CHAPTER 18
INTRODUCTION TO RECEIVERS
OLD FRIENDS AND NEW

Your first contact with radio probably was with a RECEIVER in your living room at home. Most likely your knowledge of what made the radio "tick" was limited. But you could turn it on and twist the knobs to bring in the ball game or dance band you wanted to hear.

While the home receiver is simple in design, and easy to tune in comparison to the Navy types, both are essentially the same kind of gear. Each is designed to PICK UP the electromagnetic wave sent out by a transmitter, and finally reproduce the sounds in the earphones or loud speaker.

The comparison of the home and Navy receivers is much like the relationship of the Piper Cub to the F7F Tiger Cat. Both planes are designed to fly, only one is made for slow leisurely flights, and the other is a fighter.

JOBS OF A RECEIVER

All receivers have five definite jobs to do-

If any of the five are omitted, you do not have a receiver, but just a collection of wires and vacuum tubes. But when your gear does these jobs, in the order listed, you have a radio receiver.

PICKING UP SIGNALS

The RECEIVING of the signal takes place in the ANTENNA. The antenna may be a whip rising out of the top of your car, a loop of wire built into a portable radio, or a strand of wire strung between two masts on your ship.

The antenna and the magnetic field from a transmitter act together to form an a.c. GENERATOR. Earlier in this book you learned that if you have together a conductor and a magnetic field and a relative motion exists between the two, you have an a.c. generator which will induce a voltage.

Well, the antenna is the conductor, and the carrier wave from the transmitter is the magnetic field. Thus, when a radio wave from a transmitter CUTS ACROSS the antenna, an emf will be induced in the antenna. The induced emf is of exactly the same frequency and contains the identical VARIATIONS that were present when the carrier wave left the transmitter's antenna.

FIELD STRENGTH

The size of the emf induced in an antenna depends upon the LENGTH of the antenna and the STRENGTH of the carrier wave.

When the carrier wave leaves the transmitter's antenna, it is strong. As it travels, it gradually loses its strength, eventually dying out completely. If your ship is near a

In the last chapter of this manual you will learn that factors other than distance influence the FIELD STRENGTH of a carrier wave, but for the time being you can consider distance as the only factor.

A carrier wave's FIELD STRENGTH is measured by the emf, in microvolts, that is induced in an antenna one meter (39.4 inches) long. For example-transmitter A induces an emf of 100 microvolts in an antenna one meter long. Transmitter B, which is nearer, induces an emf of 1,000 microvolts in the same antenna. By comparison, the field strength of the transmitter B is ten times that of transmitter A. Thus, if the field strength of a certain transmitter is 100 microvolts per meter, an antenna three meters long will have an induced emf of 300 microvolts.

The minimum field strength necessary to produce good reception depends upon the kind of receiver and the amount of noise interference in the neighborhood of your receiver.

RECEIVER SENSITIVITY

The sensitivity of a receiver is a measure of HOW WELL it can amplify weak signals. The average home radio can amplify the signals only a few hundred times, but the receivers used aboard your ship are capable of amplifying a signal millions of times. Because of this great amplification, a communications receiver can operate on weaker signals than a home receiver.

A receiver that STARTS with a SMALL signal and FINISHES with a LARGE signal has HIGH SENSITIVITY.

It you are in an area of strong local interference, you need strong signals to produce good reception. When the local interference has a FIELD STRENGTH of 100 my. per meter, you will need a signal strength of 500 to 1,000 my. per meter to drown-out the noise. But the same receiver, free from local interference, may give good reception when signal strength is less than 10 mv. per meter.

GETTING YOUR STATION

You TUNE your receiver by adjusting the variable condensers until the RESONANT FREQUENCY of tank circuits in the receiver is the same as the FREQUENCY of the station you wish to hear. Figure 123 is a TUNING CIRCUIT,

As shown in figure 124, the condensers are mounted (ganged) on the same shaft so that both are tuned with one twist of the knob. The greater the number of circuits used, the sharper will be the tuning. A receiver that tunes SHARP is said to be SELECTIVE.

HOW SELECTIVE

Some types of communication receivers may be more selective receivers than others. A receiver used for

Figure 125 shows the best TUNING CURVE for a broadcast receiver The top is broad and flat and the sides are steep. Most cheap broadcast receivers have tuning curves as shown by the broken lines. This design permits a lot of station interference resulting in low fidelity

The band width necessary for a satisfactory VOICE COMMUNICATION may be narrower than for the broadcast bands. Clear and intelligible messages can be obtained on bands that extend only one kc on either side of the resonant frequency. The voice may sound unnatural, but it will get through.

Transmissions for c.w. code messages contain no side-bands-just the r.f wave alone. Therefore c.w, receivers can tune very sharply.

VERNIERS AND SPREADERS

The first time you try to tune a Navy receiver you probably won't bring in a thing. You are accustomed to using broad-tuning home receivers, and you'll have to develop the touch-get that old safe-cracker's feel in your finger-tips-before you'll be able to tune a shipboard receiver. A hair's breadth movement of the dial can take you past a station without even hearing a good "bloomp."

And that brings up the tuning aids you'll find on communications receivers-VERNIERS, BAND-SPREADERS, TUNING EYES, AND TUNING METERS-all put on to help you find the station you want.

Other receivers use a system of BAND-SPREADING. You put a small variable condenser having about one-tenth the capacity of the tuning condenser in parallel with the tuning condenser, as shown in figure 126.

Some receivers have a SWITCHING ARRANGEMENT which permits preliminary tuning to be broad, and the final adjustment to be sharp.

Many receivers have TUNING EYES or TUNING METERS to indicate the presence of automatic volume control (A.V.C.) voltage, and this voltage appears only when a station is tuned in. You'll hear more about this later.

R. F. AMPLIFICATION

Look back at figure 124. In addition to the tuning circuits, you have TWO STAGES OF R.F. AMPLIFICATION. The amplifier circuits are similar to those you learned back in chapter 15. The tubes are PENTODES and the stages are COUPLED together by r.f. transformers.

THE DETECTOR STAGE

The DETECTOR follows the last r.f. amplifier stage. It is in this stage that the a.f. wave is separated from the r.f. component of the carrier wave The r.f. component is cast aside and the a.f. portion is sent on to the audio stage for more amplification.

AUDIO FREQUENCY AMPLIFIERS

Most receivers have TWO a.f. amplifier stages. The first is a voltage amplifier used to drive the output POWER AMPLIFIER stage. It is in the POWER AMPLIFIER that the power of the a.f. wave is stepped up to a strength sufficient to operate the LOUD SPEAKER or EARPHONES.

RECEIVER CIRCUITS

There are a great number of receiver circuits being used to do the five jobs listed back on page 172. But the majority of Navy receivers fall into two classes-the TUNED RADIO FREQUENCY, and the SUPERHETERODYNE. Both receivers operate by having an emf induced in the antenna and by transforming this signal to a sound from the loudspeaker. But the WAY the two circuits perform their duties between the antenna and loudspeaker is quite different.

TUNED RADIO FREQUENCY RECEIVER

The TUNED RADIO FREQUENCY receiver, T.R.F., is simpler in design than the superheterodyne.

Following the r.f. amplifiers is the DETECTOR, in which the a.f. component is separated from the r.f. portions of the carrier wave.

The a.f. wave is sent on to the third part-the audio frequency amplifier-where further amplification takes place. The last step is completed when the audio signal finally appears in the earphones (or loudspeaker) as a sound.

Look back again at figure 127 and trace the progress of the carrier wave through the receiver. In the beginning

THE SUPERHETERODYNE RECEIVER

The SUPERHETERODYNE receiver contains all the major units of the T.R.F.-with THREE ADDITIONS. In figure 128 the r.f. amplifier and detector of the T.R.F. have been cut apart, and the three additional units (MIXER, LOCAL OSCILLATOR, and INTERMEDIATE FREQUENCY AMPLIFIER) are inserted.

The operation of the r.f. detector, and a.f. stages is exactly the same as in the R.T.F. receiver, but new units change the basic operation of the circuit completely.

The object of placing the additional units in the circuit is to produce a SINGLE CONSTANT RADIO FREQUENCY. This constant frequency is called the INTERMEDIATE FREQUENCY. Here is the story-

The carrier wave from the r.f. amplifier is FED into the vacuum tube of the MIXER STAGE. A second higher r.f. is produced by a LOCAL OSCILLATOR, and fed into the SAME vacuum tube. In this tube, the r.f. signal BEATS against the local oscillator signal and produces a THIRD frequency, the INTERMEDIATE FREQUENCY.

How does all this come about? The word BEAT is the clue to the answer.

WHAT ARE BEATS?

Did you ever hear two persons playing musical instruments that were slightly out of tune with each other? Certainly you have. DISCORDS were produced, and those discords were BEAT NOTES.

Beat notes are produced when two wave motions of slightly different frequency strike, or beat, against each other. For example, suppose two notes, one of 1,200 cycles and the other of 1,500 cycles, BEAT against each other. Part of the time the two will work against each other, and part of the time they will work together. This produces TWO NEW NOTES, in addition to the two original notes. One equal to the sum of the original frequencies-

1,500 + 1,200 = 2,700 cycles

1,500 - 1,200 = 300 cycles

The 2,700- and 300-cycle notes are BEAT NOTES. In the same way, beat notes always appear when two unequal frequencies are mixed together. One of the new pates is equal to the SUM of the two frequencies and the other is equal to their DIFFERENCE.

HOW THE INTERMEDIATE FREQUENCY IS PRODUCED

Now go back to the superheterodyne, in which you wish to produce a SINGLE CONSTANT INTERMEDIATE FREQUENCY. Suppose the I.F. desired is 500 kc. You could produce it by mixing ANY two frequencies whose SUM or DIFFERENCE is equal to 500 kc. But in practice you would use only the DIFFERENCE to produce the WANTED frequency.

Remember, ANY two frequencies whose DIFFERENCE equals 500 kc. will do. Thus if the incoming CARRIER WAVE is 2,200 kc., the OSCILLATOR frequency must be 2,700 kc. to produce an I.F. of 500 kc. Or you may use any number of other combinations such as-

And you could go on and fill the rest of this manual with other combinations whose differences are equal to 500 kc.

Notice the oscillator frequency. It is 500 kc. MORE than the incoming CARRIER WAVE.

2,900 - 2,400 = 500
3,650 - 3,150 = 500

Or turn it around-the oscillator frequency is equal to the carrier frequency PLUS the intermediate frequency-

2,400 + 500 = 2,900
3,150 + 500 = 3,650

To sum it up-in a superheterodyne receiver the oscillator generates a frequency that is always the I.F.

The condenser that tunes the oscillator is connected, or ganged, to the SAME shaft that tunes the r.f. sections of the radio. And by turning a single knob, the oscillator is automatically tuned to the I.F. HIGHER than the incoming r.f. carrier wave.

Since the I.F. signal is a COMBINATION of the local oscillator and the carrier wave signals, it will be MODULATED and have the same characteristics as the carrier, only at a lower frequency.

Look back again at figure 128. The output from the mixer stage is sent into the I.F AMPLIFIER, where the voltage of the I.F. is still further strengthened. And the output of the I.F. amplifier is sent into a detector where the r.f. and a.f. components are separated, just as they are in the T.R.F. receiver.

Sometimes you will hear the MIXER stage called the FIRST DETECTOR and the other detector stage the SECOND DETECTOR. Don't let it trouble you. The term FIRST DETECTOR comes from the fact that the production of beat notes is sometimes called HETERODYNE DETECTION.

WHY THE EXTRA PARTS

You may wonder why all the extra parts are added to a T.R.F. receiver to form a SUPERHETERODYNE when the T.R.F. does a good job. That is a sensible question. The answer is-the superheterodyne does a BETTER job. Increased SENSITIVITY and SELECTIVITY make the superheterodyne a much better receiver for the reception of weak signals. That is reason enough.

BAND SWITCHING

Practically all Navy receivers are made to tune over several BANDS of frequencies. The RBB/RBC receivers have four bands; the RAK has six and the RAL has nine. To change from one band to another, it is only necessary to rotate a switch to the band you wish to use.

Some receivers, especially the T.R.F. types, have TRIMMER controls that are adjusted each time you change frequency bands. This is done by opening the TUNING condensers to their widest mesh at the high end of the frequency band, and then adjusting the trimmer controls until the noise level is maximum.

This control is necessary because, in spite of the greatest care in manufacturing, coils have slight differences in their windings. This causes variations in the resonant frequencies of the several tuning stages. The trimmer controls correct these variations.

RECEIVER CALIBRATION

The CALIBRATION of a receiver is only the RECORD of the dial settings indicating where you can find a station of a certain frequency. As an example, if you lived near Chicago, you knew that WGN could be picked up by setting the dial at 720. Maybe your receiver was a little out Of adjustment and you got the station by setting the dial at 710 or 730. You didn't write these numbers down, you just remembered them. That is a rough example of calibration.

Most Navy receivers have several dials to be set for each station you receive. To save time wasted in hunting all over the band, and in trying to remember the proper settings, you will RECORD the positions of ALL the dials for EACH STATION you listen to. The resulting chart is the calibration of your receiver.

To calibrate a receiver properly you must very carefully check the settings of the dial against known frequencies. Then, when you are instructed to listen to a station transmitting on 2,120 kc, you can turn to the chart and find the exact setting for each dial.

VOLUME CONTROLS

In addition to the TUNING knobs, all Navy receivers have several other dials and controls to help you in operating the set.

The VOLUME CONTROL is the most familiar. With it you increase or decrease the volume of sound to the desired level. Your receiver at home has one of these controls.

The r.f. GAIN CONTROL, sometimes called sensitivity control, is closely related to the volume control. You can raise and lower the output sound level with it, but that is not its prime purpose. This gain control is usually located in the first r.f amplifier stages. When a very weak station is being received, this control is turned all the way up; but if you are tuned to a strong station, the control is turned DOWN to prevent OVERLOADING the r.f. tubes. This is necessary since overloading causes SERIOUS DISTORTION in the signal.

AUTOMATIC VOLUME CONTROLS

The AUTOMATIC VOLUME CONTROL, AVC-sometimes called AUTOMATIC SENSITIVITY CONTROL, ASC-serves to keep the output volume at a constant level. This saves you the job of continually turning the manual volume control up and down each time the stations being received FADE and. REAPPEAR in strength.

Most AVC systems have two controls, an OFF-ON switch, and an AVC LEVEL regulator. It is the usual practice to turn the AVC off while tuning the receiver. When the receiver has been tuned, the switch is turned ON, and the LEVEL is adjusted for the desired operation.

The AVC system in most Navy receivers is too rapid and pronounced to permit its use with voice reception. So the AVC usually will be OFF when you are receiving a voice message.

NOISE SUPPRESSORS AND OUTPUT LIMITERS

The high sensitivity of all communication receivers causes them to pick up a lot of local interfering noise

The NOISE SUPPRESSOR works much the same as a TONE CONTROL in a home receiver. When this control is turned for DEEP or BASS reception, much of the noise is FILTERED OFF and is not permitted to reach the earphones. But the noise suppressor also reduces the volume. So on very weak, signals, it may be necessary to turn the switch that cuts it out of the circuit.

The OUTPUT LIMITER prevents sudden crashes of static from bursting your ear drums. There are several ways this can be done, but all work as a safety POP-OFF valve. When the output volume of sound reaches a certain level, the output limiter goes into action and prevents the sound from rising any higher.

Some receivers have circuits called SILENCERS, designed to keep the receiver silent when no signal is being received. This is very useful when you are standing by to receive a message.

Most output limiters and silencers have OFF-ON switches, and an OUTPUT LEVEL adjustment. The specific name used for these controls depends upon the particular make of the set.

OUTPUT METERS

Many receivers use a meter to show the level of SOUND OUTPUT. It is also useful as an aid in tuning the receiver, especially where you are SEARCHING for a station that is not on the calibration chart.

These meters are made to indicate the presence of a station even when the sound is considerably below the minimum level your ears can hear. Once the presence of a station is indicated by the meter, the volume can be brought up to audible level by turning up the sensitivity control.

Most output meters are calibrated in DECIBELS. A decibel is the SMALLEST difference in sound your ear can

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The output meter is used by the Electronic Technician's Mate when he is aligning, or tuning up, your receiver. With this meter he will be able to tell whether a station with a certain signal strength can be heard.

OTHER METERS

Some receivers have two other meters-one to indicate the FILAMENT VOLTAGE, and the other the PLATE VOLTAGE. A control accompanies each meter, so that if the voltages are incorrect, you can correct them.

BEAT FREQUENCY OSCILLATOR

The BEAT FREQUENCY OSCILLATOR, B.F.O., is a part of every communication receiver designed to receive C.W. messages. When the receiver is being used to receive I.C.W., modulated C.W., or voice messages, the B.F.O. is always turned OFF.

With each B.F.O. is a TUNING control, sometimes marked A.F. TUNING. With this control, you adjust the PITCH of the audio note to the desired frequency.

The B.F.O. is usually connected to the detector tube in the T.R.F. receiver and to the second detector in the superheterodyne.

The frequency of the B.F.O. is about 1,000 cycles less than the incoming carrier wave with the T.R.F., and 1,000 cycles less than the I.F. in the superheterodyne receiver. For example, if the carrier frequency being received by a T.R.F. is 4,720 kc., the B.F.O. will be tuned to approximately 4,719 kc., so the BEAT note produced will be-

4,720 - 4,719 = 1 kc. (1,000 cycles)

In a superheterodyne, if the I.F. is 500 kc., the B.F.O. will be tuned to about 499 kc. This also will produce a beat note of 1,000 cycles. By adjusting the B.F.O. you can raise or lower the pitch of the beat note to a frequency slightly above or below the 1,000 cycle note.

A CRYSTAL FILTER control is used in connection with many B.F.O.'s. Its purpose is to prevent interfering noises and notes from blotting out the tone of the C.W. signal. The filter has two controls, an OFF-ON switch, and a REJECTION control. Sometimes, the OFF position of the switch is marked BROAD, and the ON position, SHARP. The REJECTION CONTROL is adjusted for beat reception each time the filter is turned on.

Don't be surprised to find controls other than those just described. Almost every receiver has some special knob all its own. You can find all this specialized information in the manufacturer's instruction books.