US2166522A

US2166522A - Frequency converter

Info

Publication number: US2166522A
Application number: US196124A
Authority: US
Inventors: John F Farrington
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1938-03-16
Filing date: 1938-03-16
Publication date: 1939-07-18
Anticipated expiration: 1956-07-18

Description

J. F.V FARRxNGToN FREQUENCY CONVERTER Filed March 16, 1958 INVENTOR ATTORNEY July 18, 1939.

y F. FARRINGT 'N B Patented July 18, 1939 UNITED STATES PATENT OFFICE FREQUENCY CONVERTER Wale Application March '16, 1938, Serial No. 196,124

7 Claims.

This invention relates generally to frequency converters, and particularly to frequency converters adapted to be utilized in molulated-carrier signal receivers of the superheterodyne type.

In frequency converters of superheterodyne receivers, there is provided a local oscillator with a frequency-determining circuit for generating oscillations which are modulated with a received signal to produce an intermediate-frequency signal. There is a tendency for the modulator to affect the frequency of the local oscillations. This variation of the oscillator frequency is due to the fact that the modulator is usually coupled relatively closely to the frequency-determining circuit of the oscillator so that changes in the impedance of the modulator grid circuit to which the oscillator is coupled affect the natural resonant frequency of the frequency-determining circuit. Suoh changes in impedance may be caused by variations in the supply potentials or A. V. C.

potential applied to the modulator. Various attempts have been made to eliminate this difllculty. One such arrangement utilizes a common cathode resistor for coupling the oscillator section and the modulator section of a frequency converter.

With this arrangement the effect of variations in impedance or trans-conductance of the modulator section of the frequency converter upon the requency of the oscillator is materially reduced. However, when this circuit is utilized in a superheterodyne receiver, the eflciency of the converter is reduced by degeneration due to the cathode resistor which decreases the signal amplitude applied to the input electrodes of the modulator and which also applies to the input electrodes intermediate-frequency Wavesin such phase as to reduce the intermediate-frequency output.

It is an object of the present invention, therefore, to provide a frequency changer suitable for 4 use in superheterodyne receivers in which the oscillation frequency is substantially independent of the impedance and transconductance char-- acteristics of the modulator.

It is another object of the invention to provide l a frequency converter suitable for use in superheterodyne receivers in which the modulator is not substantially coupled with the frequency-determining circuit of the oscillator.

In accordance with preferred embodiments of -n the invention, there is provided a frequency converter including an oscillator section and a modulator section, each section having a space current path and input and output electrodes. A resistor is included in the space current path of the oscillator section across which are developedy (CL o-20) oscillation voltages which are coupled to the modulator section in a conventional manner. In one embodiment. separate cathodes are provided for the two space current paths, and the resistor is included in the cathode-ground circuit of the oscillator section. In any embodiment, the resistor or impedance across which the oscillation voltage is developed is not substantially coupled to the frequency-determining circuit of the oscillator so that changes in the operating potentials applied to the modulator, which cause variations in the impedance of the modulator grid circuit to which the oscillator is coupled, do not appreciably affect the frequency of the oscillator frequency-determining circuit,

Referring nowfto the drawing, Figs. 1 and 2 are circuit diagrams, partlyschematic, of complete superheterodyne receivers embodying different forms of the invention in the frequency converter or first detector of the receiver.

Referring now more particularly to Fig. 1, there is shown schematically a complete superheterodyne radio receiver embodying the present invention in a preferred form. In general, the receiver includes a. radio-frequency amplifier Ill having its input circuit connected to an antenna I I and ground I2 and its output circuit connected to a frequency changer or oscillator-modulator I3. Connected in cascade with the frequency changer I3, in the order named, are an intermediate-frequency amplifier Il of one or more stages, va detector and automatic amplification control supply I5, an audio-frequency,A amplifier I6 of one or more stages, and a sound reproducer I1. An automatic amplification control bias, derived from A. V. C. supply l5, is applied to the grids of one or more of the tubes of amplifier I0, oscillator-modulator I3, and one or more of the grids of intermediate-frequency amplifier I4 in order to maintain the signal input to detector I5 within a relatively narrow range for a wide range of received signal amplitudes. It will be understood that the various circuits just described, with the exception of frequency-converter circuit I3, may be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. n

Considering briefly the operation of v the receiver as a whole and neglecting for the moment the operation of frequency converter I3, per se, presently to be described, a desired modulated signal is selected and amplified by radio-frequency amplifier Il, converted to a modulated intermediate-frequency signal in frequency chang- REISSUED serves to maintain the volume output of the receiver within a relatively narrow range for a wide range of received signal amplitudes. Y

Referring now more particularly `to the parts of the system involving the present invention, frequency converter I3 comprises a Amodulator section 20 and an oscillator section 2|. The modulator section 20 comprises a mixer or modulator vacuum tube 22 having an input circuit coupled to radio-frequency amplifier I0 and an output circuit coupled to intermediate-frequency amplifier |4. The oscillator section 2| comprises a vacuum tube 23; a frequency-determining circuit connected to the grid circuit of tube 23 and including a variable tuning condenser 24 and an inductance 25; and an oscillationfeed-backcircuit comprising inductance 26 coupled to inductance 25 and connected to the anode of tube 23 through a blocking condenser 21. The tube 23 includes an un-bypassed resistor 28 in its cathode-ground circuit, Resistor 28 and condenser 29 serve to couple oscillator 2| to a second input or injector grid of tube 22. Suitable operating potentials are provided for tubes 22 and 23 from sources indicated on the drawing as -`C, -i-Sc. and +B.-

In considering the operation of the circuit just described, it will be seen that the oscillator section 2| is a conventional one, generating local oscillations at the resonant frequency of the frequency-determining circuit 24, 25, The variations in the plate current of tube 23 generate voltage waves of oscillator frequency across resistor 28 These waves are applied through condenser 28 to theinjector grid of modulator tube 22 and they modulate the received signals at this frequency. It will be seen that resistor 28 is not substantially coupled with the oscillator frequency-determining circuit 24, 25. Furthermore, the value of resistor 28 can be made low as compared to the impedance of the injector grid circuit of tube 22, which is effectively in parallel with it. Consequently, variations in the impedance of the injector grid circuit, caused by changes in the operating potentials applied to the modulator tube 22, do not materially affect the frequency of the oscillator frequency-determining circuit 24, 25, or the amplitude of the oscillator voltage applied to the injector grid.

As illustrative of a specific applicationvutilizing the circuit of Fig. 1, the following circuit constants are given:

Tube 22 'Iype 6L? Tube 23 Type 6C5 Resistor 28 ohms-.. 1,000 Resistor 28 ohms 50,000

In Fig. 2 is shown a receiver embodying another form of the invention in which the space current pat-hs of the oscillator and modulator sections are included in a common envelope, comprising a to the output circuit of radio-frequency amplier Il and the output circuit of the modulator section is coupled to the input circuit of intermediate-frequency amplifier |4. 'Ihe oscillator circuit is similar to Fig. 1 except that resistor 2l is omitted from the cathode circuit and replaced by

resistors

45 and 46 in the space current path of the oscillator section of tube 4l. Resistor 4l and condenser 4l constitute the means for coupling the oscillator to the injector grid of the modulator section of vacuum tube 4l.

The operation of the circuit of Fig. 2 will be apparent from the description given above with respect to .the operation of the circuit of Fig. 1. It is seen that oscillation voltages are developed across resistor 45 due to variations in the space current of the modulator section. Resistor 4l is not substantially coupled to the frequency-determining circuit due to the decoupling resistor 4l, Therefore, variations in the impedance of the modulator section of vacuum tube 4I or the transconductance thereof do not materially aect the resonant frequency of frequency-determining circuit 24, 25.

It will be understood that the coupling circuits, comprising elements 23, 2l', and 2l or 4l, 4l', and 41, may be designed to have a frequencyresponse characteristic which varies over the operating range of the oscillator in a complementary manner to the voltage developed across the output impedance in the space current path of the oscillator, thereby to maintain the voltage supplied to the modulator section from the oscillator section substantially constant.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modiflcations may be madetherein without departing from the invention, and it is, therefore, aimed xn the appended claims to cover all such 'changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said impedance to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequencydetermining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

2. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes., a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, a resistor included in the space current path of said oscillator section and substantially uncoupled with said frequencydetermining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said resistor to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably aifected by variations in the impedance of the circuit of said second electrode.

3. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit included in the input circuit of said -oscillator section, impedance means included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said impedance to said second input electrode to modulate the signal input to said modulator section,

whereby the frequency of said frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

4. A frequency converter comprising a modulator section and an oscillator section, each of saidsections including a space current path comprising input and output electrodes and separate cathodes, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, an

unbypassed resistor included in the cathode circuit of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said resistor to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said A frequency-determining circuit is not appreciably .in the space current path individual tosaid oscillator section and substantially uncoupled with said frequency-determining circuit. a second input electrode in the space current path of said modulator section, and means for coupling arl` oscillation voltage developed across said impedance to said second input electrode to modulate the signal input to said modulator section, whereby ther resonant frequency (of said frequencyoscillator section, a rst resistor included in the 'lator section and an oscillatowction, each of said said sections including a space current path comprising input and output electrodes, a signal-input circuit coupled to the modulatorinput electrode, a tunable frequency-determining circuit coupled to said oscillator section, a second input electrode in the space current path of said modulator section, impedance means included in the space current path of said oscillator section and coupled to said second input electrode for coupling a voltage to said second electrode to modulate the input signal to said modulator section, said impedance means being substantially uncoupled with said frequency-determining circuit, whereby the resonant frequency of said frequency-determining circuit is not substantially aected by variations inthe impedance of the circuit of said second electrode, said impedance and-its coupling to said second input electrode being so proportioned that the voltage coupled to said second input electrode from said impedance means is substantially constant over the tuning range of said frequency-determining circuit.

JOHN F. FARRINGTON.