NEETS Module 17 — Radio-Frequency Communications Principles
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controlled degenerative feedback. Use of an npn transistor, in the same configuration, would require the AGC voltage to possess a negative potential.
Figure 2-17.—Common emitter amplifier with AGC.
FORWARD AND REVERSE AGC.—When we use an AGC voltage to cause degeneration by driving the amplifiers toward cutoff, it is referred to as REVERSE AGC. Figure 2-18 shows the type of AGC circuitry normally used with this method. A second method that uses AGC is an application called FORWARD AGC. In the case of forward AGC, you'll find the amplifier is driven toward the saturation region of its characteristic curve. (Sometimes referred to as an energy diagram.)
Figure 2-18.—Reverse AGC.
Let’s look at our example. Assume the AGC voltage is negative. Under this condition, Q1 is self biased; under no-signal conditions, it is operating well up on its characteristic curve. When a signal is applied, negative AGC voltage is developed in the detector circuit and fed back to Q1, which increases
forward bias. Any increase in signal level causes an increase in AGC voltage. An increase in AGC voltage increases conduction, which in turn drives the transistor to or near saturation. As the transistor approaches saturation, its gain is correspondingly reduced.
On the other hand, if the input signal level decreases, the negative AGC voltage decreases. The forward bias is then reduced, and the transistor operates on a lower portion of its characteristic curve where gain is higher.
Forward AGC provides you with better signal-handling capabilities; however, reverse AGC is simpler to use, causes less loading of the tuned circuits, and produces smaller variations in input and output capacitance.
Q17. What does manual gain control do to strong and weak signals, respectively?
Q18. What is the purpose of AGC/avc in a receiver?
Delayed Automatic Gain Control
The disadvantage of automatic gain control, attenuating even the weak signal, is overcome by the use of delayed automatic gain control (DAGC). Let’s take a look at the typical DAGC circuitry in figure 2-19. This type of system develops no AGC feedback until an established received signal strength is attained. For signals weaker than this value, no AGC is developed. For sufficiently strong signals, the delayed AGC circuit operates essentially the same as ordinary AGC.
Figure 2-19.—Delayed AGC action.
Our circuit uses two separate diodes; one is the detector diode and the other the AGC diode. The AGC diode is connected to the primary of the last IF transformer and the detector diode to its secondary. A positive bias is applied to the cathode of the AGC diode. This keeps it from conducting until a prearranged signal level has been reached. The adjust delay control allows manual control of the AGC diode bias. Manual control allows you to select the signal level at which AGC is applied. If mostly weak stations are to
be received, the setting should be high (no AGC until the signal level is high). However, you should set it as low as possible to prevent overloading of the last IF amplifier by stronger signals.
Finally, you must have two diodes to obtain delayed AGC. If only one diode were used, the AGC would be developed from the detector diode, and there would be no delayed action. Or, if a signal diode were biased to provide the delaying action desired, no signal would pass to the audio amplifier until the bias was exceeded by the input signal.
The beat-frequency oscillator (BFO) is necessary when you want to receive CW signals. CW signals are not modulated with an audio component, you remember, so we must provide one. The action of the RF amplifier, mixer, local oscillator, and IF amplifier is the same for both CW and AM; but the CW signal reaches the detector as a single frequency signal with no sideband components. To produce an AF output, you must heterodyne (beat) any CW signal with an RF signal of the proper frequency. This separate signal is obtained from an oscillator known as a beat-frequency oscillator.
Figure 2-20 is a block diagram of a superheterodyne receiver capable of receiving and demodulating a CW signal. The BFO heterodynes at the detector and produces an AF output. The detector (second detector) is used primarily because the mixer (first detector) is normally used as the source of AGC.
Figure 2-20.—Placement of the beat frequency oscillator.
If the intermediate frequency is 455 kilohertz and the BFO is tuned to 456 kilohertz or 454 kilohertz, the difference frequency of 1 kilohertz is heard in the output. Generally, you will tune the BFO from the front panel of a receiver. When you vary the BFO control, you are varying the output frequency of the BFO and will hear changes in the tone of the output audio signal.
The sensitivity of a receiver is maximum when no signal is being received. This condition occurs, for example, when a receiver is being tuned between stations. At this time background noise is picked up by the antenna, and you will hear noise greatly amplified. This noise is highly annoying and occurs because receiver gain is maximum without a signal. You can often overcome this problem by using a circuit called a SQUELCH, NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER. All of these noise type circuits just clip the peaks of the noise spikes. Squelch will actually eliminate noise. Figure 2-21 is a
typical circuit of this type. The circuit cuts off receiver output when no input signal is being received. It accomplishes this by blocking either the detector or audio amplifier when no signal is present. Let’s take a look at the theory involved in this process.
Figure 2-21.—Squelch circuit.
The squelch diode CR1 connects the output of the first AF stage to the input of the second. Amplifier Q1 serves as the control transistor for the circuit. The anode and cathode voltages of CR1 are normally biased positive with respect to ground.
With no input signal, R9 is adjusted until Q1 draws enough collector current to reduce its collector voltage and the anode voltage of CR1 to a value below the voltage on the cathode of CR1. At this point the anode voltage of the squelch diode is negative with respect to its cathode, and conduction ceases. Audio output is now reduced to zero and the receiver is silent.
The base of Q1 is connected to the automatic volume control (AVC) line. Anytime a signal enters the receiver, a negative AVC voltage is applied to the base of Q1. This reduces the collector current and increases the collector voltage, which in turn increases the anode voltage of CR1 until the anode becomes positive with respect to the cathode. Once again diode CR1 will conduct, and the signal will be passed to the second AF amplifier. Diode CR1 is effectively a switch controlled by the AVC voltage.
Q19. What is a disadvantage of AGC?
Q20. What is the main difference between AGC and DAGC?
Q21. What is the function of the BFO?
Q22. What is the purpose of a squelch circuit?
The tone of the sound reproduced in the audio section of a receiver depends on several factors. The frequency response of the audio amplifiers determines the degree of amplification provided to different frequencies in the sound spectrum. The size and quality of any loudspeaker used will determine its response to various frequencies. Response of the human ear is the final judge of tonal quality, and that varies with the individual.
Because of these variables, some form of tone control is sometimes used in Navy receivers. Treble tones are defined as the audio frequencies above approximately 3,000 hertz and bass tones are those below approximately 300 hertz. Although several methods of tone control can be used, we are only going to mention the attenuation method. With this method, a decrease in the intensity of one tone can produce an apparent increase in the intensity of another tone. As an example, let’s look at tones of 400 and 4,000 hertz produced by a speaker with the same intensity. If we reduce the intensity of the 4,000-hertz tone, the 400-hertz tone will appear to be louder, even though its intensity has not actually changed. You should see from this example that bass emphasis can be accomplished by attenuating treble tones.
The simplest type of tone control is illustrated in figure 2-22. Fixed capacitor C1 parallels the primary winding of the output transformer, effectively shunting the higher frequencies to ground. The size of C1 determines the lowest frequency to be affected. When you select the BASS position of the tone control, C1 is connected and improves bass response by de-emphasizing the treble tones. You can often use this circuit to improve the output of a small speaker with poor treble response.
Figure 2-22.—Fixed capacitor tone control.
A continuously variable tone control is illustrated in figure 2-23. Tone control R1 and bypass capacitor C1 act as a variable RC filter. With the wiper arm of R1 in the upper (BASS) position, C1 bypasses the higher frequencies to ground and provides better bass response. When the wiper arm of R1 is in the lower (TREBLE) position, the resistance of R1 is placed in series with C1, which reduces the shunting effect of C1 to high frequencies and improves the treble response. This method gives you the advantage of smooth, continuous tone control at all points between maximum bass and maximum treble response.
Figure 2-23.—Variable tone control using an RC filter.
A switch-type, variable tone control is illustrated in figure 2-24. With this method we use a three- position switch to provide fixed degrees of tone control. When tone control S1 is in the BASS position, capacitor C1 bypasses the high frequencies and provides bass emphasis. With S1 in the normal (NORM) position, C2 acts as the bypass, and a moderate amount of high-frequency attenuation is accomplished. This position provides balanced bass and treble response. When S1 is in the TREBLE position, C3 acts as the bypass and provides minimum high-frequency attenuation and maximum treble emphasis. As a rule of thumb, you can figure the capacitance of C2 is approximately five times the value of C3, and C1 is approximately ten times the value of C3. For example with C3 at .001 microfarads, C2 would be .005 microfarads, and C1 would be .01 microfarads.
Figure 2-24.—Switch tone control.
A quartz crystal, used as a selective filter in the IF section of a communications receiver, is one of the most effective methods of achieving maximum selectivity. It is especially useful when the channel is crowded and considerable noise (both external and internal) is present.
One possible circuit arrangement is shown in figure 2-25. Let’s look at the theory involved in understanding this circuit. You can see a crystal in one leg of the bridge circuit. The secondary of the input transformer (T1) is balanced to ground through the center tap connection. The crystal acts as a high Q series resonant circuit. It allows signals within the immediate vicinity of resonance to pass through the crystal to the output coil (L3). The desired signal appears between the center tap of L3 and ground.
Figure 2-25.—Crystal filter used in the IF section of a superheterodyne receiver.
The capacitance that exists between the crystal holder plates (C3) may bypass unwanted signals around the crystal; therefore, some method must be provided to balance out this capacitance. You accomplish circuit balancing by applying out-of-phase voltages to neutralize any undesired signal voltages. The tap on L3 permits proper impedance matching and is the balanced input to the next stage.
Q23. What does a tone control circuit in a receiver do to the audio signal?
Q24. What is the function of a crystal filter in a receiver?
Automatic Frequency Control
AFC circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. Basically, this type circuit does two things: It senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference; it also uses the control voltage to change the oscillator to the desired frequency. AFC circuits are used to control the frequency of sinusoidal oscillators and nonsinusoidal oscillators. Only sinusoidal AFC circuits will be covered here. AFC circuits are used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. Figure 2-26 is a block diagram illustrating AFC operation in a receiver. Let’s run through the applicable parts of this block diagram.
Figure 2-26.—Block diagram of receiver showing automatic frequency control.
The frequency discriminator controls the varicap in this receiver. A varicap is used to keep the IF stable. You may want to review varicap theory in chapter 3 of NEETS, Module 7, Introduction to Solid- State Devices and Power Supplies at this point. The varicap application here produces an apparent reactance, which is included in the oscillator frequency control circuitry. For example, let’s assume the IF is 455 kilohertz and the local oscillator (lo) is tracking below the incoming station. When the lo output decreases slightly in frequency, the IF will rise. This causes the output of the discriminator to increase the capacitive reactance of the varicap, which increases the oscillator frequency to the desired value. Now let’s assume the lo output increases. The IF will then decrease. This causes the discriminator output to decrease the capacitive reactance of the varicap. This will cause the oscillator frequency to decrease.
Figure 2-27 shows another widely used type of AFC and its circuitry. This type is commonly referred to as a BALANCED-PHASE DETECTOR or PHASE-DISCRIMINATOR. This circuit uses fixed capacitors and the varying conductance of the diodes to achieve a variable reactance. As you have seen in the block diagram, an AFC circuit requires two sections, a frequency detector and a variable reactance. Our detector output is a dc control voltage proportional to the amount of frequency change. This dc voltage is applied directly to the oscillator. The phase inverter input signals are discriminated IF outputs fed to the two diodes 180 degrees out of phase.
Figure 2-27.—Automatic frequency control (phase discriminator).
A reference voltage is also applied to both diodes. The diodes are biased to conduct only during the peak portions of the input signals. Any change in oscillator frequency will alter the phase relationship between the sawtooth reference voltage and the incoming signals. If this happens, one diode will conduct
more than the other and produce a control signal. This system remains unbalanced at all times because any change in frequency is instantaneously corrected. The network between the diodes and oscillator is essentially a low-pass filter. This filter prevents discriminator pulses from reaching the oscillator.
In present day communications systems, long term accuracy of one part in a million is required from many of the frequency generators (local oscillators) used in communications equipment. Variable frequency oscillators cannot practically achieve this high degree of stability. Therefore, a system known as FREQUENCY SYNTHESIS has been developed to meet the stringent demands for stability. This system uses circuitry that produces a signal frequency through a heterodyning and frequency selection process. This signal is not harmonically related to any of the signals used in the heterodyning process. It is also not related to the selected crystal frequency. This makes the signal unique.
Figure 2-28 is a multiple crystal, frequency synthesizer that produces desired output frequencies by mixing frequencies from several crystal oscillators. Each oscillator uses ten or more crystals to control its operating frequency. This provides for a large number of output frequency combinations. Figure 2-29 is a practical frequency synthesizer in which the harmonics and subharmonics of a single standard oscillator are combined to provide a wide multichoice of output signals. Each of these signals is harmonically related to a subharmonic of the standard oscillator. You will find the primary difficulty encountered in the frequency synthesizers is the presence of spurious signals generated in the "combining mixers." Extensive filtering and extremely careful selection of operating frequencies are required for even the simplest circuits. Spurious frequency problems increase and channel spacing decreases as the range of operating frequencies increases.
Figure 2-28.—Multiple crystal frequency synthesizer.
Figure 2-29.—Simple crystal frequency synthesizer.
Q25. What is the primary function of an AFC circuit?
Q26. What is frequency synthesis?
AUDIO REPRODUCTION DEVICES
The purpose of audio reproduction devices, such as loudspeakers and headphones, is to convert electrical audio signals to sound power. Figure 2-30 shows you a diagram of a loudspeaker called the PERMANENT MAGNET SPEAKER. This speaker consists of a permanent magnet mounted on soft iron pole pieces, a voice coil that acts as an electromagnet, and a loudspeaker cone connected to the voice coil. The audio signal has been previously amplified (in terms of both voltage and power) and is applied to the voice coil. The voice coil is mounted on the center portion of the soft iron pole pieces in an air gap so that it is mechanically free to move. It is also connected to the loudspeaker cone; as it moves, the cone will also move. When audio currents flow through the voice coil, the coil moves back and forth proportionally to the applied ac current. As the cone (diaphragm) is attached to the voice coil, it also moves in accordance with the signal currents; in so doing, it periodically compresses and rarefies the air, which produces sound waves.
Figure 2-30.—Permanent magnet speaker.
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