Variable frequency oscillator circuit

Details Variable frequency oscillator circuit Variable frequency oscillator circuit

1999-10-04

2001-07-03

Kinkead, Arnold (Department: 2817)

Oscillators

With frequency adjusting means

Variable inductance device

C331S03600C, C331S17700V, C331S03600C, C334S014000, C334S015000, C334S047000

Reexamination Certificate

active

06255913

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to variable frequency oscillators (VFOs) and, more particularly, to a VFO having an increased tunable frequency range.
2. Description of the Related Art
Currently employed variable frequency oscillators (VFOs) are manufactured to achieve a desired target frequency (F) and provide for a limited tuning range relative to the circuit operating frequency to accommodate for shifts caused during manufacture or as required for operation. This tuning range allows the operating frequency to be adjusted downward from an actual frequency value to a value closely proximate the target frequency. Such VFOs include a tank circuit and an active device, such as a transistor, connected to a control box for adjusting the VFO frequency.
A known prior art VFO circuit
5
is shown in
FIG. 1
having a tank circuit
10
connected to an active device
20
. The tank
10
employs an LC oscillator circuit having a tank inductor L with a fixed inductance connected in parallel with a tank capacitor C having a fixed capacitance. A varactor having a variable capacitance value is provided for tuning the VFO frequency by adjusting the value of the varactor capacitance. Since the varactor is connected in parallel with tank capacitor C, the overall capacitance of the VFO circuit will be increased by a maximum value of the maximum varacator capacitance (&Dgr;C), thereby reducing the frequency F of the tank
10
by a maximum amount (&Dgr;f). In addition, a switchable capacitor C′ is connected in parallel to the load capacitor C by a switch S
1
for providing a stepped increase of the overall capacitance of the tank
10
and, therefore, a stepped decrease (&Dgr;f′) in the resulting frequency. For example, when switch S
1
is closed, thereby incorporating capacitor C′ into tank circuit
10
, the overall capacitance of the circuit will be C+C′+&Dgr;C, resulting in a frequency of F−&Dgr;f′−&Dgr;f. As such, currently employed VFO circuits, such as the circuit
5
of
FIG. 1
, are limited in that they only provide for the VFO frequency to be reduced downward from a set frequency, as opposed to also or alternatively being increased above the set frequency, thereby limiting the versatility and tuning range of these known VFO circuits. Although it may be possible, to an extent, to over-design tank circuits having an increased maximum frequency with an increased downward tuning range—in an attempt to avoid the need for an upward tuning range—such an alternative is limited. This is because the incorporation of multiple switches S
1
for the required multiple switchable capacitors needed to increase the downward tuning range add detrimental parasitic capacitance to the tank.
SUMMARY OF THE INVENTION
The present invention is directed to a variable frequency oscillator circuit having an increased tunable frequency range. The inventive VFO circuit includes a parallel connected load capacitor and load inductor having capacitance and inductance values, respectively, which determine the VFO set frequency F. A varactor is provided having a variable capacitance &Dgr;C for decreasing the operating frequency F by an amount &Dgr;f. A secondary inductor selectively affects the load inductor for reducing the overall inductance of the VFO circuit for providing a stepped increase of the operating frequency of the VFO above the set frequency. In this manner, the tunable frequency range of the VFO circuit includes attainable frequencies both below and above the set frequency F.
In a preferred embodiment, a selectively connectable fixed capacitor is included for providing a stepped decrease of the operating frequency when the fixed capacitor is connected to the VFO circuit.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.


REFERENCES:
patent: 2143693 (1985-02-01), None

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