Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Clock or pulse waveform generating
Reexamination Certificate
2000-06-16
2001-12-11
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Clock or pulse waveform generating
C327S099000, C331S018000
Reexamination Certificate
active
06329862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reference frequency signal switching circuit and, more particularly, to a reference frequency signal switching circuit capable of selectively outputting an internal reference frequency signal generated by an internal oscillator or an external reference frequency signal supplied from the outside and maintaining the output at a constant level irrespective of the level of the external reference frequency signal supplied.
2. Description of the Related Art
Hitherto, in transmitter of a cable television (CATV), a reference frequency signal switching circuit for supplying a reference frequency signal to operate the transmitter is used.
The reference frequency signal switching circuit has therein an internal oscillator for generating an internal reference frequency signal and operates so as to selectively output an internal reference frequency signal outputted from the internal oscillator or an external reference frequency signal supplied from the outside. In the case of operating a single transmitter, the internal reference frequency signal outputted from the internal oscillator is used. In the case of using a plurality of transmitters in the same system, in order to operate the transmitters at the same frequency, the external reference frequency signal supplied from the outside is used.
FIG. 5
is a diagram showing an example of the construction of a conventional reference frequency signal switching circuit.
As shown in
FIG. 5
, the conventional reference frequency signal switching circuit comprises: an external reference frequency signal input terminal
31
to which the external reference frequency signal is supplied; an input circuit
32
; an internal oscillator
33
for generating an internal reference frequency signal; a buffer amplifier
34
; a signal switching circuit
35
; a reference frequency signal output terminal
36
; a high voltage power source terminal
37
; and a low voltage power source terminal
38
. The signal switching circuit
35
has a tuned amplifier
35
1
a detecting circuit
35
2
, a smoothing circuit
35
3
, a first switching transistor
35
4
, a second switching transistor
35
5
, a first switching diode
35
6
, a second switching diode
35
7
, a coupling capacitor
35
8
, a collector resistor
35
9
, an inductor
35
10
and a bias resistor
35
11
.
The external reference frequency signal supplied to the external reference frequency signal input terminal
31
passes through the input circuit
32
and is supplied to the tuned amplifier
35
1
and the first switching diode
35
6
in the signal switching circuit
35
.
On the other hand, the internal reference frequency signal outputted from the internal oscillator
33
passes through the buffer amplifier
34
and is supplied to the second switching diode
35
7
in the signal switching circuit
35
. Either the external reference frequency signal supplied to the first switching diode
35
6
or the internal reference frequency signal supplied to the second switching diode
35
7
is outputted via the coupling capacitor
35
8
to the reference frequency signal output terminal
36
.
A voltage of 5V is supplied from the high voltage power source terminal
37
to the tuned amplifier
35
1
and is supplied to the collector of the first switching transistor
35
4
via the collector resistor
35
9
. A voltage of 3V is supplied from the low voltage power source terminal
38
to the internal oscillator
33
, the buffer amplifier
34
and the anode of the first switching diode
35
6
The cathode of the second switching diode
35
7
is connected to the ground via the bias resistor
35
11
.
The detecting circuit
35
2
is connected to the tuned amplifier
35
1
the smoothing circuit
35
3
is connected to the detecting circuit
35
2
, and the base of the first switching transistor
35
4
is connected to the smoothing circuit
35
3
. The collector of the first switching transistor
35
4
is connected to the base of the second switching transistor
35
5
via a resistor and is also connected via the inductor
35
10
to the cathode of the first switching diode
35
6
and the anode of the second switching diode
35
7
.
The internal oscillator
33
and the buffer amplifier
34
are connected to the ground via the second switching transistor
35
5
which operates as a switch of the internal oscillator
33
and the buffer amplifier
34
.
The operation of the conventional reference frequency signal switching circuit will be described as follows.
When the external reference frequency signal is not supplied to the external reference frequency signal input terminal
31
, since the external reference frequency signal is not outputted from the tuned amplifier
35
1
a DC voltage is not outputted from the detecting circuit
35
2
and, therefore, a detection voltage is not outputted from the smoothing circuit
35
3
. Consequently, the first switching transistor
35
4
is turned off. When the first switching transistor
35
4
is turned off, the high voltage (5V) supplied to the high voltage power source terminal
37
is supplied via the collector resistor
35
9
to the base of the second switching transistor
35
5
. Then the second switching transistor
35
5
is turned on and the internal oscillator
33
and the buffer amplifier
34
are connected to the ground via the second switching transistor
35
5
. The voltage (3V) is supplied from the low voltage power source terminal to the internal oscillator
33
and the buffer amplifier
34
, the internal oscillator
33
outputs the internal reference frequency signal, and the buffer amplifier
34
amplifies the signal. At this time, the signal is supplied via the collector resistor
35
9
and the inductor
35
10
to both the cathode of the first switching diode
35
6
and the anode of the second switching diode
35
7
, so that the second switching diode
35
7
is turned on. The internal reference frequency signal amplified by the buffer amplifier
34
is outputted from the reference frequency signal output terminal
36
via the second switching diode
35
7
and the coupling capacitor
35
8
On the other hand, the first switching diode
35
6
enters an OFF state, so that the internal reference frequency signal appearing at the anode side of the second switching diode
35
7
is not leaked to the external reference frequency signal terminal
31
.
When the external reference frequency signal is supplied to the external reference frequency signal input terminal
31
, the external reference frequency signal is amplified by the tuned amplifier
35
1
detected by the detecting circuit
35
2
smoothed by the smoothing circuit
35
3
, and a resultant is supplied as a detection voltage to the base of the first transistor
35
4
, thereby turning on the first switching transistor
35
4
. Then the collector of the first switching transistor
35
4
approaches ground. Accordingly, the base of the second switching transistor
35
5
, the cathode of the first switching diode
35
6
and the anode of the second switching diode
35
7
approaches ground. The second switching transistor
35
5
is therefore turned off and the voltage (3V) is not supplied from the low voltage power source terminal
38
to the internal oscillator
33
and the buffer amplifier
34
. The internal oscillator
33
stops outputting the internal reference frequency signal and the buffer amplifier
34
also stops outputting the signal. The second switching diode
35
7
is turned off, so that the internal oscillator
33
and the buffer amplifier
34
are isolated from the reference frequency signal output terminal
36
. On the other hand, the low voltage (3V) supplied from the low voltage power source terminal
38
is supplied to the anode of the first switching diode
35
6
and the cathode approaches ground. Consequently, a forward bias is applied and the first switching diode
35
6
is turned on. The external reference frequency signal supplied to the external reference frequency signal input terminal
31
is outputted to the reference frequency signal out
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Dinh Paul
Wells Kenneth B.
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