BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a conventional harmonic mixer.
FIG. 2 is a schematic diagram of conventional implementation with coupling capacitors and inductor-based loads in the RF front-end circuitry.
FIG. 3 illustrates a schematic block diagram of a radio-frequency front-end with mixers according to one embodiment of the present invention.
FIGS. 4A and 4B illustrate detailed schematic diagrams of the dividing unit in FIG. 3 according to one embodiments of the present invention.
FIG. 5 illustrates a detailed schematic diagram of the first and the second mixer units in a cascode configuration in FIG. 3 according to one embodiment of the present invention.
FIG. 6 illustrates schematic frequency spectra and corresponding amplitude at different nodes of the mixer units of the cascode configuration in FIG. 5 according to one embodiment of the present invention.
FIG. 7 shows a flow chart of down converting a radio-frequency signal with a single-stage mixer according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to the simplified mixer architecture with dynamic intermediate frequency used in a radio-frequency (RF) front-end to actively adjust the intermediate frequency by the first and the second mixer units in a cascode configuration. A dividing unit can be further employed to receive an oscillator signal to provide a first, a second, and a third frequency signals to the first and the second mixer units. Moreover, the radio-frequency front-end with simplified mixer architecture can reduce the size of the circuitry. The mixer architecture of the present invention is applicable to any kind of transceivers including receivers and transmitters, preferably for direct conversion receivers.
Referring to FIG. 3, a schematic block diagram of a radio-frequency front-end with mixers according to one embodiment of the present invention is shown. The radio-frequency front-end 300 comprises a band-pass filter 302, an amplifier 304, a first mixer unit 306 and a second mixer unit 308. The band-pass filter 302 receives a first radio frequency (RF) signal to suppress the unwanted signal that is out of the wanted frequency band to generate a second RF signal. The amplifier 304 is connected to the band-pass filter 302 to amplify the second RF signal and output a third RF signal, denoted by SRF. The first mixer unit 306 coupled to the amplifier 304 is used to mix the third RF signal (SRF) with a first frequency signal (S1) to down convert the third RF signal (SRF) to an intermediate frequency (IF) and outputs an IF signal, denoted by SIF. The second mixer unit 308 is connected to the first mixer unit 306 in a cascode configuration 316, which will be shown in detail later, and has an I-channel mixer 308a and a Q-channel mixer 308b to transform IF signal (SIF) to an I-channel signal (SI) and a Q-channel signal (SQ). Note that the single-stage structure with cascode configuration 316 can improve circuit noise. Further, some kind of active or passive components, such as coupling capacitors, between the first and the second mixer units (306, 308) in the single-stage structure are removed to reduce the size of the first and the second mixer units (306, 308) within the radio-frequency front-end circuitry. Moreover, such a cascode configuration 316 benefits lower power consumption on the radio-frequency front-end.
The I-channel mixer 308a is used to mix the IF signal (SIF) with a second frequency signal (S2) to output the I-channel signal (SI) at baseband. The Q-channel mixer 308b is used to mix IF signal (SIF) with a third frequency signal (S3) to output the Q-channel signal (SQ) at baseband. Note that, in order to improve the noise immunity, the third RF signal (SRF), the first frequency signal (S1), the second frequency signal (S2), and the third frequency signal are preferably of differential type. However, the present invention can also applied to the case that the signals are of single-ended type.
Referring to FIG. 3 again, the radio-frequency front-end 300 further comprises a dividing unit 310 connected to the first and the second mixer units (306, 308) for receiving an oscillator signal (S0), e.g. generated by a voltage-controlled oscillator (VCO), to generate the first frequency signal (S1), the second frequency signal (S2), and the third frequency signal (S3) such that the frequency of the first frequency signal (S1) substantially equals the frequency of the oscillator signal (S0) divided by two's power of a first non-negative integer (N1), the frequency of the second and the third frequency signals (S2, and S3) substantially both equals the frequency of the oscillator signal (S0) divided by two's power of a second non-negative integer (N2), and the second frequency signal (S2) is approximately 90 degree out of phase with respect to the third frequency signal (S3). In order to down convert the third RF signal (SRF) to the baseband, the frequency summation of the first and second (or third) frequency signals (S2 or S3) is substantially and preferably exactly equal to the frequency of the third RF signal (SRF). It should be noted that although the carrier frequency of the third RF signal (SRF) is preferably exactly equal to the frequency summation of the signals (S1 and S2), the frequency of the third RF signal (SRF) may not exactly equal, but substantially equal, the frequency summation of the signals (S1 and S2) due to practical limitation of the physical circuit.
In one embodiment, the first non-negative integer (N1) is 1, the second non-negative integer (N2) is 2, and the frequency of the oscillator signal (S0) substantially equals 4/3 times the carrier frequency of the third RF signal (SRF). In other words, we have:
where f0 is the frequency of the oscillator signal (S0), f1 is the frequency of the first frequency signal (S1), f2 is the frequency of the first frequency signal (S2), and fRF is the carrier frequency of the third RF signal (SRF).
Alternatively, the first non-negative integer (N1) can also be 2, the second non-negative integer (N2) is 3, and the frequency of the oscillator signal (S0) substantially equals 8/3 times the carrier frequency of the third RF signal (SRF). In other words, we have:
FIGS. 4A and 4B show detailed schematic diagrams of the dividing unit in FIG. 3 according to embodiments of the present invention. In FIG. 4A, the dividing unit 310 includes a first divider 312 and a second divider 314a. The first divider 312 is used to divide the oscillator signal (S0) to generate the first frequency signal (S1). The second divider 314a connected to the first divider 312 to further divide the first frequency signal (S1) to generate the second frequency signal (S2) and the third frequency signal (S3). In FIG. 4B, a first divider 312 is used to divide the oscillator signal (S0) to generate the first frequency signal (S1) and, on the other hand, a second divider 314b is employed for dividing the oscillator signal (S0) to generate the second frequency signal (S2) as well as the third frequency signal (S3).
FIG. 5 shows a detailed schematic diagram of the first and the second mixer units in a cascode configuration in FIG. 3 according to one embodiment of the present invention. The first mixer unit 306 includes transistors Q1 to Q6. A RF segment includes transistors Q1 and Q2. The bases of Q1 and Q2 are together used to receive the third RF signal (SRF) that is a differential type signal and the emitters of Q1 and Q2 are connected to a biasing current source (Ib). The emitters of Q3, Q4, Q5 and Q6 are coupled to the collectors of Q1 and Q2, respectively. The bases of Q3, Q4, Q5 and Q6 are used to receive the first frequency signal (S1).
The second mixer unit 308 includes an I-channel mixer 308a and a Q-channel mixer 308b, where the I-channel mixer 308a comprises Q7, Q8, Q9 and Q10, and the Q-channel mixer 308b comprises Q11, Q12, Q13 and Q14. The emitters of Q7 and Q8 in the I-channel mixer 308a as well as the emitters of Q11 and Q12 in the Q channel mixer 308b are connected to the collectors of Q3 and Q5 in the first mixer unit 306. On the other hand, the emitters of Q9 and Q10 in the I-channel mixer 308a as well as the emitters of Q13 and Q14 in the Q-channel mixer 308b are connected to the collectors of Q4 and Q6 in the first mixer unit 306. The bases of Q7, Q8, Q9 and Q10 in the I-channel mixer 308a are used to receive the second frequency signal (S2), while the bases of the Q11, Q12, Q13 and Q14 in the Q-channel mixer 308b are used to receive the third frequency signal (S3).
The collectors of Q7 and Q9 are together connected to a load, e.g. resistive component connected to a voltage source (VCC), and so are the collectors of Q8 and Q10, and thus outputs the differential I-channel signal (SI) based on the collectors in the I-channel mixer 308a. Similarly, the collectors of Q11 and Q13 are together connected to a load, e.g. resistive component connected to the voltage source (VCC), and so are the collectors of Q12 and Q14, and thus outputs the differential Q-channel signal (SQ) based on the collectors in the Q channel mixer 308b.
FIG. 6 illustrates schematic frequency spectra and corresponding amplitude at different nodes of the mixer units of the cascode configuration 316 in FIG. 5 according to one embodiment of the present invention. In one embodiment, the first RF signal is suppressed by the band-pass filter 302, such as a surface acoustic wave (SAW) filter generating a suppressed signal 502, to reject unwanted signal, e.g. image signals 500 at the frequencies (fimg and −fimg) on the opposite side at the carrier frequencies (fRF and −fRF), and thus output the third RF signal (SRF) 504. Then, the third RF signal (SRF) is inputted to the first mixer unit 306. The third RF signal (SRF) is convoluted with the frequencies (f1 and −f1) of the first frequency signal (S1) in the first mixer unit 306 to dynamically generate IF signal (SIF) 506 at the intermediate frequencies (fIF and −fIF). Finally, IF signal (SIF) is convoluted with the frequency (f2) of the second and the third frequency signals (S2 and S3) in the second mixer unit 308, which is optionally filtered by a channel filter (not shown), to form I-channel and Q-channel signals (SI and SQ) 508 at baseband.
The spectra of the first, the second, and the third frequency signals (S1, S2 and S3) generated by a dividing unit 310 in FIG. 3 also are shown in FIG. 6. As above-mentioned, the frequency (f1) of the first frequency signal (S1) is equal to the frequency (f0) of the oscillator signal divided by two's power of x, where x is a non-negative integer. The frequency (f2) of the second and the third frequency signals (S2 and S3) substantially equals the frequency (f0) of the oscillator signal divided by two's power of a second non-negative integer, and the second frequency signal (S2) is approximately 90 degree out of phase with respect to the third frequency signal (S3). The frequency (f1) of the first frequency signal (S1) is smaller than the frequency (f0) of the oscillator signal received by the dividing unit 310 for eliminating phase noise of the second RF signal (S2) to improve phase noise performance at the carrier frequency (fRF).
As a result, the second mixer unit 308 is sequentially coupled with the first mixer unit 306 in a cascode configuration 316 to construct the single-stage architecture. Namely, the first mixer unit 306 is directly stacked with the second mixer unit 308. The single-stage structure with cascode configuration 316 can improve circuit noise and current or voltage fluctuation in the mixers, and has a reasonably higher gain than that of the multi-stage structure in the prior art.
In the present invention, the frequency (f0) is suitable for any frequency or frequency bands, such as Industrial scientific Medical (ISM, frequency band), Global System for Mobile Communication (GSM), Advance Mobile Phone System (AMPS), and Digital Communication System (DCS). In one embodiment, the frequency (f0) is no greater than 5.0 GHz. Preferably, the frequency (f0) is no greater than 2.4 GHz. More preferably, the frequency (f0) has a frequency ranging from 0.8 to 2.4 GHz.
In one preferred embodiment, the size of the radio-frequency front-end circuitry is significantly reduced because the load of the LNA 302 and the second mixer unit 308 in FIG.3 is resistor-based. Preferably, the size-reduced level of the resistor-based loads is up to 100˜1000 or more times in comparison with conventional inductive loads of the amplifier or mixer to advantageously increase design capability of the radio-frequency front-end.
FIG. 7 shows a flow chart of down converting a radio-frequency signal with a single-stage mixer according to the present invention. Starting at step S700, a first radio frequency (RF) signal is filtered by a band-pass filter to generate a second RF signal. Then, in step S702, the second RF signal is amplified by a LNA and outputs a third RF signal.
Thereafter, in step S704, an oscillator signal is divided to generate a first, a second, and a third frequency signals. The frequency of the first frequency signal substantially equals the frequency of the oscillator signal divided by two's power of a first non-negative integer. The frequency of the second and the third frequency signals substantially equals the frequency of the oscillator signal divided by two's power of a second non-negative integer. The second frequency signal is approximately 90 degree out of phase with respect to the third frequency signal.
In one embodiment, during the step of dividing the oscillator signal to generate the first, the second, and the third frequency signals, the oscillator signal is divided to generate the first frequency signal and then the first frequency signal is divided to generate the second frequency signal and the third frequency signal. In another embodiment, during the step of dividing the oscillator signal, the oscillator signal is divided to generate the first frequency signal and the oscillator signal is divided to generate the second frequency signal and the third frequency signal. Next, in step S706, the radio frequency signal at a carrier frequency is mixed with the first frequency signal to down convert the RF signal to an intermediate frequency (IF) and output an IF signal using a first mixer. The frequency of the first frequency signal is preferably smaller than an oscillator signal inputted into the dividing unit to eliminate phase noise of the amplified received signal.
Finally, in step S708, the IF signal is mixed with the second and the third frequency signals using a second mixer to output an I-channel signal and a Q-channel signal at baseband, respectively, where the first mixer and the second mixers are connected in a cascode configuration with the second mixer.
The advantages of the present invention include: (a) providing a mixer architecture having first and second mixer units in a cascode configuration to dynamically adjust the intermediate frequency in a radio-frequency front-end; (b) providing a simplified mixer architecture with a dividing unit to provide a plurality of frequency signals to the first and the second mixer units to improve the efficiency of the radio-frequency front-end; (c) providing a radio-frequency front-end with a simplified mixer architecture to reduce the size of the circuitry; and (d) providing a mixer with dynamic intermediate frequency to solve the problem of self-mixing in a radio-frequency front-end.
As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative rather than limiting of the present invention. It is intended that they cover various modifications and similar arrangements be included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.
Patent Application
20080009260
