Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Reexamination Certificate
2000-04-21
2002-07-23
To, Doris H. (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
C455S191100, C455S178100, C455S195100, C455S340000, C348S733000
Reexamination Certificate
active
06424824
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an apparatus for setting the tuning voltage in tunable resonant circuits, particularly in radio receivers.
In radio receivers, frequency-determining components of resonant circuits and other frequency-selective circuit configurations are tuned to a desired frequency or to a desired frequency range.
In integrated radio receivers, the resonant circuits are set by varying the bias voltage of variable-capacitance diodes or varactor diodes, whose capacitance decreases as the tuning voltage increases. In contrast to discrete components, such as tuning capacitors, such variable-capacitance diodes have the advantage that their Technology can be integrated on a semiconductor chip during manufacture, and they are therefore more economical to manufacture and, at the same time, miniaturization of the receiver is made easier.
In radio receivers, various programs can be received, amplified and reproduced on different frequency bands. Tuning circuits are used to set a desired frequency in order to ensure reception of a particular received signal transmitted on this frequency. To this end, an oscillator circuit is used to set an oscillator frequency, which is shifted by a fixed, predetermined intermediate frequency to produce the desired reception frequency, and is supplied to a mixer. The oscillator frequency and the prefiltered received signal, which is also supplied to the mixer, are used to produce the intermediate frequency signal. The mixer therefore has preliminary and intermediate resonant circuits connected upstream of it, which are used as frequency filters for the reception frequencies.
Tuning of the resonant circuits is not carried out manually, as was previously customary, but instead, in more recent receivers based on the prior art, is performed under electronic control.
A conventional receiver whose resonant circuits are tuned using an electronic control is shown in FIG.
1
. For this purpose, the receiver has an antenna A which receives a radio signal and outputs it via a line to a first resonant circuit, the so-called preliminary circuit. The received signal is filtered by the preliminary circuit VK on the basis of the reception frequency and is then output to an amplifier V. The amplifier V amplifies the filtered received signal, which is again filtered on the basis of the reception frequency by a second, downstream resonant circuit, the so-called intermediate circuit ZK. The received signal filtered by the preliminary circuit VK and the intermediate circuit ZK is passed on to a mixing device M which filters the filtered signals onto a desired frequency range as a result of a voltage-controlled oscillator resonant circuit VCO setting an oscillator resonant frequency equivalent to the desired reception frequency. The desired intermediate frequency ZF at the output of the mixer M is 10.7 MHz, for example. The intermediate frequency ZF is given as the difference between the reception frequency f
E
and the oscillator resonant frequency f
VCO
.
f
ZF
=f
VCO
−f
E
In a typical FM receiver, the reception frequency f
E
is in the range between 87.5 MHz and 108 MHz. Accordingly, the oscillator resonant frequency of the voltage-controlled oscillator VCO is between 98.2 MHz and 118.7 MHz, that is to say increased by the intermediate frequency f
ZF
of 10.7 MHz.
The oscillator resonant frequency f
VCO
is set through the use of an oscillator tuning voltage V
T
which can be regulated.
The output signal from the voltage-controlled oscillator VCO is supplied via a feedback line to a phase locked loop PLL, which produces the oscillator tuning voltage V
T
. As the oscillator tuning voltage V
T
rises, the oscillator resonant frequency f
VCO
increases, as can be seen from the graph in FIG.
2
. The frequency spacing &Dgr;f between the resonant frequency f
VCO
of the oscillator resonant circuit VCO and the reception frequency f
E
is ideally exactly the same size as the intermediate frequency f
ZF
, for example 10.7 MHz. Ideally, the two curves f
VCO
and f
E
run parallel over the whole frequency range, i.e. the resonant circuits VK and ZK should ideally be set so that the frequency curve f
E
always runs parallel to the oscillator resonant frequency f
VCO
, offset by the intermediate frequency f
ZF
. However, theoretical considerations and component tolerances mean that such an ideal parallel curve shape, which is also called ideal synchronism, cannot be achieved.
In known receivers, the tuning circuits are iteratively adjusted in an attempt to approximate to ideal synchronism S by calculating linear coefficients for amplifying the oscillator tuning voltage V
T
.
For this purpose, the oscillator tuning voltage V
T
is supplied to a first linear amplifier circuit V
1
and to a second linear amplifier circuit V
2
for the purpose of tuning the preliminary circuit VK and the intermediate circuit ZK.
In this case, the tuning voltage V
TVK
for the preliminary circuit is produced on the basis of the following equation:
V
TVK
=Y
1
·V
T
+X
1
The tuning voltage V
TZK
for the intermediate circuit is calculated as follows:
V
TZK
=Y
2
·V
T
+X
2
The multiplication coefficient Y and the addition coefficient X are determined and stored once, during manufacture or when turning on the receiver, as a result of a maximum adjustment of the output voltage of the mixer.
FIG. 3
shows the capacitance curve for a variable-capacitance diode in a tunable resonant circuit as a function of the applied tuning voltage V
T
. The variable-capacitance diode or variable-capacitance varactor diode is a reverse-biased semiconductor diode having a hyperabrupt pn-junction or a metal-semiconductor junction, wherein the voltage dependency of the depletion-layer capacitance is utilized. As can be seen from
FIG. 3
, the capacitance of the varactor diode decreases nonlinearly as the tuning voltage increases. The variable-capacitance varactor diode is more sensitive at a low tuning voltage V
T
than at a high tuning voltage. With a voltage change &Dgr;U, the change in capacitance &Dgr;C
1
is larger than the capacitance change &Dgr;C
2
at a higher tuning voltage.
In conventional setting apparatuses, the tuning voltage for the preliminary circuit VK, for example, is linearly dependent on the tuning voltage V
T
.
FIG. 4
shows the dependency of the capacitance of the varactor diode on the oscillator tuning voltage V
T
. As can be seen from the bottom graph in
FIG. 4
, the tuning voltage V
TVK
, produced by the amplifier setting device V
1
, for the preliminary circuit VK falls linearly as the tuning voltage V
T
increases, so that a voltage change &Dgr;V
T1
results in a capacitance change &Dgr;C
1
, and a voltage change &Dgr;V
T2
results in a capacitance change &Dgr;C
2
. If the voltage change &Dgr;V
T2
is the same as the voltage change &Dgr;V
T1
,
FIG. 4
shows that the capacitance change &Dgr;C
1
at a high tuning voltage V
T
is significantly larger than the capacitance change &Dgr;C
2
at a low tuning voltage V
T
. Since the tuning voltage V
T
is set digitally by the microprocessor &mgr;P, the smallest voltage change &Dgr;V
T
is equivalent to one bit. As
FIG. 4
shows, the change in the microprocessor's control signal by the smallest unit, i.e. by one bit, produces different capacitance changes, and hence frequency changes, in the tuning circuits, depending on what point is taken on the linear amplifier curve. In the linear tuning method shown in
FIG. 4
, the nonlinear capacitance curve for the varactor diode results in falsifications, distortions or corruptions, because the signal resolutions of the control signal are constant over the whole amplification range.
The nonlinearity of the tuning component within the tunable resonant circuit therefore produces corruptions when tuning the resonant circuits, which impairs synchronism.
This problem exists in all resonant circuits adjusted by a tuning component whose setting variable has a nonlinear curve.
SUMMARY OF THE INVENTION
It is accordingly an object
Kröbel Hans-Eberhard
Stepp Richard
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