US3668572A

US3668572A - Signal generators

Info

Publication number: US3668572A
Application number: US157309A
Authority: US
Inventors: John Alexander Mchattie
Original assignee: Marconi Instruments Ltd
Current assignee: Marconi Instruments Ltd
Priority date: 1970-07-30
Filing date: 1971-06-28
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06
Also published as: DE2138235A1; DE2138235B2; AU451016B2; GB1274417A; AU3137371A

Abstract

A signal generator includes a piston-in-waveguide attenuator coupled to a tuned circuit, a control voltage whose magnitude depends on the position of the piston in the waveguide of the attenuator being produced from a potentiometer and applied to a variable capacity diode in the tuned circuit. Changes in the reactance of the tuned circuit caused by the proximity of the piston are compensated by changes in the capacity of the variable capacity diode, maintaining a constant generated frequency.

Description

United States Patent McHattie June 6, 1972 SIGNAL GENERATORS References Cited [72] Inventor: John Alexander McHattie, Dunstable, En- UNITED STATES PATENTS gland 2,701,863 2/1955 Pierce ..333/81 s [73] Assignee: Marconi Instruments Limited, London, 3,108,239 10/1963 Koueiter ..307/320 X England Primary Examiner-Eli Lieberman [22] June I971 Assistant Examiner-Marvin Nussbaum [21] Appl. No.: 157,309 Attorney-Donald M. Wight et al.

[30] Foreign Application Priority Data [57] ABSTRACT July 30, 1970 Great Britain ..36,872/7O US. Cl. ..333/8l B, 333/24 R, 333/82 B,

333/83 R, 334/15 Int. Cl ..H0lp 1/22, HOlp 7/06, H03j 3/06 Field ot'Search ..334/l5;333/81 A,8l B, 24,

6 Claims, 1 Drawing Figure 20 18 Switch.

78 Switch.

INVENTOR 2% W ms/Jm ATTORNEYS SIGNAL GENERATORS This invention relates to signal generators and more particularly to radio frequency signal generators of the kind employing a so-called piston-in-waveguide attenuator coupled to take output from a resonant circuit, e.g., a resonant cavity or line resonator forming part of the tuned output. circuit of the generator.

A piston-in-waveguide attenuator as used in signal generators consists of a piston carrying pick-up element a pick-up resistor or an inductive pick-up loop mounted in a waveguide. One end of the waveguide is connected to a resonant circuit which is part of the signal generating circuit. The piston is movable within the waveguide, and as the piston is moved closer to the tuned circuit the pick-up element picks up a signal of increasing amplitude. Conversely as the piston is moved away from the resonant circuit the pick-up element picks up a signal of decreasing amplitude. Thus the attenuation of the system is dependent upon the position of the piston in the waveguide.

A piston-in-waveguide attenuator suffers from the disadvantage that as the piston approaches close to the resonant circuit, i.e., when the attenuation of the picked up signal is at a minimum, the proximity of the piston causes a changein the reactive loading of the resonant circuit, hence causing a change in the frequency of oscillation. The frequency change, often known as attenuation reaction, is undesirable and a number of methods are in use to reduce the effect.

One way of reducing the effect is to mount a Faraday Screen of parallel wires across the mouth of the waveguide closest to the resonant circuit. The wires forming the screen must all lie in the same direction, or else the screen would form a short circuit and no electric waves would be propagated in the output waveguide. However, by utilizing a screen of parallel wires so that electric waves are still propagated in the output waveguide the screen is not completely effective and the proximity of the piston still affects the resonance frequency of the resonantcircuit albeit to a lesser extent than if the Faraday Screen were not utilized.

An alternative method of reducing the effect is to utilize a buffer amplifier between the resonant circuit and the output waveguide. This has the disadvantage that the buffer amplifier is a further active element in the system, which may increase the noise level, as well as adding to the costs of manufacture. A further method is to limit the possible travel of the piston in the waveguide so that it cannot approach close enough to the resonant circuit to affect its reactive loading and hence alter its resonance frequency. If this method is used the picked up signal will always be attenuated, since when the piston is as close to the tuned circuit as its limit of travel permits there will be a large minimum loss in the amplitude of the picked up signal. This large minimum loss reduces the available power output of the system.

The present invention seeks to provide an improved signal generator utilizing a piston-in-waveguide attenuator in which the effects of attenuation reaction is reduced by a method not suffering from the disadvantages of the methods outlined above.

According to this invention a signal generator comprises a tuned circuit, a piston-in-waveguide attenuator coupled to said tuned circuit, means for deriving a control voltage dependent upon the position of the piston in the waveguide of the attenuator, and means for utilizing said control voltage to provide a compensating change in reactance to counter change in reactance of the tuned circuit caused by the proximity of said piston to the circuit so as to maintain the output frequency of the signal generator substantially constant.

Preferably said means for deriving a control voltage comprise a potential divider including a potentiometer, the position of the wiper of the potentiometer being determined by the position of the piston in the waveguide of the attenuator.

Preferably the potentiometer has a logarithmic law.

Preferably switch means are provided to render said means for deriving a control voltage inoperative when the piston is greater than a predetermined distance away from the tuned circuit.

Preferably said switch means is controlled by a ramp mounted on said piston which engages with a switch control lever.

Preferably said means for utilizing the control voltage include a variable capacity diode, the control voltage being a biassing voltage applied to said diode so as to vary its capacity.

The invention will now be described with reference to the accompanying drawings which is a diagrammatic representation of one embodiment of the invention.

The FlGURE shows a signal generator having a piston-inwaveguide attenuator.

The tuned circuit of the signal generator includes a variable capacitor 1 and a resonant cavity 2. Coupled to the resonant cavity 2 is a length of waveguide 3. Mounted within the waveguide 3 is a piston 4 provided with a pick-up element, shown as a pick-up resistor 5, at the end of the piston closest to the cavity 2. The resistor is connected to a length of coaxial cable which extends through the piston 4 to an output terminal 7.

The piston 4 is provided with a rack 8 which engages with the teeth 9 of a gear wheel 10. It can be seen that when the gear wheel 10 is rotated in the direction indicated by the arrow the piston 4 will be driven along the waveguide 3 and the pick-up resistor will approach closer to the cavity 2, thereby reducing the attenuation of the output signal produced at output terminal 7.

input signals to the tuned circuit are applied at input terminal 11, together with a negative d.c. biassing potential which biasses a variable capacity diode 12 also forming part of the tuned circuit of the generator and which is connected in series with a capacitor 13 between the input terminal 11 and earth. The potential across the variable capacity diode 12 determines the capacity of the diode, and thus the frequency of the tuned circuit. The input signals applied to terminal 11 can be used to provide frequency modulation or calibrated incremental frequency changes of the output signal from the generator.

As the pick-up resistor 5 is driven towards the cavity 2 the proximity of the piston 4 to the cavity alters the reactive loading of the tuned circuit, thus altering its resonant frequency. This change in resonant frequency is compensated for by applying a biassing potential to the junction of the variable capacity diode 12 and the capacitor 13 such that the frequency of the tuned circuit is maintained at a substantially constant value. This biassing voltage is derived from a negative d.c. potential source 14 by means of a resistor 15 and a potentiometer 16 connected in series between the potential source 14 and earth. The control voltage is derived from the wiper of potentiometer 16, which is directly connected to the junction point of the resistor 15 and the potentiometer 16. The control voltage is applied to the junction point of variable capacity diode l2 and capacitor 13 by cable 17.

Connected across potentiometer 16 is a switch 18. This switch is nonnally closed, but it is arranged to be opened when the piston 4 approaches close enough to the cavity 2 to alter its reactive loading. A suitable means of opening the switch comprises a ramp 19 mounted on the piston which is positioned to engage with a switch control lever 20 when the piston is in a suitable position. As the piston is withdrawn from the cavity the switch control lever 20 will disengage from the ramp 19, closing the switch once more.

The wiper of the potentiometer is driven up the potentiometer resistance element by means of drive wheel 21, which in turn drives gear wheel 10. Since gear wheel 10 determines the positioning of the piston, and since the position of the potentiometer wiper is also determined by the gear wheel 10, via drive wheel 21, the positioning of the potentiometer wiper is directly related to the positioning of the piston.

As the piston 4 approaches the cavity 2 from a large distance the switch 18 is initially closed, thus shorting the resistance element of the potentiometer 16. It can be seen that the cable 17, and thus the junction of the variable capacity diode 12 and the capacitor 13, will be connected to earth. Thus the frequency of the tuned circuit depends solely upon the negative dc. bias applied to the input terminal 1 1.

When the piston 4 is so close to the cavity 2 that it affects the reactive loading of the tuned circuit the ramp 19 engages with the switch control lever 20, thereby opening the switch 18. The wiper of the potentiometer 16 is at the end of the potentiometer resistance element connected to earth, and so the control voltage applied to cable 16 is still earth voltage. As the piston is driven closer to the cavity by the gear wheel the drive wheel 21 rotates, driving the wiper of potentiometer 16 up the resistance element away from the end connected to earth potential. Thus a negative control voltage is applied via the cable 17 to the junction point of the variable capacity diode l2 and the capacitor 13. This control voltage is such that the change of frequency caused by the variation of the capacity of the variable capacity diode cancels out the change of frequency caused by the proximity of the piston to the cavity 2. It has been found that the best results are obtained when the potentiometer 16 has a logarithmic law.

As the piston 4 is withdrawn from the cavity 2 the wiper of the potentiometer is driven back to the end of the resistance element connected to earth, and then the ramp l9 disengages the switch control lever 20, thereby closing the switch 18.

I claim:

1. A signal generator comprising a tuned circuit, a piston-inwaveguide attenuator coupled to said tuned circuit, means for deriving a control voltage dependent upon the position of the piston in the waveguide of the attenuator, and means for utilizing said control voltage to provide a compensating change in reactance to counter change in reactance of the tuned circuit caused by the proximity of said piston to the circuit so as to maintain the output frequency of the signal generator substantially constant.

2. A generator as claimed in claim 1 wherein said means for deriving a control voltage comprise a potential divider including a potentiometer, the position of the wiper of the potentiometer being determined by the position of the piston in the waveguide of the attenuator.

3. A generator as claimed in claim 2 wherein the potentiometer has a logarithmic law.

4. A generator as claimed in claim 1 wherein switch means are provided to render said means for deriving a control voltage inoperative when the piston is greater than a predetermined distance away from the tuned circuit.

5. A generator as claimed in claim 4 wherein said switch means is controlled by a ramp mounted on said piston which engages with a switch control lever.

6. A generator as claimed in claim 1 wherein said means for utilizing the control voltage include a variable capacity diode, the control voltage being a biassing voltage applied to said diode so as to vary its capacity.