Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-08-18
2003-07-15
Trost, William (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S313000, C455S150100, C334S001000
Reexamination Certificate
active
06594477
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electronic tuner for use in high frequency circuit devices, such as a cable television receiver, a television receiver and a video tape recorder.
BACKGROUND OF THE INVENTION
For example, a tuner circuit of a television receiver or the like selects a signal of a desired channel from television signals in the VHF or UHF band received using an antenna, converts it into an intermediate frequency and supplies the resultant signal to a demodulation circuit located in the succeeding stage. Here, the CATV system can transmit tens to a hundred and tens of programs, and a CATV receiver for receiving these programs uses a double conversion-type electronic tuner that is particularly effective for receiving multi-channel programs.
For instance, as shown in
FIG. 18
, in the above-mentioned conventional tuner
101
, a RF (Radio Frequency) signal received by the antenna is input to an up-converter
103
via a band-pass filter
102
a,
a PIN attenuator
102
b
and a RF amplifier
102
c,
and mixed with a first local oscillator signal Lo
1
of a frequency selected according to a received channel by a first mixer circuit
103
of the up-converter
103
. Thus, the RF signal is converted into a first intermediate frequency signal IF
1
of a higher frequency.
Moreover, the first intermediate frequency signal IF
1
is input to a first IF signal amplifying circuit
105
via a band-pass filter
104
. After the first intermediate frequency signal IF
1
is amplified by the first IF signal amplifying circuit
105
, it is applied to a down-converter
107
via a band-pass filter
106
. In the down-converter
107
, the output of the band-pass filter
106
is mixed with a second local oscillator signal Lo
2
in a second mixer circuit
171
as in the up-converter
103
. Hence, the first intermediate frequency signal IF
1
is converted into a second intermediate frequency signal IF
2
of a frequency lower than the RF signal, and output via a band-pass filter
108
and a second IF signal amplifying circuit
109
.
Here, in order to make the level of the second intermediate frequency signal IF
2
output by the electronic tuner
101
constant irrespective of variations in the strength of the received signal, an automatic gain control (AGC) signal is applied to the PIN attenuator
102
b
to control the amount of attenuation. Incidentally, the first (second) local oscillator signal is generated by a first (second) local oscillator circuit
133
(
173
), amplified by a local oscillator signal amplifying circuit
132
(
172
), and then applied to the first (second) mixer circuit
131
(
171
).
The double conversion-type electronic tuner
101
with the above-described structure increases the frequency of the RF signal by the up-converter
103
and decreases the frequency by the down-converter
107
. It is therefore possible to select a desired channel even from a CATV broadcast that transmits multi-channel programs while efficiently removing disturbance.
Here, in the electronic tuner
101
, for example, the circuits following the up-converter
103
are configured as shown in FIG.
19
. Specifically, the first IF signal amplifying circuit
105
includes a bipolar transistor T
151
with a grounded emitter, and the output of the first IF signal amplifying circuit
105
is input to the second mixer circuit
171
of the down-converter
107
via the band-pass filter
106
composed of a double-tuned circuit. Moreover, the second mixer circuit
171
includes a bipolar transistor T
171
with a grounded base, and the second local oscillator signal Lo
2
is input together with the first intermediate frequency signal IF
1
to the emitter of the bipolar transistor T
171
serving as an input terminal and converted into the second intermediate frequency signal IF
2
. The second intermediate frequency signal IF
2
is input to a second IF signal amplifying circuit
109
via a band-pass filter
108
composed of a double-tuned circuit like the band-pass filter
106
.
However, in the above-described conventional structure, in order to ensure disturbance characteristics durable to the input of a greater number of channels because of the digitalization of the CATV broadcast, the power consumed by the electronic tuner is increased and the inner temperature of the electronic tuner rises considerably. Moreover, it is necessary to add a digital signal processing circuit to meet the digitalization of the CATV broadcast, and consequently the heating value of the electronic tuner is increased in comparison with an electronic tuner configuration for receiving only an analog broadcast. As a result, the range of temperature variations in the electronic tuner is increased in comparison with the electronic tuner configuration for receiving only an analog broadcast, causing a problem that the electrical performance such as the gain and distortion characteristics of the electronic tuner deteriorates.
For instance, in the electronic tuner
101
with the configuration shown in
FIG. 19
, the gain in the circuits
151
and
171
including the bipolar transistor T
151
(
171
) is varied with a change in temperature. Therefore, for example, as shown in
FIG. 20
, the level of the second intermediate frequency signal IF
2
output by the electronic tuner
101
is varied within a range of around 24 [dB] to around 35 [dB] in a frequency band from 100 [MHz] to 800 [MHz] at 10° C., 25° C. and 60° C.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electronic tuner of high quality by reducing deterioration of the electrical characteristics of the electronic tuner caused by a rise in temperature, without adding a complicated temperature compensating circuit.
In order to achieve the above object, an electronic tuner of the present invention includes:
an up-converter for converting an input high frequency signal into a first intermediate frequency signal of a frequency higher than the high frequency signal;
a down-converter for converting the first intermediate frequency signal into a second intermediate frequency signal of a frequency lower than the high frequency signal;
a high frequency amplifying circuit, disposed on a signal transmission path between an input of the high frequency signal and an output of the second intermediate frequency signal as an output signal, for amplifying an input signal; and
a temperature compensating section for compensating for deterioration of electrical characteristics caused by a change in temperature.
According to this structure, even when the ambient temperature is changed a lot as in a digital CATV broadcast receiver in comparison with an analog broadcast receiver, it is possible to achieve an electronic tuner that has less deterioration of electrical performance such as the gain and distortion characteristics of the electronic tuner and has a simple circuit structure by providing the temperature compensating section.
It is preferable that the electronic tuner further includes a filter circuit, disposed on the signal transmission path between an input of the high frequency signal and an output of the second intermediate frequency signal as an output signal, for passing frequency components in a predetermined band,
the high frequency circuit includes a field-effect transistor as an amplifying element, and
the temperature compensating section includes:
a thermistor that is disposed in a preceding stage or succeeding stage of the filter circuit and has such a characteristic that resistance is decreased in substantially proportion to a rise in temperature within a predetermined temperature range; and
a serial circuit composed of the thermistor and a coupling capacitor of the filter circuit.
According to this structure, the high frequency amplifying circuit includes the field-effect transistor as an amplifying element, and the temperature compensating section includes the thermistor, which is disposed in the preceding stage or succeeding stage of the filter circuit and has such a characteristic that resistanc
Birch & Stewart Kolasch & Birch, LLP
Sharp Kabushiki Kaisha
Tran Congvan
Trost William
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