WO2009148367A1

WO2009148367A1 - Device and method for increasing dynamic range for radio frequency diode detector

Info

Publication number: WO2009148367A1
Application number: PCT/SE2008/050646
Authority: WO
Inventors: Lars Arvidsson
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-06-02
Filing date: 2008-06-02
Publication date: 2009-12-10

Abstract

A radio frequency detector for detecting a power in a transmission line between a source signaland a load. The radio frequency detector includes a coupler device configured to extract a signal from the transmission line, an attenuator network device connected to the coupler device and configured to reduce a power of the extracted signal, where the attenuator network includes a shunt circuit connectedto ground, the shunt circuit including at least a first diodeof a limiter type; anda diode block connected to the attenuator network and configured to determine the power in the transmission line.

Description

Device and Method for increasing Dynamic range for Radio Frequency Diode

Detector

TECHNICAL FIELD

[0001] The present invention generally relates to a radio frequency (RF) detector and, more particularly, to mechanisms and techniques for increasing a dynamic range of the RF detector.

BACKGROUND

[0002] The modern wireless communication systems and other systems use a radio frequency interface for communicating, for example, between a user terminal and a base station. One or both of the user terminal and the base station may require that a power level of the RF interface is determined and maintained at a certain level. Thus, Automatic Gain Control (AGC) circuitry is a building block used in the wireless communication systems for controlling the power of the RF interface. For example, in a receiver (RX), it is desirable to maintain the same input level for the analog to digital (A/D) converter at the receiver backend despite large input RF signal variations. AGC helps the receiver to adjust the gain to obtain the desired signal level at the intermediate frequency (IF) or baseband. In a transmitter (TX), the final transmit power level may need to be regulated due to the temperature and process variations. For example, wireless systems such as CDMA dynamically adjust the transmitter's power level to improve efficiency. For simplicity, a transmitter is used to illustrate various concepts. However, those skilled in the art would recognize that these concepts also apply to any signal source. [0003] However, before the AGC controls the power level in either the TX or the RX, a way of monitoring the power level is needed. One way to monitor the power level is to use a power detector (PD). The PD may be implemented as a diode or a field-effect transistor (FET) circuit. Another way is to use a circuit with plural transistors.

[0004] The level of complexity depends on the requirements for the PD.

Some design parameters for the PD include: dynamic range, Nnear-in-dB linearity, power consumption, ease of integration, operating frequency, and cost. Design tradeoffs can be made based on these requirements. The dynamic range of the detector is function of a difference in power between the smallest and largest power that the detector has to measure with a predetermined accuracy. For the dynamic control, only a small dynamic range is needed if the purpose is to fix the final output power level when the transmitter is assumed to work at the same output power. For such an application, a simple diode or a simple FET circuit is enough to detect the RF power. If the transmitter is able to transmit at a number of different power levels, or continuously cover a large power range, the detector is desired to be able to measure all possible transmitted power levels with a predetermined accuracy. For instance, the GSM cellular phone system includes 16 specified power levels, with a total power difference from the lowest to the highest of 30 dB. Next, the diode based PD and the FET based PD are discussed. [0005] An I-V characteristic of active devices like diodes and FETs is nonlinear. These I-V characteristics may be modeled as a power series. The square term in the series is the mechanism to convert the RF power energy to the rectified direct current (DC). Based on trigonometry, the square term can be expressed as a DC voltage and second harmonics of the signal. With a low pass filter (LPF), only the DC voltage term is left for determining the power level of the original RF signal.

[0006] A diode based PD 10 is shown in Figure 1. In this figure, RF power is transmitted from a transmitter TX 12 to a load 14 along a transmission line 16. Part of the RF power is extracted by a coupler 18. The coupler 18 may be a directional coupler and may include a coil (not shown) placed next to the transmission line and a resistor R1. The coil acts as a transformer and an electrical field from the transmission line 16 is induced into the coil to produce an extracted signal. However, as would be appreciated by those skilled in the art, other methods may be used to extract the signal from the transmission line 16, for example a capacitor or a resistor connected to the transmission line. Although Figure 1 shows a simple line between the transmission line 16 and the coupler 18, that line is understood to describe the coil or the capacitor or the resistor. The extracted RF signal (which is desired to have as little power as possible because that power is wasted relative to the transmission) is passed by coupler 18 to an attenuator network 20. The attenuator network 20 may include one or more resistors and is configured to adapt the power level of the extracted signal to suit the diode block of the PD. In addition, the attenuator network 20 may be used to provide a 50 ohm impedance in reverse towards the coupler's output port, for better frequency linearity. The attenuator network 20 may provide the return path for the rectified current. The PD also includes a diode block 30, which may include a diode and a capacitor that provide filtering. The diode block 30 may include two matched diodes, for example Schottky diodes. One diode is rectifying the RF current, and the other is shunting some of the generated DC to ground and thereby stabilizes the detectors output versus temperature. The voltage detected by the diode block 30 is provided to the DC output unit 40, which provides the rectified voltage to various units inside the base station or the user terminal to determine the RF power in the transmission line 16. [0007] The implementation shown in Figure 1 exhibits reasonable performance for a low cost as no transistors are involved. Because a diode's junction changes with the temperature, the output rectified voltage will vary with temperature. Because a forward voltage drop of a diode decreases when the temperature increases, the output DC voltage of a single diode detector increases at a higher temperature at constant RF power. By introducing a second diode, of the very same type, and often in the same capsule for better temperature tracking, the warmer the circuit gets, the higher output DC voltage the detector diode produces. However, because the temperature compensating diode also becomes hotter, it shunts more DC to ground. Therefore, the detector system is temperature compensated, which results is a more accurate reading of the PD over temperature. [0008] A more integrated PD may use a single FET instead of the active diode for detecting the RF power. Figure 2 shows that the attenuator 20 and diode block 30 of Figure 1 are replaced by a transistor T1 , which is the active device. The transistor T1 is biased and the drain of the FET transistor T1 is close to zero to measure the RF power.

[0009] However, the diode based PD and the FET based PD may achieve a dynamic range of at most 35 dB. At the lower end, the diode may be at its forward conduction limit, and at the higher end the diode may be destroyed by overcurrent. A greater range may be achieved by switching the attenuator to a higher attenuation, which in turn produces transition difficulties caused by the necessary amount of hysteresis. In addition, the switching needs to be performed by an external circuit, which decides when to switch over. For fast varying output power, a system with an external switching circuit is too slow, and produces a risk of fatal damage to the detector diode if the correct attenuating level is not switched in quickly enough. [0010] One method to overcome these problems is to use a more complex device, for example a diode detector to create a DC voltage that is amplified and fed back to a Positive-Intrinsic-Negative (PIN) diode, which is a diode type used as voltage controllable resistor, and which acts as a variable RF attenuator, controlled by the amplified detector voltage. Others are using integrated circuits as power detectors. However, these alternatives become expensive as more circuitry and transistors are required.

[0011] Accordingly, it would be desirable to provide devices, systems and methods for detecting an RF power in a transmission line that avoid the afore- described problems and drawbacks. SUMMARY

[0012] According to one exemplary embodiment, there is a radio frequency detector for detecting a power in a transmission line between a signal source and a load. The radio frequency detector includes a coupler device configured to extract a signal from the transmission line; an attenuator network device connected to the coupler device and configured to reduce a power of the extracted signal, where the attenuator network includes a shunt circuit connected to ground, the shunt circuit including at least a first diode of a limiter type; and a diode block connected to the attenuator network and configured to determine the power in the transmission line. [0013] According to another exemplary embodiment, there is a communication device that includes a transmitter configured to transmit power along a transmission line; a load connected to the transmission line and configured to receive the power from the transmitter; and a radio frequency detector coupled to the transmission line and configured to detect the power in the transmission line. The radio frequency detector includes a coupler device configured to extract a signal from the transmission line, an attenuator network device connected to the coupler device and configured to reduce a power of the extracted signal, where the attenuator network includes a shunt circuit connected to ground, the shunt circuit including at least a first diode of a limiter type, and a diode block connected to the attenuator network and configured to determine the power in the transmission line.

[0014] According to still another exemplary embodiment, there is a method for increasing a dynamic range of a radio frequency detector that detects a power in a transmission line between a signal source and a load. The method includes extracting a signal from the transmission line by using a coupler device; reducing a power of the extracted signal by directing the extracted signal through an attenuator network device connected to the coupler device, where the attenuator network includes a shunt circuit connected to ground, the shunt circuit including at least a first diode of a limiter type and the shunt circuit to ground increases the dynamic range of the radio frequency detector; and determining the power in the transmission line using a diode block connected to the attenuator network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: [0016] Figure 1 is a schematic diagram of an RF diode detector;

[0017] Figure 2 is a schematic diagram of an RF transistor detector;

[0018] Figure 3 is a schematic diagram of the RF diode detector of Figure 1 in which the structures of the attenuator and diode block are shown; [0019] Figure 4 is a schematic diagram of the diode block of the RF diode detector;

[0020] Figure 5 is a schematic diagram of a temperature compensation unit;

[0021] Figure 6 is a schematic diagram of an RF diode detector according to an exemplary embodiment; [0022] Figure 7 is a graph showing various compressed curves; and [0023] Figure 8 is a flow diagram showing steps for increasing a dynamic range of the RF detector of Figure 6.

DETAILED DESCRIPTION

[0024] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of an RF diode detector. However, the embodiments to be discussed next are not limited to these systems but may be applied to other RF detectors. [0025] Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification are not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. [0026] The dynamic range of the PD may be increased, according to an exemplary embodiment, without adding complicated or expensive circuits or transistors, by adding PIN diodes of a limiter type (the concept of limiter type is explained later) to the attenuator network. Prior to discussing this exemplary embodiment, the structure of the attenuator network 20 and the diode block 30 is discussed in more detail.

[0027] As shown in Figure 3, according to an exemplary embodiment, the attenuator network 20 may include a resistor 22 connected to ground. The resistor 22 provides a reduced voltage to the diode block 30, depending on the resistance value of the resistor 22.

[0028] As also shown in Figure 3, the attenuator network 30 may include the active diode 32 and a capacitor 34. The diode 32 may transform a sinusoidal signal extracted by the coupler 18 from the transmission line 16 to a voltage or current having a single polarity. The capacitor 34 may integrate the DC signal over a chosen time depending on the capacitor value and the input resistance of the DC output 40, producing a smoother readout. The capacitor 34 may also shunt the remaining RF signal to ground, allowing only the DC signal to pass. The capacitor 34 may be connected between the diode 32 and ground. Diode 32 may be, for example, a Schottky diode or another type of diode. In order to compensate diode 32 for a temperature influence, a temperature compensation unit 36 may be connected to the diode 32, as shown in Figure 4. A resistor 38 may be inserted between one end of the diode 32 and an end of the temperature compensation unit 36. The temperature compensation unit 36 may include a resistor 38 connected to a capacitor 42 and a diode 44 as shown in Figure 5.

[0029] According to an exemplary embodiment shown in Figure 6, the attenuator network 20 may include, besides the resistor 22 shown in Figure 3, a shunt circuit 25 connected to ground. The shunt circuit 25 may include, according to an exemplary embodiment, a resistor 29 in series with two anti-parallel PIN diodes 26 and 28 of the limiter type. The shunt circuit 25 may include (instead of the two- antiparallel PIN diodes 26 and 28) other combinations of PIN diodes and resistors, for example, only one PIN diode, two pairs of PIN diodes, each pair having a corresponding resistor, or other combinations, e.g., plural PIN diodes in each direction. These diodes 26 and 28 may behave as self-biasing voltage controllable resistors, i.e., the higher the RF level across them, the lower the resistance the diodes exhibit. More specifically, the PIN diodes rectify the RF signal across them, generating a DC signal. The generated DC signal controls the RF resistance through the diodes. Higher RF levels produce higher DC across the diodes, producing lower RF resistance, which causes more RF to be shunted to ground, which, in turn, lowers the RF level across the diodes, thereby giving lower DC and enabling the process to regulate itself. Thus, the diodes themselves maintain the RF level across them at an almost constant level, with no need of external biasing. No transistors or other expensive equipment are necessary for implementing the shunt circuit 25.

[0030] Therefore, the circuit attenuation increases at higher RF power levels and the RF output at the detector diode 30 versus the RF from the coupler 18 shows a compressed transfer curve at the higher end. Thus, a point where the compression is on as well as the impact of the compression may be adjusted by the PIN diode series resistor 29. Figure 6 also shows that more than a resistor 22 may be used to connect the output of the coupler 18 to ground, i.e., resistor 24. [0031] "Compression", as that term is used herein, refers to the characteristic of exemplary embodiments to select a smaller portion of the input power to measure on (when the input power is higher) than the portion which would be measured absent the use of compression. Adjusting the compression is achieved by changing a value of resistor 29. For example, the lower the value of the resistor 29 which is chosen, the higher the RF voltage is present across the PIN diodes at a certain power level from the transmitter. Thus, the RF resistance though the PIN diodes becomes lower, and more RF power is shunted to ground, partly caused by the lower resistance of 29, and partly by the lower RF resistance of the PIN diodes. Consequently, the reading from the detector may be lower.

[0032] Because the PIN diodes may be almost inert up to a certain level of RF power, the point where the additional shunting is felt may be chosen. If it is desirable to achieve a compression that starts rather high on the curve, a large resistor 29 may be used to set a high kick-in point. To achieve a large effect above that point, more than one pair of diodes with corresponding serial resistors may be used, all together shunting to ground.

[0033] Figure 7 shows two examples of compression curves, of which the

"more compressed" one has a lower value of the resistor 29, the "compressed" one has a higher value of resistor 29, and the "uncompressed" one is the response without any compression circuit. If the detector diode has a limit at 2500 in the graph below, the detector diode still works, but the resolution at the lower end is poor. By adding a compression circuit, the coupling ratio of the coupler may be increased, giving higher readings at the lower end, which results in a better accuracy, and at maximum power level the detector diode still works properly because the PIN diode and the corresponding resistor shunts off a part of the RF power. [0034] A PIN diode of limiter type is discussed next. A passive receiver- protection limiter may include a PIN diode and an RF choke inductor, both of which are in shunt with the main signal path. In limiter circuits, the input and the output of the circuit may include DC blocking capacitors. A single-stage limiter may reduce the amplitude of a large input signal by 20 to 30 dB. A limiter PIN diode may be a three-layer device whose middle I layer is doped with gold to reduce the minority carrier lifetime. The design of the diode, specifically l-layer thickness, l-layer resistivity, and P-to-l-layer junction area is based on trade-offs to produce the desired resistance, capacitance, recovery time, and threshold level. The diode may act as an input-power-controlled RF variable resistance to produce an attenuation that is a function of the diode characteristics as well as the incident signal amplitude. The limiter circuit may include a single diode or multiple cascaded diodes separated by one-quarter wavelength, λ/4. Adding a directional coupler and a Schottky detector diode to the system may lower the threshold level. [0035] The PIN-limiter diode functions as an incident-power-controlled, variable resistor. Without a large input signal, the impedance of the limiter diode is at its maximum, resulting in an insertion loss of less than 0.5 dB. Any large input signal temporarily forces the impedance of the diode to a much lower value, which shunts a part of the signal to ground, or produces an impedance mismatch that reflects most of the input signal power back toward its source. Below the threshold level, the transfer function for the limiter stage is linear; above the threshold, the transfer function shows an increasing insertion loss as the signal amplitude increases until the diode is forced to its minimum impedance. Without the large signal and after a brief delay whose duration depends on the diode used, thermal factors, and other elements of the circuit, the impedance of the diode reverts from a low value to its maximum value.

[0036] Thus, according to one or more exemplary embodiments, adding the

PIN diode assembly in parallel with the active diode and adding the PIN diode's series resistors may provide additional kick-in points to the compression curve, offering thus the desired dynamic range to the system.

[0037] These PIN diodes are robust by their nature because a high RF level across them makes the diodes to have a low resistance, for example below 1.5 ohm, and thus, these diodes have good power handling capacity. The novel attenuator network may be used for transient protection of preamplifier frontends. Therefore, the dynamic range of a diode detector may be increased by at least 10 dB by adding a pair of PIN diodes and one or more resistors and no servo amplifiers for feedback systems is needed as these diodes are self-powered.

[0038] According to an exemplary embodiment shown in Figure 8, there is a method for increasing a dynamic range of a radio frequency detector that detects a power in a transmission line between a transmitter and a load. The method includes a step 800 of extracting a signal from the transmission line by using a coupler device, a step 802 of reducing a power of the extracted signal by directing the extracted signal through an attenuator network device connected to the coupler device, wherein the attenuator network includes a shunt circuit connected to ground, the shunt circuit including one or more anti-parallel diodes of a limiter type and the shunt circuit to ground increases the dynamic range of the radio frequency detector, and a step 804 of determining the power in the transmission line using a diode block connected to the attenuator network.

[0039] The disclosed exemplary embodiments provide an RF detector and method for increasing a dynamic range of the RF detector. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0040] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. The methods or flow charts provided in the present application may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor.

Claims

WHAT IS CLAIMED IS:

1. A radio frequency detector for detecting a power in a transmission line (16) between a signal source (12) and a load (14), the radio frequency detector comprising: a coupler device (18) configured to extract a signal from the transmission line (16); an attenuator network (20) connected to the coupler device (18) and configured to reduce a power of the extracted signal, wherein the attenuator network (20) includes a shunt circuit (25) connected to ground, characterized in that the shunt circuit including at least a first diode of a limiter type; and a diode block (30) connected to the attenuator network (20) and configured to determine the power in the transmission line (16).

2. The radio frequency detector of Claim 1 , wherein the shunt circuit further includes a second diode connected anti-parallel with the first diode and a first resistor connected in series with the first and second anti-parallel diodes.

3. The radio frequency detector of Claim 2, wherein the attenuator network further includes a second resistor shunted to ground, the second resistor being electrically connected in parallel to the shunt circuit.

4. The radio frequency detector of Claim 1 , wherein the diode block further comprises: an active diode for transforming the extracted signal from a sinusoidal signal to a rectified signal.

5. The radio frequency detector of Claim 1 , wherein the diode block does not include any transistor.

6. The radio frequency detector of Claim 1 , wherein the diode block further comprises: a resistor connected in series with the active diode; a first capacitor connected with one end between the active diode and the resistor and with the other end to ground; and a temperature compensating unit connected to the resistor.

7. The radio frequency detector of Claim 6, wherein the temperature compensating unit comprises: a second capacitor connected in parallel with a diode to the resistor.

8. The radio frequency detector of Claim 1 , wherein the at least first diode of the attenuator network is a positive-intrinsic-negative diode.

9. The radio frequency detector of Claim 1 , wherein the at least first diode is not biased by an external power source.

10. A communication device comprising: a transmitter (12) configured to transmit power along a transmission line (16); a load (14) connected to the transmission line (16) and configured to receive the power from the transmitter (12); and a radio frequency detector coupled to the transmission line (16) and configured to detect the power in the transmission line (16), the radio frequency detector including: a coupler device (18) configured to extract a signal from the transmission line (16), an attenuator network (20) connected to the coupler device (18) and configured to reduce a power of the extracted signal, wherein the attenuator network (20) includes a shunt circuit (25) connected to ground, characterized in that the shunt circuit (25) including at least a first diode of a limiter type, and a diode block (30) connected to the attenuator network (20) and configured to determine the power in the transmission line (16).

11. A method for increasing a dynamic range of a radio frequency detector that detects a power in a transmission line (16) between a signal source (12) and a load (14), the method comprising: extracting a signal from the transmission line (16) by using a coupler device (18); reducing a power of the extracted signal by directing the extracted signal through an attenuator network (20) connected to the coupler device (18), wherein the attenuator network (20) includes a shunt circuit (25) connected to ground, characterized in that the shunt circuit (25) includes at least a first diode of a limiter type and the shunt circuit (25) increases the dynamic range of the radio frequency detector; and determining the power in the transmission line (16) using a diode block (30) connected to the attenuator network (20).

12. The method of Claim 11 , further comprising: transforming the extracted signal from a sinusoidal signal to a rectified signal by the diode block.

13. The method of Claim 11 , wherein the diode block does not include any transistor.

14. The method of Claim 11 , wherein the at least first diode of the attenuator network is a positive-intrinsic-negative diode.

15. The method of Claim 11 , wherein the at least first diode is not biased by an external power source.