related to the deviation (change in carrier’ frequency) divided by the highest audio frequency to be used.
For example, if the deviation is 40 kilohertz and the highest audio frequency is 10 kilohertz, the modulation index is figured as shown below.
In this example, a modulation index of 4 equates to 14 significant sidebands. Because the audio frequency is 10 kilohertz and there are 14 side-bands, the bandwidth must accommodate a 140-kilohertz signal. You can see this is considerably wider than the 10-to-15-kilohertz bandpass used in AM transmitting.
Frequency conversion is accomplished by using the heterodyne principle of beating two frequencies together to get an intermediate frequency. So far, you have only become familiar with single conversion; however, some receivers use double or triple conversion. These methods are sometimes referred to as double or triple heterodyning.
Receivers using double or triple conversion are very selective and suppress IMAGE SIGNALS to yield sharp signal discrimination. (Image signals are undesired, modulated carrier signals that differ by twice the intermediate frequency from the frequency to which the superheterodyne receiver is tuned.) Double and triple conversion receivers also have better adjacent channel selectivity than can be realized in single conversion sets.
In military communications receivers you may sacrifice fidelity to improve selectivity. This is permitted because intelligence (voice, teletypewriter, etc.) can be carried on a fairly narrow band of frequencies. Entertainment receivers, on the other hand, must reproduce a wider band of frequencies to achieve their high-fidelity objective.