Alignment

Some of the components that make up the receiver are variable. Of these, some are used for tuning and volume adjustment and are variable at the will of the operator. Other components can be varied only with the help of special tools and special test equipment. The variable components of the IF tuned circuits belong to the latter classification. Their adjustment to satisfactory working conditions is called alignment. Alignment also includes setting the ganged-tuned circuits of both the HFO and...

Communications Receiver

The communications classification covers a multitude of receivers. Though communications receivers may be less numerous than broadcast receivers, they show much greater variety in design and purpose. Communications receivers may be TRFs or superheterodynes. They may have switching and tuning arrangements for operation on several bands, or they may be fixed tuned for operation at a single frequency. Communications receivers may incorporate manual tuning or automatic tuning and may receive AM,...

Analysis Of The Superheterodyne

To get an overall view of a typical superheterodyne receiver, examine the complete schematic diagram on page 38. Two special purpose tubes are used. The pentagrid converter functions as local oscillator and mixer. The variable tuned circuits are ganged for oscillator tracking. Trimmer capacitors CI and C2 and padder capacitor C3 are included to help in tracking. They are screwdriver adjustable. R1 provides about 3 volts grid leak bias for the local oscillator. IF transformer T2 selects the IF...

Circuit Analysis Of An Fm Communications Receiver

To analyze the complete circuit of an actual FM communications receiver, study the circuit diagram of the R-19 TRC1 on page 121. The RF and IF circuits are conventional, except that the tube cathodes are operated at ground potential. Such operation provides maximum gain, but poor amplitude linearity. Amplitude linearity is not important in FM, for the intelligence is carried by frequency modulation, and the signal is limited in amplitude before it is applied to the discriminator. A double...

AvcVAr

S-Meter in AVC Circuit Electron Ray Tube Circuit getting through), the triode section conducts and there is voltage drop across the 1-megohm resistor. The voltage drop puts the two plates at different potentials. Since the deflecting probe is connected to the triode plate, its voltage is below that of the target plate. It therefore acts as a grid between the cathode and the target plate and causes a wide deflection of the electron beam and a wide pie-shaped shadow on the target. This indicates...

Lc

You will note that resonance depends on the value of L and C. Increasing the value of either L or C decreases the resonant frequency. Decreasing the value of either L or C increases the resonant frequency. Changing the value of both may change the resonant frequency or it may not. It depends on whether the product of L and C is changed. If L and C are changed so that their product remains the same, the frequency will remain the same. Thus, for the same frequency, L may be high and C low, or L...

Performance Test

The purpose of performance testing is to determine whether the radio receiver is functioning properly for its tactical use and meets its minimum performance standards. Perform ance testing of a radio receiver may include checking the general overall performance, alignment, calibration accuracy, sensitivity, selectivity, signal-plus-noise to noise ratio, image rejection ratio, IF rejection ratio, AVC characteristic, power output characteristics and final operation. The exact procedures and...

Volume Control Methods

Since the signals intercepted by an antenna may be of widely differing strengths, the receiver should be able to handle both weak and strong signals. It is advantageous to bring both the very weak signal and the very strong signal to approximately the same level of strength for the final reproduction of the intelligence. The process of matching signal strength to desired audio output involves the use of volume controls. Three methods are in common usage manual control of audio signal, manual...

Signal Generator

For an output meter to be helpful there must, of course, be an output. If a receiver is operating normally, the signal travels from the antenna to the output. If the receiver does not operate properly, however, the pathway may be blocked somewhere along the line. There may be no output. That's where the signal generator is important. It provides a substitute signal which can be introduced at any point along the pathway, in order to make an output possible. Signal generators are available for...

Physical Layout And Wiring Problems

As you have seen, the inductance, capacitance, and resistance requirements in VHF and UHF circuits are critical. In studying the broadcast receiver, you were concerned with the lumped constants associated with coils, capacitors, and resistors. In studying VHF and UHF receivers, you are concerned with the distributed inductance and capacitances of segments of transmission lines or of resonant cavities used as tuned circuits. In addition, you must consider all stray inductances and capacitances,...

Limitations Of A Simple Radio Receiver

The simple receiver just described has its limitations. There is only one tuned circuit to improve the selectivity of the antenna. The slight step-up of the antenna transformer does little to improve the sensitivity of the receiver. This limits the effectiveness of the receiver to strong signals from nearby stations. On other signals, the output of the detector is not strong enough to develop magnetic-fields around the headset coils to produce diaphragm vibrations. An effective receiver needs...

Beat Frequency Oscillator

To enable a superheterodyne receiver to obtain intelligence from a CW transmission, a beat frequency oscillator is used. The output of the BFO heterodynes with the IF and produces a difference frequency which is audible. A BFO, set at 466 kc, can heterodyne with a 465-kc IF to produce an audio frequency of 1000 cps. Notice the typical BFO circuits at the right. The circuit at A, a modified Hartley, is widely used. The circuit at B, an Armstrong, is used in some Air Force equipment. At C is an...

Magnetron

The magnetron, shown in the simplified illustration below, uses the transit time principle to achieve ultra high frequencies, surprising efficiency, and very high output. Illustrated are the plate circuit, the cathode Positive Grid Oscillator Magnetron Simplified Circuit circuit, and the magnets which give the tube its name. The plate, tube-shaped, encircles the cathode. There is no grid. The magnetic field set up by the magnets may be thought of as acting as a grid. The cathode emits electrons...

Multimeter

A multimeter, as the name implies, incorporates several different meters in the same case, all using the same meter movement. Selector switches provide for selecting the particular meter and range that are desired. Notice the multimeters on pages 152 and 153, which are part of test equipment 1-56. They are multimeter 1-166 and multimeter 1-176. They provide measurements of resistance, current, and voltage. These measurements help to locate defective components, bad connections, improper...

Use of Miniature Tubes

An oscillator circuit using a miniature tube is shown in the diagram above. This tube is a 9002. The effect of transit time is negligible in this tube because the elements are very closely spaced. The interelectrode capacitances are small due to the fact that the elements are small. Short internal leads minimize lead inductance. This circuit, with this type of tube, operates very satisfactorily at 600 megacycles. It can be used in numerous UHF applications, including its use as a high frequency...

Double Tuned Detector

Another type of discriminator, shown in the illustration above, is called a double-tuned, detector, or Travis discriminator. The signal is coupled to the push-pull tuned circuits through a primary which is tuned to the signal resting frequency. In the secondary, one tuned circuit (L2-C3) is tuned to a frequency above the signal resting frequency, while the other (L3-C4) is tuned to a frequency an equal amount below the resting frequency. Thus, each secondary tuned circuit responds equally to...

Functional Analysis of a Typical SSB Receiver

To see the relationship of the parts in an SSB communications receiver, look at the block diagram on page 135. Basically, the receiver is a double conversion superheterodyne. It has an RF, a high IF, a low IF, and an audio section. However, to permit satisfactory SSB operation, a number of stages are added to the basic superheterodyne stages. RF section. The alternate channels in the RF section are simply for convenience and accuracy of tuning. The frequency coverage, 4-22 mc, is too broad for...

If Amplifier

An IF amplifier is basically an RF amplifier with a fixed tuned input and output. The tuned circuits act as bandpass filters, accepting the IF but rejecting other frequencies. Since they are fixed tuned, the tuned circuits have constant Q. With variable tuned circuits, like those in the TRF, Q varies with frequency. Response may be too sharp at one end of the frequency band and too broad at the other. In a superheterodyne, the response is more uniform because the IF circuits are fixed tuned. In...

Air Force Receiver BC

348 Receiver

This receiver has the usual stages of a communications receiver, as you can see in the diagram at the right. The BFO is combined with the second IF in a single stage with a single tube for both circuits. Another single tube combines the third IF, detector, and AVC circuits. Notice the bench installation of the receiver on page 78. Here, the receiver is associated with an autotune transmitter shown at the Bench Installation of Receiver BC-348 right. The main tuning dial is at the upper center of...

SMeter Circuits

The S-meter is essentially a milliammeter designed to indicate signal level. Usually the dial is calibrated from 0 to 9, with each suc ceeding point indicating a doubling of signal level. Notice the two common methods of S-meter placement. At A above, the meter is placed in the plate circuit of a variable-mu tube. A movement of 0-200 microamperes is used, and a variable shunt resistor makes possible zero adjustment for no signal response. At B, the S-meter is located between cathode and ground....

Electron Tube Considerations

At VHF and at lower communications frequencies, the transit time of electrons the time it lakes them to travel from cathode to plate in the tube is negligible. Above 400 mc, however, this time is equal to a considerable part of a cycle. Therefore, plate current lags the input grid signal. Thus, the transit time of the electrons produces an effect like an inductive lag in an ordinary circuit. This sets a high frequency limit on a tube's ability to act as an oscillator. Transit, time can be...

Slope Detector

One type of circuit which reacts to differences in frequency is a tuned circuit. Thus, discriminator circuits are essentially special applications of tuned circuits. The slope detector shown below is a simple discriminator. The detector, a diode, is connected across the output of a transformer which couples the signal from the limiter. The resonant circuits of both primary and secondary are tuned to a frequency other than the resting frequency of the signals. This means that the tuned circuits...

Vacuum Tube Voltmeter

Now look at the vacuum tube voltmeter, TS-375A U, on page 159. The vacuum tube voltmeter is one of the most useful single unit instruments. In this unit, the voltage to be measured is applied to the control grid of an electron tube. Measurements and calibration are made in terms of the plate current of the tube. Thus, this meter draws less current from the circuit being measured than does an ordinary voltmeter. Therefore, measurements made with a vacuum tube voltmeter more nearly indicate...

NWAAA

Very close to the crystal's resonant frequency. This cancellation effect is called the rejection notch. To understand why the rejection notch may be positioned as desired by varying C5, the effective impedance of the holder-crystal combination must be considered. At the crystal's series resonant frequency, its capacitive and inductive reactances are equal and effectively cancel. As the frequency is increased, the inductive reactance increases while the capacitive reactance decreases so that the...

Modulation Index

Modulation in FM is usually expressed in terms of the modulation index. The modulation index is the ratio of the frequency deviation to the modulation frequency. Thus, if the deviation is 10 kc when modulated by a 5-kc signal, the modulation index is 10 5 or 2. To see how the modulation index is related to the number of effective sidebands, look at the chart at the right. Note that the number of effective sideband pairs increases as the modulation index increases. Since the modulation index is...

The Tuned Circuit At Vhf And

The use of the ordinary tuned circuit for VHF and UHF is impractical. At such frequencies, both L and C become so small that the connecting leads of the coil may have more inductance than the coil itself, and the distributed capacitance of the coil may exceed the capacitance of the variable capacitor. Under such conditions, successful tuning is impractical. Thus, it is common practice to use resonant sections of transmission lines for tuning frequencies above 100 mc. A quarter wavelength...

Principles Of Vhf And Uhf Operation

To handle VHF and UHF successfully, a receiver usually has specially designed components and circuits, particularly in the RF section. The block diagram shows a communications receiver for operation in the VHF range. As shown in block diagram below, the receiver is not unusual except that it uses double conversion. The input frequency is converted twice, once to a high IF and again to a low IF. Double conversion is accomplished by means of two heterodyne oscillators. In this receiver circuit,...

Troubleshooting

As a radio repairman or technician, your real worth lies in your ability to analyze and isolate troubles. With extensive and complicated radio circuits, such ability helps you save considerable time and effort. Usually, if you can locate the trouble, the actual repair is merely a mechanical operation replacing a tube or a capacitor, or making a new connection. If you draw on your experience and your understanding of radio principles, each trouble suggests its own cause and its means of...

Foster Seeley Discriminator

Now look at the diagram of a Foster-Seeley discriminator above, sometimes called a phase discriminator. This type is in common use. The output of the limiter is transformer-coupled to the discriminator. The primary and secondary of the transformer are parts of resonant circuits, tuned to the IF signal resting frequency. The output of the limiter is also capacitively coupled to the discriminator. Thus, the output of the limiter is applied to the discriminator by two coupling...

RF Coils

An RF coil, such as the one shown on page 10, is designed for minimum loss. It uses a multistrand wire called Litz wire. This wire is made by weaving together a large number of fine insulated wires. Its AC resistance is low, and therefore the Q of the coil is high. Very light cardboard is used for the coil form. The form is varnished both before and after winding, to prevent absorption of moisture. The physical size of coil and form is kept as small as possible. The width of the mass of wound...

Discriminator Circuits

Though RF and IF sections of an FM receiver are very much like those of an AM receiver, the detector circuit usually called a discriminator is very much different. The discriminator must react to frequency- deviation and not to amplitude variation. If a discriminator is of a type which does react to amplitude variations, one or two limiter stages must precede it to insure that the signal supplied to the discriminator does not vary in amplitude. Since intelligence is carried in the form of...

Functional Analysis of a Typical Direction Finder

Direction Finder Azimuth Indicator

Examine the block diagram of a typical direction finder used for intelligence on page 153. It has, of course, the three basic units antenna, receiver, and indicator. In addition, it has a modulating voltage generator and an azimuth indicator. These two units are not used at the same time. One is a substitute for the other. When the modulating voltage generator unit is used, instantaneous electrical indication is provided by the cathode ray tube indicator. When the azimuth indicator unit is...

Squelch Circuit

Carrier Operated Relay Schematic Diagram

The 1st AF amplifier is the first triode section of VI15. The second triode section of VI15 serves as a squelch tube to silence the receiver when no signal is being received. In fact, the second triode section controls a switch which opens or closes the circuit of the first triode section. Note that the grid of the first AF amplifier can be shorted to ground through relay RL101. The relay, as shown, is in the no-signal condition and is shorting the grid of the first audio amplifier to ground....

Principles Of Superheterodyne Operation

The superheterodyne receiver takes advantage of two important facts of radio operation. First, amplification of a radio signal at a low frequency can be more successful than amplification at a high frequency. Second, amplification of a signal by fixed tuned circuits is more successful than amplification by variable tuned circuits. The superheterodyne converts all input frequencies to a single, fixed, lower frequency which is amplified by fixed tuned circuits. This process distinguishes the...

Local Oscillator

Pentagrid Circuit

The local oscillator must meet exacting requirements in frequency coverage, frequency stability, constant output, and correct, tracking. The local oscillator may use any of the fundamental oscillator circuits. The modified Hartley and the tuned grid are commonly used. Most VHF and UHF receivers use crystal-controlled local oscillators. To maintain frequency stability, the plate voltage of the oscillator is often regulated. Another problem of stability is the effect that the other radio...

Dual Diversity Converter

Now look at the block diagram of the dual diversity converter on page 139. This converter handles the output the IF of two receivers arranged for space diversity. The diversity arrangement is continued in the converter through the detectors. Thus, the two sets of mark and space signals are combined only in the converter output. However, a limiter, common to both diversity channels, can be used, since the limiter is both preceded and followed by stages containing filters. The inputs of both...

Principles Of Fm Operation

Now study the block diagram below of a typical FM communications receiver designed to serve as a radio link in a communications system. This receiver is a superheterodyne, employing double conversion. Though it is capable of covering a frequency range 70-100 mc, it provides only single channel operation. Changing from one operating frequency to another is essentially a bench job. The crystal of the heterodyne oscillator must be replaced, and the RF, mixer, and high IF stages must be aligned to...

Trf Amplifier Circuit

Examine the typical RF amplifier stage shown below. The tube is a remote cutoff pentode. The antenna is transformer coupled through a tuned circuit to the grid of the pentode. Cathode bias is provided. The variable resistor in the cathode circuit permits adjustment of the operating bias. The output of the stage is transformer coupled to the next stage. The RF signal is passed through several such TRF amplifier stages before it is applied to the detector stage. As the signal goes through the...

Trf Receiver

The simple receiver studied in the previous chapter is classified as a single stage receiver, since neither the antenna nor the headset can be called a stage. The TRF receiver, on the other hand, is a multistage receiver. As shown in the block diagram, the detector of the TRF is the equivalent of the single stage of the simple receiver. The TRF is the more useful receiver because it has two RF amplifier stages, an AF audio frequency voltage amplifier stage, and an AF power amplifier stage. In...

Marker Generator

The marker generator is also a special type of signal generator. It is essentially a highly accurate, calibrated RF signal generator. It is used with a sweep generator and an oscillo scope to produce a marker pip on the frequency response pattern. The pip on the scope pattern establishes a reference point. It marks the frequency of the marker generator on the response curve. Look at the markers shown in the illustrations of receiver marker response patterns. The pattern at A shows a single...

Wobbulator or Sweep Generator

A wobbulator or sweep generator is a special type of signal generator. It provides the signals simultaneously. One signal is a frequency modulated RF output. The other signal is a duplicate of or is synchronized by the audio voltage which is used to frequency modulate the RF signal. This audio output may be either a sinusoidal or a sawtooth voltage. In practice, the audio voltage is applied to the horizontal deflection system of an oscilloscope while the frequency modulated RF output is applied...

Operational Analysis Of Receiver Rtrc

R19 Trc1 Receiver

Notice the front view of receiver R-19 TRC1 which illustrates the controls on the front panel. At the upper left corner are the receiver output terminals used in four-wire radio remote control to connect the high fidelity output to a remote control unit. Most of the other controls at the left of the control panel concern the AC power circuit. The AC line brings power into the receiver. The PUSH FOR LINE CHECK switch can be used when the meter switch, at lower center of the panel, is on position...

Diode Detector

Tube Diode Detector

Examine the diode detector circuit shown on page 16. Note that no DC plate voltage is used. The RF voltage across the tuned circuit, L2-C1, is applied across the diode. Since current flows only when the plate is positive in respect to the cathode, the RF voltage is rectified. When the plate is positive, the diode conducts and the capacitor in the filter circuit, C2, charges. When the plate is negative, no current flows in the tube, and C2 partially discharges through Rl, the load resistor....

Voltage Ratio And Decibels

Voltage Ratio Decibel Chart

Decibels db are used to express a ratio between two electrical quantities. Technical order standards for alignment and performance of a receiver are stated in decibels. You must be able to use decibels in order to understand the graphs that are used in technical orders to show when a receiver is properly aligned. The use of decibels in receiver alignment and maintenance, and in sound systems, is made necessary and is convenient because the response of the human ear is not linear. To explain...

Typical Trf Receiver

Trf Tube Schematic Pictures

You have now studied, circuit by circuit, all the parts of the TRF. You have studied various methods of detection used in the TRF, and in other receivers. You have studied various methods of volume control used in the TRF, and in other receivers. It is time now to bring the various circuits together to form one typical TRF circuit diagram. To help you understand the circuit diagram, the table on page 28 gives a description of each of the circuit components. The antenna circuit includes a...

Butler Oscillator

The Butler oscillator, used in the ARC-27 receiver, deserves a separate study. The oscillator is formed by combining a grounded-grid amplifier circuit with a cathode follower circuit and a crystal. Looking at the circuit diagram, assume that both tubes are conducting. Assume, also, that a random positive pulse appears on the cathode of VI. Since the grid of VI is grounded, the cathode becomes more positive in respect to the grid, and plate current decreases. This means that the plate voltage of...

Delayed AVC

The AVC voltage normally is in direct proportion to the strength of the detected signal. Even the weakest signal produces a slight negative voltage. With AVC, this results in a slight loss in amplification. Such reduction in the amplification of weak signals is undesirable, because such signals are difficult to read at best. To correct this situation, delayed volume control is used. Delayed volume control is a method which exempts weak signals from automatic volume control and allows them full...

Tube Tester

Tube testers are designed to test the operating fitness of electron tubes. There are two types of testers emission testers and dynamic mutual conductance testers. The emission testers are rapidly becoming obsolete. They check the tube for filament emission, for open circuit, and for interelectrode short circuits. Dynamic mutual conductance testers, such as the Tube Tester 1-177 shown here, check emission, mutual conductance, and interelectrode shorts. This tube tester is often included in part...

Principles of Direction Finder Operation

Aural Null Indication

As you can see in the block diagram of a simple direction finder, the three basic units of a direction finder are a directional antenna, a radio receiver, and an indicator. Antennas. Two simple antennas which qualify as directional antennas are the loop antenna and the U-antenna. Such antennas have a directional response pattern which takes the form of a figure 8, as you can see in the illustration above. When a radio wave arrives at right angles to the plane of the two vertical arms of the...

Signal Tracer

Any device which provides the means for detecting the presence of signal at appropriate points throughout the signal path in the receiver can be used as a signal tracer. Signal tracers vary in complexity, usefulness, and frequency ranges. The signal tracer TS-303 A C shown on page 162, is one of the most versatile. Like a signal generator, it permits stage-by-stage analysis of a receiver. However, where the signal generator provides a substitute signal to be applied to the various receiver...

Noise Control Circuits

Whenever there is an electric current, whether in nature or in man-made equipment, there is some radiation of electrical energy. Whenever electricity breaks down the insulation of air, whether in the form of a bolt of lightning or a discharge across the points of a sparkplug, there is RF radiation from what is effectively a spark gap transmitter. The air is always filled with such radiations, usually without fixed frequency. When these radiations get into a communications receiver, they are...