Mixer for a receiver

Details Mixer for a receiver Mixer for a receiver

1998-12-16

2001-11-13

Chang, Vivian (Department: 2682)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S333000, C455S318000, C455S326000

Reexamination Certificate

active

06317590

ABSTRACT:

BACKGROUND OF INVENTION
The present invention relates to a mixer for a receiver for a portable radio communication apparatus, and in particular to a mixer for a direct conversion receiver.
A general trend in portable communication apparatus is the reduction in volume, weight and power consumption of such apparatus. This has led to efforts towards reducing the number of elements and devices necessary to perform the functions associated with portable communication devices. In particular, the radio frequency front end of portable apparatus, which typically comprises a number of down-converting stages, is an area in which a reduction in the number of elements and devices would be beneficial.
One approach to reduce the number of stages in the radio frequency front-end is to convert a received radio frequency carrier signal down to Van Intermediate Frequency (IF) in a single step. This is termed direct conversion and is carried out in receivers known by any one of the terms homodyne or zero IF receivers, as well as direct conversion receivers. To carry out direct conversion, a local oscillator signal (LO) having the same frequency as the radio frequency carrier signal (ie. the LO is “on-channel”) is mixed with the carrier signal in a suitable non-linear device such as a mixer diode. The output of the mixer contains the sum and difference of the LO and the carrier signal. Thus a mixer product exists at twice the carrier signal, and also at DC (zero Hz). The high frequency mixer products can be filtered by a suitable low pass filter. Once the radio frequency carrier signal has been down-converted, the modulating signal may be de-modulated using an appropriate demodulator, e.g. an I/Q demodulator for an I/Q modulating signal.
In the field of radio telephony, particularly cellular telephony, use of a direct conversion receiver is not without certain drawbacks.
Radio telephones operating in the same cell of a cellular telephone network typically receive control signals on the same signal frequency as each other. This is necessary since such control signals are often so-called “broadcast” control signals such as paging signals which need to be received by all the telephones operating in the same cell. Signals received by radio telephones are of very low power, e.g. below −100 dBm and the sensitivity of the receivers is adapted accordingly.
Against the requirement for radio telephones in the same cell to be tuned to common frequencies, the introduction of direct conversion receivers with their strong on-board LO signals present the problem of interference. More specifically, spurious LO leakage from the receiver into the antenna can cause in-band interference with other nearby receivers tuned to the same channel. Therefore, such a receiver would require a very high level of isolation between the local oscillator and the antenna in order to avoid swamping or saturating receivers of any nearby radio telephones. Such high levels of isolation are very difficult to achieve due to stray capacitances directly coupling local oscillator energy into the antenna. This is the case even when conventional mixers such as a Gilbert cell is used.
Known methods addressing the problem of LO leakage include:
providing increased reverse-isolation in the RF path to the antenna, for example introducing multi-stage RF amplifiers,
inserting an isolator,
reducing the local oscillator drive power to very low levels.
However, these methods have shortcomings, in particular, they have been found to be detrimental to dynamic range and degrading to sensitivity, as well as being expensive to implement
Against this background, the present invention aims to address the previously described drawbacks.
SUMMARY OF THE INVENTION
Accordingly, and in one aspect, the present invention provides a receiver for a portable radio communication apparatus comprising a first port for inputting a received frequency signal and a second port for inputting a local oscillator signal, means for rectifying the input local oscillator signal to provide a conductance waveform at a multiple of the local oscillator signal and means for mixing the received frequency signal with the conductance waveform at said multiple of the local oscillator signal frequency for down-converting the received frequency signal to an intermediate frequency signal.
By means of the invention, the received signal can be down converted without employing a local oscillator at the carrier frequency of the received signal.
Advantageously, the local oscillator can be fixed to operate at a sub-harmonic of the received signal frequency, and the mixing action is performed between the received frequency signal and one of the harmonics of the local oscillator, which advantageously ensures that the harmonic is not generated in the receiver. This is termed sub-harmonic mixing. Accordingly, the local oscillator signal frequency is far removed from received signal frequency, resulting in the mixer providing very high local oscillation to received signal isolation. This is because leakage of local oscillator radiation from the receiver to the antenna has negligible effect on other receivers in close proximity receiving on the same channel as the received signal. Thus, less shielding is necessary to inhibit interference signals generated by the local oscillator leaking out and interfering with adjacent apparatus than for a local oscillator near or at the radio frequency signal. Additionally, since no multiplying of the local oscillator is necessary the phase noise is not multiplied either.
The LO signal advantageously is split into two signals of opposite phase. This could be performed by a passive Balun structure or alternatively a ceramic or active splitter. A further advantage is that the local oscillator requires low power drive and thus benefits in reduced power consumption.
In a first embodiment of the present invention, the rectifying means and the coupling means comprise a balanced anti-phase output transformer and switching means, wherein a signal output from the transformer drives the switching means at a multiple of the local oscillator frequency.
The balanced anti-phase output transformer further improves isolation between the input (radio frequency) port and the local oscillator port. Optionally, the transformer may be a strip line, slot-line, co-planar microstrip and/or microstrip balanced to unbalanced transformer.
Preferably, the switching means comprises an anti-parallel diode pair, and respective diodes are electrically coupled to respective arms of the balanced output of the transformer, and suitably the diodes are Schottky diodes.
Advantageously, the switching means comprises a further anti-parallel diode pair, and respective diodes of the further anti-parallel pair are reversibly electrically coupled to respective arms of the balanced output of the transformer relative to the said anti-parallel diode pair. Such a dual anti-parallel diode pair configuration provides a higher balance between arms of the switching means than achievable with a single anti-parallel diode pair. This further improves isolation between the radio frequency and local oscillator ports.
In a second embodiment of the present invention, the rectifying means and the coupling means comprise a phase splitter and a plurality of transistors arranged in a network, wherein a signal output from the driver transistor drives the transistor network at a multiple of the local oscillator frequency.
The use of transistors in the second embodiment advantageously provides for the integration of the embodiment into an integrated circuit.
Preferably, the phase splitter provides anti-phase local oscillator drive signals to the plurality of transistors arranged in a network, and the driver transistor may comprise a unipolar transistor, conveniently an FET transistor, while the plurality of transistors arranged in a network may comprise bipolar transistors in a bridge configuration.
For this application of direct conversion receiver the local oscillator is half the received frequency.
The present invention extend

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