Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-03-20
2001-01-23
Urban, Edward F. (Department: 2746)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S333000, C327S355000
Reexamination Certificate
active
06178320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radio receiver having particular, but not exclusive, application in a digital, wide area pager.
2. Description of the Related Art
In view of the relatively small size of digital pagers there has been a desire to reduce the number of batteries carried by a pager and the objective is to have single cell operation. Accordingly the circuitry used in the pager is required to operate at about 1 volt and if a higher voltage is required then a voltage multiplier is provided. A critical part of a receiver is the frequency down-conversion stage which is normally implemented as a double-balanced mixer circuit. Double-balanced mixer circuits operating on 2 cell supplies are known per se. U.S. Pat. No 5,521,545 discloses a development of a double-balanced mixer circuit which enables it to operate at a lower voltage. An examination of the circuit disclosed in U.S. Pat. No 5,521,545 has shown that over the range of frequencies, between 150 MHz and 950 MHz, which are used by paging operators, there are frequency bands when this known double-balanced mixer circuit becomes unstable and behaves as an oscillator.
OBJECT AND SUMMARY OF THE INVENTION
An object of the invention is to eliminate this instability in low voltage double-balanced mixer circuits.
Accordingly to a first aspect of the present invention there is provided a radio receiver comprising means for receiving an input signal, local oscillator signal generating means and a frequency down conversion stage having a first input for connection to the means for receiving an input signal, a second input for receiving a local oscillator signal and an output for a frequency down-converted signal, the frequency down conversion stage comprising first, second, third and fourth transistors of the same conductivity type, emitter electrodes of the first and second transistors and of the third and fourth transistors, respectively, being coupled together to define first and second emitter junctions, collector electrodes of the first and third transistors and of the second and fourth transistors being connected to respective first and second output junctions, base electrodes of the first and fourth transistors and of the second and third transistors being coupled to respective first and second local oscillator junctions, first and second local oscillator inputs coupled to the first and second local oscillator junctions, respectively, first and second means for providing substantially identical currents to the first and second emitter junctions, first and second signal input terminals coupled to current paths between said first and second current providing means and the first and second emitter junctions, respectively, and signal damping elements coupled respectively between the first and second local oscillator inputs and the first and second local oscillator junctions and between the first and second signal input terminals and the first and second emitter junctions.
According to a second aspect of the present invention there is provided a double balanced mixer circuit comprising first, second, third and fourth transistors of the same conductivity type, emitter electrodes of the first and second transistors and of the third and fourth transistors, respectively, being coupled together to define first and second emitter junctions, collector electrodes of the first and third transistors and of the second and fourth transistors being connected to respective first and second output junctions, base electrodes of the first and fourth transistors and of the second and third transistors being coupled to respective first and second local oscillator junctions, first and second local oscillator inputs coupled to the first and second local oscillator junctions, respectively, first and second means for providing substantially identical currents to the first and second emitter junctions, first and second signal input terminals coupled to current paths between said first and second current providing means and the first and second emitter junctions, respectively, and signal damping elements coupled respectively between the first and second local oscillator inputs and the first and second local oscillator junctions and between the first and second signal input terminals and the first and second emitter junctions.
The present invention is based on the recognition of the fact that by providing small amounts of damping at selected points in the local oscillator and rf feeds to the switch stages of the double-balanced mixer circuit, frequency instability is eliminated over the band of frequencies of interest. The damping means may be purely resistive and be implemented as equal value resistors in the respective signal feed paths. The value of resistance is low and a typical value is of the order of 25 ohms.
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Richards Anthony H.
Wichern Andreas
Perez-Gutierrez Rafael
Slobod Jack D.
U.S. Philips Corporation
Urban Edward F.
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