Modulators – Frequency modulator – Including stabilization or alternatively distortion – noise...
Patent
1991-04-30
1992-08-18
Pascal, Robert J.
Modulators
Frequency modulator
Including stabilization or alternatively distortion, noise...
332136, 331177V, 307503, 307565, H03C 300, H03K 500
Patent
active
051402874
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the reduction of flicker noise in diode circuits, including varactor diode circuits. It has applications, for example, in diode amplitude-limiting circuits (sometimes termed limiter or clipper circuits), in modulating circuits, in modulation-enhancement circuits, and in varactor diode circuits for varying the frequency of tuned circuits. 2. Discussion of Prior Art
Diode limiters have threshold levels below or beyond which the transfer characteristic between input and output changes relatively suddenly. This threshold level would ideally be defined by a reference voltage but, due to the nature of the required non-linearity, it exhibits flicker-noise effects or, more generally, a higher level of low-frequency noise than would be expected from purely resistive-thermal considerations. Such noise produces unwanted modulation of the output. Varactor diodes do not have a sharp threshold level, but are operated in regions where non-linear effects produce flicker noise; this results in flicker-noise-frequency noise-modulation of any voltage-controlled oscillator (VCO) controlled by the varactor. The present invention provides diode circuits is which this unwanted noise modulation is reduced.
SUMMARY OF THE INVENTION
The present invention consists of a diode circuit characterized in that it comprises:
a diode connected in series with a bias resistor having a by-pass capacitor connected in parallel therewith;
an inverting amplifier having an input which is connected to the side of the diode remote from said bias resistor;
a negative feedback connection from the inverting amplifier output to the junction of said diode and said bias resistor whereby said bias resistor forms the load of said inverting amplifier;
a low-pass filter in the connections between said diode and said inverting amplifier for excluding higher frequencies from the inverting amplifier;
and means for applying a bias voltage across the diode;
whereby the flicker-type noise contributed by the diode when in use is reduced.
The inverting amplifier is preferably a DC differential amplifier having said diode connected to one input thereof and means for applying a control voltage to the other input thereof whereby the control voltage determines the bias voltage on the diode.
An amplitude-limiting circuit according to the present invention comprises a diode circuit as aforesaid having a series input resistor connected to the side of the diode remote from said bias resistor, said input resistor having a resistance which is large compared with that of the series resistance of the diode when conducting plus the resistance of said bias resistor.
A frequency-control circuit according to the present invention comprises a diode circuit as aforesaid wherein said diode is a varactor diode and wherein said low-pass filter is connected between said diode and the input of the amplifier.
A modulating circuit according to the present invention comprises a frequency-control circuit as aforesaid wherein a further low-pass filter is included in the connections between said diode and said amplifier to prevent negative feedback of the modulating frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described, by way of example, with reference to the accompanying drawings wherein:
FIG. 1 is a circuit diagram of a diode limiter embodying the present invention.
FIG. 1a shows a waveform across part of the circuit of FIG. 1.
FIG. 2 is a circuit diagram of a modulation enhancement circuit embodying the present invention.
FIG. 3 is a circuit diagram of a modulator embodying the present invention.
FIG. 4 is a circuit of a varactor circuit embodying the present invention.
DETAILED DISCUSSION OF PREFERRED EMBODIMENTS
In the limiter of FIG. 1, the input is an AC voltage of peak amplitude V+.delta., where .delta. is an amplitude modulation on a sinusoid of peak amplitude V. It is fed from a low-impedance source (from both the AC and DC aspects, not shown), to a resistor R1 c
REFERENCES:
patent: 3569866 (1971-03-01), Hearn
patent: 3671884 (1972-06-01), Denny, Jr.
patent: 4490681 (1984-12-01), Turner
patent: 4503402 (1985-03-01), Englund, Jr.
Hampel et al., "Hochlinearer, breitbandiger Prazisions-Hf-Demodulator", vol. 31, no. 2, Jan. 29, 1982, Munchen DE pp. 57-59, Elektronik.
Turner, Electronics, "Schottky diode pair makes an rf detector stable", vol. 47, no. 9, May 2, 1974, pp. 94-95.
Pascal Robert J.
The Secretary of State for Defence in Her Britannic Majesty's Go
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