Telecommunications – Receiver or analog modulated signal frequency converter – Convertible to different type
Reexamination Certificate
1999-04-07
2001-03-27
Le, Thanh Cong (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Convertible to different type
C455S143000, C455S280000
Reexamination Certificate
active
06208847
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a receiving circuit for receiving signal inputted from the antenna with changing the path of the received signal in a receiver.
2. Description of the Prior Art
There is known a receiving circuit in which the signal path of the received signal can be changed in order to receive broadcasting waves in different frequency bands, such as the AM and FM bands, by a single antenna and switch the receiving circuits for those different frequency bands, or in order to operate a single receiving circuit with switching multiple antenna inputs, like the case of a diversity receiver.
Such a receiving circuit generally includes a pair of diodes connected in series. The connection point of those two diodes is connected to both an antenna output and an AM circuit, and the other terminal of one of the diodes is connected to an FM circuit. By appropriately changing the direction of the bias voltage applied to the serially-connected diodes, the signal received by the antenna is supplied either the AM circuit or the FM circuit. Thus, in a receiving circuit including a single antenna for a common use by an AM circuit and an FM circuit, the switching operation of the received signal can be readily controlled.
In such a receiving circuit, when the serially-connected diodes are applied with the reverse direction bias, a constant current flows the connection point of the two diodes if the characteristics of those diodes are ideal, i.e., ideally identical to each other. However, actually the characteristics of those diodes are not completely identical and hence the leak currents of different amount flow therethrough. This introduces the fluctuation of the reverse direction bias voltage. As a result, the AM circuit cannot receive constant voltage, and the distortion of the signal may take place in the AM circuit. In addition, the FM circuit becomes susceptible to the intermodulation interference due to the multiple FM transmission waves.
As described above, in a conventional receiving circuit, there are problems that the received signal is likely to be distorted due to the irregularity in characteristics of the diodes, such as the leak currents, and that it is difficult to maintain the desired signal receiving characteristics in both AM and FM circuits under the switching operation of them.
SUMMARY OF THE INVENTION
In order to overcome the above mentioned problems, it is an object of the present invention to provide a receiving circuit capable of avoiding the distortion of and/or interference to the received signal due to the irregularity in the characteristics of the individual diodes during the switching operation of the receiving signal frequency.
According to one aspect of the present invention, there is provided a receiving circuit including: an antenna for receiving signal waves of two different frequency hands; a first receiving circuit for receiving a first receiving signal having a frequency in one of the frequency bands; a second receiving circuit for receiving a second receiving signal having a frequency in the other one of the frequency bands; a switch element for switching a signal path from the antenna to either one of the first receiving circuit and the second receiving circuit; a first diode having an anode connected to an output terminal of the antenna and a cathode connected to an input terminal of the first receiving circuit; a second diode having a cathode connected to the output terminal of the antenna and an anode connected to the switch element; and reverse bias voltage applying elements, each connected in parallel with one of the first diode and the second diode for maintaining substantially identical reverse bias voltage between the anode and the cathode for each of the first diode and the second diode, wherein the switch element is connected to a direct current power supply, and the switch element controls the connection of the direct current power supply such that the first diode and the second diode are biased in the forward direction when the first receiving signal is received and the first diode and the second diode are biased in the reverse direction when the second receiving signal is received.
In accordance with the receiving circuit thus configured, when the first receiving signal having the frequency in one of the different frequency bands, such as FM transmission wave, is supplied from the antenna, the switch element controls the connection of the power supply such that the first diode and the second diode are biased in the forward direction. Thus, the first received signal is guided from the antenna to the first receiving circuit. On the other hand, when the second receiving signal having the frequency in the other one of the different frequency bands, such as AM transmission wave, is supplied from the antenna, the switch element controls the connection of the power supply such that the first diode and the second diode are biased in the reverse direction. At this time, the reverse bias applying elements apply substantially same reverse bias voltages between the anode and the cathode of the diodes. Therefore, the variation of the voltage level due to the irregularity in characteristics of the diodes, such as the leak currents, may be absorbed and the distortion in the received signal may be avoided. In addition, the interference of the received signal due to the intermodulation of the FM transmission waves may be reduced.
In a preferred embodiment, each of the reverse bias voltage applying elements may include a resistor connected in parallel with one of the first diode and the second diode, and the resistor may have a sufficiently large resistance value such that a current sufficiently larger than a leak current, which flows through the diode biased in the reverse direction, flows through the reverse bias voltage applying element. By this, the voltage of the power supply is divided by the resistors having appropriate resistance values, and thus the voltages applied between the anode and the cathode of the respective diodes are determined and stabilized. Therefore, if the individual characteristics, such as the leak current, of the diodes are different, the variation of the applied voltages of the diodes ay be avoided, and excellent reception performance may be ensured.
According to another aspect of the present invention, here is provided a receiving circuit including: a plurality of antennas including a common antenna for receiving signal waves of two different frequency hands; a first receiving circuit for receiving a first receiving signal having a frequency in one of the frequency bands; a second receiving circuit for receiving a second receiving signal having a frequency in the other one of the frequency bands; a switch element for switching a signal path from the common antenna to either one of the first receiving circuit and the second receiving circuit and for switching the connection of a signal path from one of the plurality of antennas other than the common antenna to the first receiving circuit; a first diode having an anode connected to an output terminal of the common antenna and a cathode connected to an input terminal of the first receiving circuit; a second diode having a cathode connected to the output terminal of the common antenna and an anode connected to the switch element; and reverse bias voltage applying elements, each connected in parallel with one of the first diode and the second diode for maintaining substantially identical reverse bias voltage between the anode and the cathode for each of the first diode and the second diode, wherein the switch element is connected to a direct current power supply, and the switch element controls the connection of the direct current power supply such that the first diode and the second diode are biased in the forward direction when the first receiving signal is received from the common antenna and the first diode and the second diode are biased in the reverse direction when the second receiving signal is received from the comm
Cong Le Thanh
Ladas & Parry
Pioneer Electronic Corporation
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