US2316927A - Frequency modulation - Google Patents

Description

April zo, 1943.

Filed Sept. 26, 1941 FIG. P1 o cHYs'mL 45 .STABLE FILTER #Erwan/r 12 SEL g V' l L Exc/mm osa.

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Snnentor Gttomeg Patented Apr. 20, l

miam i raEQUENcY Monm'rroN Eamonn s. winnend, to Radio Corpora of Delaware Moorestown, N. J., assigner i onofAmerica, acorpcratlcn application september ze, mi, sum No. 41am 1 calms. (ci. 11s-111.5)

My invention relates to the production of a frequency modulated Signal and particularly to the production of a frequency modulated signal which has a stable carrier frequency or mean frequency.

An object of the invention is to provide an iman improved method of and means for stabilizing the carrier wave oscillator of a frequency modulated transmitter and minimizing frequency Udrift.

In practicing a preferred embodiment of the invention, the carrier wave oscillator is frequency modulated by varying the mutual conductance of reactance tubes in accordance with the modulating signal while, at the same time, frequency drift of the oscillator due to changes in line voltage, etc., is held to a minimum by means of an automatic phase shift control obtained through a feed-back stabilizing circuit.

In one embodiment of the invention a stable self-excitedoscillator is frequency modulated by a pair of reactance tubes so connected that one of them constitutes an inductive reactance and the other a capacitive reactance load on the oscillator tank circuit, the values of reactance varying in accordance with the modulating signal. Thus a frequency modulated signal is produced. The above-mentioned reactance changes are obtained by applying the modulating signal to the grids of the reactance tubes whereby their impedances and, therefore, their reactive current outputs, are changed.

The stabilizing feed-back circuit preferably includes a piezoelectric crystal. The crystal is series resonant at the carrier wave frequency whereby thephase of its voltage output is'either lagging or leading depending upon whether the frequency of oscillation is above or below the said resonant frequency. The connections are such that the radio frequency voltage fed back to the reactance tube grids (the radio frequency voltages on these grids having a fixed relative phase relation of 90 degrees in the example assumed) is shifted in phase with any phase shift in the crystal output. This shift in phase and, in turn, the change in reactance load on'the oscillator, is such as to tend to hold the oscillator frequency constant.

It will 'be apparent that the two above-described frequency control actions, i. e., the stabilizing action and the modulating action, oppose each other. The modulating signal circuit, however. has a stronger control action than does the crystal feed-back circuit whereby the desired modulation of the oscillator signal may be obtained.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a block diagram showing one embodiment of the invention,

Fig. 2 is a vector diagram illustrating the operation of the circuit shown in Fis. 1,

Fig. 3 is a circuit diagram showing. by way of example, a specific circuit for the embodiment of Figli,

Fig. 4 is a circuit diagram illustrating a second embodiment of the invention,

Fig. 5 shows the equivalent circuit for a piezoelectric crystal such as that employed in the above-mentioned embodiments of the invention,

Fig. 6 shows the reactance characteristics of a piezoelectric crystal, and

Fig. 'I is a vector diagram illustrating the operation of the circuit shown in Fig. 4.

Like parts in the several figures are indicated by the same reference characters.

Referring to Fig. 1, there is shown an embodiment of the invention that may be applied to a radio transmitter or the like comprising a stable self-excited oscillator indicated at ill and having a tuned circuit I l.

Reactance tubes i2 and I3 connected in balanced relation with respect to thetuned circuit Ii, and with respect to the input circuit for the modulating signal, are, provided for frequency modulating the oscillator output. As will be explained below, the reactance tube circuit is of the type wherein one tube provides inductive reactance while the other tube provides capacitive reactance.

The radio frequency voltages that are fed back to the control grids Gi and G2 of the tubes i2, and i3, respectively, have a fixed phase relation that is obtained'by means of any suitable phase shifting network Il. This is shown as a network for explicltness. These voltages before being applied to the network I6, however, have been already shifted in phase to some extent such as 45 degrees by means of a suitable phase shifting network I1.

Thus it will be apparent that the radio frequency voltage as it appears on the control grids GI and G2 may be represented by the vector diagram of Fig. 2 where the voltage Ear on grid GI is leading the voltage Eri on the plate PI by 45 degrees and where the voltage Ecs is lagging the voltage En on the plate P2 by 45', the voltages la; and Eos being 90 degrees displaced inphase.

lThe circuit thus far described will operate to frequency modulate the oscillator in accordance with the audio signal or other modulating signal in a well known manner, the modulating signal on the grids GI and G2 4causing the reactive current output of each tube to vary with said signal.

In accordance with the present invention, frequency drift-of the oscillator Il is prevented by means of a phase shifting device, such as a piezoelectric crystal indicated at Il, in the feedback circuit. This feed-back circuit from the oscillator Il may be traced through a conductor, the crystal I I, the 45 degree phase shifting network l1. and the 90 degree phase shifting network Il.

The crystal Il, the equivalent circuit of which is shown in Fig. 5, is series resonant at the carrier wave frequency or mid-frequency ofthe oscillator output (frequency fz in Fig. 6) whereby it is an inductive impedance element when the frequency of oscillation shifts to one 4side of the mid-frequency and a capacitive impedance element when the frequency shift is tothe other side of the mid-frequency.

As a result, the crystal output will shift the phase of the radio frequency grid voltages Em and Een when there is a shift in the oscillator frequency, and this phase shift is in the direction to oppose any change in oscillator frequency. As previously noted, the modulating signal "over rides this degenerative stabilizing action during the circuit operation. Otherwise the modulating signal could not change the oscillator frequency.

The above-described phase shift produced by the feed-back circuit is shown in Fig. 2 where the dotted vectors OA and B represent 'the vectors Ear and Een, respectively, shifted in phase. It will be noted that the capacitively reactive component of the voltage Em has been reduced from the value EclD to the value BC while the inductively reactive component of the voltage Ec: has been increased from the value EczF to the `value AE. Such a phase shift is caused when the oscillator frequency becomes lower than the mid-frequency (frequency fz in Fig. 6) whereby the crystal presents capacitive impedance as shown by the curve 25 in Fig. 6.

It will be apparent that the above-described -phase shift has decreased the capacitive current in the oscillator tuned circuit Il and has increased the inductive current in the same circuit whereby the feed-back circuit tends to raise the oscillator frequency towards the original midfrequency value. Thus, the oscillator is stabilized so that changes in line voltage, humidity, temperature, etc. have very little effect upon its frequency of oscillation.

A specific circuit for the embodiment ofFig. 1 is shown in Fig. 3 merely by way of example.

In Fig. 3 the radio frequency signal is fed back Yfrom. the plate Pl of the tube I2 through the piezoelectric crystal Il and through a 45 degree phase shift network i1 comprising a condenser 28 and a resistor 21, both preferably adjustable foradJusting the phase shift. Since a by-pass condenser 2l holds the lower end of resistor 21 at ground potential for R. F. voltages, the voltage Em on the grid GI is the voltage across resistor 21. 'I'he R. F. voltage is next fed to the grid G2 through a 90 degree phase shift network Il com- VEm leads the voltage Ea:

asuma? ance coil I2 that is shunted by a resistor Il. the units Il. 22 and 22 preferably being variable for adjusting the phase.

The lower end of the LR combination=22ll is held at ground potential for the R. F. voltages by means of a by-pass condenser 24. Bince the grid G2 of the tube I2 is connected to the Junction point of the condenser Il and the coil I2, the-R. F. voltage Ecs on grid'G2 is the voltage across the LR. combination 32-22. This voltage by 90 degrees in the speciilcexample assumed.

The Yabove-described phase relations are as illustrated in Fig. 2 and as described in connection with Fig. 1. Y

'Ihe audio-,signal or other modulating signal 1s applied through a transformer 2l to the grids GI and G2 in balanced or push-pullA relation. the center point of the transformer secondary being connected through a suitable biasing source such as a by-passed cathode resistor 2l and to the cathodes ofthe tubes I2 and-I2.

Fig. 4 shows another specific embodiment of the invention wherein the plates of the tubes I2 prising a condenser 3| in series with an induct- '75 and i2 are connected in parallel. The vector diagram for this circuit is shown in Fig. 7.

In Fig. 4 the input circuit of the tubes i2 and il -includes a tuned circuit 4I tuned to the carrier wave frequency and comprising inductance' coils I2 and 42 and a tuning condenser 44. The

midpoint of circuit 4I is connected to ground Ithrough by-pass condensers 40 and 41.

The modulating voltage is applied Ato the tubes in push-pull relation as in Fig. 3.

The R. F. voltage is fed back from the plates Pi and P2 through a piemelectric crystal 48 which may be a two-terminal unit as in Fig.: 3

or a four-terminal unit as illustrated. `As in Fig. 3, the crystal preferably resonates at the carrier wave frequency. From the crystal 4l the R. F. voltage passes through a 45 degree phase shift network comprising a condenser 49 and a resistor 5| putting a voltage Enron the grid Gl leading the voltage En on the tube plates by 45 degrees in the example illustrated.

The R. F. voltage at the other end of tuned circuit Ii is 180 degrees displaced from Ecu. It is shifted an additional degrees by a network comprising an inductance coil l2 `in series with a condenser 53 shunted by a resistor 54. The voltage appearing across the RC combination 54-53 is applied to the grid G2. This voltage shown as Ec: in Fig. 7 is now 45 degrees lagging the plate voltage Em.

It will be apparent that the operation of the circuit of Fig. 4 is the same as previously described in connection with Fig. l.

It may be noted that the piezoelectric crystal in the feedback circuit preferably is operated over a substantially linear portion of its phase shift characteristic `in order to minimize distortion of the transmitted signal.

While I have referred specifically to 45 degree and 90 degree phase shifting networks, it should be understood that other amounts of phase shift may be employed. Just by way of example, the first phase shift may be l0 degrees and the second phase shift degrees, but this, would require more audio swing. I claim as my invention :V

l. In combination, an oscillator, a reactance tube circuit coupled to said oscillatorto control its frequency, said circuit including a vacuum tube and means for feeding back a high frequency voltage from said oscillator to said tube and for also shifting the phase of said voltage during' said feed-back by an amount which is effectively less than ninety degrees plus or minus, means for applying a modulating voltage to said tube for frequency modulating the output of said oscillator, and a phase shifting unit in said feedback means for shifting the phase of the high frequency voltage fed back in response to a change in the frequency thereof and in such direction as to tend to hold constant the frequency of said oscillator output.

2. In combination, an oscillator, a reactance tube having its plate circuit connected to said oscillator to control its frequency, said tube having an input circuit, means for feeding back a high frequency voltage from said oscillator to the input circuit of said reactance tube and for also shifting its phase duringpsaid feed-back by an amount which is effectively less than ninety degrees plus or minus, means for applying a modulating voltage to the input circuit'of said reactance tube for frequency modulating the output of said oscillator, and a phase shifting unit in said feed-back means for shifting the phase of the high frequency voltage fed back in response to a change in the frequency thereof and in such direction as to tend to hold constant thc frequency of said oscillator output.

3.1In combination, an oscillator, a reactance tube having its plate circuit connected to said oscillator to control its frequency, said tube having an input circuit, means for feeding back a high frequency voltage from said oscillator to the input circuit of said reactance tube and for also shifting its phase during said feed-back by an amount which is effectively less than ninety degreen plus or minus, means for applying a modulating voltage to the input circuit of said reactance tube for frequency modulatina the output of said oscillator, and b piezoelectric crystal in said feed-back means for shifting the phase.

of the high frequency voltage fed back in response to a change in the frequency thereof and in such direction as to tend to hold constant the frequency of said oscillator output.

4. In combination, a stable self-excited oscillator,

to said oscillator to control its frequency, said tube having an input circuit, means for feeding back a high frequency voltage from said oscillator to the input circuit of said re- .actance tube and for also shifting its phase dur-y ing said feed-back by an amount which is effectively less than ninety degrees plus or minus, means for applying a modulating voltage to the input circuit of said reactance tube for frequency modulating the output of said oscillator, and a pieaoelect crystal in said feed-back means for shifting the phase of the high frequency voltage fedbackinresponseto achangeinthe frequency tbeseofandinnichdirectionastotendtohold constant the frequency of said oscillator output,

a reactance tube having its plate circuit' said crystal being series resonant at substantially the mid-frequency of said frequency modulated output.

5. In combination, a stable self-excited oscillator, a palrof reactance tubes having plate circuits connected to said oscillator to control its frequency, said tubes each having input circuits, means for feeding back a high frequency voltage from said oscillator to the input circuits of said reactance tubes in such phase relation that one of said tubes simulates inductive reactance while the other tube simulates capacitive reactance, means for so applying a modulating voltage to said reactance tubes that the output of said oscillator is frequency modulated in accordance therewith, and a phase shifting unit in said feedback means for shifting the phase of said high frequency voltage at the input circuits of said tubes in response to a change in the frequency thereof and in such direction as to tend to stabilize the frequency of said oscillator output.

6. In combination, a stable self-excited oscillator, a pair of reactance tubes having their plate circuits connected to said oscillator to control its frequency, said tubes each having an input circuit, means for feeding back a high frequency voltage from said oscillator to the input circuits of said reactance tubes injsuch phase relation that one of said tubes simulates inductive reactance while the other tube simulates capacitive reactance, means for so applying a modulating voltage to said reactance tubes that the output of said oscillator is frequency modulated in accordance therewith, and a piezoelectric crystal in said feedback means for shifting the phase of the said high frequency voltage at the input circuits of said tubes in response to a change in the frequency thereof and in such direction as to tend to stabilize the frequency of said oscillator output.

7. In combination, a stable self-excited oscillator, a pair of reactance tubes having their plate circuits connected to said oscillator to control its frequency, said tubes each having an input circuit, means for feeding back a high frequency voltage from said oscillator to the input circuits of said reactance tubes in such phase relation that one of said tubes introduces inductive reactance while the other tube introduces capacitive reactance, means for so applying a modulating voltage to said reactance tubes that the output of said oscillator is frequency modulated in accordance therewith, and a piezoelectric crystal in said feedback means for shifting the phase of said high frequency voltage at the input circuits of said tubes in response to a change in the frequency thereof and in such direction as to tend to stabilize the frequency of said oscillator output, said crystal being series resonant at the mld-frequency of said frequency modulated out- Put.

IDHOND B. WINLUND.

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