Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Compensation for variations in external physical values
Reexamination Certificate
1999-05-11
2001-04-17
Nuton, My-Trang (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Compensation for variations in external physical values
C327S558000, C327S357000
Reexamination Certificate
active
06218885
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to FM receivers and more particularly relates to a circuit for providing temperature stability of an FM quadrature detector.
BACKGROUND OF THE INVENTION
Typical FM receivers, such as the receiver
10
shown in
FIG. 1
, include a quadrature detector
12
to demodulate frequency shift keyed (FSK) signals and other types of FM signals. As is well known in the art, a quadrature detector
12
, as shown in
FIG. 2
, accepts an input signal on line
14
into a mixer
16
. The input signal is also input to an LC circuit, comprising capacitor
18
and inductor
20
, and which may also include a resistor
21
. Inductor
20
is typically a tunable inductor. The LC circuit feeds an input to the mixer on line
22
, which is phase shifted 90 degrees from the input signal on line
14
. The mixer generates an output signal on line
24
, which is then typically input to a low pass filter
26
, which generates an output signal on line
28
.
The LC circuit of the quadrature detector has a resonant frequency that is adjusted to match the center frequency of the Intermediate Frequency (IF) of the receiver. For purposes of the description herein, an IF of 10.7 MHz, which is a standard intermediate frequency for an FM receiver, will be used as an example. For an IF of 10.7 MHz, the values of L and C are set as shown in
FIG. 2
so that the resonant frequency of the LC circuit is 10.7 MHz. As is well known in the art, the resonant frequency of an LC circuit is determined by the following equation:
f
res
=
1
2
π
LC
where L is the inductance of the circuit in henries, and C is the capacitance of the circuit in farads.
However, if the temperature of the LC circuit changes, the value of the inductance L of inductor
20
and the value of the capacitance C of capacitor
18
may vary, thereby causing the resonant frequency of the LC circuit to shift. Typically, when the temperature increases, the resonant frequency decreases, and vice versa. When the resonant frequency shifts from the center frequency, then the phase shift of the signal on line
22
is not 90 degrees, but shifts between zero and 180 degrees. If the resonant frequency shifts in the quadrature detector, then an undesirable de voltage is introduced on output line
24
and output line
28
, in a manner well known to those skilled in the art.
FIG. 4
shows the effect of the dc offset causing distortion on the output signal.
Therefore, a need exists in the art for a circuit that provides automatic temperature stability for FM quadrature detectors so that the resonant frequency does not shift, thereby ensuring that the quadrature detector operates in the middle of its linear region.
SUMMARY OF THE INVENTION
The present invention includes a circuit and method for providing temperature stability in an FM quadrature detector. The circuit includes a feedback branch that feeds a portion of the dc output voltage to a varactor diode that is connected in parallel with a capacitor of an LC circuit in the quadrature detector. When the ambient temperature of the LC circuit of the quadrature detector changes, the resonant frequency shifts from the desired center value and a dc voltage is introduced at the output of the quadrature detector. The dc voltage is input to the varactor diode via the feedback circuit branch, and the capacitance of the varactor diode, which is dependent on the dc voltage applied to it, causes the overall capacitance of the LC circuit to change. The change in overall capacitance of the LC circuit caused by the capacitance of the varactor diode causes the resonant frequency of the quadrature detector to shift to be more closely maintained at the desired center frequency. In this manner, unwanted distortion caused by a change in the dc voltage, due to temperature, is reduced or eliminated.
REFERENCES:
patent: 4633316 (1986-12-01), Burke et al.
patent: 4662001 (1987-04-01), Cruz et al.
patent: 4667170 (1987-05-01), Lofgren et al.
patent: 5708394 (1998-01-01), Karlquist
patent: 5821818 (1998-10-01), Idei et al.
Antec Corporation
Boss Gerald R.
Nuton My-Trang
Troutman Sanders LLP
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