Telecommunications – Receiver or analog modulated signal frequency converter – With wave collector
Reexamination Certificate
2000-06-05
2001-12-04
Chang, Vivian (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
With wave collector
C455S317000, C455S192100, C455S192200
Reexamination Certificate
active
06327464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency converting circuit comprising a PLL (Phase-Locked Loop) circuit, and, in particular, relates to a damping circuit for locally oscillated electric signals.
This application is based on Patent Application No. Hei 11-160256 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
It is general practice to convert input signals mixed with signals which are generated by a locally oscillating circuit (hereinafter, the abbreviation “OSC” indicates oscillating) into output signals with a predetermined frequency. A frequency converting circuit ideally outputs signals with only output frequency, however in practice, output signals contain leaked components originating from the input signals or the OSC signals. It is required that the amount of signal leakage be reduced to as low as possible since the output signals can be undesirably affected if the leaked components from the input signals or the OSC signals are excessive. Furthermore, in the case of a frequency converting circuit comprising a known double-balanced mixing circuit, the leaked components from the OSC signals are greater than the ones from the input signals, and the leaked components must be reduced.
In order to meet the above requirements, a circuit, shown in
FIG. 5
, for example, has been proposed. In this circuit, a signal being input from a terminal
51
and the OSC signal being output from the OSC circuit
54
, which is controlled by a PLL circuit
57
fed by a buffer amplifier
56
, are mixed in a mixer
55
, then a differential frequency is derived. The derived signals are amplified by buffer amplifier
58
passed from terminal
52
through a capacitor
59
, a filter
60
(such as a band-pass filter or a low-pass filter) and a further capacitor
61
in order to eliminate undesired leak signals, so that only necessary output signals can be obtained at terminal
53
.
SUMMARY OF THE INVENTION
The method described above, however, has some drawbacks when the output signal has relatively low frequency or the output frequency and the OSC frequency are too close to each other. If the output frequency is relatively low, the inductance of the coil and the capacitance of the capacitor must be increased in order to construct a filter through which low frequency signals pass. A coil of a large size will be necessary when relatively high inductance is to be achieved. If the capacitance of the capacitor is increased, the Q-factor of the filter will be improved, while in contrast, the gain at the resonant frequency will be lowered.
It is required that the Q-factor of the filter be improved in order to reduce the OSC leak amount when the output frequency and the OSC frequency are close to each other, however this leads to an undesired loss of gain.
Another method to separate output signals from OSC signals whose frequency is close to the output frequency is to use a crystal filter, however, a drawback with this is that crystal filters are expensive. Another method is to connect a trap filter whose resonant frequency is fixed. A drawback of this method is that the trap filter is effective only at its resonant frequency.
It is therefore an object of the present invention to resolve the aforementioned drawbacks of the prior art by providing a signal frequency converting circuit which is capable of reducing the amount of leakage of OSC signal to the output signals.
According to one aspect of the invention, there is provided a signal frequency converting circuit comprising a local oscillator; a mixer for mixing output signals from the local oscillator and input signals and consequently generating a signal with a differential frequency between both signals; and a variable trap circuit connected to the output line of the mixer for reducing undesired signals leaked into the signal with the differential frequency.
According to another aspect of the invention, there is provided a signal frequency converting circuit according to the first aspect further comprising a PLL circuit, wherein the variable trap circuit comprises a coil; and a varactor diode; and wherein the variable trap circuit resonates at the output frequency of the local oscillator by means of controlling the capacity of the varactor diode by the PLL circuit, and traps the undersigned signals having a frequency close to the output frequency.
According the present invention, the amount of leakage of the OSC signal to the output signal can be reduced as a result of inserting a variable trap circuit comprising a coil and a varactor diode in the output line of the signal frequency converting circuit having a PLL circuit. A controlling voltage, output from the PLL circuit to control the OSC frequency in the signal frequency converting circuit, is used to define the resonant frequency of the variable trap circuit.
Referring now to
FIG. 1
, a variable trap circuit
37
having a coil
31
and a varactor diode
32
is connected in the output line of the frequency converting circuit
11
including a PLL circuit
14
. The coil
31
and the varactor diode
32
are so selected that undesired signals in the OSC signals
42
can be effectively trapped. The OSC signals
42
are generated by a coil
18
and a varactor diode
19
which consist of a tank circuit
17
in the frequency converting circuit
11
.
A voltage, output from the PLL circuit
14
to control the OSC frequency
42
in the signal frequency converting circuit
11
and given to the tank circuit
17
, is used to define the resonant frequency of the variable trap circuit
37
. The variable trap circuit
37
is therefore a filter consistently having a trap effect around the frequency of the OSC signal, with which leaked components from the OSC signal
42
included in the output signals of the frequency converting circuit
11
can be reduced.
REFERENCES:
patent: 4215372 (1980-07-01), Suzuki
patent: 4849721 (1989-07-01), Matsuura
patent: 5991609 (1999-11-01), Marrah
patent: 6031878 (2000-02-01), Tomasz
patent: 59-112784 (1984-06-01), None
patent: 59-188242 (1984-10-01), None
patent: 62-278820 (1987-12-01), None
patent: 2-302113 (1990-12-01), None
patent: 4-72728 (1992-06-01), None
patent: 9-74362 (1997-03-01), None
Chang Vivian
Moore James K
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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