Basic Principles of Frequency Modulation (1944)

[Music] morning [Music] what the hell goes on ah some jerk shaving hey cut off that lawnmower relax Junior I've got a date with a Queen of Sheba she's allergic to beards are you silly take it easy I'll fix it frequency modulation or FM is a big step forward in radio when you tune in you hear what the studio mic [Music] is there's no static no heterodon squeals the guy with the electric Razer can use it all he wants a GI can dial his girl's number all these things in the same area but they won't bother reception well you've seen what FM means to ordinary everyday reception but what's its military value just this until the Army turned to FM for some of its installations radio interference was a communication headache sets out on the battlefield picked up static static that hashed up the incoming signal tank cracks created static that made reception Dam near impossible then there are teletypes battery chargers Armored Cars tanks the material of Modern War as well as such natural phenomena storms created enormous amounts of electrical interference frequency modulation licks this problem and that's why we want you to know something about it now the best way we can explain FM is to compare it with am meaning of course amplitude modulation the kind that's never been able to comb static out of its hair now this oscillator generates a radio wave we call the Carrier it's then fed into the amplifier where it's strengthened but it's un modulated unchanging it carries no message or as we say no intelligence it's something like the steady monotonous note of a trumpet in order to carry a message the wave must have the intelligence impressed on it an am it's done like this the intelligence goes into the mic is sound the mic changes the sound into electrical energy that varies at an audio rate according to the sound the electrical energy is fed into the modulator where it's Amplified from here it goes into the amplifier where the fluctuations and electrical energy are impressed on the carrier or radio wave generated by the oscillator now what's that mean well just this the carrier is modulated it has its amplitude of strength Changed by the fluctuating electrical energy from the modulator then the wave that comes from the amplifier looks like this in this way the carrier has its amplitude Changed by the intelligence the wave that's transmitted is an AM wave the varying strength or amplitude of the wave carries the intelligence and AM radio these waves are radiated by the antenna now what happens in an AM receiver first let's take the am wave as it approaches the antenna of Our receiver the am receiver picks up the waves first it amplifies them this way then it feeds them to the demodulator the demodulator changes the RF amplitude variations back into audio frequency energy this audio energy is built up or Amplified by Audio frequency stages then it is finally fed into a speaker which turns it back into sound that's how amplitude modulation works of course we've gone over parts of it pretty sketchily because we're taking for granted you're fairly well grounded in AM radio by now all is fine in am everything works perfectly when operating conditions are ideal but suppose there's electrical interference of some sort for instance here's our transmitter and here's our receiver electrical impulses of All Sorts produce amplitude variations in a radio wave much the same as the audio signal and the am transmitter over there to the left lightning for instance hitchhikes onto the transmitted wave messing it all up confusing the intelligence whenever it strikes notice how fuzzy the RF way between the sets becomes at these times the am receiver can't separate the amplitude variations that carry intelligence from those that carry just noise noise that's the problem now what are we going to do about it well we know that a radio wave has both frequency and amplitude so far the intelligence represented by this audio wave has been transmitted by holding the frequency of the carryer constant and varying its amplitude its strength this way but as we've seen earlier lightning and other electrical disturbances also vary the amplitude and upset or interfere with the original intelligence but Engineers have discovered that lightning and other electrical disturbances have a negligible effect on the frequency of a radio wave and so they started reasoning why not hold the amplitude constant and Vary the frequency or wavelength and make it carry the message where am carries its intelligence by increasing and decreasing the strength of its carrier this new type would do the same job by varying a mod cating the frequency of its carrier any static that jumps onto the wave affects only the amplitude not the frequency the intelligence therefore is not affected FM radio is based on this idea the changing frequencies carry the message as clear as a bell there are several types of of FM transmitters used by the Army to get a good clear picture of frequency modulation and its general characteristics we better consider a simple set first now just as an am the carrier wave is produced or generated if you like by an oscillator in FM this carrier wave is called the rest or resting frequency when it's unmodulated that is when there's no sound being produced the frequency depends on the values of the coil and capacitor in the tank circuit change the value of either the coil or the capacitor and the frequency changes now if there were any way for the audio signal to change the coil of capacitor values the result would be an FM wave at the output you can accomplish this by placing a capacitor microphone and parallel with the capacitor in the tank circuit this way capacitor microphones contain two plates one of which vibrates when struck by sound waves of course this rhythm is greatly slowed down in reality it could be anything from 16 to perhaps 16,000 vibrations per second when the space in here between the plates varies due to the vibration the capacity of the mic will also vary at the same rate now take this in because it's the way the transmitter operates the capacity of the mic affects the capacitance in the tank circuit the capacitance of the tank circuit affects the frequency of the oscillator the frequency of the oscillator determines the frequency of the RF wave the RF wave is therefore frequency modulated in accordance with the vibrations of the mic plate in other words the frequency now varies at an audio rate when sound hits the mic when there isn't any sound the diaphragm of the capacitor mic keeps straight and motionless the oscillator produces its original or resting frequency when the first vibration of a sound wave hits the mic it pushes the diaphragm plate closer to the stationary plate that action increases the capacitance of the mic this increased capacitance decreases the frequency of the oscillator the closer the plates in the mic the less the frequency and the farther apart the waves when the diaphragm moves away from the stationary plate the oscillation increases and the waves squeeze closer together two complete Cycles cause the FM waves to squeeze stretch squeeze stretch the higher the notor sound the higher its frequency now what effect does that have on the FM wave just this I take a look at this diagram this Center Line with a figure 40 Mega Cycles written below it represents the rest frequency of our oscillator now keep that in mind rest frequency represented by this line is 40 Meyes that is without being modulated it oscillates the rate of 40 million cycles per second now what will cause it to vary from rest frequency any sound impressed on the capacitor microphone of course for example let's say we modulate the carrier with a 500 cycle note we get a frequency that swings back and forth above and below the rest frequency 500 times per second suppose we increase the frequency of the note to 1,000 cycles per second see the difference this th cycle note causes the FM wave to swing back and forth twice as fast as did the 500 cycle note I know you're wondering why these two notes of different pitch swing back and forth the same amount across the rest frequency well that brings us to a very important point or rather two points rate of change and amount of change you know now that frequency affects rate of change let's see what affects amount of change take the 500 cycle note again but make it louder then this happens see it we've got the same rate of change across rest frequency but the distance of the Swing to either side of the rest frequency becomes greater the distance is called deviation so the frequency of a note or sound determines how many times the swing takes place and the loudness determines the amount of Swing or deviation now you can see that a great amount of deviation might cause a little trouble that is the deviation would go too far and interfere with another FM transmitter so the army sets the maximum deviation for any channel at 40 kilocycles on either side of this rest frequency in other words the strongest audio signal that can be used for modulating a transmitter is one that allows only a deviation of 40 kilocycles on either side of the rest frequency this whole thing 40 kilocycles on one side and the same on the other is the carrier swing th here we have a carrier swing of 80 kilocycles altogether one more thing there has to be a separation between channels that is we've got to have some method of protecting one channel from possible slop over from another so here's what we do we provide guard bands on each side of the maximum deviation each band is 10 kilocycles wide making a total of 20 kilocycles the channel allotted to each station consists of two deviation ranges of 40 kilocycles each plus a 10 kilocycle guard band on each side that's a total of 100 kilocycles remember what we've seen so far is how we get the FM signal we showed you an FM transmitter being modulated by a capacitor type microphone because that was the easiest way of getting the point across however the Army uses two other systems of getting FM they're called reactance tube and phase modulation there's one big difference between the two reactants tube modulates in the oscillator stage while faas does it in some succeeding stage but no matter what method we use an FM receiver will pick up the signal and that gets us to the FM receiver naturally because we're dealing with FM waves we can't use the ordinary am receiver but the two are fairly similar as a matter of fact there are only three main differences number one is band pass all band pass means is we've got to have a receiver that will pass the wide band or range of frequencies we're bound to get with FM taking care of that is pretty easy because it's just a matter of circuit design now here's a difference we can't brush off as easily as we did the band pass problem a frequency modulation receiver has to have some means of cutting or clipping off amplitude variations which in FM carry noise not intelligence so we get rid of them how we do it we'll see in a minute and here's the third difference the FM receiver has to be able to change the frequency variations back into audio amplitude variations now let's see what a block diagram of an AM Superhead receiver looks like to change the am receiver to an FM receiver we have to make changes in the set first in the circuit to take care of band pass now the amplitude limiting part in place of this if amplifier we put a device called a limiter this takes care of clipping off the amplitude variations that could hash up reception all right the limiter takes care of this now something to take care of our third condition in place of the demodulator in the AM receiver we put a device called a discriminator so now we've satisfied all three conditions the band pass by means of circuit design the amplitude limiting by means of the limiter and the translation of frequency variations back into audio amplitude Variations by the discriminator now we have an FM receiver but there's a little more to it than just saying we put this here and that there we want to understand what happens all right here's a simplified diagram of a limiter it consists of a tuned grid circuit a resistor a tune plate circuit and bypass capacitors also this sharp cut off tube it operates at zero initial grid bias and low plate voltage let's see what the action of this limiter circuit is here is our FM wave as it leaves the transmitter it's free of any amplitude variations but as you saw before the best laid plans of men and radio go astray and our wave doesn't stay nice and clean amplitude variations creep in caused by electrical disturbances so the wave looks like this when it reaches the antenna of Our receiver the frequency hasn't changed but there are amplitude variations on the positive portion of the wave and variations on the negative portion now let's take this W through the limiter the incoming wave induces a voltage in the first tune circuit as you see here the grid of the tube is connected directly to the tune circuit now with no initial bias it stands to reason that any positive amplitude swing on the signal will make the grid positive therefore since the grid is positive it attracts electrons from the cathode of the tube the electrons move along as a grid current the more positive the amplitude variations of the wave the more positive the grid will become and the greater the grid current flow but the grid current flow through this resistor produces a voltage drop which tends to Buck the positive signal now you can see what's going to happen as the positive amplitude of the wave increases more and more electrons are attracted to the grid more and more current flows through the grid circuit this increased current flowing through the grid resistor develops an increasing negative voltage that acts against the incoming positive signal the voltage drop across the resistor finally becomes so great that it prevents any further increase in positive amplitude of the signal from getting to the grid only a certain amount gets through and what happens to the wave just this the amplitude variations on the positive side of the wave are clipped and with them go the noises they carry but we've still got these to worry about the negative amplitude variations the clipping of the negative amplitude swings is simpler than clipping the positive you've just seen that a positive swing and Signal amp ude turns the grid positive so we'll naturally get a negative grid when the swing of amplitude is negative and the more that swing the more negative the grid will become and what effect does that have say the negative amplitude swing is just beginning as soon as it begins the grid becomes negative that means that the grid will repel electrons that try to hop over from the cathode of the tube Lo just a small negative charge in the grid means that not all the electrons from the cathode will be repelled some will flow through and get to the plate thus causing a plate current to flow but as the negative amplitude swing of the signal becomes larger the grid becomes more negative it repels more and more electrons that try to get over from the cathode now remember I said we used a sharp cutof tube in the limiter the tube is biased very quickly Beyond cut off therefore it will quickly reach the point where its grid becomes so negative that it will repel any electrons emitted by the cathode we get this effect on the incoming signal here's the incoming signal just the negative amplitude swing of it as it becomes more and more negative the action we just went over on the grid takes place suddenly because of the sharp cut off point of the tube plate current ceases to flow and any further negative amplitude variations won't get by the negative portion of the wave now looks like this of course the negative amplitude swings back up this means that the grid will become less negative until it reaches a point where the electrons could once more flow through to the plate plate current flows again now combine the positive swing clipping action with the negative and we get this here is our FM signal coming in with all its amplitude variations it gets to the limiter the two clipping actions we saw take place first a positive the swing goes up and up until it's stopped clipped off eventually it starts down again toward the negative swing it reaches the start of the negative swing and causes the grid to become negative when the cut off point of the tube is reached the negative swing is clipped the result of these clipping actions is this kind of a wave now we have what we went after a wave with constant amplitude with all amplitude variations clipped off the intelligence is still with us carried in the frequency variations but we still have a little trouble to get rid of notice that the tops and bottoms are squared off that will mean Distortion of the signal the tune plate circuit takes care of that it's able to smooth off any irregularities or sharp corners of a wave by what is called flywheel effect so the wave that leaves the limiter will look like this and that's it now the wave is ready for the discriminator the device that interprets these frequency variations has audio voltage variations now here's a simplified diagram of a discriminator it consists of a tune circuit a diod detector tube and a load resistor with a bypass capacitor across it now as you know a tune circuit can pass a maximum voltage at its resonant frequency that is when the frequency of the incoming wave is the same as the resonant frequency of the circuit now let's see the response curve of the tune circuit we'll say it's resonant frequency is here now here's a line that represents voltage varying amounts of it any one point along this line represents the amount of voltage at that particular spot let this line represent rest frequency the discriminator is purposely tuned off resonance with the incoming frequency all right as long as an unmodulated carrier is coming in say at this frequency it means that the output at the discriminator is a steady DC voltage but now Watch What Happens is the frequency of an FM wave with its rest frequency here gets closer to the resonant frequency of the discriminator now don't forget here's the resonant frequency of the discriminator and here's the voltage at the rest frequency of our FM wave now when the frequency of the incoming wave is changed so that it moves closer to the resonant frequency of the discriminator More Voltage is passed by the discriminator this means that the voltage output of the discriminator will rise and step when the frequency of the incoming FM wave reaches its peak the vol volage reaches its peak now as the frequency of the wave gets farther away from the resonant frequency of the discriminator less voltage is passed here's the entire action watch it so you can see that the voltage output of the discriminator is an audio frequency voltage which is exactly in step with the frequency variations this wave is then fed to the AF amplifiers and on to the speaker exactly as in the AM receiver FM is the answer to the static problem static that hashes up perception now let's give it a quick once over and call it a day okay first the FM wave is generated or formed at the transmitter by one of several methods the simplest the one we showed in this picture was by means of a capacitor mic this capacitor mic you remember increased or decreased in capacitance according to the sound that hit it increasing or decreasing the capacitance of the mic affected the frequency of the oscillator the two Army methods of getting FM are reactant tube and phase modulation but no matter what method is used the FM wave is radiated by the transmitting antenna the receiving antenna picks it up it's Amplified and then fed into the device called the limiter the limiter Clips off any amplitude modulations that would come out of the speaker as noise from the limiter the now cleaned up wave is fed into the discriminator this device changes frequency variations back into audio voltage variations from there it's into the amplifier and out of the speaker so briefly and simply you've seen the how why and what of frequency modulation electrical disturbances don't bother FM as far as the static they cause is concerned when frequency modulation is used you can be sure that electrical interference won't hash up your messages