Abstract:
The purpose of the invention is to provide an accurate measurement of RF signal power transfer between a power amplifier circuit and an antenna in the presence of supply voltage variations, temperature variations and VSWR mismatch. Knowing the VSWR mismatch enables modification of a control loop for the PA and thus allows for output power adjustment in order to make the PA more efficient and robust against VSWR changes. Having an indication of power delivered to the load and the VSWR is desirable for many wireless applications especially in those applications where the PA can generate emissions that are out of band and all emissions subject to industry standards. In particular, the embodiments of the invention are applicable to wireless LANs (WLANs).

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of measuring of output signal power and more specifically to the area of measuring of output signal power for wireless systems in the presence of supply voltage variations, temperature variations, and voltage standing wave ratio (VSWR) mismatch. 
   BACKGROUND OF THE INVENTION 
   In wireless systems, power amplifier circuits (PAs) are used to amplify a RF input signal prior to providing an amplified RF output signal to a load. In delivering of RF power from the PA to a load coupled thereto, an impedance match between the PA, feedline circuit, and the load is important in order to facilitate maximum power transfer therebetween. Any portion of the signal provided to the load that is not transferred reflects back into an output port of the PA and results in the PA producing unwanted signal emissions and lower efficiency. For wireless appliances, the FCC imposes strict radiation emission standards. If a wireless device does not fall within these standards, then such a wireless device is not salable, since broadcasting of RF signals outside a designated frequency band for the wireless devices is known to cause interference to surrounding electrical devices. 
   Furthermore, if the amplified RF output signal reflects back from the load, then less than a maximum transfer of RF signal power occurs and this results in unnecessary energy consumption by the PA. In addition to reduced power efficiency, amplified signals reflected back into the PA can damage the PA as relatively high voltages build on the output terminals of the circuit. The reflected signal is, effectively, energy that is not transferred from the PA to the load. Rather, this energy can be absorbed by the PA circuit resulting in a rise in junction temperatures concomitant with the loss of net power amplifier efficiency. 
   A Voltage Standing Wave Ratio (VSWR) is an indicator that is used with RF antenna systems to measure the coupling efficiency between the PA output port and an antenna. Typically, most antennas are not directly connected to a PA. The antenna is usually located some distance from the transmitter and PA and uses a feedline to transfer power therebetween. If the feedline has no loss and is impedance matched to both the PA output impedance and the antenna input impedance then maximum RF signal power is delivered to the antenna. In this case the VSWR is 1:1 and the voltage and current are constant over the whole length of the feedline. Any deviation from this situation causes a “standing wave” of voltage and current to exist on the feedline therebetween. This standing wave results in wasted energy and thus leads to wireless system inefficiencies. 
   Various techniques for measuring of RF signal power transfer between a PA output port and a load coupled thereto are known to those of skill in the art. For example, voltage sensing is performed at a final power amplification stage of the PA. Typically, a peak voltage detection scheme is utilized and it is therefore directly affected by a VSWR mismatch error. Unfortunately, with this technique, no indication of a level of VSWR mismatch is provided because the rise in peak voltage can be attributed to an increase in output power from the PA or a change in the VSWR. In effect, the peak voltage detection scheme can be ambiguous since voltage is not indicative of power transferred. 
   Another technique for measuring of RF signal power transfer utilizes voltage sensing at a penultimate stage of the PA. Unfortunately, this technique implements a peak voltage detection scheme and is therefore directly affected by VSWR mismatch error. Due to buffering of a final amplification stage of the PA, errors in the VSWR measurement are reduced. A disadvantage however, is that the final amplification stage is typically outside a control loop of the PA. Thus, the RF output signal is typically susceptible to supply voltage and temperature changes in the final amplification stage of the PA. Additionally, with this technique, there is no indication of VSWR mismatch provided. 
   A third technique for measuring of RF signal power transfer, which is known to those of skill in the art, is to provide the RF output signal through an off-chip directional coupler for forming a coupled signal. The coupled signal is connected to a detector circuit, which is usually a diode. Unfortunately, this approach has a number of off-chip components and often the detectors used for such an approach do not have the temperature stability of on-chip detectors. 
   Measuring of the VSWR is advantageous in that an amount of amplified RF signal power that is not coupled to the load is known and thus corrections to vary the RF output signal power are performable as a result thereof. A need therefore exists for measuring of power coupling between a PA output port and a load in the presence of supply voltage variations, temperature variations, and VSWR mismatch. It is therefore an object of the invention to provide a method and system of measuring power coupling between a circuit for emitting an amplified RF signal and a load coupled thereto. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention there is provided a circuit for measuring power transfer between a first node and a second node comprising: a coupler circuit comprising: a first port directly connected to the first node for receiving a RF output signal and a second port directly connected to the second node for providing the RF output signal therefrom, the first transmission line for propagating the RF output signal between the first port and the second port, a third port and a fourth port, the fourth port for providing a first signal indicative of forward propagating RF energy propagating from the first port to the second port and the third port for providing a second signal indicative of backwards propagating RF energy propagating from the second port to the first port; and, detection circuitry comprising: a first input port coupled to the third port for receiving the first signal indicative of forward propagating RF energy; a second input port coupled to the fourth port for receiving the signal indicative of backwards propagating RF energy; a first detected energy output signal port for providing a first detected energy output signal; and, a second detected energy output signal port for providing a second detected energy output signal. 
   In accordance with the invention there is provided a method of measuring power transfer between a power amplifier circuit for emitting a RF output signal and a first load for receiving the RF output signal comprising the steps of: providing a coupling circuit having a main signal path and a coupled signal path, the main signal path disposed between the power amplifier circuit and the first load; propagating a RF output signal along the main signal path to the first load; coupling forward propagating RF energy and backward propagating RF energy into the coupled signal path; detecting the forward propagating RF energy to form a first detected energy output signal; detecting the backward propagating RF energy to form a second detected energy output signal; and, 
   providing two detected output signals, a first detected signal relating to the forward propagating RF energy and a second detected signal relating to the backward propagating RF energy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the drawings in which: 
       FIG. 1  illustrates a coupler and detector circuit having an internal main signal path and an internal coupled signal path in accordance with a first embodiment of the invention; and, 
       FIG. 2  illustrates a coupler and detector circuit having an external main signal path and an internal coupled signal path in accordance with a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a coupler and detector circuit (CDC)  100  having an internal signal path, in accordance with a first embodiment of the invention. A power amplifier (PA) circuit  120  is disposed for providing an amplified RF output signal via an output port thereof  120   a  to a first node  100   a  of the CDC  100 . Within the CDC the amplified RF output signal propagates through a RF coupler, in the form of a quarter wave coupler circuit  103 , from a first port thereof  103   a  to a second port thereof  103   b , along a main signal path  151 , disposed between a first node  100   a  directly connected to the PA output port  120   a , and a second node  100   b  directly connected, at an input of the feedline circuit  101 . The feedline circuit  101  is disposed between a RF antenna  102  and the second node  100   b  of the CDC  100 , where the antenna  102  is used to broadcast the amplified RF output signal. The second port  103   b  of the quarter wave coupler is directly connected to the second node  100   b  of the CDC  100 . The energy of the amplified RF output signal broadcast by the antenna is dependent upon an impedance transformation along the feedline circuit  101  and main signal path between the first node  100   a  and the second node  100   b . If the impedance transformation along the feedline circuit and main signal path is one that allows for maximum power transfer from the PA output port  120   a  to the antenna  102 , then maximum power transfer occurs therebetween and the VSWR ratio is 1:1 with the voltage and current being substantially constant over this length. Any deviation from the VSWR ratio of 1:1 situation, resulting from improper impedance matching along the feedline circuit  101  and main signal path  151 , causes a “standing wave” of voltage and current, having peaks and troughs, to exist therealong. 
   In this embodiment, the quarter wave coupler  103  is manufactured within a same die as the CDC  100  as either a distributed quarter wave coupler or lumped quarter wave coupler. Alternatively, another form of quarter wave coupler is envisioned. The manufacturing technique implemented for manufacturing of this quarter wave coupler  103  is dependent upon the frequency of the amplified RF signal, integrated circuit process, and geometry of the coupler. For lower frequency RF signals, the quarter wave coupler is typically implemented using lumped components, where a quarter of a wavelength results in a physically large size and thus integration thereof becomes expensive. For higher frequencies, transmission line or distributed structures are typically utilized. A quarter wave coupler in the form of an edge coupler is also useable. 
   Under proper impedance transforming conditions, the feedline  101  and main signal path  151  only propagate forward propagating RF energy, propagating from the PA  120  to the antenna  102 . Under mismatched conditions there is a backreflection of RF energy at the antenna  102 , so a portion of the RF signal, the backward-propagating RF energy, propagates from the antenna  102  and along the feedline circuit  101  and the main signal path  151  to the PA  120  output port  120   a . These forward and backward propagating RF energies interact to form a standing wave along the feedline  101  and main signal path  151 , a quality of which is indicated by the VSWR. Conventional means for measuring the RF signal power of this standing wave, having peaks and troughs using voltage detection, provides a misleading VSWR measurement. VSWR measurement is known to be phase dependent and in performing of this measurement, it is unknown whether the measurement is being performed at a peak or in a trough of the standing wave. 
   The quarter wave coupler circuit  103 , in conjunction with associated circuitry, described hereinbelow, is used to measure the VSWR in such a manner that prior knowledge of the phase of the RF signal propagating along the feedline circuit  101  and main signal path  151  is not utilized. The function of the quarter wave coupler circuit  103  is to couple RF signal energy, from the forward propagating RF energy and backward propagating RF energy, along the main signal path  151  into a coupled signal path  152 . To those of skill in the art it is known that a quarter wave coupler  103  has a property, where, RF energy entering at the first port  103   a  appears at the second port  103   b  and a fourth port  103   d , but not at a third port  103   c . Likewise, RF energy entering the quarter wave coupler  103  at the second port  103   b  appears at the first port  130   a  and the third port  103   c , but not at the fourth port  103   d . Thus, the third port  103   c  provides a signal indicative of backwards propagating RF energy and the fourth port  103   d  provides a signal indicative of forward propagating RF energy. The main signal path  151  of the quarter wave coupler  103  is disposed between ports  103   a  and  103   b . A coupled signal path  152  of the quarter wave coupler is disposed between ports  103   c  and  103   d . Coupling between the main signal path  151  and the coupled signal path  152 , and hence the ports of the quarter wave coupler  103 , is achieved using a first coupling capacitor  141  and a second coupling capacitor  142 . Preferably, the first and second coupling capacitors are small value capacitors. 
   A second detector circuit  112 , coupled to the third port  103   c  of the quarter wave coupler circuit  103 , is disposed for receiving of a second signal indicative of backwards propagating RF energy to form a second intermediate detected signal. A first detector circuit  111 , coupled to the fourth port  103   d  of the quarter wave coupler circuit  103 , is disposed for receiving of a first signal indicative of forward propagating RF energy to form a first intermediate detected signal. A second difference amplifier  132  is disposed for receiving the second intermediate detected signal and a second reference signal emitted from an output port of a second reference circuit  114  in the form of a detector circuit. A first difference amplifier  131  is disposed for receiving the first intermediate detected signal and a first reference signal emitted from an output port of a first reference circuit  113  in the form of a detector circuit. The first difference amplifier  131  has an output port thereof coupled to a first output port  100   c  for providing a first detected energy output signal, which is indicative of the forward-propagating RF energy. The second difference amplifier  132  has an output port thereof coupled to a second output port  100   d  for providing a second detected energy output signal, which is indicative of the backward-propagating RF energy. 
   Thus, the level of the VSWR is therefore determined by measuring the level of the backwards-propagating RF energy as detected by the second circuit  112 . For an impedance transformation that allows for maximum power transfer between the PA output port  120   a  and the antenna  102 , the second detected energy output signal is substantially zero and first detected energy output signal is substantially maximized, otherwise, for an impedance transformation that allows for other than maximum power transfer, the second detected energy output signal is substantially other than zero and the first detected energy output signal is substantially other than maximized. The reference circuits  113  and  114  provide first and second reference signals to the first and second difference amplifiers  131  and  132  so that voltage offsets are subtracted from the first and second intermediate detected signals. These reference circuits  113  and  114  serve to decrease systematic errors and to reduce pedestals in the first and second detected energy output signals. The first and second difference amplifiers  131  and  132  perform signal subtraction, where these difference amplifiers are optionally in the form of operational amplifiers. 
   Preferably, first and second detector circuits, as well as the first and second reference circuits, are disposed within a same substrate as the remainder of the CDC  100  along with a regulator circuit  121 . A supply voltage input port  100   e , for receiving a supply voltage from a voltage source (not shown), is coupled to the regulator circuit  121 . Further preferably, the regulator circuit  121  includes a temperature sensing circuit  121  a for sensing a temperature of a die forming the CDC  100 . The regulator circuit  121  is for providing a regulated supply voltage to the two detector circuits, to the two reference circuits, and to the difference amplifiers. The regulated supply voltage thus provides reduced temperature sensitivity and increased supply voltage fluctuation immunity for the CDC  100 . Further preferably, the quarter wave coupler circuit is also disposed within a same semiconductor substrate as the detector circuits,  111  through  112 , the reference circuits  113  and  114 , and the regulator circuit  121 . Additionally, a first impedance  161  is disposed between the third port  103   c  and a ground potential and a second impedance  162  is disposed between the fourth port  103   d  and the ground potential. 
     FIG. 2  illustrates a coupler and detector circuit (CDC)  200  having an external signal path, in accordance with a second embodiment of the invention. The external signal path, in the form of a main signal path  251 , is not disposed on a same semiconductor die as the CDC  200 . The main signal path  251  is preferably in the form of a PCB trace, with a first coupling capacitor  241  and a second coupling capacitor  242  disposed between the main signal path and a coupled signal path  252 . The off-chip main signal path  251  and the on-chip coupled signal path form the coupler circuit, in the form of a quarter wave coupler circuit  203 . Disposed on opposing ends of the main signal path are a first node  203   a  and a second node  203   b . A power amplifier (PA) circuit  220  provides an amplified RF output signal via an output port thereof  220   a  to the first node  203   a  of the CDC  100 . The amplified RF output signal propagates through the main signal path  251  to the second node  203   b . The second node  203   b  is directly connected in series with a feedline circuit  201  and an input port of a RF antenna  102 . The RF antenna  202  is used to broadcast the amplified RF output signal. Of course, energy of the amplified RF output signal broadcast by the antenna is dependent upon an impedance transformation along the feedline circuit  201  and the main signal path  251  formed between the PA output port  220   a  and the input port of the antenna  202 . If the impedance transformation along the feedline circuit  201  and the main signal path  251  is such that maximum power transfer occurs therealong, then a VSWR ratio of 1:1 is obtained. During maximum power transfer, VSWR ratio of 1:1 is obtained along the feedline circuit  251  and main signal path  201 , where voltage and current are substantially constant. Any deviation from this state, such as when the antenna  202  is disconnected or places in proximity of a metal surface, results in an improper impedance transformation to occur along the feedline circuit and main signal path and a “standing wave” of voltage and current, having peaks and troughs, forms therealong. 
   Under properly impedance matched conditions, the RF signal path along the feedline circuit  201  and main signal path  251  only propagates forward propagating RF energy, propagating from the PA  220  to the antenna  202 . Under mismatched conditions there is a reflection of RF energy at the antenna  202 , so a portion of the RF signal, the backward-propagating RF energy, propagates from the antenna  202  to the PA input port  220   a . These forward propagating and backward propagating RF energies interact to form a standing wave along the feedline  201  and main signal path  251 , a quality of which is indicated by the VSWR. Conventional means of measuring the RF signal power of this standing wave having peaks and troughs using voltage detection provide a misleading measurement. VSWR measurement is phase dependent and in performing of this measurement, it is unknown whether the measurement is being performed at a peak or in a trough of the standing wave. 
   The quarter wave coupler circuit  203 , in conjunction with associated circuitry, described hereinbelow, is used to measure the VSWR in such a manner that prior knowledge of the phase of the RF signal along the feedline is not required. The function of the quarter wave coupler circuit  203  is to couple a portion of forward propagating RF energy and backward propagating RF energy, propagating along the main signal path  251  into the coupled signal path  252 . To those of skill in the art it is known that a quarter wave coupler  203  has a property that RF energy entering at the first RF signal port  230   a  appear at the second RF signal port  203   b  and a fourth port  203   d , but not at a third port  203   c . Likewise, energy entering the quarter wave coupler  203  at the second RF port  203   b  appears at the first RF port  230   a  and the third port  203   c , but not at the fourth port  203   d . Thus, the third port  203   c  provides a signal indicative of backward propagating RF energy and the fourth port  203   d  provides a signal indicative of forward propagating RF energy. 
   A first conductor is disposed in series proximate the first RF signal port  203   a  and a first input port of the CDC  200   a . A second conductor is disposed in series proximate the second RF signal port  203   b  and a second input port of the CDC  200   b . A first coupling capacitor  241  is disposed in series between the first input port  200   a  the coupled signal path and a second coupling capacitor is disposed in series between the second input port  200   b  the coupled signal path  252 . Coupling between the main signal path  251  and the coupled signal path  252  of the quarter wave coupler  203  is achieved using the first coupling capacitor  241  and the second coupling capacitor  242 . 
   A second detector circuit  212 , coupled to the third port  203   c  of the quarter wave coupler circuit  203 , is disposed for receiving of the signal indicative of backward-propagating RF energy to form a second intermediate detected signal. A first detector circuit  211 , coupled to the fourth port  203   d  of the quarter wave coupler circuit  203 , is disposed for receiving of the signal indicative of forward propagating RF energy to form a first intermediate detected signal. A first difference amplifier  231  is disposed for receiving the first detected signal and a first reference signal emitted from a first reference circuit  213 . A second difference amplifier  232  is disposed for receiving the second intermediate detected signal and a second reference signal emitted from a second detector circuit  214 . The first difference amplifier  231  has an output port thereof coupled to a first output port  200   c  for providing a first detected energy output signal, indicative of the forward-propagating RF energy. The second difference amplifier  232  has an output port thereof coupled to a second output port  200   d  for providing a second detected energy output signal indicative of the backward-propagating RF energy. 
   Thus, the level of the VSWR is therefore determined by measuring a level of the reflected RF energy as detected by the second detector circuit  212 . For a substantial impedance match between the PA output port  220   a  and the antenna  202 , the second detected energy output signal is substantially zero and first detected energy output signal is substantially maximized, otherwise, for an other than substantial impedance match, the second detected energy output signal is substantially other than zero and the first detected energy output signal is substantially other than maximized. The first and second reference circuits provide the first and second reference signals to the first and second difference amplifiers so that offsets are subtracted from the first and second intermediate detected signals. These reference detectors serve to decrease systematic errors and also to decrease pedestals in the first and second detected energy output signals. The first and second difference amplifiers perform a signal subtraction process in order to obtain a measure of the VSWR. Optionally, the first and second difference amplifiers are in the form of operational amplifiers. 
   Preferably, first and second detector circuits, as well as the first and second reference circuits, are disposed within the CDC  200  along with the regulator circuit  221 . Further preferably, the regulator circuit  221  includes a temperature sensing circuit  221   a  for sensing a temperature of the CDC  200  and for providing a regulated voltage to the quarter wave coupler circuit  203  and to the first and second detector circuits, as well as the first and second reference circuits. The regulated voltage thus provides reduced temperature sensitivity for the CDC  200 . Additionally, a first impedance  261  is disposed between the third port  203   c  and a ground potential and a second impedance  262  is disposed between the fourth port  203   d  and the ground potential. 
   The coupled signal path of the quarter wave coupler  203  is manufactured within the CDC  200  as either a distributed quarter wave coupler or lumped quarter wave coupler. The manufacturing technique implemented for manufacturing of this quarter wave coupler is dependent upon the frequency of the RF output signal, integrated circuit process and geometry of the coupler. For lower frequency RF signals the quarter wave coupler is typically implemented using lumped components since a quarter of a wavelength results in a physically large size. For higher frequencies, typically, transmission line or distributed structures are used. Of course, the use of an edge coupler is another possibility. Thus, for lower RF signal frequencies an external main signal path is preferred because of the costs associated with manufacturing of the main signal path on a same substrate as the CDC  200 . 
   Advantageously, the use of an off-chip main signal path  251  and an on-chip coupled signal path  252 , as shown in  FIG. 2 , a lower insertion loss is provided since the quarter wave section of the coupler does not significantly attenuate the signal propagating along the main signal path. 
   The measuring of the backward propagating RF signal is useful for providing a feedback signal to the transceiver relating to potential VSWR mismatch issues. Optionally, the transceiver modifies the power control loop process or feedback signals for the PA in order to compensate for the VSWR mismatch. 
   Advantageously, the embodiments of the invention are suitable for use with wireless applications, where accurate RF output signal power detection is preferred and VSWR mismatch is an issue. In particular, wireless local area networks (WLANs) typically require RF amplified signal power detection and are vulnerable to VSWR mismatch. The embodiments of the invention further provide temperature stability and RF amplified signal power detection without phase mismatch problems. Furthermore, a measure of VSWR mismatch is attainable. The embodiments of the invention advantageously offer the aforementioned advantages with a reduced component count. 
   Numerous other embodiments may be envisaged without departing from the spirit or scope of the invention.