Broadband detecting circuit

FIELD OF THE INVENTION

The present invention relates to a circuit, transmitter module, and method of for detecting an impedance.

BACKGROUND OF THE INVENTION

In general, the overall performance of a radio frequency (RF) power amplifier (PA) can be optimized by suitably choosing its load impedance. However, due to environmental influences the impedance at the driven load (e.g. antenna) is not constant and can put the RF PA in a non-optimal operating condition. This causes performance degradation of the RF PA in power efficiency, output power and linearity.

To solve this problem it has been suggested to place an isolator or circulator between the power amplifier and the load (e.g. antenna) . The properties of such an isolator are such that the signal can only propagate in one direction and that any reflected power is absorbed in a load that is connected to a third terminal of the device. Thus, the impedance mismatch of the antenna does not affect the performance of the power amplifier anymore. However, an isolator is big, expensive, and power inefficient. Moreover, it is not suitable for use in low cost, low power, portable communication systems.

As explained for example in WO2006/038167A1 a way to solve the above antenna mismatch problem is by sensing the RF load impedance and dynamically correcting a matching network provided between the load (e.g. antenna) and the RF PA.

I. Yokoshima, "RF Impedance Measurements by Voltage-Current Detection," IEEE Trans. Instrumentation and Meas . , vol. 42, no. 2, April 1993 discloses a method of sensing an RF impedance by using V/I detection. The RF voltage and current

are sensed through a resistor and a pick-up coil, respectively. Using this method, the RF current is measured indirectly by sensing the voltage across a reference impedance, which is connected to the pick-up coil (secondary winding) . Due to non-idealities of a practical transformer, this method introduces a phase shift and a frequency dependency in the voltage read-out due to the self-inductance of the transformer windings, which makes the detection non- trivial. Furthermore, the current and voltage are measured across a resistive impedance, which means that some RF power is absorbed, and thus this sensing method is not loss-less.

The WO2006/038167A1 discloses a method of current sensing, wherein the voltage difference across an impedance is measured by using a differential amplifier. Also this method introduces frequency dependency and phase shift in the voltage read-out when the impedance is reactive. On the other hand, a series sensing resistor (which introduces no phase shift) is undesirable, since it introduces additional losses in the path between the antenna and the PA. Making the resistor very small is not a feasible solution, since it sets tremendous requirements on the common mode rejection ratio (CMMR) of the differential voltage amplifier.

A further method of sensing the complex impedance is described in D. Qiao, et al, "Antenna Impedance Mismatch Measurements and Correction for Adaptive CDMA Transceivers," IEEE MTT-S Digest, pp. 783-786, June 2005. Using this method, the impedance can be determined by measuring two independent voltage ratios, i.e. the voltages at three different points on the transmission line. For this method, knowledge is needed of the characteristic impedance of the line. This method requires three measurements along a transmission line, which has a relatively long length (quarter wavelength) and is not really suitable for integration. One could think of making a lumped equivalent of this transmission line, but this technique still remains a narrow-band solution. Moreover, knowledge is needed about the value of the lumped components, making the measurement procedure non-trivial.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a detection method and circuit, which enable broadband and loss-less current sensing.

This object is achieved by a detection circuit as claimed in claim 1, and by a detection method as claimed in claim 13.

Accordingly, by measuring or detecting the current in this way, a frequency-independent and loss-less current read-out can be established. The proposed circuit can directly sense the short-circuit current through a pick-up coil by using the transimpedance amplifier. Using this method, no CMRR requirement exists on the current detector as in conventional circuits and no frequency dependency is introduced, since magnetic coupling factor between the pick-up coil and the transmission line can be assumed constant. Although the use of transimpedance amplifiers may add to total DC-power consumption, the signal power supplied to the impedance is not compromised.

In an exemplary implementation, the detection circuit may be adapted to detect an impedance, wherein the detection circuit may further comprise another transimpedance amplifier for measuring a voltage at said impedance via a sensing impedance. Thereby, an impedance sensor can be provided, which does not suffer from bandwidth limitations and does not introduce additional losses in the signal path in which the impedance is sensed. The proposed detection circuit can be designed in such a way that it does not disturb or loads the sensing node itself. The outputs of the transimpedance amplifiers can be fed into any kind of impedance detector circuit. Another possibility is to sample the outputs of the transimpedance amplifiers, which could lower the total power consumption of the proposed sensor/detector circuit. This is possible since the impedance changes relatively slowly (e.g. for adaptive antenna matching or the like) , so that the impedance could be measured at relatively large intervals.

In another exemplary implementation, the detection circuit may comprise a differential circuit, wherein the transformer arrangement may comprise a first primary winding, through which a first differential component of the current is caused to flow, and a second primary winding, through which a second differential component of the current is caused to flow, and wherein the transformer arrangement may comprise a first secondary winding, grounded at one end and connected to a first differential input of the transimpedance amplifier, and a second secondary winding, grounded at one end and connected to a second differential input of the transimpedance amplifier. Such a differential implementation provides better rejection of external noise or interference and is especially of interest for detection of currents of at differential amplifiers or the like.

In a further exemplary implementation, the differential circuit may be arranged to detect an impedance, wherein first and second differential components of a voltage at the impedance may be applied to respective first and second differential inputs of another transimpedance amplifier via a sensing impedance. Thus, the proposed solution can be used for differential impedance measurements in connection with impedance matching purposes.

In the above exemplary implementations, the sensing impedance may comprise a sensing capacitor and the other transimpedance amplifier may comprise at least one voltage-to-current feedback capacitor. Thereby, sensing amplification can be controlled by suitable selection of the capacitance ratio between the sensing capacitor and the feedback capacitor. Moreover, the capacitor-based implementation allows incorporation of sensing capacitor (s) into the design of transmission lines.

Alternatively, in the above implementations, the sensing impedance may comprise a sensing resistor and the other transimpedance amplifier may comprise a voltage-to-current feedback resistor. Sensing amplification can now be controlled

by suitable selection of the resistance ratio between the sensing resistor and the feedback resistor.

According to another exemplary implementation, the transformer arrangement may be implemented by coupled transmission lines which have a small electrical length with respect to an operation wavelength of the circuit. As an example, the coupled transmission lines may be configured as lumped coupled inductors. In this or another example, the circuit may be configured in a manner so that all parasitic and sensing capacitors are connected in parallel to ground or to virtual ground of the first and second transimpedance amplifiers. In the above or other examples, the circuit may be configured as an L-type matching network in an impedance transformation network between an amplifier and a load. Such a design can thus be easily incorporated as part of existing impedance matching network (s) and will thus not add any additional chip or circuit area due to additional passive components apart from the sensing electronics needed to process the sampled voltage and current signals.

Further advantageous embodiments are defined in the dependent claims .

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greater detail based on embodiments with reference to the accompanying drawings in which:

Fig. 1 shows a general schematic circuit diagram of a working principle of the proposed detection circuit;

Fig. 2 shows a schematic circuit diagram of a detection circuit according to a first embodiment;

Fig. 3 shows schematic circuit diagram of a detection circuit according to a second embodiment;

Fig. 4 shows a schematic circuit diagram of a detection circuit according to a third embodiment; and

Fig. 5 shows a practical embodiment of a detection circuit according to the second embodiment using a lumped capacitor and coupled transmission line.

DESCRIPTION OF THE EMBODIMENT

The embodiments of the present invention will now be described in greater detail based on a broadband complex RF impedance detector based on voltage/current (V/I) sensing. It is however noted that the present invention is not restricted to such a specific implementation and could as well be used in any kind of current ant/or voltage detection for any purpose.

In the following embodiment, a broadband and loss-less voltage and current sensor or detection circuit is proposed, which can be used e.g. for detecting an impedance.

Fig. 1 shows a general and principle structure of the proposed detection circuit for RF impedance detection by measuring voltage v _in and current I _1n between an output of a power amplifier (PA) 10 and a load Z _L (e.g. antenna load) directly via an impedance Z ₃ and a transformer 30, respectively.

The input voltage v _in is sensed by or via the impedance Z ₃ and amplified using a first transimpedance amplifier 22. The input current I _1n is sensed by measuring the short circuit current in the secondary winding L ₃ of the transformer 30 and amplified using a second transimpedance amplifier 24. By measuring the current in this way, a frequency-independent and loss-less current read-out can be established. It is noted that the use of the first and second transimpedance amplifiers 22, 24 will add to the total DC-power consumption, but the RF signal power from the PA 10 to the load Z _L is not compromised.

A transimpedance amplifier may be implemented based on an operational amplifier which has a very high "inherent" gain.

However, if a feedback resistor (or impedance) is connected to it, i.e., from the output to the input and some input current is supplied into the input (also known as the summing point) , the gain is so high that all of the current must flow through the feedback resistor and the output voltage will be V _O uτ = -(IIN ^x RF) • Such an amplifier circuit is then called a "current-to-voltage converter" or "transimpedance amplifier, " where the "gain" or "transimpedance" is equal to the feedback resistor .

In Fig. 1, the outputs v _v and V ₁ of the first and second transimpedance amplifiers 22 and 24 can be fed into an analogue complex impedance detector circuit such as described for example in the WO2006/038167, where the peak voltage, peak current, and phase difference between the measured voltage and current are determined. Of course other known impedance detector circuits could be used as well. Another possibility is to sample the outputs v _v and V ₁ , which could lower the total power consumption of the overall sensor/detector circuit. This is possible since the impedance changes relatively slowly (for adaptive antenna matching) and could therefore be measured at relatively large intervals (e.g. 1 s) .

Fig. 2 shows a schematic circuit diagram of the first embodiment of the voltage and current sensor for RF impedance measurements .

The input voltage V _1n is sensed using a sensing capacitor C _s and is amplified through a first transimpedance amplifier 22 having a voltage-to-current feedback capacitor C _F . The input current I _1n is sensed through the secondary winding L ₃ of a transformer 30, which is grounded at one side. The secondary measurement current is amplified through a second transimpedance amplifier 24 having a voltage-to-current feedback resistor R _F .

Fig. 3 shows a schematic circuit diagram of the second embodiment of the voltage and current sensor for RF impedance measurements, where the input voltage v _in is sensed using a

sensing resistor R ₃ and amplified through a first transimpedance amplifier 22 having a voltage-to-current feedback resistor R _F . Additionally, input current sensing is implemented as shown in Fig. 2.

For the above first and second embodiments input voltage sensing can be expressed by:

V C V R (ϊ) ^{F m s}

The current sensing can be expressed by

[I)

where n = k _m I—- , and k _m is the magnetic coupling coefficient, which are tne ^s main non-idealities of the transformer 30. It proves that both embodiments are broadband solutions, when assuming ideal (broadband) transimpedance amplifiers 22, 24.

Fig. 4 shows a schematic circuit diagram of a detection circuit according to a third embodiment which is a differential implementation of the voltage and current detection circuit, which is especially interesting for differential PA or front-end module (FEM) implementations. Differential components vin+ and vin- of the differential voltage are measured by connecting the two differential inputs "+" and "-" of a first differential transimpedance amplifier 26 via respective sensing capacitors CS to respective lines of the differential transmission line between the differential PA 12 and a symmetrizing or balancing, respectively, transformer 36. The symmetrizing transformer 36 enables connection of the grounded load ZL to the differential transmission line. Furthermore, a differential transformer arrangement comprising a first transformer 32 and a second transformer 34 is provided, wherein one end of both secondary windings LS is connected to ground. The other end of the secondary windings LS is connected a respective one of the two differential

inputs "+" and "-" of a second differential transimpedance amplifier 28 having respective feedback resistors RF. The respective primary windings of the first and second transformers 32, 34 are connected into the respective lines of the differential transmission line in order to measure differential components iin+ and iin- of the differential input current to the load ZL.

In the third embodiment, the transformer arrangement can be implemented by two coupled transmission lines, which have a small electrical length with respect to the wavelength (typically < l/10λ) . This allows to model the coupled transmission lines as two lumped coupled inductors.

Fig. 5 shows a practical embodiment of a detection circuit according to the second embodiment using a lumped capacitor and coupled transmission lines MLl and ML2 (left-hand side) and its equivalent schematic circuit (right-hand side) .

A closer look to the equivalent circuit in Fig. 5 reveals that the voltage and current sensing network can be designed in such a way that there is minimum disturbance (undesired loading) at a point A of an upper main transmission line MLl due the impedance read-out. This is because in theory all parasitic capacitances CPl and CP2 and the sensing capacitor

CS are all connected in parallel to ground (due to the virtual ground of the transimpedance amplifiers) and can be incorporated in the design of the upper transmission line MLl. Moreover, as an example, the sensing network could even be incorporated as an L-section in a total impedance transformation network from the load ZL (e.g. antenna) to the PA 10, since the upper transmission line MLl (i.e. primary winding LP) and the capacitor parallel circuit CS//CP1//CP2 form an L-type matching network. For this reason, the circuit of the second embodiment shown in Fig. 2 is especially suitable for this implementation.

The above embodiments can be used for example in hand-held mobile phones, especially with the trend towards multi-mode

and multi-band PA front-end modules, where broadband sensing and correcting the load impedance is required (load-line adaptation) . Another interesting area is that of the so-called "picoradio", for instance used in low-power wireless sensor networks. Since these sensors are highly influenced by their environment, antenna impedance mismatch could well be a problem. Therefore, detection and correction of the RF load impedance is desirable in these applications.

In summary, a circuit, transmitter front-end module, and method of sensing an impedance have been described, wherein an input current of an impedance is routed through a primary winding of a transformer arrangement. A short circuit current of a secondary winding of the transformer arrangement is sensed to measure the input current of the impedance.

However, in general, the present invention is not restricted to the above embodiments or application examples and can be implemented in any discrete circuit arrangement or integrated architecture. The proposed current and voltage detection according to the first to third embodiments can be applied in any transmitter, transceiver, receiver system or even other systems (architectures) which requires broadband and/or lossless detection. The above embodiments may thus vary within the scope of the attached claims.

Finally, it is noted that the term "comprises" or "comprising" when used in the specification including the claims is intended to specify the presence of stated features, means, steps or components, but does not exclude the presence or addition of one or more other features, means, steps, components or group thereof. Further, the word "a" or "an" preceding an element in a claim does not exclude the presence of a plurality of such elements. Moreover, any reference sign does not limit the scope of the claims.