US2390777A - Frequency modulation system - Google Patents

Description

Dec. 11, 1945. QLE 2,390,777

FREQUENCY MODULATION SYSTEM Filed March lO, 1942 2 Sheets-Sheet l WITNESSES: INVENTOR I f D. k/PC or); 0 e

- awM ATTORNEY Dec; 11, 1945. 7

D. P. COLE FREQUENCY MODULATION SYSTEM Filed March 10, 1942 AAAA B essgq II I 56/680 arid Volts WITNESSES:

2' Sheets-Sheet, 2

Screen Vo/rs Eelac fance l l l 6.5 7.5 85 9.5 I05 INVENTOR DQNO/d/QCO/G.

ATTORNEY Patented Dec. 11, 1945 UNITED STATES PATENT OFFICE FREQUENCY MODULATION SYSTEM Donald P. Cole,v Catonsville, Md., asslgnor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 10, 1942, Serial No. 434,033 rolaim. (01. 179-1715) to the intensity of the modulation signals, such for example, as speech or music, and at a rate of the frequency of the modulation signal. The energy radiated is not changed in amplitude, only the carrier frequency changes with the instantaneous variations of the intensity of the signals to be transmitted.

There are various methods known whereby the frequency output of a vacuum tube oscillator may be varied. It is important, however, that .the unmodulated carrier shall alwayshave the exact predetermined value which is set for the operation of the transmitter. The greatest acparent from the followingdescription of the invention, pointed out in particularity by the appended claim. and taken in connection-with the accompanying drawings, in which:

curacy in frequency stability of an oscillator is obtained when the frequency determining element is a vibratile device, such for example, as a piezo-electric crystal. In view of the fact that a crystal tends to maintain the oscillations generated at a constant frequency, determined by the physical size of the crystal element, the frequency deviation may be obtained only within very narrow limits from the crystal frequency.

Various attempts have been made in the prior art to cause frequency deviation of a crystal controlled oscillator for the purpose ofmodulation. The narrow limits of frequency deviation made such attempts unsuccessful from a practical standpoint.

A particular feature of this invention is that a An advantage of the modulation system herein described is that within thepresent range of frequency modulation transmission in the neighborhood of 50 megacycles, crystals of such physical parameters may be chosen which give the maximum useful range of crystal frequency deviation.

Other features and advantages will be ap- Figure 1 is a schematic circuit diagram of the oscillator and modulator portion of. the system; Fig. 2 is a schematic circuit diagram of the complete modulation system Fig. 3 is a. simplified reference diagram to be used in connection with the theoretical consideration of the system;

Figs. 4, 5 and 6 are curves illustrating certain relations forming the bases of the theoretical consideration with reference to Fig. 3;

Fig. 7 shows the variation of frequency obtained with the modulation system shown in Fi 1 by means of changes of the screen grid voltage of the modulator tube; and

Fig. 8 shows the linearrelation of the useful range of frequency shift obtained from the combination of the two oscillators shown in Fig. 2

,when the screen voltage of each modulator tube is varied.

Referring to Fig. 1 of the drawings, vacuum tube l is connected in an oscillating circuit having as the sole frequency determining element, a piezo-electric crystal 2 placed between anode 3 and control electrode 4. The anode circuit of the oscillator includes the inductance 5 connected between the anode l and cathode 6 in series with the anode potential source shown here by the battery 1. The screen-grid 8 6f the tube I is supplied with the required potential from the battery I through a screen-grid resistor 9. The high, frequency currents are suitably bypassed by the condenser ID in the screengrid circuit and by the condenser II in the anode circuit. The grid 4 returns to the cathode by means of the grid-load'resistor II.

The circuit so far described is a conventional type of crystal controlled oscillator in which the crystal forms the coupling means between the anode and the grid circuits, as well as the frequency determining element. In this manner, the oscillator does not depend upon feedback either through external coupling between the anode and the grid circuits, or by coupling provided by the inherent inter-electrode capacity of including two identical oscillator and modulator portions; 10 K the cathode 6, and the grid it to the grid 6, the latter by means of a coupling condenser IS. The grid It returns to the cathode l5 through grid resistor 20. The screen grid 2| of the tube l3 is utilized as an additional control electrode, be-

ing connected to the secondary winding 22 of the transformer 23. An initial screen-grid potential is supplied from the battery 23 connected between the cathode l5 and the return terminal of the winding 22. Condenser 25 forms the bypass for the modulation frequency currents.

The terminals of the primary winding 26 of the transformer 23 may be connected to any suitable source of modulation frequency currents, such as the output of a speech amplifier if voice transmission is contemplated. Any suitable manner of coupling may be utilized as long as the voltage input between the terminals of the primary winding 26 is of suitable magnitude to impress upon the screen-grid circuit sufficient voltage to vary the effective potential on the screengrid electrode 2| within the required limits, as will .be described later.

The circuit of Fig. 1 represents the. oscillator and modulator portion of the combined system shown in Fig. 2 of the drawings. In the latter, two complete oscillator and modulator portions, as shown in Fig. 1, are combined in one circuit in a push-pull type arrangement as to the modulation voltage input. The output voltages of each, on the other hand, are mixed in a-common output circuit. For a better understanding of the function of the frequency modulation system shown in Figs. 1 and 2, a series of elementary theoretical diagrams are included in the drawings, to which reference shall be had prior to discussing the complete system of Fig. 2.

Referring to Fig. 3, let it be assumed first that the tube 21, which is in parallel with the tube 28, is omitted and we are considering only the tube 28 and its associated circuits. We may also regard, for the purpose of analysis, tube 28 basically as a three-electrode vacuum tube although a tetrode is shown here. The purpose of the screen grid electrode 29 is merely to reduce the grid-to-plate capacity. The effective grid-toplate capacity, on the other hand, is now supplied by the crystal element 30, which is connected between the anode 3| and grid 32 of the tube 28. The circuit betweenthe anode 3| and the cathode 33 is similar as to its components to the one. shown in Fig. 1, in that plate potential is applied from the battery 34 through the inductance 35 between anode 3| and cathode 33. Screen grid potential is derived from the battery 36 and the screen grid electrode 29 is suitably bypassed by the condenser v3?. The grid circuit includes the bias battery 38 and the grid resistor 39.

Neglecting for the moment the effect of the resistance 39 and applying a voltage which varies with time between grid 32 and cathode 33, it will of plate resistance.

be found that the impedance presented to this voltage is a negative value of input resistance. The value of this resistance can be calculated by referring to the equation (57) on page 209 in the book "Thermionic Vacuum Tubes by Van der Bijl. This gives for a pure inductance in the output circuit the following value for the input resistance.

R L..C3

whereRg=the input resistance, Rp=the anode resistance, Lo=the anode circuit inductance, C1=the grid cathode capacity and Ca=the grid connected tube.

aseavvr anode capacity. Now, if the value of this input resistance is lowerthan that of the parallel positive resistance, the net input resistance will be negative andthe tube will oscillate.

Referring now to Fig. 4, curve A shows the reactance which a piezo-electric crystal presents when a voltage which varies with time is applied to it. From this, we can calculate the apparent capacity or inductance of the crystal circuit as the frequency varies. This is shown in Fig; 5. It is seen here that at some frequenc the capacity as presented by the crystal circuit will .be of the value required to make the negative input resistance of the tube 28 and its associated circuit equal to the positive resistance, This point on the frequency ordinate is shown by the dotted line. The net losses will now be zero, permitting the circuit to oscillate, If the plate resistance of the tube 28 is caused to vary by some means, it is apparent from the equation above referred to that the grid plate capacity must change in order that the negative input resistance shall be exactly equal to the positive circuit resistance. This holds true because the positive circuit resistance cannot change since it is a circuit ele- We may now consider the second tube 21 in parallel with the tube 28. A .parallel connection, as

shown in Fig. 3, will change the plate resistance of the circuit. This is evident since the total amplification, factor remains unchanged. Furthermore, .the grid-to-filament capacity of tube 21 being always in parallel with th grid-to-filament capacity of the tube 28, does not change with variations of screen grid potential on the parallel It is readily seen that if we change the screen grid potential of the tube 21, as indicated by the adjustable connection on the battery 36, the plate resistance of the tube, as well as the grid plate capacity, must change in order to maintain equilibrium. We have seen in Fig. 5 that the capacity of the crystal will vary with changes of frequency. Hence,'by the same consideration the frequency of the oscillationswill vary. This variation follows directly in accordance with the calculations employing the equation above referred to. The resultant variation of frequency with variation of screen grid potential is shown in Fig. 6 of the drawings, in which curve B indicates a change of frequericy with a change From the above theoretical analysis, it is seen that in the modulation system in accordance with .this invention the frequency deviation of a crystal ance with signal amplitudes the modulator tube is efi'ectively connected in parallel with the oscillator tube and an auxiliary control electrode thereof, such as the screen-grid electrode, is utilized for modulation signal input.

When the screen-grid circuit of the modulator tube l3 in Fig." 1 or tube 21 in Fig. 2'receives no excitation, a static condition exists and the oscil- 2,890,777 "lator frequencyrwill have a certain value depending upon the physical parameters of the crystal 2. The expression "static frequency" is chosen to define the frequency from which any departure is caused b modulation. The output of the oscillator may be taken off from either the anode cirsuit or the grid circuit to be utilized in the transstraight-line portion of frequency shift may be observed. The useful range represented by the straight portion gives in total shift for one crystal approximately 400 to 500 cycles. In present practice of frequency modulation transmission, this order of frequency shift would not be practical since it is generally desired that the carrier output frequency deviation should be approximately 200 kilocycles with a carrier frequency between 40 and 50 megacycles. With only 400 cycles available frequency shift of one oscillator, the frequency would have to be multiplied 500 times and the static oscillator frequency would have to be between 80 to 100 kilocycles to cover the proper range. Piezo-electric crystals in this range are dimcult to make and are sluggish in response.

- In order to effectively employ the simple frequency modulation circuit in accordance with this invention, in a transmitting system operating in the 40 megacycle wave band with wide band frequency deviation, :two oscillators are combined each with its associ .ted modulator tube in such manner that the output of each will be additive as to total frequency deviation. This arrangement is shown in Fig. 2 of the drawings, in Which identical components in the oscillator circuits with respect to Fig. 1 bear similar reference characters. The first oscillator modulator stage is indicated by G and the second similar stage by H. In the first stage, vacuum tube I comprises the oscillator proper having its anode 3 connected to suitable impedance 5 which terminates at the high potential side of the power supply source represented by the battery I, which is bypassed by means of the condenser I I. The screen-grid electrode 8 bypassed by the condenser I0, returns in series with the screen-grid resistor 9 to a suitable voltage tap of the battery I. -The cathode 6 of the oscillator tube is connected to ground, which is indicated by a heavy line throughout in the figure. Between the anode 3 and grid 4 is connected the crystal 2. The grid circuit returns to ground through the grid resistor I2. The modulator tube I3 is effectively in parallel with the I oscillator tube having its anode I4 connected to i the anode 3 and its cathode I5 to ground. The grid I5 thereof is capacitively coupled by means of the condenser I8 to the grid 4 of the oscillator one previously described it is not believed necessary to repeat the circuit description The combined function of the modulated oscillator stages H and G will be considered in detail when referring to the operation of the system.

The modulation voltage is fed to the primary winding 26 of the transformer 23 from a preceding pushpull amplifier stage shown here by the dual triode tube 40. The anodes M and 42 thereof terminate in the free ends of the winding 23, the center tap of which is supplied with the required anode potential from the high potential side of the battery I through the conductor 43. A series resistor 44 is included and the center tap of the winding 26 is bypassed by the condenser 45. The grids '46 and 41 of the tube 40 are excited with modulation frequency currents from the output of another amplifying stage, the grid 46 being coupled to the anode 48 of the tube 49, through coupling condenser 58, whereas the grid 41 is coupled to the anodeBI by means of coupling condenser 52. Resistor 53 is connected to the grid 46 and returns to ground. The resistor 54 similarly connects to the grid Q7 and returns to ground. The cathode 55 and the cathode 56 of the tube It each return to ground through suitable bias voltage resistors 5l and 58, respectively, bypassed by the condensers 59 and 6b.

The anode circuit of the tube 48 includes the load resistor ti for the anode 38 and the load resistor $2 for the anode 5i terminating in the anode supply sourc by means of the series resistance til which is bypassed by the condenser 6d. The input circuit of the tube 39 between grids 55 and 56 includes the secondary winding 6? of the input transformer 68. The center tap thereof returns to ground. Bias voltage for the grids is derived by the cathode resistor G9 which connects to the cathode ,IIl of the tube 49 and ground. The latter is bypassed by the condenser I I-. The primary winding I2 of the transformer 68 is shown here as being connected to a microphone 93 which is the ultimate source of modulation voltage. While there are only two audio frequency stages shown between the microphone and the modulator tubes, it is to be understoodthat these are merely by way of example to indicate that suitable amplification ofthe microphone currents must be provided in order to raise the output voltage to the level necessary for swinging the screengrid electrodes 2I within the limits of screen voltage change giving linear responsive to the frequency variation of the oscillator. The screengrid electrodes 2I of the tubes I3 in stages G and H, are of course supplied with the acquired operating potentials from the top on the battery I which also supplies thescreen-grid electrodes 8 of the oscillator tubes I. A connection from the midpoint of secondary winding'22 of the modulation input transformer 23 is connected through filter resistor I4 to the screen protective top of th battery I. ulation frequency by pass between ground and the midpoint of the winding 22.

Further analysis of the circuit will show that both oscillator modulator stages G and H are connected in push pull relation so that the modulation excitation voltage is out of phase between screen grids 2|. Consequently, while in one stage the voltage is increasing from the initial voltage supplied by the battery; in the other tage it will be proportionately descreasing in the same amount provided that balance is maintained in the push pull circuit. From this it is also seen that in one stage the oscillator output frequency The condenser I5 forms the modwill be increasing from a predetermined value whereas in the other stage, it will be decreasing from another predetermined value. Thi condition is better understood when referring to Fig. 8, which shows the frequency deviation of oscillators G and H plottedagainst change in screen voltage. The effective change in each within the voltage limits for the screen grid is identical and linear giving approximately 400 cycles total shift. The

output frequency of each oscillator may be socombined that the frequency shift of one oscillatorwill add to the frequency shift of the other with respect to the ultimaely derived ouput frequency deviation,

Now then, if such combination is achieved as will be seen with further reference to Fig. 2, a

' total shift of approximately 800 cycles may be obtained.- The push pull excitation bf the two modulator tubes provides for the proper phase displacement between the oscillator voltages which are derived from th grid circuit of each oscillator tube. In the oscillator stage G the grid is coupled to the first control grid '50 of the mixer tube ll by means of the condenser 58 terminating at the grid load resistor 13 which returns to ,ground. The tube ll is of the multi-grid type widely used for mixing purposes. The cathod 79 thereof returns to ground through the conventional biasresistor 80 bypassed by the condenser Bi. 'The screen grid 82 bypassed by the condenser 83 is supplied with suitable potential from the screen-grid voltage top of the battery '5. The second grid, generally referred to as the injector grid 86, is coupled to the grid d of the oscillator tube l of the stage II in the same manner as the control grid d of the oscillator stage G,

namely by means of the coupling condenser 18 and grid load resistor F3.

The anode circuit of the tube ll between the anode as and cathode 19 includes the parallel y and the ground terminal 9 l In the operation of the system, the static frequency of the oscillator tube l and in stage G of tube I in stage H is so chosen as to give a suitable value ofdiiference frequency in the output circuit of the mixer tube which, when further multiplied by means of frequency multiplying stages in accordance with conventional practice, will result in the designated operating frequency for the transmitting system. Therefore, in the mixer tube the output circuit is designed to be resonant to the differencefrequency of the two oscillators, and is tuned to resonance by means of the condenser Bl.

In a practical embodiment of the modulation system herein described, the static crystal frequency for the oscillator tube of stage G was chosen to be 2942.5 kilo cycles and for the oscillator tube of stage H 2557.5 kilocycles. The resultant frequency difference was 385 kilocycles. The mod-- ulation of both oscillators resulted in a linear frequency shift of approximately 500 cycles for each, which, by means of the push pull circuit arrangement in combining the oscillator frequency, doubled to 1000 cycles. In this manner a multiplioation in the order of '250 times was only necessary to operate in the 40 or 50 megacycle band. Itis to be noted here that without the push pull circuit and mixer stage, the useful frequency shift would be only 400 cycles and this would have to be multiplied 500 times to cover the assigned frequency range. This means also that the oscillator static frequency would have to be between to kilocycles. In this frequency range, crystals are also sluggish in operation and are difficult to make.

I claim as my invention:

In a frequency modulationsystem, a vacuum tube oscillator having anode, cathode and control electrodes, a vibratile frequency determining element connected between said anode and control electrode, means for causing frequency deviation of said oscillator in proportion to modulation signal intensities and at a signal frequency rate, said ineans including a second vacuum tube having an anode, a cathode and a plurality of control electrodes, the last mentioned anode and cathode being connected respectively to the corresponding electrodes of said oscillator tube and one of the last-mentioned control electrodes lbeingcapacitively coupled to the control electrode thereof, an input circuit between the other control electrode and cathode of said second vacuum tube, said circuit being energizedwith modulation signals. DONALD P. COLE.

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