NEETS Module 17 — Radio-Frequency Communications Principles
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above the tuned frequency. As you approach the tuned frequency, the input level required to maintain a given output level will fall. As you pass the tuned frequency, the required input level will rise. Input voltage levels are then compared with frequency. They can be plotted on paper or you might view them on an oscilloscope. They would appear in the form of a response curve. The steepness of the response curve at the tuned frequency indicates the selectivity of the receiver.
The fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that appears at its input. You will usually find the broader the band passed by frequency selection circuits, the greater your fidelity. You may measure fidelity by modulating an input frequency with a series of audio frequencies; you then plot the output measurements at each step against the audio input frequencies. The resulting curve will show the limits of reproduction.
You should remember that good selectivity requires that a receiver pass a narrow frequency band. Good fidelity requires that the receiver pass a broader band to amplify the outermost frequencies of the sidebands. Receivers you find in general use are a compromise between good selectivity and high fidelity.
Q11. What four basic functions must a receiver perform?
Q12. What are the four basic receiver characteristics?
The superheterodyne 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
Figure 2-9 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. (Heterodyning will be further discussed later in this chapter and was covered in NEETS, Module 12, Modulation Principles.) 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.
Figure 2-9.—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.)
HETERODYNING.—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 in this chapter.
DETECTION.—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. Figure 2-10 is a block diagram showing waveforms of a typical FM superheterodyne receiver. Comparison of block diagrams in figures 2-9 and 2-10 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.
Figure 2-10.—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.
ADVANTAGES.—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. At this point you may want to review chapter 2 of NEETS, Module 12, Modulation Principles. 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.—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) 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.
Q13. What frequency conversion principle is used to develop the IF?
Q14. What is the function of the detector?
Q15. What is the major disadvantage of an FM signal as compared to an AM signal?
You know from studying the single-sideband transmitter material in this chapter you may transmit only one sideband of an AM signal and retain the information transmitted. Now you will see how a single-sideband signal is received.
Figure 2-11 illustrates the transmitted signal for both AM and SSB. SSB communications has several advantages. When you eliminate the carrier and one sideband, all of the transmitted power is concentrated in the other sideband. Also, an SSB signal occupies a smaller portion of the frequency spectrum in comparison to the AM signal. This gives us two advantages, narrower receiver bandpass and the ability to place more signals in a small portion of the frequency spectrum.
Figure 2-11.—Comparison of AM and SSB transmitted signals.
SSB communications systems have some drawbacks. The process of producing an SSB signal is somewhat more complicated than simple amplitude modulation, and frequency stability is much more critical in SSB communication. While we don’t have the annoyance of heterodyning from adjacent signals, a weak SSB signal is sometimes completely masked or hidden from the receiving station by a stronger signal. Also, a carrier of proper frequency and amplitude must be reinserted at the receiver because of the direct relationship between the carrier and sidebands.
Figure 2-12 is a block diagram of a basic SSB receiver. It is not significantly different from a conventional superheterodyne AM receiver. However, a special type of detector and a carrier reinsertion oscillator must be used. The carrier reinsertion oscillator must furnish a carrier to the detector circuit. The carrier must be at a frequency which corresponds almost exactly to the position of the carrier used in producing the original signal.
Figure 2-12.—Basic SSB receiver.
RF amplifier sections of SSB receivers serve several purposes. SSB signals may exist in a small portion of the frequency spectrum; therefore, filters are used to supply the selectivity necessary to adequately receive only one of them. These filters help you to reject noise and other interference.
SSB receiver oscillators must be extremely stable. In some types of SSB data transmission, a frequency stability of ±2 hertz is required. For simple voice communications, a deviation of ±50 hertz may be tolerable.
These receivers often employ additional circuits that enhance frequency stability, improve image rejection, and provide automatic gain control (AGC). However, the circuits contained in this block diagram are in all single-sideband receivers.
The need for frequency stability in SSB operations is extremely critical. Even a small deviation from the correct value in local oscillator frequency will cause the IF produced by the mixer to be displaced from its correct value. In AM reception this is not too damaging, since the carrier and sidebands are all present and will all be displaced an equal amount. Therefore, the relative positions of carrier and sidebands will be retained. However, in SSB reception there is no carrier, and only one sideband is present in the incoming signal.
The carrier reinsertion oscillator frequency is set to the IF frequency that would have resulted had the carrier been present. For example, assume that a transmitter with a suppressed carrier frequency of 3 megahertz is radiating an upper sideband signal. Also assume that the intelligence consists of a 1-kilohertz tone. The transmitted sideband frequency will be 3,001 kilohertz. If the receiver has a 500-kilohertz IF, the correct local oscillator frequency is 3,500 kilohertz. The output of the mixer to the IF stages will be the difference frequency, 499 kilohertz. Therefore, the carrier reinsertion oscillator frequency will be 500 kilohertz, which will maintain the frequency relationship of the carrier to the sideband at 1 kilohertz.
Recall that 1 kilohertz is the modulating signal. If the local oscillator frequency should drift to 3,500.5 kilohertz, the IF output of the mixer will become 499.5 kilohertz. The carrier reinsertion oscillator, however, will still be operating at 500 kilohertz. This will result in an incorrect audio output of 500 hertz rather than the correct original 1-kilohertz tone. Suppose the intelligence transmitted was a complex signal, such as speech. You would then find the signal unintelligible because of the displacement of the side frequencies caused by the local oscillator deviation. The local oscillator and carrier reinsertion oscillator must be extremely stable.
Q16. What two components give a SSB receiver its advantages over an AM superheterodyne receiver?
RECEIVER CONTROL CIRCUITS
This section deals with circuits that control receiver functions. We will explain how some of the basic manual and automatic receiver control functions work.
Manual Gain Control (MGC)
You learned previously that high sensitivity is one of the desirable characteristics of a good receiver. In some cases high sensitivity may be undesirable. For example, let’s suppose the signals received from a nearby station are strong enough to overload the RF sections of your receiver. This may cause the audio output to become distorted to the point of complete loss of intelligibility. To overcome this problem, you can use manual gain control of the RF section. By using the manual gain control, you can adjust the receiver for maximum sensitivity and amplify weak input signals. When you receive a strong input signal,
the RF gain may be reduced to prevent overloading. A typical manual gain control circuit for a receiver is illustrated in figure 2-13. Let’s go through the basic circuitry.
Figure 2-13.—Typical RF gain control.
C1 is an emitter bypass capacitor. Resistors R1 and R2 develop emitter bias for the amplifier. C2 provides dc isolation between the tank and the base of transistor Q1. You should recall from your studies of NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, and Module 8, Introduction to Amplifiers, that amplifier gain may be varied by changing bias. Potentiometer R2, the RF gain control, is nothing more than a manual bias adjustment. When the wiper arm of R2 is set at point B, minimum forward bias is applied to the transistor. This causes the amplifier to operate closer to cutoff and reduces gain. When you move the control toward point A, the opposite occurs. R1 limits the maximum conduction of Q1 when R2 is short circuited. You may run into an alternate biasing method when the transistor is operated near saturation. In that case, a large change in gain would again be a function of bias.
Manual Volume Control (MVC)
Figure 2-14 shows the circuitry for a common method of controlling volume in a superheterodyne receiver. C1 and R1 form an input signal coupling circuit and are also the means of controlling the level applied to the audio amplifier. R1, R2, and R3 develop forward bias and set the operating point for the transistor amplifier. R4 is the collector load resistor for Q1, and C3 is the output coupling capacitor. Potentiometer R1 in the circuit shown causes the input impedance of the stage to remain fairly constant. The signal from the preceding stage is felt across R1. By adjusting R1, you can change the input level to Q1 and vary the output amplitude.
Figure 2-14.—Typical manual gain/volume control.
Automatic Gain/Volume Control (AGC/AVC)
Output volume variations of a receiver often result from variations in the input signal strength. Changes in input signal strength occur when we change stations or when we experience fading because of changing atmospheric conditions. The function of an AUTOMATIC GAIN CONTROL, also referred to
as an AUTOMATIC VOLUME CONTROL, is to limit unwanted variations in the output of the receiver caused by variations in strength of the received signal input. A receiver without AGC would require continuous manual readjustment to compensate for received signal changes so that it could maintain a constant output level.
Signals from stations operating at the same power level may not reach the receiver antenna with the same power. This is because of differences in transmission distances, carrier frequencies, atmospheric conditions, and obstructions between the transmitter and receiver antennas.
You might draw the conclusion that an AGC network is not necessary when the receiver is operating on a single station. However, this is not true; atmospheric conditions may cause the signal strength to vary (fade in and out), or the antenna may receive components of the signal which have traveled along different paths. For example, one component may travel directly from the antenna, and another may have been reflected from a distant object. The two signals will sometimes be in phase and at other times be out of phase, thus tending to reinforce or cancel each other. The result is a variation in signal strength at the receiver antenna. This variation in signal strength is often referred to as FADING. The effect of fading in the output signal voltage of an RF stage is best demonstrated by the following example: An RF amplifier connected to a receiving antenna has a voltage gain of 100. If the antenna receives an input signal of 10 microvolts, the output voltage is computed as follows:
With the output voltage equal to 1 millivolt, and if fading is to be avoided, the output voltage must remain at 1 millivolt. However, if a reflected signal is received that is approximately one-half the strength (5 microvolts) of the original and is in phase with the original signal, the total input signal to the receiving
antenna will increase to 15 microvolts. To maintain the desired 1 millivolt of output signal, you must somehow reduce the gain of the RF amplifier. With an input of 15 microvolts and a desired output of 1 millivolt (1,000 microvolts), the gain of the amplifier must be reduced to:
When the 10-microvolt original signal and the 5-microvolt reflected signal are out of phase with each other, the signal strength at the receiving antenna will decrease to 5 microvolts. If we want to maintain our original 1,000-microvolt output signal, the voltage gain of the amplifier must be increased as follows:
A variation of amplifier gain, similar to the example, is necessary if we are going to compensate for input signal strength variations. The required amplifier gain variations can be accomplished automatically by the addition of an AGC circuit within the receiver. Let’s take a look at the methods and circuits used to produce AGC and the manner in which AGC (AVC) controls receiver gain.
CIRCUITRY.—Figure 2-15 is a block diagram representing AGC feedback to preceding stages. The detector circuit has a dc component in the output that is directly proportional to the average amplitude of the modulated carrier. The AGC circuitry uses this dc component by filtering the detector output to remove the audio and IF components and by applying a portion of the dc component to the preceding stages. This AGC voltage controls the amplification of any or all of the stages preceding the detector stage. Solid-state receivers may use either positive or negative voltage for AGC. The type of transistors used and the elements to which the control voltage is applied determine which type we will have.
Figure 2-15.—Block diagram showing AGC application.
The circuit shown in figure 2-16 produces a positive AGC voltage. Transformer T1, diode CR1, capacitor C1, and resistor R1 comprise a series diode detector. The AGC network is made up of R2 and C2. With normal detector operation and the positive (+) potential shown at the input, CR1 conducts. Conduction of the diode will cause a charging current (shown by the dashed line) to flow through AGC capacitor C2 and AGC resistor R2. This charging current develops a voltage across C2. When the potential across T1 reverses, the diode will be reverse biased and will not conduct. When this happens, the charging current ceases and C2 begins to discharge. The discharge path for C2 is shown by the solid arrows. The
discharge path time constant of C2, R1, and R2 is chosen to be longer than the period (1/f) of the lowest audio frequency present in the output of the detector. Because of the longer time constant, C2 will not discharge much between peaks of the modulating signal, and the voltage across C2 will be essentially a dc voltage. This voltage is proportional to the average signal amplitude. Now, if the signal strength varies, C2 will either increase or decrease its charge, depending on whether the signal increases or decreases. Since the charge on the AGC capacitor responds only to changes in the average signal level, instantaneous variations in the signal will not affect the AGC voltage.
Figure 2-16.—Series diode detector and simple AGC circuit.
You should remember that, depending on transistor types, the receiver may require either a positive or a negative AGC voltage. A negative AGC voltage could be easily obtained by reversing CR1. Once the values for R2 and C2 have been selected, the voltage divider action of the components is fixed, and the circuit operates automatically without further adjustment. If the average amplitude of the signal increases, the charge on C2 will also increase. If the signal amplitude decreases so does the charge on C2.
The AGC voltages in a receiver provide controlled degenerative feedback. By adjusting the operating point of an amplifier, you can control the gain. Under no-signal conditions, bias of the RF and IF amplifiers is developed by standard means, such as self bias. With an applied signal, an AGC voltage is developed, which in conjunction with normal biasing methods develops the operating bias for the amplifiers.
TRANSISTOR AMPLIFIER GAIN.—You have seen how a dc voltage that is obtained at the output of the AGC network is proportional to, and will reflect, the average variations of the average signal level. Now all we have to do is use this AGC voltage to control the amplification of one or more of the preceding amplifiers. Figure 2-17 illustrates a common-emitter amplifier with AGC applied to the base element. A change in the AGC voltage will change the operating point of the transistor and the dc emitter current. In this circuit, R1 and R4 form a voltage divider and establish no-signal (forward) bias on the base. Since a PNP transistor is used, the base has a negative potential. The AGC voltage from the detector is positive with respect to ground and is fed to the base through dropping resistor R2. You will find when the dc output of the detector increases (because of an increase in the average signal level) the AGC voltage will become more positive. This increased positive potential is applied to the base of Q1, which decreases the forward bias of Q1 and decreases the gain of the amplifier. AGC, in this application, works with
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