Abstract:
A double balanced active mixer is used for compensating an asymmetric characteristic of complementary radio frequency signals, to thereby improve linearity of the double balanced active mixer. The double balanced active mixer includes 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 amplified first and second radio frequency signals.

Description:
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 amplified first and second radio frequency signals, wherein the input transistor part comprises: a) a first transistor for receiving the first radio frequency signal from the external circuit through a gate of the first transistor and for amplifying the first radio frequency signal and outputting a amplified radio frequency signal to a drain of the first transistor; b) a second transistor for receiving the second radio frequency signals from the external circuit through a gate of the second transistor and for amplifying the second radio frequency signal and outputting the amplified signal to a drain of the second transistor; c) a third transistor for receiving the first radio frequency signal through a source and for amplifying the first radio frequency signal and outputting a amplified signal to the common drain of the second and third transistor; and d) a fourth transistor for receiving the second radio frequency signal at a source and for amplifying the second radio frequency signal and outputting a fourth resulting signal to the common drain of the first and fourth transistor. 
     In accordance with another aspect of the present invention, there is provided a double balanced active mixer, comprising: a) a first input port for receiving a first radio frequency having a positive phase; b) a second input port for receiving a second radio frequency having a negative phase; c) a third input port for receiving a first local oscillation signal having a positive phase; d) a fourth input port for receiving a second local oscillation signal having a negative phase; e) a first output port for outputting a first intermediate frequency having a positive phase; f) a second output port for outputting a second intermediate frequency having a negative phase; g) a first transistor which has a gate connected to the first input port and a source connected to the ground voltage level; h) a second transistor which has a gate connected to the second input port and a source connected to the ground voltage level; i) a third transistor which has a source connected to the first input port, a drain connected to a first node and a gate connected to the virtual RF ground terminal; j) a fourth transistor which has a source connected to the second input port, a drain commonly connected to a second node and a gate connected to the virtual RF ground terminal; k) a fifth transistor which has a gate connected to the third input port and a source connected to the first node; l) a sixth transistor which has a gate connected to the third input port and a source connected to the second node; m) a seventh transistor which has a gate connected to. the fourth input port and a source connected to the first node and a drain connected to a drain of the sixth transistor at the second output port; and n) a eighth transistor which has a gate connected to the fourth input port and a source connected to the second node and a drain connected to a drain of the fifth transistor at the second output node. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given with conjunction to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating a double balanced active mixer using Gilbert Cell according to the prior art; 
     FIG. 2 is a schematic diagram illustrating a double balanced active mixer in accordance with the present invention; and 
     FIG. 3 is a graph illustrating linear characteristic comparing the prior art with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail referring to the accompanying drawings. 
     Referring to FIG. 2, an input transistor part  400  receives RF+ and RF− input signals complementary to each other from an external circuit, amplifies each of the RF+ and RF− input signals and transfers the amplified RF+ and RF− signals. An output transistor part  500  outputs IF+ and IF− signals switching the amplified RF+ and RF− signals from the input transistor part  400  in response to LO+ and LO− signals complementary to each other from an external circuit. Here, the input transistor part  400  includes FETs  203  and  204 , whose gates receive the RF+ and RF− input signals, respectively, and amplify each of the RF+ and RF− input signals and FETs  208  and  209 , whose sources receive the RF+ and RF− input signals, respectively, and amplify each of the RF+ and RF− input signals. 
     Preferably, the FETs can be replaced with p-channel FETs or n-channel FETs. It is also possible to replace them with bipolar transistors. 
     The structure of the double balanced active mixer will be described in detail. 
     The FET  203  has a gate and a source connected to an input port  201  and the ground voltage level  205 , respectively, and the FET  204  has a gate and a source connected to an input port  202  and the ground voltage level  205 , respectively. The FET  208  has a source connected to the input port  201 , a drain commonly connected to a drain of the FET  204  at a first node  230  and a gate connected to the virtual RF ground terminal  206 . The FET  209  has a source connected to the input port  207  and a drain commonly connected to a drain of the FET  203  at a second node  240  and a gate connected to the virtual RF ground terminal  207 . A FET  212  has a gate and a source connected to an input port  210  and the first node  230 , respectively, and a FET  215  has a gate and a source connected to an input port  211  and the second node  240 , respectively. A FET  213  has a gate and a source connected to an input port  216  and the first node  230 , respectively, and a drain commonly connected to a drain of the FET  215  at an output node  218 . A FET  214  has a gate and a source connected to the input node  216  and the second node  240 , respectively, and a drain commonly connected to a drain of the FET  217  at an output node  217 . The output nodes  217  and  218  are connected to the power supply voltage  221  through loads  219  and  220 . 
     In above-mentioned structure, an asymmetry of the complementary RF+ and RF− signals is compensated using a pair of common-source and common-gate FETs, thereby providing a double balanced active mixer with improved linearity. 
     Referring again to FIG. 2, the operation of the double balanced active mixer will be described in detail. 
     Some of the RF+ input signal inputted to the gate of the FET  203  is amplified and the amplified signal is outputted as a RF− signal at the drain of the FET  203 . The other of the RF+ input signal simultaneously inputted to the source of the FET  208  is amplified and the amplified signal is outputted as a RF+ signal at the drain of the FET  208 . Similarly, some of the RF− input signal inputted to the gate of the FET  204  is amplified and the amplified signal is outputted as a RF+ signal at the drain of the FET  204 . The other of the RF− input signal simultaneously inputted to the source of the FET  209  is amplified and the amplified signal is outputted as a RF− signal at the drain of the FET  209 . The RF+ signal outputted from the drains of the FETs  204  and  208  is transferred to the common source of the FETs  212  and  213 . Similarly, the RF− signal outputted from the drains of the FETs  203  and  209  is transferred to the common source of the FETs  214  and  215 . 
     The LO+ signal is inputted to the gate of the FET  212  through the input port  210  and the RF+ signal of the common node  230  transferred from the source to the drain of the FET  212  is switched in response to the LO+ signal. The LO+ signal is inputted to the gate of the FET  215  and the RF− signal of the common node  240  transferred from the source to the drain of the FET  215  is switched in response to a LO+ signal. The RF+ and RF− signals of the sources of the FETs  213  and  214 , respectively, are transferred to the drains of the FETs  213  and  214  in response to the LO− signal inputted to the common gate of the FETs  213  and  214 . 
     Through the above-mentioned switching operations, an IF+ signal is generated at the common drain of the FETs  212  and  214  and an IF− signal is generated at the common drain of the FETs  215  and  213 . 
     At this time, in case where the RF+ input signal inputted through the input port  201  is larger than the RF− input signal inputted through the input port  202  due to the asymmetry of the RF+ and RF− input signals, some of the large RF+ input signal inputted to the gate of the FET  203  through the input port  201  is amplified and outputted as a large RF− signal at the drain of the FET  203 . The other of the large RF+ input signal inputted to the source of the FET  208  is outputted as a large RF+ signal at the drain of the FET  208 . Similarly, some of the small RF− input signal inputted to the gate of the FET  204  through the input port  202  is amplified and outputted as a small RF+ signal at the drain of the FET  204 . The other of the small RF− signal inputted to the source of the FET  209  is amplified and outputted as a small RF− signal at the drain of the FET  209 . 
     Accordingly, the signal inputted to the source of the FET  213  corresponds to a sum of the large RF+ signal and the small RF+ signal from the drains of the FETs  208  and  204 , respectively. Similarly, the signal inputted to the source of the FET  214  corresponds to a sum of the large RF− signal and small RF− signal from the drains of the FETs  203  and  209 . Therefore, the asymmetric signals inputted through the input ports  201  and  202  are compensated. 
     As shown in FIG. 3, an 1 dB gain compress point is improved by 3 dB compared with the prior art. 
     While the present invention has been described with respect to certain preferred embodiments only, other modifications and variation may be made without departing from the spirit and scope of the present invention as set forth in the following claims.