An AM-PM distortion calibration method applied to a UWB transmitter

Abstract

The application provides an AM-PM distortion calibration method applied to an UWB transmitter and belongs to the field of UWB communication. The method adds a sampling resistor between a linear voltage stabilizing power supply LDO and a digital power amplifier DPA driving stage of the UWB transmitter using a digital power amplifier, so as to simulate the instantaneous change of the power supply voltage of the DPA driving stage when the UWB pulse is transmitted; an adjustable delay unit is added between a carrier input port LO and the DPA driving stage; the continuous sine wave is transmitted, the phase information under each digital baseband amplitude control word is collected to form original data of the AM-PM distortion of the UWB transmitter, the digital baseband output AM-PM control word is used to control the adjustable delay unit to output the phase which is opposite to the original data and has the same size, so that the phase predistortion of the transmitted UWB pulse is realized, and the AM-PM distortion calibration is completed. The application has the advantages of simple implementation and strong compensation capacity.

Description

Technical Field

[0001] This invention belongs to the field of UWB communication in wireless communication technology, and specifically proposes an AM-PM distortion calibration method for UWB transmitters. Background Technology

[0002] Ultra Wideband (UWB) is a carrier-free communication technology that transmits data using narrow pulses on the order of nanoseconds. UWB signals have a low duty cycle, and compared to other communication methods, the static power consumption of a UWB transmitter accounts for the majority of its total power consumption. Radio frequency transmitters mainly consist of two core modules: an analog / digital baseband and a power amplifier. Traditional transmitters use linear power amplifiers (Class A, Class B, Class AB) with large static bias currents, resulting in high power consumption. Therefore, in recent years, many studies have used digital power amplifiers (DPAs) with no static current to reduce the transmitter's static power consumption.

[0003] Figure 1 This is a circuit diagram of a UWB transmitter using DPA. Figure 1 In this diagram, LO is the carrier input port of the transmitter. The carrier input signal undergoes phase modulation and power amplification in the DPA driver stage module before entering the final stage of the DPA. The final stage of the DPA modulates the phase-modulated carrier signal, amplifies the power, and outputs it to the antenna. The microcontroller (MCU) communicates with the on-chip digital baseband (DBB) to control the DBB to generate the phase modulation control word PM required by the DPA driver stage and the amplitude modulation control word AM required by the final stage of the DPA. The DPA driver stage is typically powered by an internal linear regulated power supply (LDO). Because the UWB RF power amplifier requires a large current and a fast settling time, the LDO usually needs to include an external capacitor C1 for auxiliary power supply. Inductor L1 and resistor R1 are the parasitic inductance and parasitic resistance between the chip and the external capacitor when the LDO is connected to the external capacitor.

[0004] Poor linearity of DPA affects the quality of the transmitted signal. The main causes of poor linearity are amplitude distortion (AM-AM distortion) and phase distortion (AM-PM distortion). Therefore, transmitters using DPA typically require additional linearity compensation measures to ensure system communication quality. Transmitter output quality is usually defined by the channel's spectral mask. According to the latest IEEE 802.15.4z protocol standard (hereinafter referred to as the "Standard"), in f... LO ±0.65f BW The output spectrum outside the frequency range needs to be attenuated by 18dBr, at f LO ±0.8f BWThe frequency spectrum outside the specified frequency range needs to have an attenuation of 10dBr, where f LO It is the channel center frequency, f BW It is the channel bandwidth. Figure 2 The spectrum represents the standard-defined channel spectrum mask (IEEE MASK), the pulse spectrum with positive / inverted AM-PM distortion, and the spectrum of an ideal pulse signal without distortion. The horizontal axis represents frequency, and the vertical axis represents the normalized power integral at the corresponding frequency. With positive AM-PM distortion, the in-band spectrum is tilted towards lower frequencies; with inverted AM-PM distortion, the in-band spectrum is tilted towards higher frequencies; and ideally, the in-band spectrum is very flat.

[0005] AM-AM and AM-PM distortions in DPA can be reduced using predistortion techniques. However, improving linearity with predistortion requires first sampling the original AM-PM distortion data before performing distortion compensation that is inversely proportional to the original data. When the signal bandwidth is small, the AM-PM distortion of DPA is mainly due to the variation of the parasitic capacitance of the final stage power transistor under different output powers, and the original AM-PM distortion data can be obtained by directly measuring the output signal spectrum. However, in UWB systems, the standard specifies a baseband pulse signal bandwidth of 0.5GHz to 1.35GHz, which is much larger. The AM-PM distortion of DPA is mainly caused by the rapidly changing power supply voltage affecting the switching speed of the driver stage circuit. Therefore, the original AM-PM distortion data of DPA in UWB transmitters cannot be directly measured. Thus, existing predistortion techniques for AM-PM distortion are mainly used for transmitters with bandwidths less than 100MHz and cannot be used for UWB transmitters. AM-PM predistortion techniques for transmitters with bandwidths greater than 500MHz are still under active research. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and propose an AM-PM distortion calibration method for UWB transmitters. This invention can solve the problem of significant AM-PM distortion caused by the large bandwidth UWB signal acting on a non-ideal power network in UWB transmitters using digital power amplifiers (including but not limited to Class D, E, and F), which leads to a decrease in the output spectrum quality of the UWB transmitter. This invention features simple implementation and strong compensation capability.

[0007] This invention proposes an AM-PM distortion calibration method for UWB transmitters, comprising:

[0008] 1) Add a sampling resistor between the linear regulated power supply (LDO) and the digital power amplifier (DPA) driver stage of the UWB transmitter using a digital power amplifier, and add an adjustable delay unit between the carrier input port (LO) and the DPA driver stage; wherein, the digital baseband (DBB) of the UWB transmitter controls the delay of the adjustable delay unit through the AM_PM control word;

[0009] The sampling resistor is used to simulate the instantaneous change of the power supply voltage of the DPA drive stage when the UWB transmitter transmits a continuous sine wave mode.

[0010] The adjustable delay unit is used to adjust the carrier phase to compensate for AM-PM distortion when transmitting UWB pulses;

[0011] 2) The UWB transmitter emits a continuous sine wave with constant amplitude and phase to test the phase of the output signal;

[0012] 3) Initialize the amplitude control word AM of the digital baseband DBB to 0;

[0013] 4) By acquiring the phase of a continuous sine wave, the phase information under the current amplitude control word can be obtained;

[0014] 5) Configure the amplitude control word AM of the digital baseband DBB to increment by 1, and then return to step 4) until the amplitude control word AM reaches the set maximum value, so as to obtain the original data of the UWB transmitter AM-PM distortion composed of the phase information under each amplitude control word;

[0015] 6) The digital baseband DBB outputs an AM-PM control word to control the adjustable delay unit to output a phase that is opposite in magnitude but the same in magnitude as the original data of the UWB transmitter's AM-PM distortion, so as to achieve phase pre-distortion of the transmitted UWB pulse and thus complete AM-PM distortion calibration.

[0016] In one specific embodiment of the present invention, the resistance value of the sampling resistor is equal to |R1+j2Πf BB L1|, where j is the imaginary unit of the real number, π is pi, and f BB It is the baseband signal frequency. R1 and L1 are the parasitic resistance and inductance of the circuit between the DPA driver stage and the external capacitor C1, respectively.

[0017] When the UWB transmitter emits a continuous sine wave, the supply current of the DPA driver stage is I2, and the supply voltage of the DPA driver stage is V. LDO -I2*R2, where R2 is the resistance value of the sampling resistor.

[0018] In one specific embodiment of the present invention, the adjustable delay unit includes: an inverter and N capacitors C. Pi and N NMOS switches M i , i = 1, 2, ..., N; where each capacitor C pi The upper ends of each capacitor C are connected to the output terminals of the inverter. pi The lower end is connected to the corresponding NMOS switch M i The drains are connected, M i The source level is connected to ground, M i The gate of the inverter is connected to the i-th bit of the N-bit AM_PM control word output by the digital baseband DBB; the input of the inverter is connected to the carrier input port LO, and the output is connected to the DPA driver stage.

[0019] In one specific embodiment of the present invention, the method further includes:

[0020] The adjustable delay unit's capacitor and NMOS switch form an adjustable capacitor array; when the AM_PM control word is high, the NMOS switch is turned on, the total capacitance of the inverter increases, and phase lag occurs at the DPA driver stage; when the AM_PM control word is low, the NMOS switch is turned off, the total capacitance of the inverter decreases, and phase lead occurs at the DPA driver stage.

[0021] The features and beneficial effects of this invention are as follows:

[0022] This invention samples the AM-PM distortion caused by the power network by adding a sampling resistor to the power supply terminal of the digital power amplifier driver stage in a UWB transmitter, and compensates for the sampled AM-PM distortion by adding a delay unit controlled by the baseband amplitude signal to the driver stage, with the compensation magnitude controllable by a register. Its advantages include at least:

[0023] 1. By adding a sampling resistor, the phenomenon of power supply voltage reduction caused by large bandwidth signal when transmitting UWB pulse can be simulated, and AM-PM distortion caused by power network can be sampled. The sampling circuit structure is simple and easy to implement.

[0024] 2. When compensating the output phase of a UWB transmitter based on the AM-PM distortion data obtained from sampling, the specific implementation of the adjustable delay unit can use different delay unit circuits according to different circuit designs, and it is applicable to most UWB transmitters, thus having universality.

[0025] 3. The compensation effect of the present invention is less affected by external factors such as chip manufacturing process, ambient temperature and power supply voltage, and has reliability.

[0026] 4. By using this invention to improve the linearity of the UWB transmitter, the spectral efficiency of the transmitted signal can be greatly improved, thereby increasing the UWB communication distance while meeting the power spectral density requirements specified in the 802.15.4z communication standard. Attached Figure Description

[0027] Figure 1 This is a circuit diagram of an existing UWB transmitter that uses a digital power amplifier.

[0028] Figure 2 This is a schematic diagram illustrating the effect of different AM_PM distortions on the UWB output spectrum.

[0029] Figure 3 This is an overall flowchart of an AM-PM distortion calibration method applied to a UWB transmitter according to an embodiment of the present invention.

[0030] Figure 4 This is a circuit diagram of a UWB transmitter with a sampling resistor and an adjustable delay unit added in a specific embodiment of the present invention.

[0031] Figure 5 This is a circuit diagram of an adjustable delay unit in a specific embodiment of the present invention. Detailed Implementation

[0032] This invention proposes an AM-PM distortion calibration method for UWB transmitters, which is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] This invention proposes an AM-PM distortion calibration method for UWB transmitters, the overall process of which is as follows: Figure 3 As shown, it includes the following steps:

[0034] 1) Add a sampling resistor between the LDO and DPA driver stages of the UWB transmitter using a digital power amplifier, and add an adjustable delay unit between the carrier input port LO and the DPA driver stage.

[0035] In this embodiment, the circuit of the UWB transmitter with the addition of a sampling resistor and an adjustable delay unit is as follows: Figure 4 As shown. Figure 4 In the middle, in such Figure 1 A sampling resistor R2 is inserted between the LDO and DPA driver stages in the UWB transmitter circuit shown, and an adjustable delay unit is added between the carrier input port LO and the DPA driver stages. The digital baseband (DBB) of the UWB transmitter controls the delay of the adjustable delay unit via the AM_PM control word.

[0036] The sampling resistor is used to simulate the instantaneous change of the DPA drive stage power supply voltage when transmitting UWB pulses in the mode of transmitting continuous sine waves by the UWB transmitter.

[0037] The adjustable delay unit is used to adjust the carrier phase to compensate for AM-PM distortion when transmitting UWB pulses.

[0038] In a specific embodiment of the present invention, the resistance value of the sampling resistor R2 should be equal to |R1+j2Πf|. BB L1|, where j is the imaginary unit of the real number, and π is pi; f BB This is the baseband signal frequency, which is 0.5-1.3GHz in the UWB transmitter. R1 and L1 are the parasitic resistance and inductance of the circuit between the DPA driver stage and the external capacitor C1, respectively. When the UWB transmitter transmits UWB pulses, the DPA driver stage is mainly powered by the external capacitor C1, with a supply current of I1. The instantaneous supply voltage of the DPA driver stage is V. LDO -I1*(R1+j2Πf BB L1). When the UWB transmitter transmits a continuous sine wave, the DPA driver stage is mainly powered by the LDO, with a supply current of I2. The sampling resistor R2 is used to simulate the instantaneous voltage changes caused by L1 and R1. At this time, the power supply voltage of the DPA driver stage is V. LDO -I2*R2. At this point, by measuring the phase of the output signal at the antenna port under different output powers, the raw data of AM-PM distortion when transmitting UWB pulses can be obtained.

[0039] 2) Configure the register to enable the UWB transmitter to transmit a continuous sine wave with constant amplitude and phase, for testing the phase of the output signal.

[0040] 3) Initialize the amplitude control word AM of the digital baseband DBB to 0.

[0041] 4) The phase of a continuous sine wave is acquired using a network analyzer to obtain the phase information under the current amplitude control word.

[0042] 5) Increment the amplitude control word AM of the digital baseband DBB by 1, and then return to step 4) until the amplitude control word AM reaches the set maximum value, so as to obtain the raw data of UWB transmitter AM-PM distortion composed of the phase information under each amplitude control word.

[0043] In one specific embodiment of the present invention, the amplitude control word AM is a 6-bit binary signal, therefore the maximum value of the amplitude control word is 63, and there are a total of 64 amplitude control words. It should be noted that the amplitude control words applicable to the present invention include, but are not limited to, 6-bit binary signals.

[0044] 6) The digital baseband DBB outputs an AM-PM control word to control the adjustable delay unit to output a phase that is opposite in phase but the same in magnitude as the original data of the AM-PM distortion of the UWB transmitter, so as to achieve phase pre-distortion of the transmitted UWB pulse and thus complete the AM-PM distortion calibration.

[0045] One embodiment of the adjustable delay unit of the present invention has the following structure: Figure 5 As shown, it includes: one inverter and N capacitors C Pi and NMOS switch M i , i = 1, 2, ..., N. Where each capacitor C pi The upper ends of each capacitor C are connected to the output terminals of the inverter. pi The lower end is connected to a corresponding NMOS switch M i The drains are connected; M i The source level is connected to ground, M i The gate of the inverter is connected to the i-th bit of the N-bit AM_PM control word output by the digital baseband DBB. These capacitors and the NMOS switch form an adjustable capacitor array. When the AM_PM control word is high, the NMOS switch is turned on, the total capacitance of the inverter increases, and phase lag occurs at the DPA driver stage. When the AM_PM control word is low, the NMOS switch is turned off, the total capacitance of the inverter decreases, and phase lead occurs at the DPA driver stage. The size of the capacitors and the dimensions of the NMOS switches can be adjusted according to actual needs. The input of the inverter is connected to the carrier input port LO, and the output is connected to the DPA driver stage. Different AM-PM control words will affect the output phase of the inverter. When the output phase of this adjustable delay unit is opposite to and the same as the AM-PM distortion value obtained by the UWB transmitter test, the AM-PM distortion compensation effect can be achieved.

Claims

1. An AM-PM distortion calibration method applied to a UWB transmitter, characterized in that, include: 1) Add a sampling resistor between the linear regulated power supply (LDO) and the digital power amplifier (DPA) driver stage of the UWB transmitter using a digital power amplifier, and add an adjustable delay unit between the carrier input port (LO) and the DPA driver stage; wherein, the digital baseband (DBB) of the UWB transmitter controls the delay of the adjustable delay unit through the AM_PM control word; The sampling resistor is used to simulate the instantaneous change of the power supply voltage of the DPA drive stage when the UWB transmitter transmits a continuous sine wave mode. The adjustable delay unit is used to adjust the carrier phase to compensate for AM-PM distortion when transmitting UWB pulses; The adjustable delay unit includes: an inverter and N capacitors C. Pi and N NMOS switches M i , i=1,2,…,N; where each capacitor C pi The upper ends of each capacitor C are connected to the output terminals of the inverter. pi The lower end is connected to the corresponding NMOS switch M i The drains are connected, M i The source level is connected to ground, M i The gate of the inverter is connected to the i-th bit of the N-bit AM_PM control word output by the digital baseband DBB; the input of the inverter is connected to the carrier input port LO, and the output is connected to the DPA driver stage. The adjustable delay unit's capacitor and NMOS switch form an adjustable capacitor array; when the AM_PM control word is high, the NMOS switch is turned on, the total capacitance of the inverter increases, and phase lag occurs at the DPA driver stage; when the AM_PM control word is low, the NMOS switch is turned off, the total capacitance of the inverter decreases, and phase lead occurs at the DPA driver stage. 2) The UWB transmitter emits a continuous sine wave with constant amplitude and phase to test the phase of the output signal; 3) Initialize the amplitude control word AM of the digital baseband DBB to 0; 4) By acquiring the phase of a continuous sine wave, the phase information under the current amplitude control word can be obtained; 5) Configure the amplitude control word AM of the digital baseband DBB to increment by 1, and then return to step 4) until the amplitude control word AM reaches the set maximum value, so as to obtain the original data of the UWB transmitter AM-PM distortion composed of the phase information under each amplitude control word; 6) The digital baseband DBB outputs an AM-PM control word to control the adjustable delay unit to output a phase that is opposite in magnitude but the same in magnitude as the original data of the UWB transmitter's AM-PM distortion, so as to achieve phase pre-distortion of the transmitted UWB pulse and thus complete AM-PM distortion calibration.

2. The method according to claim 1, characterized in that, The resistance value of the sampling resistor is equal to |R1+j2Πf BB L1|, where j is the imaginary unit of the real number, π is pi, and f BB It is the baseband signal frequency. R1 and L1 are the parasitic resistance and inductance of the circuit between the DPA driver stage and the external capacitor C1, respectively. When the UWB transmitter emits a continuous sine wave, the supply current of the DPA driver stage is I2, and the supply voltage of the DPA driver stage is V. LDO -I2*R2, where R2 is the resistance value of the sampling resistor.

Images