Patent Application
20080125073
The present disclosure generally relates to the processing of radiofrequency (RF) signals, and has applications particularly but not exclusively in integrated circuits which are receivers for RF signals comprising a multiplier, also called a mixer.
Such integrated circuit devices are found in mobile telephones for example.
A mixer is a circuit adapted to multiply a signal corresponding to a received RF signal of frequency FRF and an oscillation signal of frequency FLO generated by a local oscillator in the receiver, in order to extract a component of the resulting signal at a×FRF+b×FLO where a and b are integer non-zero numbers. For example, the extracted component corresponds to (a;b)=(1;−1) or (−1;1).
The mixer 3 is a passive mixer, meaning it operates without bias current.
The low noise amplifier 2 is adapted to receive a radiofrequency signal S and deliver a signal S′ resulting from a first amplification of the signal S by the receiver, with little degradation to the signal-to-noise ratio.
In the case shown in
The mixer 3 comprises an input terminal b adapted to receive when operational the signals delivered by the signal generator 4, and a multiplier 9 which comprises a mixing module 10, 11 for each of the channels I and Q. The mixer 3 additionally comprises two capacitors 12, 13 of the same capacitance C, with one of the capacitors placed parallel to the output of the mixing module 10, and the other placed parallel to the output of the mixing module 11. Each capacitor acts as a low pass filter.
The oscillation signal generator 4 is adapted to provide to the mixing module 10 of the mixer 3, via the input terminal b, on channels 5, 6, a differential oscillation signal TI of frequency FLO, and to provide to the mixing module 11, via the terminal b, on channels 7, 8, a differential oscillation signal TQ of frequency FLO.
The differential signal TI is equal to the difference TIP−TIM, where TIM and TIP are approximately square signals of duty cycle α=0.5, in phase opposition, of frequency FLO, delivered by the generator 4 on channel 5 and 6 respectively. The duty cycle α designates the ratio between the time (τ) at the high state during a period (T) and the period (T), or
The differential signal TQ is equal to the difference TQP−TIM, where TQM and TQP are approximately square signals of duty cycle α=0.5, in phase opposition, of frequency FLO, delivered by the generator 4 on channel 5 and 6 respectively.
The oscillation signals TI and TQ of the respective channels I and Q are generally out of phase by 90°.
In the case in question, the mixer 3 is a passive voltage-to-voltage mixer (dependent on the ratio between the output impedance and input impedance), meaning that it is characterized by a transfer of the input voltage to an output voltage.
The voltage of the signal S′ is delivered to the inputs of each mixing module 10, 11.
This voltage is mixed by the mixing module 10 with the voltage of the signal TI (meaning that the voltage of the signal S′ is multiplied by the voltage of the signal TI) and it is mixed by the mixing module 11 with the voltage of the signal TQ (meaning that the voltage of the signal S′ is multiplied by the voltage of the signal TQ).
The mixing module 10 delivers a signal SI″ of a voltage equal to the mixed voltages of signals S′ and TI.
The mixing module 11 delivers a signal SQ″ of a voltage equal to the mixed voltages of signals S′ and TQ.
The capacitors 12 and 13 eliminate the high frequencies of signals SI″ and SQ″, meaning the signal frequencies exceeding the frequency ±(FRF−FLO), in particular the components located at frequency FRF+FLO, and provide as input to the post-mixer amplifier with filter 14 the components of signals SI″ and SQ″ at frequency ±FIF=±(FRF−FLO). The post-mixer amplifier with filter 14 extracts the desired component from each channel I and Q for subsequent processing (not represented) in the receiver 1. For example, the component at frequency (+FIF) is extracted and the component at the opposite frequency is eliminated. In another example, the frequency (−FIF) is extracted and the component at the opposite frequency is eliminated. The value of FIF is positive or negative depending on the case.
Such processing allows creating a mixer 3 with image rejection because during the processing sequence it differentiates between the signal S at frequency FRF=FLO+FIF and the image signal of frequency FLO−FIF.
Such a mixer has a voltage gain which is equal to
As an illustration, in one embodiment the voltage signal S′ is a signal of type A cos(FRFt) as represented by the thin line in
The signals on channel I at the output of the mixer 3 are represented as a thick line in
The gain in the mixer 3 corresponds to the mean of the corrected signal SI,Q″ divided by the amplitude of the input signal.
There is a need for a mixer presenting a better gain than the gain of −3.92 dB.
For this purpose, a first aspect of the invention proposes a mixer presenting an improved gain.
A radiofrequency mixer according to the first aspect of the invention comprises:
a terminal from which a first radiofrequency signal is available, when operational,
an input for receiving a second radiofrequency signal,
a means of multiplication for multiplying the first radiofrequency signal and the second radiofrequency signal.
In a mixer of one embodiment of the invention, the first radiofrequency signal is:
either a differential signal corresponding to the difference between a first component of the signal and a second component of the signal, each corresponding to a respective signal which is approximately square and of a duty cycle strictly below 0.5,
or a single-ended signal which is a non-differential approximately square signal of a duty cycle strictly below 0.5.
Such a mixer allows obtaining an output gain which is better than the gain presented by mixers of the prior art.
In one embodiment, the duty cycle of the approximately square signal corresponding to each signal component of the first radiofrequency signal, the respective duty cycles of the non-differential approximately square signal, is less than 0.25, which allows obtaining a further increase in the gain.
In one embodiment, the mixer comprises an output for delivering a voltage indicating the result of the multiplication of the first radiofrequency signal and the second radiofrequency signal, and comprising a capacitor parallel to said output.
This allows either decreasing the cutoff frequency of the low-pass filter corresponding to the capacitor, or decreasing the size of the capacitor necessary at an equal cutoff frequency.
In one embodiment, the first radiofrequency signal has a first frequency and the second radiofrequency signal has a second frequency, and the absolute value of the quotient of the second frequency divided by the first frequency is between 0.5 and 2 (not including 0.5 and 2).
In one embodiment, the first signal is a differential signal and the means of multiplication comprises two differential input terminals and two differential output terminals, and:
the first input terminal is connected to the common sources of a first and second transistor, the second input terminal is connected to the common sources of a third and fourth transistor,
the common gates of the first and fourth transistors receive the first component of the first signal and the common gates of the second and third transistors receive the second component of the first signal.
the first output terminal is connected to the common drains of the first and third transistors, the second output terminal is connected to the common drains of the second and fourth transistors.
A second aspect of the invention proposes a system-on-chip comprising a mixer according to the first aspect of the invention.
A third aspect of the invention proposes a method for processing signals in a mixer adapted to multiply a first radiofrequency signal with a second radiofrequency signal.
In the process of one embodiment of the invention, the first radiofrequency signal is:
either a differential signal corresponding to the difference between a first signal component and a second signal component, each respectively corresponding to an approximately square signal for which the duty cycle is strictly below 0.5,
or a single-ended signal which is a non-differential approximately square signal for which the duty cycle is strictly below 0.5.
Other features of one or more non-limiting and non-exhaustive embodiments of the invention will be further illustrated by the description which follows. This is purely illustrative and should be read with reference to the attached drawings, where:
FIGS. 2.1-2.2 show voltage signals for the receiver of
FIGS. 3.1-3.2 also show voltage signals for the receiver of
In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
In one embodiment of the invention, in a radiofrequency receiver of a system-on-chip of the type represented in
The mixer 100 comprises a terminal B adapted to interface with the signal generator 101 and to receive the oscillation signals delivered by this generator, a mixing module 102 for channel I, and a mixing module 103 for channel Q. The mixer 100 additionally comprises the capacitor 109 which is parallel to the output of the mixing module 102, and the capacitor 110 which is parallel to the output of the mixing module 103.
The capacitors 109 and 110 both have a capacitance value of C1.
The oscillation signal generator 101 provides to the mixing module 102, via the input terminal B of the mixer 100, a differential oscillation signal LOI of frequency FLO on channels 105, 106, and provides to the mixing module 103 a differential oscillation signal LOQ of frequency FLO on channels 107, 108.
The differential signal LOI is equal to the difference LOIP−LOIM, where LOIM and LOIP are approximately square signals of frequency FLO, respectively delivered by the generator 101 on channels 105, 106.
The voltage of the LOIM signal over time appears in
The differential signal TQ is equal to the difference LOQP−LOQM, where LOQM and LOQP are approximately square signals in phase opposition of frequency FLO, respectively delivered by the generator 101 on channels 107, 108.
The voltage of the signal LOQM over time is shown in
Thus each oscillation signal LOIP, LOIM, LOQP and LOQM is an approximately square signal of frequency FLO, of a duration τ at the high state and a duration T-τ at the low state (T being the period of these signals, equal to 1/FLO), which corresponds to a duty cycle of
In one embodiment of the invention, the duty cycle for each of these oscillation signals is strictly below 0.5.
In one embodiment, the oscillation signals LOIM and LOIP are out of phase with each other by 180°. Similarly, the oscillation signals LOQM and LOQP are out of phase with each other by 180°.
The oscillation signals LOI and LOQ for the respective channels I and Q are out of phase with each other by 90°.
In the embodiment in question, the mixing module 102 for the channel I comprises two differential input terminals I1 and I2 between which the voltage S′ issuing from LNA 2 is applied, and two differential output terminals OI1 and OI2 between which the output voltage SI″ of the module is delivered, with this output voltage indicating the result of the mixing (meaning the multiplication) of signal S′ and signal LOI.
The mixing module 102 of the channel I additionally comprises four CMOS transistors M1, M2, M3 and M4 used as switches.
As is shown in
It is also possible to set the bias of transistors M1, M2, M3 and M4 using an impedance similar to the impedance ZPMA and provided upstream from the mixer, I on the side of the low noise amplifier.
The input terminal I1 is connected to the common sources of transistors M1 and M2, and the input terminal I2 is connected to the common sources of transistors M3 and M4. The common gates for transistors M1 and M4 are connected to the channel 105 for delivering the voltage of the oscillation signal LOIM, while the gates for transistors M2 and M3 are connected to the channel 106 delivering the voltage of the oscillation signal LOIP.
The output terminal O1I is connected to the common drains of transistors M1 and M3. The output terminal O2I is connected to the common drains of transistors M2 and M4.
The mixing module 103 for the channel Q comprises the differential input terminals I1 and I2, between which the voltage S′ issuing from the LNA 2 is applied. It additionally comprises two differential output terminals OQ1 and OQ2, between which the output voltage SQ″ of the module is delivered, with this output voltage indicating the result of the mixing of signal S′ and signal LOQ.
The mixing module 103 of the channel Q additionally comprises four CMOS transistors M1′, M2′, M3′ and M4′, assembled similarly to the respective transistors M1, M2, M3 and M4 of the mixing module 102.
Additionally represented in
The operation of an embodiment of the mixing module 102 during a period T is described below. The operation of the mixing module 103 is similar.
Based on their being assembled as represented in
In the described embodiment, the radiofrequency receiver comprising the mixer 100 is a NZIF receiver. The frequency FRF of the radiofrequency signal S′ is equal for example to 2.4 GHz+FIF, where FIF is the frequency of the signal of interest, and the frequency FLO of the oscillation signal LO is for example equal to 2.4 GHz. One embodiment of the invention can also be implemented in a ZIF receiver, where the frequencies FLO and FRF are equal.
In general, an embodiment of the invention is implemented for frequencies FRF and FLO such that the absolute value of the ratio
is between 0.5 and 2 (not including 0.5 and 2).
As shown in
Phases τ2M, τ3M etc., similar to this phase τ1M, occur every period of duration T.
Then during a phase t1, the signals LOIM and LOIP are both in the low state. The transistors M1, M2, M3 and M4 are non-conducting. The signal RI″ provided during the phase t1 by the mixing module 102 (case where C1=0) is then equal to 0. The signal SI″ provided during the phase t1 by the mixing module 102 (case where C1≠0) is then equal to the voltage delivered to the terminals of the capacitor 109 which discharges into the load impedance ZPMA of the post-mixer amplifier 14, thus achieving in phase t1 an averaging of the values assumed by the voltage −S′ during the phase τ1M, until a pair of transistors becomes conductive once again. The mean of the signal SI″ delivered during phase t1 is ultimately equal to the mean of the signal S′ over a period of time corresponding to the phase τ1M because in this embodiment the capacitance discharge into the impedance ZPMA is negligible.
Phases t3, t5, etc., similar to this phase t1, occur every period of duration T.
Then, during a first phase τ1P of duration τ, the signal LOIP is at the high state and the signal LOIM is at the low state. The transistors M2 and M3 are conductive while the transistors M1 and M4 are non-conductive. The signal SI″ provided during phase τ1P by the mixing module 102 (case where C1≠0), or the signal RI″ provided during phase τ1P by the mixing module 102 (case where C1=0), is then equal to S′.
Phases τ2P, τ3P etc., similar to this phase τ1P, occur every period of duration T.
Then during a phase t2 (such that T=2τ+t1+t2), the signals LOIM and LOIP are both in the low state. The transistors M1, M2, M3 and M4 are non-conductive. The signal RI″ provided during phase t2 by the mixing module 102 (case where C1=0) is then equal to 0. The signal SI″ provided during phase t2 by the mixing module 102 (case where C1≠0) is then equal to the voltage delivered to the terminals of the capacitor 109, which once again discharges into the impedance ZPMA, thus realizing in phase t2 an averaging of the values assumed by the voltage S′ during phase τ1P. The mean of the signal SI″ delivered during phase t2 is ultimately equal to the mean of the signal S′ in phase τ1P because in this embodiment, the capacitance discharge into the impedance ZPMA is negligible.
Phases t4, t6, etc., similar to this phase t2, occur every period of duration T. The operation of the mixing module 103 for the channel Q is similar to the mixing module 102 for the channel I, except the signals LOI and LOQ are out of phase by 90°.
The graphs discussed above correspond to a capacitance value C1 which is low or zero for the capacitors 109, 110. In practice, the capacitance value C1 is chosen to be sufficient in particular to filter out the second-order harmonics of the output signal from the mixing module 102. On the left side of
Represented in the right part of
The lower the value of the duty cycle α, the closer the mean value of the output signal SI″ or SQ″ to the maximum value of the signal S′.
The voltage gain of a mixer 100 is represented in
In actuality, the duty cycle α is calculated by considering a part of the rising and falling phases of the oscillation signals over time to be in the high state (as a function of the value of the common mode, meaning as a function of the bias voltage of the transistors corresponding to the switch), and therefore corresponds to a lower effective duty cycle, therefore to a greater gain.
The curve L1 corresponds to the case where the stage of the LNA module 2 represented in
The curve L2 corresponds to the case where the stage of the LNA module 2 upstream from the mixer 100 presents an output module of the LNA which delivers the voltage S′ in a common manner to the mixing modules 102, 103 for channels I and Q, such as the module 120 represented in
An output module of the LNA, for example a buffer circuit, is adapted to output a voltage which copies or even amplifies the voltage it receives as input, while presenting at the output to the mixer 100 a lower impedance than the impedance ZPMA presented by the PMA amplifier 14 at the output of the mixer. An LNA can comprise such a circuit for each channel or one common to both channels, or such modules in a cascading arrangement.
For each of the curves L1, L2, the lower the duty cycle below 0.5, the greater the gain of the mixer 100. The increase in the gain is particularly noticeable when the duty cycle α is below 0.25. In such a case, no time period exists during which the oscillation signals LOIP and LOQP are both in the high state. Similarly, the oscillation signals LOIM and LOQM are never in the high state at the same time.
In fact, for the curve L1, the gain is about −3.8 dB for α=0.5, −0.9 dB for α=0.25, and progressively increases as a decreases, until it reaches −0.1 dB for α=0.1.
For the curve L2, the gain is about −7.4 dB for a =0.5, −0.9 dB for α=0.25, and progressively increases as a decreases, until it reaches −0.1 dB for α=0.1.
As can be seen by comparing curves L1 and L2, implementing the invention in a mixer eliminates the constraint of two separate modules at the output from the LNA, disconnecting channel I from channel Q when the duty cycle is less than 0.25. In fact, in the prior art, the use of two separate output modules for the LNA was advantageous because this avoided additional degradation in the gain due to a discharge of one of the two channels I, Q into the other channel when the channels I and Q were conductive at the same time.
When the duty cycle is less than 0.25 as described in the invention, there is no longer any moment when the channels I and Q are conductive simultaneously. There is therefore no more interference from one channel to the other. A common output module of the LNA shared by the channels I and Q allows an increased gain in comparison to two separate modules when the value of the duty cycle is less than 0.25. This also saves space in the integrated circuit of the system-on-chip.
In fact, the case considered in
The signals SI″ and SQ″, indicating the result of multiplications made by the mixing modules 102, 103 for channels I and Q, are then delivered to the PMA amplifier 14 as represented in
The load impedance ZPMA must be sufficiently large not to discharge the capacitors 109, 110 when a pair of transistors (M1, M4) or (M2, M3) is conductive.
Each capacitor 109, 110 eliminates the high frequencies at the output from the mixer 100, particularly the components at ±(FRF+FLO). The more the duty cycle α decreases, the more the cutoff frequency Fc of the filter achieved by the capacitor 109, 110, beyond which the frequencies are eliminated, decreases. When a pair of transistors (M1, M4) or (M2, M3) are conductive, the pair R1.C1 defines the filtering. However, when all transistors are non-conductive, the resistance R1 as seen from the capacitor becomes infinite. Therefore the mean value of R1 as seen by the capacitor 109, 110 is
This enables a lower cutoff frequency while retaining the component situated at ±FIF.
Reducing the value of the duty cycle also uses less space on the integrated circuit at the same cutoff frequency: by decreasing the duty cycle by a factor of two, the size of the capacitors 109, 110 is decreased by a factor of two.
In one embodiment of the invention, a generator 101a of approximately square oscillation signals LOIM, LOIP, LOQM, LOQP of frequency FLO and presenting a duty cycle α=0.25 is constructed by serially connecting, as shown in
In other embodiments, NAND logic gates and inverters are used in place of NOR gates.
In one embodiment of the invention, in order to construct a generator 101b of approximately square oscillation signals LOIM, LOIP, LOQM, LOQP of frequency FLO and presenting a duty cycle α strictly below 0.25, as shown in
An embodiment of the invention was discussed above in a voltage-to-voltage mixer application, which means that the input source (the last stage of the low noise amplifier) is of relatively low impedance in comparison to the output load ZPMA.
One embodiment of the invention was implemented above in an image rejection mixer. It can also be implemented in a mixer without image rejection and therefore not comprising a Q channel.
One embodiment of the invention was described above with a differential oscillation signal LOI, LOQ and a differential input signal S′. An embodiment can be implemented in a mixer adapted to multiply a signal received as input by a non-differential oscillation signal, called a single-ended signal, with the input signal itself being differential or single-ended.
One embodiment of the invention was described above with the use of CMOS technology. An embodiment can be implemented in integrated circuits comprising components based on other technologies such as AsGa or BiCmos.
The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06 10288 | Nov 2006 | FR | national |
