US2394393A

US2394393A - Frequency modulation transmitter

Info

Publication number: US2394393A
Application number: US488506A
Authority: US
Inventors: Harry F Mayer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1943-05-26
Filing date: 1943-05-26
Publication date: 1946-02-05
Anticipated expiration: 1963-02-05

Description

Feb. 5, 1946. H F. MAYER 2,394,393

FREQUENCY MODULATION TRANSMITTER Filed May 26, 1943 5 FREQUENCY l6 MULTOISLIER AMPLIFIER POWER QR AMPLIFIER BOTH FREQUENCY 8 MULEIRPLIER H AMPLIFIER 9 7 0R -3= 25; BOTH CRYSTAL 28 Z6, Z5 OSCILLATOR Inventor: Harry F. Mayer,

His Attorney.

Patented Feb. 5, 1946 FREQUENCY MODULATION TRANSMITTER Harry F. Mayer, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May 26, 1943, Serial No. 488,506

3 Claims.

The present invention relates to a freq ency modulation transmitter and is particularly concerned with a system for stabilizing or preventing drift of the mean frequency of the transmitter.

In prior systems it has been customary to apply both signal and frequency control voltages to a reactance tube in the resonant circuit of the transmitter oscillator with the result that the frequency control voltage prevents operation of the reactance tube at the optimum bias for low distortion.

In the present transmitter this disadvantage is overcome by applying the frequency control voltage to the reactance tube of a separate oscillator and mixing the output of this oscillator with a frequency modulated oscillator to produce a heterodyned output which is fed to the transmitting antenna.

An object of my invention is to provide an improved arrangement for stabilizing the mean frequency of a frequency modulation transmitter.

The novel features which I believe to be char-.

acteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying rawing in whichwFig. 1 is a diagram of a frequency modulation transmitter embodying my invention; and Fig. 2 is a diagram of a modification for substitution in the Fig. 1 transmitter.

Referring to Figfl of the drawing there is shown a modulator oscillator I having a resonant circuit 2 in parallel with a reactance tube 3 having a control grid 4 inductively coupled to the resonant circuit so that the voltage applied to the control grid is in quadrature with the voltage applied to the anode 5. The tube 3 draws a reactive current in an amount varying with the bias applied to the control grid from an audio input 6 and thereby causes modulation of the output frequency of the oscillator in accordance with the audio signal. Because the output frequency of the modulator oscillator I is not controlled for frequency drift, no trimmer condenser is necessary in the resonant circuit 2 which permits the maximum L/C ratio for this circuit and the tube 3 can be adjusted for operation at the bias for minimum distortion. The output of the oscillator accordingly has the maximum linear frequency modulation.

Associated with the modulated oscillator I is a similar oscillator I having a resonant circuit 8 in parallel with a reactance tube 9 having a control grid I0 inductively coupled to the resonant circuit 8 to supply a voltage in quadrature with the voltage applied to the anode II. The tube 9 draws a reactive current varying in magnitude with a frequency control voltage applied through a conductor I2 to the grid III. The frequency of the oscillator varies in accordance with the frequency control voltage. After amplification or frequency multiplication or both in apparatus indicated at I3 and I4, the output of the oscillators I and I is fed to a mixer I5, the heterodyned output of which is fed through a power amplifier I6 to transmitting antenna I'I.

While the frequency radiated by the antenna is continually varying in accordance with the the output of a crystal or other frequency stabilized oscillator II! to produce a beat frequency proportional to the deviation of the mean frequency of the transmitter output from the frequency of the crystal oscillator I9. Oscillations of this beat frequency are amplified in an amplifler 20 and fed to a discriminator 2I which produces a frequency control voltage proportional to the deviation or drift of the mean frequency of the transmitter output. The output of the amplifier 20, appearing in a resonant circuit comprising a condenser 22 and an inductance 23, is coupled through a condenser 24 to the midpoint of an inductance 25 in parallel with a condenser 26. The condenser 24 and inductance 25, which comprise the discriminator circuit, are tuned to a frequency proportional to the desired mean transmitter frequency and are designed for maximum frequency stability and carefully temperature compensated. Because there is no mutual inductance between the inductances 23 and 25, the discriminator may be located in any convenient position, for example in a temperature controlled compartment. The quadrature voltage in the discriminator circuit, which in the 'conventional design is supplied by mutual inductance from the output of amplifier 20 is supplied by an unbalancing condenser 21 connected between ground and the lower end of the inductance 25. The current flowing through the condenser 21 induces a voltage in the inductance 25 which is degrees out of phase with the volt- 1 2 tage applied to the inductance when the frej quency of the voltage is exactly equal to the resonant frequency of the discriminator circuit and which has a greater or less phase angle for frequencies above and below the resonant frequency.

The vector sums of the voltages in the inductf ance 25 above and below the midpoint are equal at the frequency for which the disciminator circuit is tuned and are unequal at frequencies above and below that frequency. These voltages are applied to

diodes

28 and 29 connected in opposition across a condenser 30. The difference voltage appearing across the condenser, which varies in magnitude and sign with the deviation or drift of the mean transmitted frequency is fed through If less accuracy is required in the frequency stabilization, the crystal oscillator I9 andmi'xer I8 may be omitted and the transmitted frequency may be fed to the discriminator through an amplifier 3| (Fig. 2) substituted in Fig. 1 in place I of the converter I8.

The automatic frequency control circuit per se utilizing the discriminator apparatus 2| is described and claimed in a divisional application, j Serial No. 597,039, filed June 1, 1945, and asj signed'to the same assignee as plication.

the instant ap- While I have shown particular embodiments of my invention, it will be understood that many 1 modifications may be made without departing s from the spirit thereof, and I contemplate by the appended claims to cover any such modifications I as fall within the true spirit and scope of my in- 1 vention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A frequency modulation transmitter, comprising two oscillators, means for heterodyning cordance with the signal, a discriminator responsive to deviation of the mean frequency, of said carrier from a desired value, and a reactance tube responsive to the discriminator for varying the frequency of said other oscillator in the direction to maintain a constant mean carrier frequency.

2. A frequency modulation transmitter, com-. prising a frequency modulated oscillator, another oscillator having a reactance tube for varying the frequency, means for heterodyning the output of said oscillators'to produce a carrier, and a discriminator responsive to the deviation of the mean frequency of the carrier from apredetermined value for producing a control voltage for controlling said reactance tube so as to maintain aconstant average carrier frequency.

3. A frequency modulation transmitter, comprising a source of high frequency waves subject to slow variations in mean frequency, means for frequency modulating said waves in accordance with a signal, another source of high frequency waves, means for heterodyning the outputs of said sources to produce a carrier, and means responsive to slow variations of the mean frequency Y of the carrier from a predetermined value for changing the frequency of said other source to reduce said deviations. A

HARRY F. MAYER.