Double balanced active mixer

Details Double balanced active mixer Double balanced active mixer

1999-09-21

2002-06-25

Cumming, William (Department: 2684)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

Reexamination Certificate

active

06411801

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a frequency mixer and, more particularly, to a double balanced active mixer for compensating for an asymmetric characteristic of complementary radio frequency signals, thereby improving linearity of the double balanced active mixer.
DESCRIPTION OF THE PRIOR ART
A frequency mixer is used to modulate or mix a radio frequency (hereinafter, referred to as RF) signal with a local oscillation (hereinafter, referred to as LO) signal to produce a different signal having a new frequency component.
As is well known to the art, an MMIC (microwave monolithic integrated circuit) is a circuit integrated together with active and passive elements on a same semiconductor substrate. Therefore, compared with a circuit which is implemented with each individual unit element, the MMIC configuration may reduce a distance between each individual unit element so that a size and weight of the circuit is reduced. In addition, parasitic components caused by a packaging of the individual unit elements can be fundamentally eliminated, thereby highly improving a frequency bandwidth performance. With high demands on a light and small-size wireless/mobile communication equipment and a mass production at a low cost, it is inclined that the MMIC configuration is used to implement microwave parts of the recent wireless/mobile communication by the MMIC configuration. The manufacturing cost of the MMIC is generally proportional to the size, so that it is important to scale down the size of the MMIC.
Meanwhile, a microwave frequency mixer is the most useful in RF parts used for receivers and transmitters. The microwave frequency mixer is an apparatus for modulating or mixing the RF signal with the LO signal to produce an intermediate frequency (hereinafter, referred to as IF) signal, wherein the IF signal corresponds to a difference and sum of the RF and LO signal. A receiving station and a transmitting station widely use a down conversion mixer for converting the RF signal into the IF signal and an up conversion mixer for converting the IF signal into the RF signal, respectively.
Generally, requirements for the microwave frequency mixer used in the receiving station include a low noise, a high gain, an excellent linearity such as a low intermodulation distortion, an excellent signal isolation between input and output terminals of the mixer, a low manufacturing cost and a small size, a low power. consumption and so on. Since a low-capacity battery is used in order to reduce the weight of the mobile station, the use of parts operable at a low power source is essential for an increase of available time. As the number of registers is increasing, it is more desirable for the mobile station to have improved receiving and transmitting characteristics (in particular, the low noise and the high linearity).
The frequency mixer is generally classified by a single ended mixer and a balanced mixer, wherein the balanced mixer is again classified by a single balanced mixer and a double balanced mixer. The balanced mixer will be described in detail.
When the large LO signal used for a frequency conversion is leaked to an output port of the mixer, an normal operation of an amplifier circuit connected back to the mixer may be disturbed. In the down conversion mixer, since a frequency difference between the LO signal and an output signal is large, only the LO signal can be eliminated using a filter. However, in the up conversion mixer, since the frequency of the LO signal is close to the frequency of the output signal, it is difficult to eliminate only the LO signal using the filter.
In this case, a structure known as Gilbert Cell is widely used. The structure of the Gilbert Cell has a double balanced structure, which uses an offset effect between the complementary signals. However, since a balun circuit is used to generate the complementary signals in the structure of the Gilbert Cell, an asymmetry of the signals due to an imperfectness of the balun circuit may occur, therefore resulting in degrading the mixer performance.
FIG. 1
is a schematic diagram illustrating a mixer using the Gilbert Cell according to the prior art. Referring to
FIG. 1
, the conventional mixer includes an input transistor part
100
for receiving and amplifying the RF+ and RF− signals complementary to each other and an output transistor part
200
for receiving the LO+ and LO− signals complementary to each other and for outputting an IF+ and IF− signals by switching the amplified RF+ and RF− signals from the input transistor part
100
.
A positive-phase radio frequency (hereinafter, referred to as RF+) input signal is inputted to a gate of a field effect transistor (hereinafter, referred to as FET)
103
through an input port
101
. The RF+ input signal is amplified and outputted as a negative-phase. radio frequency (hereinafter, referred to as RF−) signal at a drain of the FET
103
and the RF− signal from the FET
103
is transferred to a common source of FETs
109
and
110
. Similarly, a RF− input signal is inputted to a gate of a FET
104
through an input port
102
. The RF− input signal is amplified and outputted as a RF+signal at a drain of the FET
104
and the RF+ signal from the FET
104
is transferred to a common source of FETs
111
and
112
.
A positive-phase local oscillation (hereinafter, referred to as LO+) input signal is inputted to a gate of the FET
109
through an input port
106
and the RF− signal which is transferred from the source to a drain of the FET
109
is switched in response to the LO+ input signal. Similarly, the LO+ signal is inputted to a gate of the FET
112
through an input port
107
and the RF+ signal which is transferred from the source to a drain of the FET
112
is switched in response to the LO+ input signal.
Furthermore, a negative-phase local oscillation (hereinafter, referred to as LO−) input signal is inputted to a common gate of FETs
110
and
111
through an input port
108
and the RF− signal which is transferred from the source to a drain of the FET
110
is switched in response to the LO− signal. Similarly, the RF+ signal which is transferred from the source to a drain of the FET
111
is switched in response to the LO− signal.
Through the switching operations as described above, a positive-phase intermediate frequency (hereinafter, referred to as IF+) signal at the common drain of the FETs
109
and
111
is outputted to an output port
113
and a negative-phase intermediate frequency (hereinafter, referred to as IF−) signal at the common drain of the FETs
110
and
112
is outputted to an output port
114
.
At this time, in case where the RF+ and RF− input signals inputted to the FETs
101
and
102
, respectively, are ideally complementary signals each other, the IF+ and IF− signals also have a complementary characteristic. However, in case where the RF+ and RF− signals is generated using the balun circuit, an asymmetry of the signals may occur, resulting in the asymmetry of the IF+ and IF− signals in the phase and amplitude.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a double balanced active mixer for compensating for an asymmetric characteristic of complementary radio frequency signals, thereby improving linearity of the double balanced active mixer.
In accordance with an aspect of the present invention, there is provided a double balanced active mixer for compensating for asymmetry of radio frequency signals, comprising an input transistor part for amplifying first and second radio frequency signals having complementary phase each other which are inputted from external circuit and for transferring the amplified first and second radio frequency signals and an output transistor part for outputting first and second intermediate frequency signals which are complementary each other by switching the

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