Oscillators – Plural oscillators – Selectively connected to common output or oscillator...
Reexamination Certificate
1999-12-02
2001-07-10
Mis, David (Department: 2817)
Oscillators
Plural oscillators
Selectively connected to common output or oscillator...
C331S1160FE, C331S158000, C331S173000, C327S298000
Reexamination Certificate
active
06259329
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reference-frequency-signal switching circuits, and more particularly, to a reference-frequency-signal switching circuit which selectively outputs either the internal reference-frequency signal generated by an internal reference-frequency-signal oscillator or the reference-frequency signal generated by an external reference-frequency-signal oscillator and which stops the operation of the internal reference-frequency-signal oscillator when the external reference-frequency signal is output.
2. Description of the Related Art
In a cable television (CATV) transmitter, a reference-frequency-signal switching circuit for sending a reference-frequency signal to a reference-frequency-signal source in a head end has been conventionally used.
This reference-frequency-signal switching circuit includes an internal reference-frequency-signal oscillator for generating a reference-frequency signal in its inside. The reference-frequency-signal switching circuit selectively outputs the reference-frequency signal output from the internal reference-frequency-signal oscillator or a reference-frequency signal input externally.
FIG. 2
shows an outlined structure of a conventional reference-frequency-signal switching circuit.
As shown in
FIG. 2
, the conventional reference-frequency-signal switching circuit is formed of an internal reference-frequency-signal oscillator
21
, a reference-frequency-signal input terminal
22
, a reference-frequency-signal output terminal
23
, a detection circuit
24
, a relay
25
having one circuit with two contacts, and a power source terminal
26
.
The internal reference-frequency-signal oscillator
21
generates an internal reference-frequency signal having a reference frequency, for example, a frequency of 10 MHz. The internal reference-frequency-signal oscillator
21
is connected between the power source terminal
26
and the ground, and its output end is connected to one fixed contact of the relay
25
. At the reference-frequency-signal input terminal
22
, an external reference-frequency signal having the reference frequency, for example, a frequency of 10 MHz, is given externally. The reference-frequency-signal input terminal
22
is connected to the input end of the detection circuit
24
and to the other fixed contact of the relay
25
. Either the internal reference-frequency signal or the external reference-frequency signal is selectively sent to the reference-frequency-signal output terminal. The terminal is connected to a movable contact of the relay
25
. The detection circuit
24
detects the external reference-frequency signal input to the input end, and generates a DC driving voltage at the output end. Its output end is connected to one end of the driving coil of the relay
25
. In the relay
25
, the other end of the driving coil is grounded.
The reference-frequency-signal switching circuit having the above structure operates in the following way.
In a period when the external reference-frequency signal is not input to the reference-frequency-signal input terminal
22
, since the external reference-frequency signal is not sent to the input end of the detection circuit
24
and the DC driving voltage is not generated at the output end of the detection circuit
24
, the driving coil of the relay
25
is not driven and the movable contact of the relay
25
is switched to one fixed contact side as shown by a solid line in FIG.
2
. The internal reference-frequency signal generated by the internal reference-frequency-signal oscillator
21
is sent to the reference-frequency-signal output terminal
23
from the output end of the internal reference-frequency-signal oscillator
21
through the switched contact of the relay
25
. The internal reference-frequency signal having a frequency of 10 MHz is taken out at the reference-frequency-signal output terminal
23
.
In a period when the external reference-frequency signal is input to the reference-frequency-signal input terminal
22
, since the external reference-frequency signal is sent to the input end of the detection circuit
24
and the DC driving voltage is generated at the output end of the detection circuit
24
, the driving coil of the relay
25
is driven and the movable contact of the relay
25
is switched to the other fixed contact side as shown by a dotted line in FIG.
2
. The external reference-frequency signal input to the reference-frequency-signal input terminal
22
is sent to the reference-frequency-signal output terminal
23
through the switched contact of the relay
25
. The external reference-frequency signal having a frequency of 10 MHz is taken out at the reference-frequency-signal output terminal
23
.
As described above, the conventional reference-frequency-signal switching circuit automatically switches the output of the reference-frequency signal such that the internal reference-frequency signal generated by the internal reference-frequency-signal oscillator
21
is output when the external reference-frequency signal is not input to the reference-frequency-signal input terminal
22
and the external reference-frequency signal is output unmodified when the external reference-frequency signal is input to the reference-frequency-signal input terminal
22
.
In the conventional reference-frequency-signal switching circuit, when the input external reference-frequency signal is output unmodified, since the internal reference-frequency-signal oscillator
21
operates to output the internal reference-frequency signal, a part of the internal reference-frequency signal may be combined for some reason with the external reference-frequency signal being output. In this case, although the external reference-frequency signal having a frequency of 10 MHz and the internal reference-frequency signal having a frequency of 10 MHz have the identical frequency, if a frequency difference occurs even slightly between them, a beat happens between the two reference-frequency signals. This beat component reduces the quality of the output reference-frequency signal. In addition, since the internal reference-frequency-signal oscillator
21
is operating unnecessarily, power consumption increases.
In the conventional reference-frequency-signal switching circuit, only when the external reference-frequency signal is input, the contact of the relay
25
is switched and the input external reference-frequency signal is output. Since the external reference-frequency signal is sent or is not sent to the reference-frequency-signal input terminal
22
usually by a manual operation, even if an external signal other than the determined external reference-frequency signal is erroneously input to the reference-frequency-signal input terminal
22
, the detection circuit
24
responds to the signal and the contact of the relay
25
is switched to output the signal from the reference-frequency-signal output terminal
23
.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a reference-frequency-signal switching circuit which always outputs a high-quality external reference-frequency signal or a high-quality internal reference-frequency signal selectively and which reduces power consumption.
Another object of the present invention is to provide a reference-frequency-signal switching circuit which, when a signal other than the desired one is erroneously input to a reference-frequency-signal input terminal, does not respond to the input signal and does not output the input signal.
One of the foregoing objects is achieved in one aspect of the present invention through the provision of a reference-frequency-signal switching circuit including an internal reference-frequency-signal oscillator for generating an internal reference-frequency signal; a reference-frequency-signal input terminal to which an external reference-frequency signal is input; a reference-frequency-signal output terminal; and a reference-frequency-signal output switching section for setting the internal reference-frequency-signal oscillat
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Mis David
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