WO2019015756A1

WO2019015756A1 - Single balanced voltage mode passive mixer with symmetrical sideband gain

Info

Publication number: WO2019015756A1
Application number: PCT/EP2017/068306
Authority: WO
Inventors: Shuja Hussain Andrabi; Hashem ZARE HOSEINI
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2017-07-20
Filing date: 2017-07-20
Publication date: 2019-01-24
Also published as: CN110945781B; CN110945781A

Abstract

A single balanced voltage mode mixer that comprises a compensation component for reducing the negative effects of a reactance presented by a pre-mixing circuit on the frequency response of the mixer. Specifically, the single balanced voltage mode mixer includes a pre-mixing circuit; and a passive mixer circuit configured to convert an input signal at a first frequency (fin) to an output signal at a second frequency (fout) by mixing the input radio frequency signal with a local signal. The passive mixer circuit includes: a first input terminal configured to receive the input signal via the pre-mixing circuit, the pre-mixing circuit presenting a first impedance comprising a first reactance to the first input terminal; and a second input terminal connected to a predetermined potential by a compensation component that presents a second impedance comprising a second reactance to the second input terminal, the second reactance being a conjugate reactance to the first reactance.

Description

SINGLE BALANCED VOLTAGE MODE PASSIVE MIXER WITH SYMMETRICAL

SIDEBAND GAIN

FIELD

[0001] This application relates to single balanced voltage mode passive mixers. BACKGROUND [0002] An analog mixer (which may also be referred to as a frequency mixer) is a device configured to change the frequency of an input signal. Specifically, an analog mixer converts an input signal (e.g. radio frequency (RF) signal) at a first frequency (fin) to an output signal (e.g. an intermediate frequency (IF) or baseband signal) at a second frequency (f_out) by mixing the input signal with a local signal (which may be referred to as the local oscillator (LO) signal) with a predetermined frequency (fi₀). The input signal (e.g. RF signal) typically carries information and during the frequency conversion the information carried by the input signal (e.g. RF signal) is frequency translated to the output signal (e.g. IF signal).

Accordingly, an analog mixer translates information content from a signal at a first frequency to a signal at a second frequency. [0003] When an analog mixer translates a higher frequency signal (e.g. RF signal) to a lower frequency signal (e.g. IF signal) the mixer is referred to as a downconverter, and when an analog mixer translates a lower frequency signal (e.g. IF signal or baseband signal) to a higher frequency signal (e.g. RF signal) the mixer is referred to as an upconverter.

Upconverters are typically used in transmitters for translating information to a frequency (e.g. RF) appropriate for transmission, and downconverters are typically used in receivers for translating information in an RF signal down to a fixed (e.g. intermediate) frequency for processing.

[0004] Analog mixers may be implemented using active or passive circuits. Active mixer circuits, which typically include an amplification component, consume a DC bias current. Passive mixer circuits, which typically comprise one or more diodes and/or transistors configured to function as switches, do not consume a DC bias current thus consume less power than active mixer circuits. Passive mixer circuits can generally achieve linearity and noise level performance levels comparable, or better, to those of active mixer circuits while consuming less power. Accordingly, passive mixers are widely used for their inherent linearity, low noise and simplicity. [0005] Passive mixers may be configured to operate in current mode or voltage mode. A voltage mode mixer is configured to operate on the voltage of the input signal. In other words, a voltage mode mixer is configured to receive a voltage as input. In contrast, a current mode mixer is configured to operate on the current of the input signal. In other words, a current mode mixer is configured to receive a current as input. While current mode mixers generally have good linearity and noise figures, they are not suitable for wideband applications as they require a transimpedance amplifier (TIA) after the mixer to provide a virtual ground and such closed loop operational amplifiers are very difficult to operate for a wide bandwidth signal range and consume a significant amount of power. [0006] Mixers can be classified as unbalanced, single balanced (SB) or double balanced (DB). In an unbalanced analog mixer both the input signal (e.g. input RF signal) and the LO signal are "unbalanced" so there is no isolation between the input signal and the LO signal allowing both to appear in the output signal. As a result, unbalanced analog mixers tend to have a high level of noise. In a single balanced (SB) mixer one of the input signal and the LO signal is "balanced" which provides either LO or input signal (e.g. RF) rejection in the output signal (e.g. IF signal). Specifically, in a SB mixer either the LO signal or input signal is applied to a balanced (differential circuit) which provides LO or input signal (e.g. RF) rejection in the output signal (e.g. IF signal). For example, in a SB mixer the LO signal may be applied to a device, such as a balun (a type of transformer used to convert an unbalanced signal to a balanced signal or vice versa), which translates the non-differential LO signal (i.e. a single-ended signal) to a differential LO signal (i.e. a differential pair of signals, such as, an in-phase signal and a quadrature-phase signal). In a double balanced (DB) mixer both the input signal (e.g. input RF signal) and the LO signal are "balanced" (i.e. applied to a balanced (differential circuit)). [0007] Single balanced mixers typically provide a good balance between noise reduction and complexity compared to unbalanced and double balanced mixers. Specifically, single balanced mixers have reduced noise compared to unbalanced mixers but are less complex than double balanced mixers. In particular, since in a single balanced mixer the input signal is not a differential signal the circuit driving the mixer (e.g. an amplification circuit) is simplified, and the power consumption and area of the system comprising the mixer is reduced.

[0008] Accordingly, single balanced voltage mode passive mixers are suitable for wideband, low power, low noise applications, such as 5G applications (e.g. in a 5G receiver). However, known single balanced voltage mode passive mixers suffer from asymmetrical sideband gain. [0009] The embodiments described below are provided by way of example only and are not limiting of implementations which solve any or all of the disadvantages of known single balanced voltage mode passive mixers.

SUMMARY

[0010] It is an object of the invention to generate a single balanced voltage mode passive mixer that has substantially symmetrical sideband gain.

[0011] The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the detailed description and the figures.

[0012] According to a first aspect there is provided a single balanced voltage mode mixer comprising: a pre-mixing circuit; and, a passive mixer circuit configured to convert an input signal at a first frequency to an output signal at a second frequency by mixing the input signal with a local signal, the passive mixer circuit comprising a first input terminal configured to receive the input signal via the pre-mixing circuit, the pre-mixing circuit presenting a first impedance comprising a first reactance to the first input terminal; and a second input terminal connected to a predetermined potential by a compensation component that presents a second impedance comprising a second reactance to the second input terminal, the second reactance being a conjugate reactance to the first reactance. The first reactance generally causes the mixer to have an asymmetrical frequency response and thus an asymmetrical sideband gain. The second reactance substantially negates the negative effect of the first reactance on the frequency response and thus the sideband gain so that the single balanced voltage mode mixer has a substantially symmetrical sideband gain.

[0013] In a further implementation of the first aspect, the first reactance is capacitive and the second reactance is inductive. If the first reactance is capacitive then by having the second reactance be inductive the second reactance negates or counteracts the negative effects of the first reactance on the frequency response and thus the sideband gain.

[0014] In a further implementation of the first aspect, the compensation component comprises an active inductor. An active inductor, compared to a passive inductor, is area efficient and less sensitive to interference thus resulting in a singled balanced voltage mode passive mixer that has reduced noise compared to other mixers, such as double balanced and/or current mode passive mixers. [0015] In a further implementation of the first aspect, the compensation component comprises a passive inductor. Passive inductors are simple and easy to implement and in contrast to active inductors do not consume power.

[0016] In a further implementation of the first aspect, the second impedance further comprises a resistance.

[0017] In a further implementation of the first aspect, the first reactance is inductive and the second reactance is capacitive. If the first reactance is inductive then by having the second reactance be capacitive the second reactance negates or counteracts the negative effects of the first reactance on the frequency response and thus the sideband gain. [0018] In a further implementation of the first aspect, the input signal is single ended signal. Using a non-differential input signal (e.g. a single ended signal) allows the pre-mixing circuit (e.g. low-noise amplifier (LIMA)) to be simpler (e.g. less complex)).

[0019] In a further implementation of the first aspect, the output signal is a differential signal comprising a differential in-phase component and a differential quadrature-phase

component.

[0020] In a further implementation of the first aspect, the second input terminal is further connected to the predetermined potential by a dummy resistor in parallel with the

compensation component. The dummy resistor may provide impedance matching.

[0021] In a further implementation of the first aspect, the passive mixer circuit has a substantially symmetrical upper sideband and lower sideband frequency response. Having a substantially symmetrical upper sideband and lower sideband frequency response produces a symmetrical sideband gain.

[0022] In a further implementation of the first aspect, the input signal is a radio frequency signal and the output signal is a baseband signal. [0023] In a further implementation of the first aspect, a signal path of the input signal does not include the compensation component. By ensuring the compensation component is not in the signal path of the input signal the compensation component may include passive components without affecting the linearity of the mixer.

[0024] In a further implementation of the first aspect, the pre-mixing circuit comprises a capacitor that causes at least a portion of the first reactance. If the pre-mixing circuit comprises a capacitor that performs a specific function having the second reactance coupled to the second input terminal instead of the first input terminal allows the capacitor to continue to perform its function while still negating the effect of the capacitor on the frequency response.

[0025] In a further implementation of the first aspect, the first input terminal is connected to the predetermined potential by the capacitor.

[0026] In a further implementation of the first aspect, the first reactance is at least partially caused by a parasitic capacitance of the pre-mixing circuit.

[0027] In a further implementation of the first aspect, the predetermined potential is ground and/or the same potential to which at least a part of the pre-mixing circuit, in particular a component of the pre-mixing circuit that causes the first reactance, is connected.

[0028] In a second aspect, there is provided a radio frequency receiver comprising the single balanced voltage mode mixer of the first aspect. A mixer with an asymmetrical sideband gain may degrade the error vector magnitude (EVM) and/or signal to noise (SNR) of a receiver in which such as mixer is implemented. Accordingly, a receiver comprising a mixer with a substantially symmetrical sideband gain may have an improved EVM and/or SNR.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention is described by way of example with reference to the accompanying drawings. In the drawings: [0030] FIG. 1 is a block diagram of a known single balanced voltage mode passive mixer;

[0031] FIG. 2 is a graph of an example frequency response of the input side of the single balanced voltage mode passive mixer of FIGS. 1 and 4;

[0032] FIG. 3 is a graph of an example frequency response of the output side of the single balanced voltage mode passive mixer of FIGS. 1 and 4; [0033] FIG. 4 is a block diagram of an example single balanced voltage mode passive mixer with a substantially symmetrical sideband gain; and

[0034] FIG. 5 is a block diagram of an example RF receiver comprising the single balanced voltage mode passive mixer of FIG. 4. DETAILED DESCRIPTION

[0035] The following description is presented by way of example to enable a person skilled in the art to make and use the invention. The present invention is not limited to the embodiments described herein and various modifications to the disclosed embodiments will be apparent to those skilled in the art. Embodiments are described by way of example only.

[0036] As described above, while single balanced voltage mode passive mixers are well- suited for wide bandwidth, low power and low noise applications, known single balanced voltage mode passive mixers suffer from asymmetrical sideband gain. In particular, the frequency response of the upper and lower sidebands in known single balanced voltage mode passive mixers is asymmetrical about the carrier frequency of the input signal. This is due to the fact that in known single balanced voltage mode passive mixers the input signal is received at the passive mixer circuit via a pre-mixing circuit that has a reactance associated with it. Specifically, the passive mixer circuit comprises two input terminals - a first input terminal that receives the input signal via the pre-mixing circuit and a second input terminal that is connected to a predetermined potential (e.g. ground). The reactance of the pre- mixing circuit causes the asymmetrical frequency response.

[0037] Accordingly, described herein are single balanced voltage mode passive mixers wherein the second input terminal of the passive mixer circuit is connected to the

predetermined potential (e.g. terminated) via a compensation component that presents a substantially conjugate reactance to that of the pre-mixing circuit. The compensation component substantially counteracts or negates the reactance of the pre-mixing circuit with respect to the passive mixer circuit which corrects or reverses the asymmetry in the frequency response and thus the asymmetry in the sideband gain. [0038] The single balanced voltage mode passive mixers described herein may be configured to operate as a downconverter or an upconverter. For example, the single balanced voltage mode passive mixers described herein may be configured to receive a radio frequency (RF) signal and output an intermediate frequency or baseband signal; or the singled balanced voltage mode passive mixers described herein may be configured to receive a baseband signal or an intermediate frequency signal and output an RF signal.

[0039] To more clearly describe the improved single balanced voltage mode passive mixer, reference is first made to FIG. 1 which illustrates a single balanced voltage mode passive mixer 100 that is known to the applicant. The fact that the single balanced voltage mode passive mixer 100 is known to the applicant is not an admission that the single balanced voltage mode passive mixer 100 is well-known. The known single balanced voltage mode passive mixer 100 comprises a passive mixer circuit 102 which is configured to convert an input signal 104 at a first frequency (fin) to an output signal 106a, 106b at a second frequency (fout) by mixing (e.g. multiplying) the input signal 104 with a local signal (i.e. local oscillator (LO) signal 108a, 108b) with a predetermined frequency (fi₀). Since the mixer is a single- balanced mixer the passive mixer circuit 102 receives a single-ended input signal 104, a differential local oscillator signal 108a, 108a, and generates a differential output signal 106a, 106b. The passive mixer circuit 102 is shown in FIG. 1 as being implemented by a set of field effect transistors (FETs) configured to act as switches. However, it will be evident to a person of skill in the art that this is only an example of a passive mixer circuit and the passive mixer circuit 102 may be implemented using any suitable techniques.

[0040] The passive mixer circuit 102 comprises an input side 107 for receiving the input signal 104 and an output side 109 for outputting the output signal 106a, 106b. The input side 107 of the passive mixer circuit 102 comprises a first input terminal 1 10 and a second input terminal 1 12 for capturing the voltage of the input signal. As described above, since the mixer 100 is a single balanced mixer the input signal 104 is a single-ended, or unbalanced, signal. As is known to the art, a single ended signal is transmitted via two wires wherein one wire carries a varying voltage that represents the signal (the input signal in this case) and the other wire is connected to a predetermined potential (e.g. ground). Accordingly, the voltage varying signal propagates down the signal path and returns through ground. The wire carrying the voltage varying signal is referred to as the active wire and the wire connected to the predetermined potential is referred to as the dummy wire.

[0041 ] In the example shown in FIG. 1 the voltage varying input signal 104 is received at the first input terminal 1 10 via a pre-mixing circuit 1 14 and the second input terminal 1 12 is connected to a predetermined potential (e.g. ground). The second input terminal 1 12 may be connected to the predetermined potential by a purely resistive component, such as a dummy resistor (Rdum) for impedance matching (as shown in FIG. 1 ), however, it will be evident to a person of skill in the art that the purely resistive component is optional. Since the first input terminal 1 10 is connected to the active wire and the second input terminal 1 12 is connected to the dummy wire, the first input terminal 1 10 may be referred to as the active input terminal and the second input terminal 1 12 may be referred to as the dummy input terminal.

[0042] In contrast, if the mixer 100 was a double balanced mixer 100 the input signal would be a differential signal. As is known to those of skill in the art a differential signal is transmitted via two wires which carry two complementary signals (i.e. a differential pair of signals). The two complementary signals are often referred to as a positive signal and a negative signal where the negative signal is both equal in magnitude to, and opposite in polarity from the positive signal. In a double balanced mixer one signal of the pair of differential signals (e.g. the positive signal) is received at the first input terminal via a first pre- mixing circuit and the other signal of the pair of differential signals (e.g. the negative signal) is received at the second input terminal via a second pre-mixing circuit.

[0043] The output side 109 of the passive mixer circuit 102 comprises a first output terminal 1 17 and a second output terminal 1 18 for outputting the output signal 106a, 106. As described above the output signal 106a, 106b is a differential signal which comprises two complementary signals 106a, 106b (e.g. an in-phase signal 106a and a quadrature signal 106b). Each of output terminals 1 17, 1 18 is configured to output one of the complementary signals. FIG. 1 shows that the output terminals 1 17, 1 18 are connected to a predetermined potential (e.g. ground), or terminated, via a capacitor C_out. However, it will be evident to a person of skill in the art that this is an example only and that the output terminals 1 17, 1 18 may be terminated in any suitable manner. [0044] The pre-mixing circuit 1 14 presents a first impedance Zi comprising a first reactance Xi to the first input terminal 1 10. As is known to those of skill the art, impedance, denoted Z, is the measure of the opposition that a circuit presents to a current when a voltage is applied. Impedance Z is a complex quantity defined as Z = R+ jX where the real part of impedance is the resistance R and the imaginary part is the reactance X. The impedance of an ideal resistor is purely real and is called resistive impedance. Ideal inductors and capacitors have purely imaginary reactive impedance. Specifically, an inductor has an inductive reactance and a capacitor has a capacitive reactance. In many cases the first reactance Xi is capacitive, however, in some cases the first reactance Xi may be inductive. The phrase "A presents an impedance of B to C" is used herein to mean that from the perspective of C it appears that A has an impedance of B.

[0045] The first reactance Xi may be at least partially caused by an inductive or capacitive component (e.g. a passive or active inductor or capacitor) in the pre-mixing circuit 1 14 and/or the parasitic (or stray) inductance or capacitance of the pre-mixing circuit 1 14. For example, in FIG. 1 the pre-mixing circuit 1 14 comprises a low-noise amplifier (LNA) 1 16, a feedback resister Ri_n and a capacitor Cm. The LNA 1 16 receives the input signal 104 and amplifies (using the feedback resistor R_n) the input signal to generate an amplified input signal that has an appropriate level of gain for processing by the passive mixer circuit 102. In this example, the first reactance Xi may be at least partially caused by the capacitor Cm. It will be evident to a person of skill in the art that FIG. 1 only shows an example of a pre-mixing circuit and the pre-mixing circuit 1 14 may be any circuit that is connected to the first input terminal 1 10. For example, where the mixer 100 is used as a downconverter the pre-mixing circuit may include additional and/or different RF components that precede the mixer, such as, but not limited to an antenna and/or a filter, such as a SAW filter.

[0046] The first reactance Xi (i.e. the reactance of the pre-mixing circuit) causes the mixer 100 to have an asymmetrical frequency response for the upper and lower sidebands around the carrier frequency. Specifically, the input signal carries information (e.g. is modulated with information) at a certain carrier frequency (f_c). In general, the carrier frequency (f_c) can be understood as the frequency of the input signal (fin) (i.e. f_c=fm). As is known to those of skill in the art, the band of frequencies in the modulated signal higher than the carrier frequency (f_c) is referred to as the upper sideband (USB) and the band of frequencies in the modulated signal lower than the carrier frequency is referred to as the lower sideband (LSB). Ideally the frequency response on the input side 107 of the passive mixer circuit 102 is symmetrical about the carrier frequency (f_c) and the frequency response on the output side 109 of the passive mixer circuit 102 is the same for the upper and lower sidebands (i.e. the sideband gain is symmetrical). For example, where the carrier frequency is 5 GHz, ideally the frequency response on the input side 107 of the passive mixer circuit 102 is symmetrical about 5 GHz.

[0047] However, due to the reactance of the pre-mixing circuit 1 14 (i.e. the first reactance Xi) the frequency response of the mixer 100 on the input side 107 of the passive mixer circuit 102 is asymmetrical about the carrier frequency (i.e. the frequency response is not centred at the carrier frequency (f_c)). For example, FIG. 2 shows the frequency response 202 at the input side 107 of an example of the known single balanced voltage mode passive mixer 100 where the input signal is an RF signal with a carrier frequency of 5 GHz. It can be seen from FIG. 2 that the frequency response 202 is not centred at 5 GHz (i.e. the peak of the frequency response is not at 5 GHz).

[0048] This asymmetrical frequency response on the input side 107 of the passive mixer circuit 102 results in the frequency response on the output side 109 of the passive mixer circuit 102 being different for the upper sideband and the lower sideband. In other words, the asymmetrical frequency response on the input side 107 of the passive mixer circuit 102 results in the mixer 100 having an asymmetrical sideband gain. In other words, the sideband gain ratio (i.e. the ratio of the gain for the upper sideband and the gain for the lower sideband is not one). For example, FIG. 3 shows the frequency response 302, 304 at the output side 109 of the passive mixer circuit 102 of the known single balanced voltage mode passive mixer 100 where the input signal is an RF signal with a carrier frequency (f_c) of 5 GHz and the output signal is a baseband signal. Specifically, the curve 302 shows the frequency response for the upper sideband of the received RF signal at the output side 109 of the passive mixer circuit 102, and the curve 304 shows the frequency response for the lower sideband of the received RF signal at the output side 109 of the passive mixer circuit 102. It can be seen from FIG.3 that while curve 302 is angled upward, curve 304 is angled downward. Accordingly, it can be seen from FIG.3 that the frequency response at the output side 109 (the baseband side in this example) of the passive mixer circuit 102 is different for the upper sideband and the lower sideband. As a result, the sideband gain of the mixer 100 is not symmetrical.

[0049] Accordingly, it is desirous to counteract or negate the effect of the reactance of the pre-mixing circuit (i.e. the first reactance Xi) on the frequency response of such a single balanced voltage mode passive mixer 100. Since every reactance has an opposite or conjugate reactance, the effect of the reactance of the pre-mixing circuit (i.e. the first reactance X1) could potentially be negated, or counteracted, by inserting a substantially opposite or conjugate reactance on the input side 107 of the passive mixer circuit 102 since an opposite or conjugate reactance will have an opposite or counter effect on the frequency response. For example, the conjugate reactance to an inductive reactance is a capacitive reactance of the same magnitude and the conjugate reactance to a capacitive reactance is an indunctive reactance of the same magnitude. Therefore, if the reactance of the pre- mixing circuit (i.e. the first reactance Xi) is inductive a compensation component with a capacitive reactance could be added to the input side of the passive mixer circuit 102.

Similarly, if the reactance of the pre-mixing circuit (i.e. the first reactance Xi) is inductive a compensation component with an inductive reactance could be added to the input side of the passive mixer circuit 102.

[0050] However, the placement of the compensation component in the input side of the passive mixer circuit 102 is significant. If the compensation component is placed in the signal path connected to the first input terminal (e.g. the active input terminal) and the reactance of the pre-mixing circuit (i.e. the first reactance Xi) is at least partially caused by a physical inductive or capacitive component (such as Cm) which was included in the pre- mixing circuit for a specific purpose (e.g. impedance matching) then the compensation component will not only negate the effect of the reactance of the pre-mixing circuit (i.e. the first reactance Xi) on the frequency response of the mixer, but it will also negate the effect of the inductive or capacitive component for its intended purpose (e.g. impedance matching). For example, the capacitor Cm may have be included in the pre-mixing circuit 114 to create an inductive impedance at the input of the LNA 116 for impedance matching. If an inductor with a conjugate reactance to the capacitor is added in parallel with the capacitor Cm the effect of the capacitor Cm on the input to the LNA is effectively negated which will degrade the Sii scattering parameter. The Sn scattering parameter may be subsequently improved by adding a bulky inductor or balun at the input of the LNA 1 16. Accordingly, placing the compensation component in the signal path connected to the first input terminal may eliminate one problem (asymmetrical sideband gain) but may cause another (e.g. the problem initially solved by the inductive or capacitive component).

[0051] Even if the reactance of the pre-mixing circuit is completely parasitic, placing the compensation component in the signal path connected to the first input terminal places the compensation component in the path of the input signal. This means that if the

compensation component is an active component (which is non-linear) the compensation component may affect the linearity of the mixer.

[0052] Accordingly, described herein are single balanced voltage mode passive mixers wherein the second input terminal of the passive mixer circuit is connected to the

predetermined potential (e.g. terminated) via a compensation component that presents a conjugate reactance to that of the pre-mixing circuit (i.e. a conjugate reactance to the first reactance Xi). The compensation component effectively counteracts or negates the reactance of the pre-mixing circuit with respect to the passive mixer circuit which corrects or reverses the asymmetry in the frequency response. Furthermore, since the compensation component is not in the signal path of the received input signal the compensation component does not affect the linearity of the mixer.

[0053] Reference is now made to FIG. 4 which illustrates an example of a single balanced voltage mode passive mixer 400 with substantially symmetrical sideband gain. The mixer 400 comprises the passive mixer circuit 102 of FIG. 1 with a first input terminal 1 10 and a second input terminal 1 12 wherein the first input terminal 1 10 is configured to receive the input signal 104 via a pre-mixing circuit 1 14 and the second input terminal 1 12 is connected to a predetermined potential (e.g. ground). However, the single balanced voltage mode passive mixer 400 of FIG. 4 also comprises a compensation component 402 between the second input terminal 1 12 (e.g. the dummy terminal) and the predetermined potential (e.g. ground). The compensation component 402 presents a second impedance Z2 to the second input terminal 1 12. The second impedance Z2 comprises a second reactance X2 which is a conjugate reactance to the reactance presented by the pre-mixing circuit (i.e. the first reactance Xi) to the first input terminal 1 10. The conjugate reactance of a particular reactance is a reactance that has the same magnitude but opposite sign to that particular reactance. Accordingly, the second reactance X2 has the same magnitude but opposite sign to the first reactance Xi. [0054] Since a capacitive reactance is the opposite of an inductive reactance (i.e. an inductive reactance has the opposite sign as a capacitive inductance), if the first reactance Xi is inductive then the second reactance X2 will be a capacitive reactance with substantially the same magnitude as the first reactance Xi. Accordingly, where the first reactance Xi is inductive then the compensation component may comprise one or more capacitors in parallel that present the second reactance X2 to the second input terminal. Similarly, if the first reactance Xi is capacitive then the second reactance X2 will be an inductive reactance with substantially the same magnitude as the first reactance Xi. Accordingly, where the first reactance Xi is capacitive then the compensation component may comprise one or more inductors (e.g. Ld_Um) in parallel that present the second reactance X2 to the second input terminal.

[0055] Since the compensation component 402 is not in the path of the input signal the compensation component 402 may comprise an active inductor/capacitor or a passive inductor/capacitor without affecting the linearity of the system. A compensation component that comprises an active inductor/capacitor to provide at least a part of the second reactance X2 is generally more area efficient and less sensitive to interference than a compensation component that comprises only a passive inductor(s)/capacitor(s) to provide the second reactance X2. Accordingly, the noise of the mixer 400 may be reduced if the compensation component comprises an active inductor/capacitor to provide at least a part of the second reactance X2.

[0056] The second reactance X2 substantially negates or counteracts the negative effect of the first reactance Xi on the frequency response of the mixer 400 resulting in a mixer 400 with a substantially symmetrical sideband gain. For example, FIG. 2 shows the frequency response 204 at the input side 107 (the RF side in this example) of the passive mixer circuit 102 of FIG. 4 when the input signal is an RF signal with a carrier frequency (f_c) of 5 GHz. It can be seen from FIG. 2 that the frequency response 204 is substantially symmetrical about the carrier frequency (f_c) of 5 GHz.

[0057] The symmetrical frequency response at the input side 107 of the passive mixer circuit 102 results in the frequency response at the output side 109 of the passive mixer circuit 102 being substantially the same for the upper sideband and the lower sideband. In other words, the symmetrical frequency response at the input side of the passive mixer circuit 102 results in the mixer 400 having a substantially symmetrical sideband gain. In other words, the sideband gain ratio (i.e. the ratio of the gain for the upper sideband and the gain for the lower sideband) is substantially one. For example, FIG. 3 shows the frequency response 306, 308 at the output side 109 (the baseband side in this example) of the passive mixer circuit 102 of FIG. 4 when the input signal is an RF signal with a carrier frequency (f_c) of 5 GHZ and the output signal is a baseband signal. Specifically, the curve 306 shows the frequency response of the upper sideband of the received RF signal at the output side 109 (the baseband side in this example) of the passive mixer circuit 102 of FIG. 4 and the curve 308 shows the frequency response of the lower sideband of the received RF signal at the output side 109 (the baseband side in this example) of the passive mixer circuit 102 of FIG. 4. It can be seen from FIG. 3 that the curves 306 and 308 are substantially similar. Accordingly, it can be seen from FIG. 3 that the frequency response at the output side 109 of the passive mixer circuit 102 of FIG. 4 is substantially the same for the upper sideband and the lower sideband. As a result, the sideband gain of the mixer 400 of FIG. 4 is said to be substantially symmetrical.

[0058] The curves 306 and 308 are not likely to be identical unless the second reactance X2 is the exact conjugate to the first reactance Xi. In some cases it may be difficult to implement a compensation component that presents a second reactance X2 that is the exact conjugate to the first reactance Xi. For example, in some cases, it may be difficult to determine the first reactance Xi with exact precision, and in some cases, it may be difficult to implement the compensation component 402 to achieve an exact conjugate reactance to the first reactance Xi due to the components available to implement the compensation component. Although these cases will result in a frequency response at the output side 109 (the baseband side in this example) of the passive mixer circuit 102 that is not identical for the upper sideband and the lower sideband the curves will likely be close enough for most practical purpose if the second reactance X2 is substantially the conjugate of the first reactance Xi.

[0059] Accordingly, introducing a compensation component 402 in the signal path connected to the second input terminal 1 12 that presents a conjugate reactance to the second input terminal relative to the impedance of the pre-mixing circuit 1 14 generates a mixer 400 that has a substantially symmetrical sideband gain without having to modify the passive mixer circuit 102 itself.

[0060] Since the negative effects of an asymmetrical sideband gain are more pronounced in wideband applications, such as 5G, the single balanced voltage mode passive mixer 400 described above with respect to FIG. 4 is particularly suitable for wideband applications.

[0061] Analog mixers are often used in radio frequency receivers to convert a received RF signal to a lower frequency signal (e.g. intermediate frequency signal or a baseband signal) for processing. Radio frequency receivers that use single balanced voltage mode passive mixers typically have lower noise, lower power consumption and lower complexity than receivers that use other mixers. In particular, double balanced mixers, since they require a differential input signal, typically have more complex and power consuming pre-mixing circuits (e.g. the pre-mixing circuit may comprise a balun and a differential LNA which is more complex and consumes more power than a single-ended LNA); and, as described above, current mode mixers consume a significant amount of power. However, the asymmetrical sideband gain of known single balanced voltage mode passive mixers can reduce the Error Vector Magnitude (EVM) and//or signal to noise (SNR) of the receiver. Accordingly, the single balanced voltage mode passive mixer 400 described herein can be used to generate an RF receiver with the mentioned benefits (e.g. lower noise, lower power consumption and lower complexity) with an improved EVM and/or SNR since the sideband gain is substantially symmetrical.

[0062] Reference is now made to FIG. 5 which illustrates a block diagram of an example radio frequency receiver 500 in which the single balanced voltage mode passive mixer 400 described herein may be implemented. The receiver 500 comprises an antenna 502 for capturing/receiving an RF signal. The RF signal captured by the antenna 502 may be provided to a single balanced voltage mode passive mixer 400 as described with reference to FIG. 4. The IF signal output by the mixer 400 may be provided to a filter 504 which generates a filtered IF signal. The filtered IF signal may then be provided to an analog to digital converter 506 which converts the filtered signal to a digital signal for processing. The digital signal may then be processed by a processing unit 508. It will be evident to a person of skill in the art that this is an example only and that other radio frequency receivers in which the single balanced voltage mode passive mixer 400 described herein may be implemented may comprise additional and/or different components. [0063] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1 . A single balanced voltage mode mixer (400) comprising: a pre-mixing circuit (1 14); and a passive mixer circuit (102) configured to convert an input signal (104) at a first frequency (fin) to an output signal (106a, 106b) at a second frequency (f_out) by mixing the input signal (104) with a local signal (108a, 108b), the passive mixer circuit (102) comprising: a first input terminal (1 10) configured to receive the input signal (104) via the pre-mixing circuit (1 14), the pre-mixing circuit (1 14) presenting a first impedance (Zi) comprising a first reactance (Xi) to the first input terminal (1 10); and a second input terminal (1 12) connected to a predetermined potential by a compensation component (402) that presents a second impedance (Z2) comprising a second reactance (X2) to the second input terminal (1 12), the second reactance (X2) being a conjugate reactance to the first reactance (Xi).

2. The single balanced voltage mode mixer (400) of claim 1 , wherein the first reactance (Xi) is capacitive and the second reactance (X2) is inductive.

3. The single balanced voltage mode mixer (400) of claim 2, wherein the compensation component (402) comprises an active inductor.

4. The single balanced voltage mode mixer (400) of claim 2, wherein the compensation component (402) comprises a passive inductor (Ldum).

5. The single balanced voltage mode mixer (400) of claim 3 or claim 4, wherein the second impedance (Z2) further comprises a resistance.

6. The single balanced voltage mode mixer (400) of claim 1 , wherein the first reactance (Xi) is inductive and the second reactance (X2) is capacitive.

7. The single balanced voltage mode mixer (400) of any preceding claim, wherein the input signal (104) is a single-ended signal.

8. The single balanced voltage mode mixer (400) of any preceding claim, wherein the output signal (106a, 106b) is a differential signal comprising a differential in-phase component (106a) and a differential quadrature-phase component (106b).

9. The single balanced voltage mode mixer (400) of any preceding claim, wherein the second input terminal (1 12) is further connected to the predetermined potential by a dummy resistor (Rdum) in parallel with the compensation component (402).

10. The single balanced voltage mode mixer (400) of any preceding claim, wherein the passive mixer circuit (102) has a substantially symmetrical upper sideband and lower sideband frequency response.

1 1 . The single balanced voltage mode mixer (400) of any preceding claim, wherein the input signal (104) is a radio frequency signal and the output signal (106a, 106b) is a baseband signal.

12. The single balanced voltage mode mixer (400) of any preceding claim, wherein a signal path of the input signal (104) does not include the compensation component (402).

13. The single balanced voltage mode mixer (400) of any preceding claim, wherein the pre-mixing circuit (1 14) comprises a capacitor (Cm) that causes at least a portion of the first reactance.

14. The single balanced voltage mode mixer (400) of claim 13, wherein the first input terminal (1 10) is connected to the predetermined potential by the capacitor (Cm).

15. The single balanced voltage mode mixer (400) of any preceding claim, wherein the first reactance (Xi) is at least partially caused by a parasitic capacitance of the pre- mixing circuit (1 14).

16. The single balanced voltage mode mixer (400) of any preceding claim, wherein the predetermined potential is ground and/or the same potential to which at least a part of the pre-mixing circuit (1 14), in particular a component (Cm) of the pre-mixing circuit (1 14) providing the first reactance (Xi), is connected.

17. A radio frequency receiver (500) comprising the single balanced voltage mode mixer (400) of any preceding claim.