# Patent

## Title
Mixer used in direct conversion transceiver and related method

## Abstract

A frequency conversion apparatus for converting an input signal having an input frequency into an output signal having an output frequency. The frequency conversion apparatus includes a first multiplier and a gm stage. The first multiplier multiplies a first local signal having a first frequency with a second local signal having a second frequency to generate a differential current signal having a third frequency. The third frequency is substantially the same as the input frequency or the output frequency, and the first frequency and the second frequency are different from the input frequency or the output frequency. The gm stage is electrically connected to the first multiplier and receives the differential current signal and the input signal to generate the output signal.

## Background
BACKGROUND 
This invention relates to mixers, and more particularly, to mixers used in direct conversion transceivers with the local oscillator (LO) self-mixing problem or the LO leakage problem being minimized.
In every kind of receiver or transmitter, mixers are always used for frequency conversion from an RF (radio frequency) to an IF (intermediate frequency) or a BB (baseband), and vice versa. A mixer used in a receiver for down frequency conversion is called a “down mixer”; and a mixer used in a transmitter for up frequency conversion is called an “up mixer”. Both up mixer and down mixer have similar structure.
Taking receivers as an example, there are numerous receiver topologies commonly used today. One of which is called a heterodyne receiver (or superheterodyne receiver), which down converts an input signal into a rather high intermediate frequency to generate an IF signal first. Then the IF signal is further filtered, amplified, and down converted again into another lower frequency (typically called a second IF or BB) for demodulation and data reconstruction. It is important that an IF filter must be used in the signal path of the heterodyne receiver, and the using of the IF filter always increases the cost and area of the whole mixer.
A direct conversion receiver (DCR) is another commonly used receiver topology, which substantially generates no IF signals. In other words, a mixer of a DCR converts the RF signals directly into baseband. FIG. 1 shows a block diagram of a conventional double-balanced mixer, which is also referred to as a “Gilbert-Cell”, used in a DCR. The double-balanced mixer 100 shown in FIG. 1 includes a current source IBIAS, six transistors Q1 ̃Q6, and two resistors R1 and R2. The transistor pair Q1 and Q2 converts an input signal VRF into a differential current signal IDIFF. The differential current signal IDIFF includes a first single-ended current signal IDIFF+ and a second single-ended current signal IDIFF−. With the two resistors R1 and R2 being used as loads, the four transistors Q3 ̃Q6 mix the differential current signal IDIFF with a LO signal VLO to generate an output signal VBB.
An advantage of the DCR is that it has a simple circuit layout. Besides, since a DCR avoids the use of IF filters, the cost of using DCR can be much lower than using heterodyne receiver with additional IF filters.
However, a DCR employing the standard mixer (such as the double-balanced mixer 100 shown in FIG. 1) suffers from a technical problem, which is called “LO self-mixing problem”. This problem is induced because the frequency of the LO signal used by the mixer is substantially the same as the frequency of the RF signal inputted into the mixer. The LO signal with voltage form might leak into the port of the mixer for receiving the RF signal and then the leaked LO signal will self-mix with the original LO signal during the mixing process. A resulted random DC offset then appears at the output port of the mixer. The random DC offset degrades the baseband signal quality and deteriorate the performance of the DCR.
Similarly, a direct conversion transmitter employing the standard mixer also suffers from a technical problem, which is called “LO leakage problem”. This problem is induced because the frequency of the LO signal used by the mixer is substantially the same as the frequency of the RF signal outputted by the mixer. The LO signal with voltage form might leak into the port of the mixer for outputting the RF signal, and then the leaked LO signal will mix with the outputted RF signal. This LO leakage problem declines signal quality of the outputted RF signal.
FIG. 2 shows a subharmonic double-balanced mixer disclosed by Rebeiz, et al. in U.S. Pat. No. 6,348,830. The subharmonic double-balanced mixer 200 shown in FIG. 2 includes a current source IBIAS, eight transistors Q1 ̃Q8, and two resistors R1 and R2. The four transistors Q1 ̃Q4 form a LO section 210; with four LO signals having four different phases and a bias current generated by the current source IBIAS being inputted, the LO section 210 generates a differential current signal IDIFF. The differential current signal IDIFF includes a first single-ended current signal IDIFF+ and a second single-ended current signal IDIFF−. The four transistors Q5 ̃Q8 form a RF section 220, with the two resistors R1 and R2 being used as loads, the RF section 220 mixes the differential current signal IDIFF with a RF signal VRF to generate an output signal VBB.
The subharmonic double-balanced mixer (such as that shown in FIG. 2) provides a great solution to the abovementioned LO self-mixing problem. The reason is that the frequency of the LO signals used by the subharmonic double-balanced mixer is different from that of the received RF signal. Even if the LO signals mixed with the received RF signal, since these two signals have different frequencies, DC offset problems will not be generated. Similarly, the subharmonic double-balanced mixer also provides a great solution to the abovementioned LO leakage problem. The reason is that the frequency of the LO signals used by the subharmonic double-balanced mixer is different from that of the RF signal generated by the mixer. Even if the LO signals mixed with the outputted RF signal, since these two signals have different frequencies, the leaked LO signal will not influence the signal quality of the outputted RF signal.
SUMMARY OF INVENTION 
It is an objective of the claimed invention to provide a mixer used in a direct conversion receiver or a direct conversion transmitter with the LO self-mixing problem or the LO leakage problem being minimized.
According to the claimed invention, a frequency conversion apparatus for converting an input signal having an input frequency into an output signal having an output frequency is disclosed. The frequency conversion apparatus includes a first multiplier and a gm stage. The first multiplier multiplies a first local signal having a first frequency with a second local signal having a second frequency to generate a differential current signal having a third frequency. The third frequency is substantially the same as the input frequency or the output frequency, and the first frequency and the second frequency are different from the input frequency or the output frequency. The gm stage is electrically connected to the first multiplier for receiving the differential current signal and the input signal to generate the output signal.
In addition, according to the claimed invention, a method for converting an input signal having an input frequency into an output signal having an output frequency is also disclosed. The method includes multiplying a first local signal having a first frequency with a second local signal having a second frequency to generate a differential current signal having a third frequency and receiving the differential current signal and the input signal to generate the output signal. The third frequency is substantially the same as the input frequency or the output frequency, and the first frequency and the second frequency are different from the input frequency or the output frequency.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF DRAWINGS 
FIG. 1 shows a block diagram of a conventional double-balanced mixer used in a DCR.
FIG. 2 shows a subharmonic double-balanced mixer disclosed by Rebeiz, et al. in U.S. Pat. No. 6,348,830.
FIG. 3 shows a block diagram of a mixer according to an exemplary embodiment of the present invention.
FIG. 4 shows a circuit diagram illustrating an example of the gm stage of FIG. 3.



DETAILED DESCRIPTION 
FIG. 3 shows a block diagram of a mixer according to an exemplary embodiment of the present invention. In this embodiment, the mixer 300 is a down mixer used in a direct conversion receiver. However, in another embodiment of the present invention, the mixer could also be an up mixer used in a direct conversion transmitter.
Since the mixer 300 in this embodiment is a down mixer used in a direct conversion receiver, it converts an input signal VIN (which is an RF signal) having an input frequency fIN into an output signal VOUT (which is an IF signal or a baseband signal) having an output frequency fOUT. The mixer 300 includes a local oscillator (LO) 310, a first divider 320, a second divider 330, a first multiplier 340, a gm stage 350, and a load circuit 360. The local oscillator 310 generates a preliminary local signal LO0 (which is a voltage signal in this embodiment) having a basic frequency fBASIC. The first divider 320 is electrically connected to the local oscillator 310 and divides the basic frequency fBASIC of the preliminary local signal LO0 by a first divisor N1 to obtain a first local signal LO1 (which is a voltage signal in this embodiment) having a first frequency f1, where f1=fBASIC/N1. The second divider 330 is electrically connected to the local oscillator 310 and divides the basic frequency fBASIC of the preliminary local signal LO0 by a second divisor N2 to obtain a second local signal LO2 (which is also a voltage signal in this embodiment) having a second frequency f2, where f2=fBASIC/N2. Both the first divisor N1 and the second divisor N2 could be integers. The first multiplier 340 is electrically connected to the first divider 320 and the second divider 330 and multiplies the first local signal LO1 with the second local signal LO2 to generate a differential current signal IDIFF having a third frequency f3, where f3=[(N1+N2)/N1N2]×fBASIC or f3=[(N1−N2)/N1N2]×fBASIC (assume that N1>N2). The unwanted frequency component [(N1−N2)/N1N2]×fBASIC or [(N1+N2)/N1N2]×fBASIC in the differential current signal IDIFF could be filtered by a filter (not shown) set between the first multiplexer 340 and the gm stage 350. Apparently, the function of the first divider 320, the second divider 330, and the first multiplier 340 as a whole is similar to the function of the LO section 210 of FIG. 2. In FIG. 2, the LO section 210 generates a differential current signal IDIFF according to the four LO signals. In FIG. 4, the first divider 320, the second divider 330, and the first multiplier 340 as a whole generates a differential current signal IDIFF according to a preliminary local signal LO0.
Since in this embodiment the mixer 300 is a down mixer used in a direct conversion receiver, to avoid the LO self mixing problem, the basic frequency fBASIC of the preliminary local signal LO0, the first frequency f1 of the first local signal LO1, and the second frequency f2 of the second local signal LO2 are all different from the input frequency fIN of the input signal VIN. The third frequency f3 of the differential current signal IDIFF is substantially the same as input frequency fIN of the input signal VIN.
The function of the gm stage 350 is similar to that of the RF section of subharmonic double-balanced mixer 200 shown in FIG. 2. In FIG. 2, with the two resistors R1 and R2 being used as loads, the four transistors Q5 ̃Q8 mix the differential current signal IDIFF with the RF signal VRF to generate the output signal VBB. In FIG. 3, with the load circuit 360, the gm stage 350 mixes the differential current signal IDIFF with the input signal VIN to generate the output signal VOUT. More particularly, the gm stage 350 converts the input signal VIN having the input frequency fIN into the output signal VOUT having the output frequency fOUT according to the differential current signal IDIFF having the third frequency f3. Since the third frequency f3 of the differential current signal IDIFF is substantially the same as input frequency fIN of the input signal VIN, the output frequency fOUT of the output signal VOUT will be substantially in baseband, so the follow-up demodulation and data reconstruction process would be performed on the resulted output signal VOUT. An example of the gm stage 350 is the gm stage of a conventional Gilbert cell mixer, and an example of which is illustrated in FIG. 4. Since the operational principle of the gm stage shown in FIG. 4 is well known to a person skilled in the art, further explanation on the gm stage 350 will be omitted.
Please note that the if the mixer 300 is used in a direct conversion transmitter as an up mixer, the input frequency fIN of the input signal VIN would be in baseband, and the output frequency fOUT of the output signal VOUT will be substantially the same as the third frequency f3 of the differential current signal IDIFF. Besides, the basic frequency fBASIC of the preliminary local signal LO0, the first frequency f1 of the first local signal LO1, and the second frequency f2 of the second local signal LO2 are all different from the output frequency fOUT of the output signal VOUT. The third frequency f3 of the differential current signal IDIFF is substantially the same as output frequency fOUT of the output signal VOUT.
For implementing the first multiplier 340, which multiplies two voltage signals (the first local signal LO1 and the second local signal LO2) together to generate a current signal (the differential current signal IDIFF), two documents could be referenced. One is “Low-Voltage BiCMOS Four-Quadrant Multiplier Using Triode-Region Transistors,” IEEE Transactions on Circuits and Systems-I: Fundamental Theory and Applications, vol. 46, no. 7, July 1999, included herein by reference. The other one is “A Parallel Structure for CMOS Four-Quadrant Analog Multipliers and Its Application to a 2-GHz RF Downconversion Mixer,” IEEE Journal of Solid-State Circuits, vol. 33, no. 6, June 1998, included herein by reference.
In a conventional mixer used in a direct conversion receiver or a direct conversion transmitter, the LO self-mixing problem or LO leakage problem occurs because the frequency of the local oscillating signal(s) (in voltage form) used by the mixer is substantially equal to the frequency of the input signal or the output signal of the mixer. However, in the exemplary embodiment of the present invention, the basic frequency fBASIC of the preliminary local signal LO0, the first frequency f1 of the first local signal LO1, and the second frequency f2 of the second local signal LO2 are all different from the input frequency fIN of the input signal VIN (or different from the output frequency fOUT of the output signal VIN when the mixer 300 is an up mixer), hence the LO self-mixing problem (or the LO leakage problem) will be minimized. The differential current signal IDIFF still induces voltage swings with a frequency being substantially the same as the input frequency fIN of the input signal VIN occur at a node A and a node B of FIG. 4. However, since both these two nodes are low impedance points in the gm stage 350, voltage swings induced by the differential current signal IDIFF at these two nodes will be small, and the LO self-mixing problem (or the LO leakage problem) caused by the voltage swings at node A and B could be ignored.
Take a look at an example, assume that the first divisor N1 is 4, the second divisor N2 is 2, and the input signal VIN is a RF signal having the input frequency fIN lie between 1805 ̃1990 MHz. Since the third frequency f3 of the differential current signal IDIFF equals to [(N1+N2)/N1N2]×fBASIC=(3/4)×fBASIC and should be substantially the same as input frequency fIN of the input signal VIN, fBASIC should be set as (4/3)×fIN and lie between 2406.6 ̃2653.3 MHz. An advantage of using ‘4’ as the first divisor is that local signals used for the in-phase (I) channel and the quadrature (Q) channel could be generated at once easily.
Aside from the dividing scheme illustrated in FIG. 3, a multiplying scheme could also be used by the present invention. Under the multiplying scheme, the first divider 320 and the second divider 330 should be replaced by a second multiplier and a third multiplier respectively. Complying with the restrictions that basic frequency fBASIC, the first frequency f1, and the second frequency f2 being all different from the input frequency fIN (or the output frequency fOUT) and the third frequency f3 being substantially the same as input frequency fIN (or the output frequency fOUT), the second multiplier and the third multiplier multiply the basic frequency of the preliminary local signal with a second and a third multiplicator to obtain the first local signal having the first frequency and the second local signal having the second frequency respectively. Both a second multiplicator used by the second multiplier and a third multiplicator used by the third multiplier could be integers, and more particularly, the second multiplicator could be 4 and the third multiplicator could be 2.
Please note that in FIG. 3, the preliminary local signal LO0, the first local signal LO1, and the second local signal LO2 could be single-ended or differential signals, and the gm stage 350 could have a single balance or a double balance structure accordingly, and the present invention is not limited by this disclosure.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A frequency conversion apparatus for converting an input signal having an input frequency into an output signal having an output frequency, the frequency conversion apparatus comprising:
a first multiplier for multiplying a first local signal having a first frequency with a second local signal having a second frequency to generate a differential current signal having a third frequency, wherein the third frequency is substantially the same as the input frequency or the output frequency, and the first frequency and the second frequency are different from the input frequency or the output frequency; and
a gm stage electrically connected to the first multiplier, for receiving the differential current signal and the input signal to generate the output signal.

2. The frequency conversion apparatus of claim 1, further comprising:
a local oscillator (LO) for generating a preliminary local signal having a basic frequency;
a first divider electrically connected to the local oscillator and the first multiplier, for dividing the basic frequency of the preliminary local signal by a first divisor to obtain the first local signal having the first frequency; and
a second divider electrically connected to the local oscillator and the first multiplier, for dividing the basic frequency of the preliminary local signal by a second divisor to obtain the second local signal having the second frequency.

3. The frequency conversion apparatus of claim 2, wherein the first divisor of the first divider and the second divisor of the second divider are integers.

4. The frequency conversion apparatus of claim 3, wherein the first divisor of the first divider is 4 and the second divisor of the second divider is 2.

5. The frequency conversion apparatus of claim 2, wherein the preliminary local signal, the first local signal, and the second local signal are voltage signals, and the basic frequency, the first frequency, and the second frequency are different from the input frequency of the input signal or the output frequency of the output signal.

6. The frequency conversion apparatus of claim 1, further comprising:
a local oscillator (LO) for generating a preliminary local signal having a basic frequency;
a second multiplier electrically connected to the local oscillator and the first multiplier, for multiplying the basic frequency of the preliminary local signal with a second multiplicator to obtain the first local signal having the first frequency; and
a third multiplier electrically connected to the local oscillator and the first multiplier, for multiplying the basic frequency of the preliminary local signal with a third multiplicator to obtain the second local signal having the second frequency.

7. The frequency conversion apparatus of claim 6, wherein the second multiplicator of the second multiplier and the third multiplicator of the third multiplier are integers.

8. The frequency conversion apparatus of claim 6, wherein the second multiplicator of the second multiplier is 4 and the third multiplicator of the third multiplier is 2.

9. The frequency conversion apparatus of claim 6, wherein the preliminary local signal, the first local signal, and the second local signal are voltage signals, and the basic frequency, the first frequency, and the second frequency are different from the input frequency of the input signal or the output frequency of the output signal.

10. The frequency conversion apparatus of claim 1, further comprising:
a filter electrically connected between the first multiplier and the gm stage, for filtering out an unwanted frequency of the differential current signal.

11. The frequency conversion apparatus of claim 1, wherein the input signal and the output signal are differential signals, the gm stage comprises:
a first transistor pair electrically connected to the first multiplier, for generating a first single-ended output signal of the output signal according to the input signal and a first single-ended current signal of the differential current signal; and
a second transistor pair electrically connected to the first multiplier, for generating a second single-ended output signal of the output signal according to the input signal and a second single-ended current signal of the differential current signal.

12. A method for converting an input signal having an input frequency into an output signal having an output frequency, the method comprising:
multiplying a first local signal having a first frequency with a second local signal having a second frequency to generate a differential current signal having a third frequency, wherein the third frequency is substantially the same as the input frequency or the output frequency, and the first frequency and the second frequency are different from the input frequency or the output frequency; and
receiving the differential current signal and the input signal to generate the output signal.

13. The method of claim 12, further comprising:
generating a preliminary local signal having a basic frequency;
dividing the basic frequency of the preliminary local signal by a first divisor to obtain the first local signal having the first frequency; and
dividing the basic frequency of the preliminary local signal by a second divisor to obtain the second local signal having the second frequency.

14. The method of claim 13, wherein the first divisor and the second divisor are integers.

15. The method of claim 13, wherein the first divisor is 4 and the second divisor is 2.

16. The method of claim 13, wherein the preliminary local signal, the first local signal, and the second local signal are voltage signals, and the basic frequency, the first frequency, and the second frequency are different from the input frequency of the input signal or the output frequency of the output signal.

17. The method of claim 12, further comprising:
generating a preliminary local signal having a basic frequency;
multiplying the basic frequency of the preliminary local signal with a second multiplicator to obtain the first local signal having the first frequency; and
multiplying the basic frequency of the preliminary local signal with a third multiplicator to obtain the second local signal having the second frequency.

18. The method of claim 17, wherein the second multiplicator and the third multiplicator are integers.

19. The method of claim 17, wherein the second multiplicator is 4 and the third multiplicator is 2.

20. The method of claim 17, wherein the preliminary local signal, the first local signal, and the second local signal are voltage signals, and the basic frequency, the first frequency, and the second frequency are different from the input frequency of the input signal or the output frequency of the output signal.

21. The method of claim 12, further comprising:
filtering out an unwanted frequency of the differential current signal before it get into the receiving step.