FIELD

This application relates to electronic circuits, and more particularly, to single-ended to differential signal converters.

BACKGROUND

Radio receivers receive a single-ended signal and typically convert the received single- ended signal to a differential signal (i.e. a differential pair of signals) for processing. It is generally advantageous to convert the received single-ended signal to a differential signal (i.e. a differential pair of signals) as early as possible because differential interfacing can suppress external interference noise; supress even-order distortion components (which is very important in direct conversion receivers because even-order components appearing in a low-frequency signal are difficult to filter out); and improve gain since the output voltage can be twice that of a single-ended signal.

There are many known techniques for converting a single-ended signal to a differential signal, but many suffer from one or more of: asymmetry, costly area, noise, linearity and high-power consumption.

For example, one common technique for converting a single-ended signal to a differential signal is to use one or more on-chip inductor baluns. As is known to those of skill in the art an inductor balun is a circuit that converts a single-ended signal to a differential one using one or more inductors. On-chip inductor baluns are generally costly in terms of area.

Furthermore, since on-chip inductor baluns are very narrow-band a plurality of inductor baluns are typically used to provide coverage for wide-band operations. A receiver typically includes a gain stage, such as a low-noise amplifier (LNA). As shown in FIGS. 1 and 2 the inductor baluns 102, 202 may be placed before or after the gain stage 104, 204. If the inductor balun(s) 102 are placed before the gain stage (e.g. LNA) 104 as shown in FIG. 1 the output of the LNA 104 will be fully differential and will benefit from reduced even order distortion components at its output. This helps in setting the input matching conditions in a straightforward way. However, placing the inductor balun(s) 102 before the gain stage 104 can adversely affect the noise figure and special attention may need to be paid to any local oscillator (LO) leakage. If, instead, the inductor balun(s) 202 are placed after the gain stage (e.g. LNA) 204 as shown in FIG. 2 the inductor balun(s) 202 cannot correct for any nonlinearities introduced by the gain stage (e.g. LNA) 204.

Another common technique for converting a single-ended signal to a differential signal is to use an active balun. While most baluns comprise passive components, such as inductors, an active balun comprises an amplifying device. Accordingly, active baluns can be integrated with the gain stage (e.g. LNA) of a receiver. FIGS. 3 and 4 illustrate two example implementations of active baluns incorporated into an LNA. FIG. 3 illustrates a resistor- feedback inverter differential LNA and FIG. 4 illustrates a common source-common gate differential LNA. Active baluns are advantageous over inductive baluns because they are inherently wideband, can be implemented in significantly less area and can nearly double the amplitude of the output signal. However, since they have active components they consume power. It is also difficult to obtain a symmetrical fully differential signal from two parts of the circuit with asymmetric functionality. Such active baluns also tend to contribute to the signal noise and non-linearity.

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-ended to differential signal converters.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Described herein are single-ended to differential signal converters which use an N-path filter to perform the conversion. Specifically, the single-ended to differential signal converters comprise an N-path filter that comprises a plurality of paths wherein each path comprises a low pass filter circuit that is selectively connected to an input port of the converter, selectively connected to a first output port of the converter to generate a first output signal, and selectively connected to a second output port of the converter to generate a second signal that is 180 degrees out of phase with the first output signal. In this manner the first and second output signals form a differential pair of signals corresponding to the single-ended input signal. A first aspect provides a single-ended to differential signal converter comprising: an input port for receiving a single-ended signal; a first output port for outputting a first signal of a differential pair of signals; a second output port for outputting a second signal of the differential pair of signals; and an N-path filter comprising a plurality of paths, each path comprising a low pass filter circuit which is selectively connected to the input port, selectively connected to the first output port and selectively connected to the second output port.

Such a single-ended to differential signal converter has a negligible impact on noise, and does not consume any additional DC power. Furthermore, an N-path filter can also be used in a radio receiver to perform baseband filtering. Thus, the incremental increase in size and complexity of the receiver chip is negligible as converting a single-ended N-path filter to a single-ended to differential converter only requires an additional switch (or two) in each path.

Each path may further comprise: a first switching circuit which, when activated, connects the first output port to the low pass filter circuit; and a second switching circuit which, when activated, connects the second output port to the low pass filter circuit; wherein the first switching circuit is activated by a first control signal and the second switching circuit is activated by a second control signal that is out of phase with the first control signal.

Each path may further comprise a third switching circuit which, when activated, connects the input port to the low pass filter circuit.

The first switching circuits of the plurality of paths may be activated in a sequence, and the third switching circuits of the plurality of paths may be activated in the same sequence.

The third switching circuit of a path may be activated by a third control signal which has a different phase from the first and second control signals.

The phase of the third control signal may differ from the phase of one of the first and second control signals by 360/n wherein n is the number of paths in the n-path filter. The third switching circuit of a path and the first or second switching circuit of another path may be activated by a same control signal.

The input port may be shorted to the first output port such that when a first switch of a path is activated the input port and the first output port are connected to the low pass filter circuit of that path. The first switching circuit of a path and the second switching circuit of another path may be activated by the same control signal.

The first switching circuit of at least one path may comprise a single switch.

The second switching circuit of at least one path may comprise a single switch. When a low pass filter circuit is connected to the input port at a same frequency as the single-ended signal that low pass filter may receive a baseband version of the single-ended signal which is converted to a filtered baseband signal by the low pass filter circuit.

When a first switching circuit is activated by a control signal having the same frequency as the single-ended signal the first switching circuit may up-convert the filtered baseband signal to an original band of the single-ended signal.

When a second switching circuit is activated by a control signal having the same frequency as the single-ended signal the second switching circuit may up-convert the filtered baseband signal to an original band of the single-ended signal.

The single-ended signal may be a radio frequency signal.

The first and second signals of the differential pair of signals may be radio frequency signals.

At least one low pass filter circuit may comprise a capacitor in parallel with a resistor.

A second aspect provides a front-end signal-processing circuit comprising: an amplifier configured to output a single-ended signal; and the single-ended to differential signal converter of the first aspect wherein the input port of the single-ended to differential signal converter is coupled to an output port of the amplifier.

A third aspect provides a method of converting a single-ended input signal to a differential pair of signals comprising: connecting the single-ended input signal to a plurality of low pass filter circuits in sequence; generating a first signal of the differential pair of signals by outputting signals generated by the low pass filter circuits in sequence via first switching circuits associated with the low pass filter circuits; and generating a second signal of the differential pair of signals by outputting signals generated by one of the low pass filter circuits in sequence via second switching circuits associated with the low pass filter circuits; wherein the first and second signals of the pair of differential signals include the signal generated by a same low pass filter circuit for periods that are out of phase.

The above features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the examples described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described in detail with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of a circuit comprising an inductive balun followed by a differential LNA;

FIG. 2 is a block diagram of a circuit comprising an LNA followed by an inductive balun;

FIG. 3 is a circuit diagram of a first example circuit implementing an active balun;

FIG. 4 is a circuit diagram of a second example circuit implementing an active balun;

FIG. 5 is a circuit diagram of an example single-ended N-path filter;

FIG. 6 is a timing diagram of example control signals for the single-ended N-path filter of FIG. 5;

FIG. 7 is a circuit diagram of an example differential N-path filter;

FIG. 8 is a circuit diagram of a first example single-ended to differential signal converter comprising an N-path filter;

FIG. 9 is a circuit diagram of a second example single-ended to differential signal converter comprising an N-path filter;

FIG. 10 is a graph illustrating the frequency response of a front-end circuit comprising an LNA followed by the single-ended to differential signal converter of FIG. 9 and a front-end circuit comprising an LNA followed by a single-ended N-path filter; and

FIG. 1 1 is a flow chart of an example method of converting a single-ended signal to a differential pair of signals. The accompanying drawings illustrate various examples. The skilled person will appreciate that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the drawings represent one example of the boundaries. It may be that in some examples, one element may be designed as multiple elements or that multiple elements may be designed as one element. Common reference numerals are used throughout the figures, where appropriate, to indicate similar features.

DETAILED DESCRIPTION

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.

Described herein are single-ended to differential signal converters that use a single-ended N- path filter to generate a differential signal (i.e. a different pair of signals) from a single-ended input signal.

An N-path filter (which may also be referred to as a channel selection filter) comprises N identical parallel signal paths wherein N is an integer greater than or equal to two. Each path comprises an input modulator which down-converts the input signal to baseband, a low pass filter circuit which filters the baseband signal to generate a filtered baseband signal, and an output modulator which up-converts the filtered baseband signal to the original band of the input signal. At any given time the low pass filter circuitry is connected between the input and output through a single path. The low-pass filtering performed at baseband translates to band-pass filtering once up-converted. The centre frequency of the filter is determined by the mixing frequency. N-path filters have proven to provide band-pass filters having high Q- factors and a wide centre-frequency tuning range.

Reference is now made to FIGS. 5-7 which illustrate features of N-path filters. FIG. 5 illustrates an example of a single-ended N-path filter 500 which receives a single-ended input signal and generates a single-ended output signal which is a filtered version of the input signal. The single-ended N-path filter 500 comprises an input port 502 for receiving a single- ended input signal (VJN) and an output port 504 for outputting a filtered version of the input signal (V OUT). In some cases, the single-ended input signal (VJN) and the single-ended output signal (V OUT) are radio frequency signals (RF) signals. However, in other cases the input signal (V_IN) and the output signal (V OUT) may be in a different band.

The N-path filter 500 also comprises N identical signal paths 506-1 to 506-N where N is an integer greater than or equal to 2. Common values for N are 4 and 8, but it will be evident to a person of skill in the art that these are examples only and other values of N may be used. Each path 506-i comprises a low pass filter circuit (R+Ci) and a switching circuit (Si) in series. In the example shown in FIG. 5 each low pass filter circuit is formed by a capacitor C i and a resistor R (which represents the resistance between the input voltage source and the input port 102). However, it will be evident to a person of skill in the art that this is only an example low pass filter circuit and that other low-pass filter circuits may be used. In the example shown in FIG. 5 each switching circuit comprises a single switch Si, however, in other examples one or more of the switching circuits may comprise more than one switch.

The switching circuit (Si) of a path i is situated between the input port and the corresponding low pass filter circuit such that when the switching circuit (Si) is activated (i.e. closed) the input port 502 is connected to the corresponding low pass filter circuit (R+Ci). Since the input port 502 and the output port 504 are short-circuited in the N-path filter 500 of FIG. 5 activating a switching circuit (Si) also connects the output port 504 to the corresponding low pass filter circuit (R+Ci). In this configuration the periodic activation of a switching circuit (Si) of a path i down-converts the input signal to baseband, the low pass filter circuit (R+Ci) filters the baseband signal to generate a filtered baseband signal, and the same switching circuit (Si) up-converts the filtered baseband signal to the original band of the input signal which is provided to the output port 504. The components of the low pass filter circuits (e.g. C1 , C2 ... CN) are selected to provide a desired channel filtering according to the input signal bandwidth.

Typically, the switching circuits (S1 , S2 ... SN) are activated in a sequence such that only one switching circuit (Si) is active at a time and each switching circuit (S1 , S2, ... SN) is active for the same amount of time. For example, the switching circuits may be activated in the sequence S1 , S2, ... SN. In some cases, each switching circuit (Si) is activated by a corresponding control signal (Pi). Specifically, control signal P1 controls the activation of the first switching circuit (S1 ), control signal P2 controls the activation of the second switching circuit (S2) and so on. In some cases, the control signals are based on a local oscillator (LO) signal. FIG. 6 illustrates an example set of control signals (P1 , P2, ... PN) for the N-path filter 500 of FIG. 5 wherein each control signal represents a phase-shifted version of the LO signal. Specifically, the i ^th control signal Pi represents a ((i-1 ) ^* 360/N) degree phase shifted version of the LO signal. For example, where N=4 there will be four control signals (P1 , P2, P3 and P4) wherein P1 represents a 0 degree phase shifted version of the LO signal, P2 represents a 90 degree phase-shifted version of the LO signal, P3 represents a 180 degree phase shifted version of the LO signal and P4 represents a 270 degree phase shifted version of the LO signal. In this example, each control signal has a duty cycle equal to 1/N of the LO period (T_LO). This results in each switching circuit (Si) being activated for 1/Nth of the LO period. The LO signal may be set to the centre frequency of the input signal which makes the N-path filter 500 of FIG. 5 well suited for direct conversion receivers.

The low-pass filtering performed at baseband by the plurality of paths 506-1 to 506-N translates to a band-pass filtering effect once up-converted.

FIG. 7 illustrates an example differential N-path filter 700. The differential N-path filter 700 of FIG. 7 receives a differential pair of signals as inputs and outputs a differential pair of signals that represent filtered versions of the input signals. The differential N-path filter 700 of FIG. 7 is similar to the single-ended N-path filter 500 of FIG. 5 except instead of there being a single input port and a single output port there are two input ports 702, 703 and two output ports 704, 705. Each input port 702, 703 receives one of the signals of a differential pair of signals and each output port 704, 705 outputs one signal of a differential pair of signals.

Furthermore, instead of each path 706-1 to 706-4 comprising a single switching circuit that connects the input port and output port to the corresponding low pass filter, each path comprises two switching circuits S1 -1 to S4-1 , and S1 -2 to S4-2. Specifically, each path 706 -i comprises a first switching circuit SM which, when activated, connects the first input port 702 and the first output port 704 to the corresponding low pass filter circuit (C i + R/2); and a second switching circuit Si-2 which, when activated, connects the second input port 703 and the second output port 705 to the corresponding low pass filter circuit (Ci + R/2).

The first and second switching circuits Si-1 and Si-2 of a path i are controlled by control signals that are out-of-phase with each other (i.e. each control signal represents a 180- degree phase shifted version of the other control signal).

For example, if the first switching circuits S1 -1 to S4-1 are activated by control signals which represents 0 degree, 90 degree, 180 degree and 270 degree phase shifted versions of a LO signal, then the second switching circuits S1 -2 to S4-2 are activated by control signals which represent 180 degree, 270 degree, 0 degree and 90 degree phase shifted versions of the LO signal as shown in Table 1 . This means that the control signals used to control the first switching circuits can be reused to control the second switching circuits. For example, the control signal used to control the first switching circuit of the first path S1 -1 can also be used to control the second switching circuit of the third path S3-2, the control signal used to control the first switching circuit of the second path S2-1 can also be used to control the second switching circuit of the fourth path S4-2 and so on.

Table 1

However, the differential N-path filter of FIG. 7 requires a differential input.

Described herein are single-ended to differential signal converters which use a single-ended N-path filter to generate a different signal (i.e. a differential pair of signals) from a single- ended input signal. The differential signal is generated by selectively connecting the input port of the N-path filter to each low pass filter circuit, selectively connecting the low pass filter circuits to a first output port to generate a first output signal, and selectively connecting the low pass filter circuits to a second output port to generate a second output signal such that the second output signal is 180 degrees out of phase with the first output signal. In this manner the first and second output signals form a differential pair of signals that represent a filtered version of the single-ended input signal. This may be implemented by adding one or more switching circuits to each path of the single-ended N-path filter. For example, in an example embodiment each path has a first switching circuit, such as the switching circuits S1 to SN of the N-path filter 500 of FIG. 5, which, when activated, connects the input port and a first output port to the corresponding low pass filter circuit; and a second switching circuit which, when activated, connects the corresponding low pass filter circuit to a second output port. In another example

embodiment, each path may have a first switching circuit which when, activated, connects the input port and the corresponding low pass filter circuit; a second switching circuit which, when activated, connects the corresponding low pass filter circuit to a first output port; and a third switching circuit which, when activated, connects the corresponding low pass filter circuit to the second output port. In either case the switches of the same path that connect the low pass filter circuit to the first and second output ports respectively are activated by control signals that are out-of-phase (i.e. each control signal represents a 180-degree phase shift of the other control signal).

Such a single-ended to differential signal converter has a negligible impact on noise, and does not consume any additional DC power. Furthermore, due to the desire to achieve low- cost tunable filtering many radio receivers may comprise an N-path filter to perform baseband filtering meaning the incremental increase in size and complexity of the receiver chip is negligible as converting a single-ended N-path filter to a single-ended to differential converter only requires an additional switch (or two) in each path.

Reference is now made to FIG. 8 which illustrates a first example single-ended to differential signal converter 800 that comprises an N-path filter. The single-ended to differential signal converter 800 comprises an input port 802 for receiving a single-ended input signal (VJN), a first output port 804 for outputting a first output signal (V OUT P) of a differential pair of signals, and a second output port 805 for outputting a second output signal (V OUT N) of a differential pair of signals. In this example the input port 802 is shorted to the first output port 804. In contrast, the second output port 805 is not shorted to the input port 802. In some cases, the input and output signals may be RF signals, however, in other cases the input and output signals may be in a different band.

The single-ended to differential signal converter 800 also comprises an N-path filter with N paths 806-1 to 806-4. In this example, N=4, however it will be evident to a person of skill in the art that this is an example only and that N may be any integer greater than or equal to two. Each path 806-i of the N-path filter comprises a low-pass filter circuit 808-i, a first switching circuit 810-i and a second switching circuit 812-i. In the example shown in FIG. 8 each low pass filter circuit 808-i is formed by a capacitor C i in parallel with a resistor R i. However, it will be evident to a person of skill in the art that this is only an example of a low pass filter circuit and that other low-pass filter circuits may be used. In the example shown in FIG. 8 each of the first and second switching circuits 810-i and 812-i comprises a single switch Si-1 or Si-2, however, in other examples one or more of the switching circuits 810-1 to 810-4, 812-1 to 812-4 may comprise more than one switch. The components of the low pass filter circuits (e.g. R1 -R4 and C1 -C4) are selected to provide a desired channel filtering according to the input signal bandwidth.

Each first switching circuit 810-1 to 810-4 is situated between the input port 802 (and the first output port 804) and the corresponding low pass filter circuit 808-1 to 808-4 such that when the first switching 810-i circuit of a path i is activated (i.e. closed) the input port 802 and the first output port 804 are connected to the corresponding low pass filter circuit 808-i. In this configuration, periodic activation of the first switching circuit 810-i of a path i down-converts the input signal to baseband and provides the baseband signal to the low pass filter circuit 808-i. The low pass filter circuit 808-i generates a filtered baseband signal from the baseband signal. The first switching circuit 810-i then up-converts the filtered baseband signal to the original band of the input signal and provides the up-converted signal to the first output port 804.

Each second switching circuit 812-1 to 812-4 is situated between the second output port 805 and the corresponding low pass filter circuit 808-1 to 808-4 such that when a second switching circuit 812-i of a path i is activated (i.e. closed) the second output port 805 is connected to the corresponding low pass filter circuit 808-i. In the example shown in FIG. 8 one side of each second switching circuit 812-i is connected to the second output port 805 and the other side of the second switching circuit 812-i is connected to the wire or trace between the corresponding first switching circuit 810-i and the corresponding low pass filter circuit 808-i. When the second switching circuits 812-1 to 812-4 are situated between the second output port 805 and the corresponding low pass filter circuit 808-i, the periodic activation of the second switching circuit 812-i of a path i causes the second switching circuit 812-i to act as an up-converter. Specifically, periodic activation of the second switching circuit 812-i up-converts the filtered baseband signal generated by the corresponding low- pass filter circuit 808-i to the original band of the input signal and provides the up-converted signal to the second output port 805. In this arrangement, when the second switching circuit 812-i of a path i is activated (i.e. closed) there will be substantially no current flowing through the second switching circuit 812- i. Since there will be substantially no current flowing through the second switching circuits 812-1 to 812-4 the switches of the second switching circuits 812-1 to 812-4 can be smaller than the switches of the first switching circuits 810-1 to 810-4. In some cases, the switches of the second switching circuits may be an order of magnitude smaller than the switches of the first switching circuits. The size of the switch may be the physical size of the switch which may be defined by the length (L) and width (W) of the switch. For example, the size of a switch may be defined by the area of the switch which is equal to the product of the length and the width (L x W). In some cases, the minimum size of the switches of the second switching circuits may be limited by the maximum acceptable noise. For example, the switches of the second switching circuits 812-1 to 812-4 may be reduced to any size so long as their contribution to the overall noise is acceptable. Generally, the lower the frequency, the smaller the switches can be. Reducing the size of the switches of the second switching circuits may reduce the area to implement the single-ended to differential signal converter 800.

The first switching circuits 810-1 to 810-4 are activated in a sequence such that only one first switching circuit 810-1 to 810-4 is active at a time and each first switching circuit 810-1 to 810-4 is active for the same amount of time. For example, the first switching circuits 810-1 to 810-4 may be activated in the sequence 810-1 , 810-2, 810-3 and 810-4. In some cases, each of the first switching circuits 810-1 to 810-4 is activated by a corresponding control signal that is a phase-shifted version of a local oscillator signal (LO) wherein each control signal has a duty cycle equal to 1/N of the LO period.

For example, the first switching circuits 810-1 to 810-4 may be controlled by the example control signals P0 to P4 respectively illustrated in FIG. 6. As described above, in the example shown in FIG. 2 the i ^ih control signal Pi represents a ((i-1 ) ^* 360/N) degree phase shifted version of the LO signal. For example, where N=4 there will be four control signals (P1 , P2, P3 and P4) wherein P1 represents a 0 degree phase shifted version of the LO signal, P2 represents a 90 degree phase-shifted version of the LO signal, P3 represents a 180 degree phase shifted version of the LO signal and P4 represents a 270 degree phase shifted version of the LO signal. It will be evident to a person of skill in the art that this is an example only and the control signals for the first switching circuits 810-1 to 810-4 may represent different phase shifts of the LO signal, however, they are typically 360/N degrees apart. The second switching circuits 812-1 to 812-4 are activated in the same sequence as the corresponding first switching circuits 810-1 to 810-4. For example, if the first switching circuits 810-1 to 810-4 are activated in the sequence 810-1 , 810-2, 810-3, 810-4, then the second switching circuits 812-1 to 812-4 are activated in the sequence 812-1 , 812-2, 812-3, 812-4. However, the second switching circuits 812-1 to 812-4 are activated 180 degrees out of phase with the corresponding first switching circuit. Specifically, each second switching circuit 812-1 to 812-4 is activated by a control signal that is phase shifted by 180 degrees, with respect to the control signal used to activate the corresponding first switching circuit.

For example, where N is equal to 4 and the first switching circuits 810-1 to 810-4 are activated by control signals that represents 0 degree, 90 degree, 180 degree, and 270 degree phase shifted versions of the LO signal respectively, the corresponding second switching circuits 812-1 to 812-4 are activated by control signals that represent 180 degree, 270 degree, 0 degree, and 90 degree phase shifted versions of the LO signal as shown in Table 2.

Table 2

It will be evident to a person of skill in the art that these are example phase-shifts for the control signals and that other phase shifts may be used. For example, in another example the first switching circuits 810-1 to 810-4 may be activated by control signals that represent 5 degree, 95 degree, 185 degree and 275 degree phase shifted versions of the LO signal and the second switching circuits 812-1 to 812-4 may be activated by control signals that represent a 185 degree, 275 degree, 5 degree and 95 degree phase shifted versions of the LO signal.

The converter 800 of FIG. 8 may be preceded by an amplifier 820, such as an LNA, to form a front-end circuit for a radio receiver. Specifically, the amplifier (e.g. LNA) 820 may be configured to receive the single-ended RF signal captured by an antenna and generate a single-ended amplified version of the received signal and provide it to the input port 802 of the converter 800. The output V OUT (V OUT N and V OUT P) of the converter 800 may then be provided to a mixer, and/or baseband processing components. By placing the amplifier (e.g. LNA) 820 before the converter 800 the amplifier can perform the impedance matching with the input circuit (e.g. antenna), which relaxes the constraints on the converter 800 components (e.g. relaxes the constraints on the component of the switching circuits and the low-pass filter circuits). For example, in some cases, placing the amplifier (e.g. LNA) 820 before the converter relaxes the constraints on the size of the switches and the low-pass filter circuits. In such a configuration it can be advantageous for the amplifier to have a high output impedance as this will allow the capacitors of the low-pass filter circuits to be smaller. However, an LNA typically has a low output impedance which not only causes the capacitors of the low-pass filter circuits to be larger, but also limits the amount of out-of-band rejection achievable. While having the gain stage (e.g. amplifier) precede the converter 800 can provide some advantages it means that the gain stage (e.g. amplifier) doesn’t receive a filtered signal at its input (which would be the case if the converter 800 were placed before the gain stage (e.g. amplifier)). As a result, the gain stage (e.g. amplifier) is presented with the full strength of out-of-band blockers which increases the risk of saturation.

The single-ended to differential converter 800 of FIG. 8 can be generated from a single- ended N-path filter by adding a single switching circuit to each path without adding any complexity to the control circuity (the circuitry that generates the control signals that control activation of the switching circuits) since the same control signals used to activate the first switching circuits can be used to activate the additional switching circuits. However, since one of the output ports 804 is shorted to the input port 802 and the other output port 805 is not shorted to the input port 802, the driving capabilities of the two output ports 804 and 805 may be different. Accordingly, it is possible that the first and second output signals

(V OUT P and V OUT N) are not symmetric. This may be undesirable for some applications depending on how sensitive the down-stream components (i.e. the components following the single-ended to differential signal converter) are to the asymmetry of the two signals forming the differential pair of signals. Accordingly, reference is now made to FIG. 9 which illustrates a second example single- ended to differential converter 900 in which the direct short between the first output port and the input port is removed, and a third switching circuit is added to each path which, when activated, connects the first output port to the corresponding low pass filter circuit. The single-ended to differential signal converter 900 comprises an input port 902 for receiving a single-ended input signal (VJN), a first output port 904 for outputting a first output signal (V OUT P) of a differential pair of signals, and a second output port 905 for outputting a second output signal (V OUT N) of a differential pair of signals. In this example neither of the output ports 904 and 905 are shorted to the input port 902.

The single-ended to differential signal converter 900 also comprises an N-path filter with N paths 906-1 to 906-4. In this example, N=4, however it will be evident to a person of skill in the art that this is an example only and that N may be any integer greater than or equal to two. Each path 906-i of the N-path filter comprises a low-pass filter circuit 908-i, a first switching circuit 910-i, a second switching circuit 912-i and a third switching circuit 914-i. In the example shown in FIG. 9 each low pass filter circuit 908-i is formed by a capacitor C i in parallel with a resistor R i. However, it will be evident to a person of skill in the art that this is only an example of a low pass filter circuit and that other low-pass filter circuits may be used. In the example shown in FIG. 9 each of the first, second and third switching circuits 910-i, 912-i and 914-i comprises a single switch Si-1 , Si-2 or Si-3, however, in other examples one or more of the switching circuits 910-1 to 910-4, 912-1 to 912-4, or 914-1 to 914-4 may comprise more than one switch. The components of the low pass filter circuits (e.g. R1 -R4 and C1 - C4) are selected to provide a desired channel filtering according to the input signal bandwidth.

Each first switching circuit 910-1 to 910-4 is situated between the input port 902 and the corresponding low pass filter circuit 908-1 to 908-4 such that when the first switching 910-i circuit of a path i is activated (i.e. closed) the input port 902 is connected to the

corresponding low pass filter circuit 908-i. In this configuration, periodic activation of the first switching circuit 910-i of a path i down-converts the input signal to baseband and provides the baseband signal to the low pass filter circuit 908-i. The low pass filter circuit 908-i then generates a filtered baseband signal from the baseband signal.

Each second switching circuit 912-1 to 912-4 is situated between the second output port 905 and the corresponding low pass filter circuit 908-1 to 908-4 such that when a second switching circuit 912-i of a path i is activated (i.e. closed) the second output port 905 is connected to the corresponding low pass filter circuit 908-i. In the example shown in FIG. 9 one side of each second switching circuit 912-i is connected to the second output port 905 and the other side of the second switching circuit 912-i is connected to the wire or trace between the corresponding first switching circuit 910-i and the corresponding low pass filter circuit 908-i. When the second switching circuits 912-1 to 912-4 are situated between the second output port 905 and the corresponding low pass filter circuit, periodic activation of the second switching circuit 912-i of a path i up-converts the filtered baseband signal to the original band of the input signal and provides the up-converted signal to the second output port 905.

Each third switching circuit 914-1 to 914-4 is situated between the first output port 904 and the corresponding low pass filter circuit 908-1 to 908-4 such that when a third switching circuit 914-i of a path i is activated (i.e. closed) the first output port 904 is connected to the corresponding low pass filter circuit 908-i. In the example shown in FIG. 9 one side of each third switching circuit 914-i is connected to the first output port 904 and the other side of the second switching circuit 912-i is connected to the wire or trace between the corresponding first switching circuit 910-i and the corresponding low pass filter circuit 908-i. When the third switching circuits 914-1 to 914-4 are situated between the first output port 904 and the corresponding low pass filter circuit, periodic activation of a third switching circuit 914-i of a path i up-converts the filtered baseband signal generated by the corresponding low-pass filter circuit 908-i to the original band of the input signal and provides the up-converted signal to the first output port 904.

In this arrangement, when the second or third switching circuit 912-i or 914-i of a path i is activated (i.e. closed) there will be substantially no current flowing through the second switching circuit 912-i or the third switching circuit 914-i. Since there will be substantially no current flowing through the second and third switching circuits 912-1 to 912-4 and 914-1 to 914-4 the switches of the second and/or third switching circuits 912-1 to 912-4 and/or 914-1 to 914-4 can be smaller than the switches of the first switching circuits 910-1 to 910-4. In some cases, the switches of the second and/or third switching circuits may be an order of magnitude smaller than the switches of the first switching circuits. In some cases, the minimum size of the switches of the second and third switching circuits may be limited by the maximum acceptable noise. For example, the switches of the second and third switching circuits 912-1 to 912-4 and 914-1 to 914-4 may be reduced to any size so long as the maximum acceptable noise is achieved. Generally, the lower the frequency, the smaller the switches can be. In a single-ended N-path filter (such as that shown in FIG. 5) the on-resistance of the switching circuits are in the signal path and thus have current running through them.

Accordingly, in a single-ended N-path filter the on-resistance of the switching circuits can limit the out-of-band rejection. However, since there is substantially no current flowing through the second and third switching circuits 912-1 to 912-4 and 914-1 to 914-4 the on- resistance of the second and third switching circuits does not limit the out of band rejection. Accordingly, reducing the size of the switches of the second switching circuits may reduce the area to implement the single-ended to differential signal converter and/or ease the linearity requirements on downstream components connected to the output ports 904 and 905 of the converter 900.

The first switching circuits 910-1 to 910-4 are activated in a sequence such that only one first switching circuit 910-1 to 910-4 is active at a time and each first switching circuit 910-1 to 910-4 is active for the same amount of time. For example, the first switching circuits 910-1 to 910-4 may be activated in the sequence 910-1 , 910-2, 910-3 and 910-4. In some cases, each of the first switching circuits 910-1 to 910-4 is activated by a corresponding control signal that is a phase-shifted version of a local oscillator signal (LO) wherein each control signal has a duty cycle equal to 1/N of the LO period. For example, the first switching circuits 910-1 to 910-4 may be controlled by the example control signals P0 to P4 respectively illustrated in FIG. 6. As described above, in the example shown in FIG. 6 the i ^ih control signal Pi represents a ((i-1 ) ^* 360/N) degree phase shifted version of the LO signal. For example, where N=4 there will be four control signals (P1 , P2, P3 and P4) wherein P1 represents a 0 degree phase shifted version of the LO signal, P2 represents a 90 degree phase-shifted version of the LO signal, P3 represents a 180 degree phase shifted version of the LO signal and P4 represents a 270 degree phase shifted version of the LO signal. It will be evident to a person of skill in the art that this is an example only and the control signals for the first switching circuits 910-1 to 910-4 may represent different phase shifts of the LO signal, however, they are typically 360/N degrees apart.

The second and third switching circuits 912-1 to 912-4 and 914-1 to 914-4 are activated in the same sequence as the corresponding first switching circuits 910-1 to 910-4. For example, if the first switching circuits 910-1 to 910-4 are activated in the sequence 910-1 , 910-2, 910-3, 910-4, then the second switching circuits 912-1 to 912-4 are activated in the sequence 912-1 , 912-2, 912-3, 912-4 and the third switching circuits 914-1 to 914-4 are activated in the sequence 914-1 , 914-2, 914-3 and 914-4. However, the second and third switching circuits 912-1 to 912-4 and 914-1 to 914-4 are activated at a different phase offset from the corresponding first switching circuit and at a 180-degree offset from each other. Specifically, each second switching circuit 912-1 to 912-4 is activated by a control signal that is phase shifted by a predetermined amount, with respect to the control signal used to activate the corresponding first switching circuit; and is phase shifted 180 degrees, with respect to the control signal used to activate the corresponding third switching circuit.

For example, where N is equal to 4 and the first switching circuits 910-1 to 910-4 are activated by control signals that represents 0 degree, 90 degree, 180 degree, and 270 degree phase shifted versions of the LO signal respectively; the second switching circuits 912-1 to 912-4 may be activated by control signals that represent 270 degree, 0 degree, 90 degree, and 180 degree phase shifted versions of the LO signal respectively; and the third switching circuits 914-1 to 914-4 may be activated by control signals that represent 90 degree, 180 degree, 270 degree and 0 degree phase-shifted versions of the LO signal respectively as shown in Table 3.

Table 3

It will be evident to a person of skill in the art that these are example phase-shifts for the control signals and that other phase shifts may be used. In particular, while it may be advantageous to have the control signals that activate/control the first switching circuits to be related to the control signal that activates the corresponding second or third switching circuit by a factor of 360/N so that the same N control signals can be used to control all switching circuits, it is possible for the control signals that activate/control the first switching circuits to be related to the control signals that activate the corresponding second or third switching circuits by other amounts. For example, in another example the first switching circuits 910-1 to 910-4 may be activated by control signals that represent a 0 degree, 90 degree, 180 degree and 275 degree phase shifted versions of the LO signal; the second switching circuits 912-1 to 912-4 may be activated by control signals that represent a 185 degree, 275 degree, 5 degree and 95 degree phase shifted versions of the LO signal; and the third switching circuits 914-1 to 914-4 may be activated by control signals that represent a 185 degree, 275 degree, 5 degree and 95 degree offset version of the LO signal as shown in Table 4. Table 4

As with the converter 800 of FIG. 8 the converter 900 of FIG. 9 may be preceded by an amplifier 920, such as an LNA, to form a front-end circuit for a radio receiver. Specifically, the amplifier (e.g. LNA) 920 may be configured to receive the single-ended RF signal captured by an antenna and generate a single-ended amplified version of the received signal and provide it to the input port 902 of the converter 900. The output V OUT (V OUT P and V OUT N) of the converter 900 may then be provided to a mixer, and/or baseband processing components. Reference is now made to FIG. 10 which illustrates the frequency response 1002 of a front- end circuit comprising a single-ended LNA followed by the single-ended to differential signal converter 900 of FIG. 9 and the frequency response 1004 of a front end circuit comprising an LNA following by a single-ended N-path filter (e.g. the single-ended N-path filter 500 of FIG. 5). In these examples the centre-frequency of the N-path filters was 2.0 GHz (i.e. the LO signal used to control the switching units of the N-path filters had a frequency of 2.0 GHz). It can be seen from FIG. 10 that using the single-ended to differential signal converter 900 of FIG. 9 instead of a single-ended N-path filter increased the gain at the centre frequency (2.0 GHz) by 6 dB and the out-of-band rejection was significantly improved.

Accordingly, the single-ended to differential signal converters 800, 900 described herein can provide a set of symmetrical differential outputs from a single-ended input signal at no cost to DC power consumption, minimal increase in area to implement, minimal complexity to the control circuitry and minimal effect on the noise. In addition, in some cases (e.g. the example of FIG. 9) a gain boost may be achieved with respect to a corresponding single- ended output and/or the out-of-band rejection may be improved. When the converters are used in a radio receiver the improved out of band rejection may ease the linearity

requirements for stages and/or components that following the converter.

Reference is now made to FIG. 1 1 which illustrates an example method 1 100 of converting a single-ended signal to a differential pair of signals. The method 1 100 begins at blocks 1 102,

1 104, and 1 106. At block 1 102 the input signal is sequentially connected to each of a plurality of low pass filter circuits (e.g. low pass filter circuits 808-1 to 808-4, or 908-1 to 908- 4). At block 1 104, a first signal of the differential pair of signals is generated by outputting a signal generated by the low pass filter circuits in the same sequence via first switching circuits (e.g. switching circuits 810-1 to 810-4 or 914-1 to 914-4) associated with the low pass filter circuits. At block 1 106, a second signal of the differential pair of signals is generated by outputting a signal generated by the low pass filter circuits in the same sequence via second switching circuits (e.g. switching circuits 812-1 to 812-4 or 912-1 to 912-4) associated with the low pass filter circuits. The first and second signals of the pair of differential signals include the signal generated by a same low pass filter circuit for periods that are out of phase.

In some cases, the input signal may be connected to the plurality of low pass filter circuits using the first switching circuits associated with the low pass filter circuits. In other cases, the input signal may be connected to the plurality of low pass filter circuits using third switching circuits associated with the low pass filter circuits.

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.