Patent Description:
Improving the radar is a key aspect for the next generation Frequency-Modulated Continuous-Wave (FMCW) imaging radar systems. The improvement of the range detection can be addressed by using a multi-chip transceiver using beam steering techniques. It consists in dynamically adapting the beam pattern of the antennas by changing the signal phase in real time without changing the antenna elements or other hardware. The beam steering performance depends on the accuracy of the control of the phase of each antenna. Phase and amplitude of the radiation pattern are digitally controlled through a phase shifter. Any distortions from the programmed phase and amplitude will degrade the radar performance.

<CIT> discloses a phased array transmission device. Transmission outputs of a plurality of transmission branches are extracted by coupler sections. Branch detectors respectively detect the levels of the extracted signals of the respective transmission branches and a combination detector detects an output obtained by combining two extracted outputs from the transmission branches by a signal combining section. A phase error is detected and corrected by an output level of the combination detector.

<CIT> discloses a phased array transmission device including: a plurality of transmission branches, each being provided with a phase shift unit that applies a phase rotation to a baseband signal, a DC offset correction unit that adds a first correction value to an output signal of the phase shift unit, and a mixer that subjects an output signal of the DC offset correction unit to a frequency conversion to a high frequency band; and a correction control unit that calculates a second correction value with which a carrier leak component included in an output signal of the mixer is minimized, for each of a plurality of candidates for a phase rotation amount that is set for the phase rotation, and determines the first correction value on the basis of the second correction value.

<CIT> discloses an electronic circuit includes a first PLL circuit including a first frequency divider whose frequency-division ratio is variably controlled, a second frequency divider configured to divide a frequency of a signal input into the first frequency divider, a delay circuit configured to delay an output signal of the second frequency divider, a second PLL circuit configured to receive an output signal of the delay circuit as a reference signal, and a mixer circuit configured to receive as inputs an oscillating signal of the first PLL circuit and an oscillating signal of the second PLL circuit.

<CIT> discloses a compensator for compensating mismatches, and methods for such compensation, the compensator compensates for mismatches in output signals of a system with mismatches during normal operation of the system with mismatches.

According to a first aspect of the invention, there is provided a method for calibrating a multi-channel radio frequency transmitter according to claim <NUM>.

According to a second aspect of the invention, there is provided a multi-channel radio frequency transmitter according to claim <NUM>.

Optional and/or preferable features are laid out in the dependent claims.

Other aspects and advantages of embodiments of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, depicted by way of example of the principles of the invention.

Thus, the following detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments.

The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment.

Reference throughout this specification to "one embodiment," "an embodiment," or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment. Thus, the phrases "in one embodiment," "in an embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

<FIG> is a diagram of a multi-channel radio frequency (RF) transmitter system <NUM> in accordance with an embodiment of the invention. As explained below, the multi-channel RF transmitter includes mechanisms to calibrate phase rotators in the transmitter, perform phase alignment and check for phase drift to ensure that the phase rotators are performing properly. Although the multi-channel RF transmitter can be used for various application, one application of interest is a radar system for autonomous vehicles.

As shown in <FIG>, the multi-channel RF transmitter <NUM> includes an I/Q coupler module <NUM>, a first channel transmitter module TX1, a second channel transmitter module TX2, digital-to-analog converters (DACs) <NUM>, <NUM>, <NUM> and <NUM>, a mixer <NUM>, an analog-to-digital converter (ADC) <NUM> and a fast Fourier transform (FFT) module <NUM> and a digital processing system that includes a phase error controller <NUM> and a direct digital synthesizer (DDS) <NUM>. The multi-channel RF transmitter has an input <NUM>, which receives an input signal from a local oscillator (not shown), and outputs <NUM> and <NUM>, which transmit output signals TX1_OUT and TX2_OUT from the first and second channel transmitter modules TX1 and TX2, respectively.

The I/Q coupler module <NUM> is connected to the input <NUM> to receive the input signal and convert it to a complex (I/Q) RF signal consisting of a first (I) signal component cos(ωLO * t) and a second (Q) signal component sin(ωLO * t), where ωLO is the frequency of the local oscillator, which are received by the first and second channel transmitter modules TX1 and TX2.

The first channel transmitter module TX1 includes a phase shifter (also known as a phase rotator) <NUM> and a power amplifier <NUM>. The phase shifter receives the complex (I/Q) RF signal from the I/Q coupler module <NUM>. The phase shifter <NUM> also receives an intermediate frequency (IF) signal from the DDS <NUM> via the DACs <NUM> and <NUM>, which in the example shown in <FIG> is dynamically applied to the phase shifter <NUM> and consists of a first IF signal component cos(ωIF * t) and a second IF signal component sin(ωIF * t), where ωIF is the frequency of IF signal, cos(<NUM>πf<NUM> * t), where f<NUM> is in the order of MHz, which can be defined by the equation <MAT>, where FSDDS is the sampling frequency of the DDs, cmd is the digital control of the DDS <NUM>, which is used to program the wanted IF frequency, FS_ADC is the sampling frequency of the ADC <NUM>, npts_fft is the number of points used in the discrete Fourier transform, and k is an integer. In the example shown in <FIG>, the first and second channel transmitter modules TX1 and TX2 are both being calibrated at the same time. The output of the first channel transmitter module TX1 in this example is a signal cos(ωLO * t + ωIF * t), which is amplified by the power amplifier <NUM>, and transmitted to the mixer <NUM> for calibration. During normal operations, the output of the power amplifier <NUM> would be transmitted to the output <NUM>.

The second channel transmitter module TX2 includes a phase shifter or rotator <NUM> and a power amplifier <NUM>. The phase shifter <NUM> also receives the complex (I/Q) RF signal from the I/Q coupler module <NUM>. The phase shifter <NUM> also receives a phase code signal from the DDS <NUM> via the DACs <NUM> and <NUM>, which in the example shown in <FIG> is statically applied to the phase shifter <NUM> and consists of a first phase shift signal component cos(φ<NUM>) and a second phase shift signal component sin(φ<NUM>), where φ<NUM> is the wanted phase code that is applied on the phase rotator. The output of the second channel transmitter module TX2 is a signal cos(ωLO*t+φ<NUM>), which is amplified by the power amplifier <NUM>, and transmitted to the mixer <NUM> for calibration. Again, during normal operations, the output of the power amplifier <NUM> would be transmitted to the output <NUM>.

The mixer <NUM> is connected to the outputs of the power amplifiers <NUM> and <NUM> to receive the amplified signals from the first and second channel transmitter modules TX1 and TX2. The output of the mixer <NUM> is connected to the ADC <NUM>, where mixed analog signal cos(ωIF * t+φ<NUM>) from the mixer in the illustrated example is converted to a digital signal. The FFT module <NUM> is connected to the ADC <NUM> to receive the digital signal and execute FFT on the digital signal. In an embodiment, the FFT module <NUM> executes <NUM> points FFT on the digital signal. As an example, the FFT module may use a frequency bin of <NUM> for a <NUM> range. The output of the FFT module is used by the phase error controller <NUM>, as explained below. The phase error controller <NUM> can be any digital signal processing device, such as a microcontroller or a digital processor. The operations performed by the phase error controller <NUM> will be described below.

The first and second channel transmitter modules TX1 and TX2 provide output signals with phase rotations. There are various influences on the phase rotations provided by the first and second channel transmitter modules TX1 and TX2, in particular, the phase shifters or rotators <NUM> and <NUM>, that may cause phase error, as described below.

The first influence of interest is the influence of phase rotator gain imbalance on phase error. This influence is illustrated in <FIG>, which illustrates phase errors of +/- <NUM> degrees caused by gain imbalance of <NUM>. With this gain imbalance, the output phase is defined by [cos(LO) cos(φ) - (<NUM>. 8dB) * sin(LO) sin(φ)]. The left graph in <FIG> is a graph of phase errors over different phase codes from <NUM> to <NUM> degrees, which shows phase errors of +/- <NUM> degrees. The right graph in <FIG> is a polar coordinate graph showing points for an ideal signal versus points for a signal with phase rotator gain imbalance at the mixer <NUM> of the multi-channel RF transmitter <NUM>.

The second influence of interest is the influence of phase rotator phase imbalance on phase error. This influence is illustrated in <FIG>, which illustrates phase errors of +<NUM> degrees maximum caused by phase imbalance of <NUM> degrees. With this phase imbalance, the output phase is defined by [cos(LO) cos(φ) - sin(LO) sin(φ + <NUM>)]. The left graph in <FIG> is a graph of phase errors over different phase codes from <NUM> to <NUM> degrees, which shows phase errors of + <NUM>/<NUM> degrees maximum. The right graph in <FIG> is a polar coordinate graph showing points for an ideal signal versus points for a signal with phase rotator phase imbalance at the mixer <NUM> of the multi-channel RF transmitter <NUM>.

The third influence of interest is the influence of phase rotator LO-to-RF leakage on phase error. Phase rotator LO-to-RF leakage is the resultant leakage from the I and Q signals in the phase rotators <NUM> and <NUM>, which involves the leakage from the LO, which is input to each of the phase rotators, to the RF signal, which is the output from each mixer in each of the phase rotators. This influence is illustrated in <FIG>, which illustrates phase error of + <NUM> degrees caused by LO-to-RF leakage of the I and Q mixer of the phase rotator, where the leakage for I is 26dB and the leakage for Q is 26dB. The left graph in <FIG> is a graph of phase errors over different phase codes from <NUM> to <NUM> degrees, which shows the phase errors of +<NUM> degrees to -<NUM> degrees. The right graph in <FIG> is a polar coordinate graph showing points for an ideal signal versus points for a signal with phase rotator LO-to-RF leakage at the mixer <NUM> of the multi-channel RF transmitter <NUM>.

The combined effect of these influences or phase influencing factors is illustrated in <FIG>, which illustrates phase error of +/-<NUM> degrees caused by gain imbalance of <NUM>. 8dB, phase imbalance of <NUM> degrees and LO-to-RF leakage of 23dB. The left graph in <FIG> is a graph of phase errors over different phase codes from <NUM> to <NUM> degrees, which shows phase errors of +<NUM> degrees to -<NUM> degrees. The right graph in <FIG> is a polar coordinate graph showing points for an ideal signal versus points for a signal with phase rotator gain imbalance, phase imbalance and LO-to-RF leakage at the mixer <NUM> of the multi-channel RF transmitter <NUM>.

In order to calibrate the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 to reduce these influences, the multi-channel RF transmitter <NUM> is designed to perform three successive calibrations, which may be done in any order, in what will be referred to herein as a three-step calibration process. These three calibrations include (<NUM>) calibration of LO-to-RF leakage, (<NUM>) calibration of phase rotator gain imbalance and (<NUM>) calibration of phase rotator phase imbalance. These calibrations involve selecting compensation values to adjust or modify the signals from the DDS <NUM> to the phase shifters <NUM> and <NUM> to compensate for these influences. This is illustrated in <FIG>, which shows a modified IF signal that is applied to the phase rotator <NUM> of the first second channel transmitter module TX1 and a modified phase code signal that is applied to the phase rotator <NUM> of the second channel transmitter module TX2 (DACs <NUM>, <NUM>, <NUM> and <NUM> are not illustrated in this figure). As shown in <FIG>, the modified signals include a DC offset value Offset_predist to compensate for phase rotator LO-to-RF leakage, a phase value Φ_predist to compensate for phase rotator phase imbalance, and a gain value G_predist to compensate for phase rotator gain imbalance. The basis for using these values on the signals on the phase rotators is illustrated in <FIG> and <FIG>.

<FIG> shows the phase rotator <NUM> of the first channel transmitter module TX1 that receives the complex (I/Q) RF signal consisting of (I) signal component cos(ωLO * t) and (Q) signal component sin(ωLO * t) from the I/Q coupler module <NUM>. In addition, the phase rotator <NUM> received an IF signal consisting of a phase shift signal component cos(ωIF * t) and a phase shift signal component sin(ωIF * t). In an ideal case, the output of the phase rotator would be cos(LO) cos(φ) - sin(φ) sin(LO). However, if there is phase and gain imbalances, the actual output of the phase rotator <NUM> would be cos(LO) cos(φ) - G_imb * sin(φ) sin(LO + Δφ_imb), where G_imb is the gain imbalance and Δφ_imb is the phase imbalance, as illustrated in <FIG>. Thus, the actual output of the phase rotator <NUM> would include phase error with respect to the ideal case. This phase error can be theoretically canceled by modifying the IF signal applied to the phase rotator <NUM>.

As illustrated in <FIG>, the IF signal applied to the phase rotator <NUM> has been modified to a signal consisting of a phase shift signal component cos(ωIF * t) and a second phase shift signal G_predist * sin(ωIF * t + φ_predist), where Gpredist = <NUM>/G_imb and ΔΦ_roredist = ΔΦ_imb. Using this modified IF signal, the output of the phase rotator would be cos(LO) cos(φ) - G_imb * G _predist * sin(Φ + ΔΦ_predist) sin(LO + ΔΦ_imb) so that the phase error caused by the gain imbalance and the phase imbalance would be canceled out.

The three-step calibration process performed by the multi-channel RF transmitter <NUM> in accordance with an embodiment is described with reference to a process flow diagram of <FIG>, <FIG> and <FIG>. In this embodiment, the three-step calibration process involves the following order of performing calibrations: LO-to-RF leakage calibration (<FIG>), gain imbalance calibration (<FIG>) and phase imbalance calibration (<FIG>). However, as previously mentioned, these three calibrations may be performed in any order, such as gain imbalance calibration, LO-to-RF leakage calibration and then phase imbalance calibration.

As shown in <FIG>, the three-step calibration process begins at block <NUM>, where a first LO-to-RF leakage compensation value ("DC offset value") is selected. A LO-to-RF leakage compensation value is a value to produce an opposite offset of an offset caused by LO-to-RF leakage. In an embodiment, the LO-to-RF leakage compensation value may represent an offset value in mV. Next, at block <NUM>, a phase code corresponding to a first phase is selected. Next, at block <NUM>, an IF signal with the selected LO-to-RF leakage compensation value is applied to the phase shifter <NUM> of the first channel transmitter module TX1. Next, at block <NUM>, the phase code with the same selected LO-to-RF leakage compensation value is applied to the phase shifter <NUM> of the second channel transmitter module TX2. Next, at block <NUM>, the resultant phase error due to the phase errors in the outputs of the phase shifters <NUM> and <NUM> is computed using the FFT results from the FFT module <NUM> and stored in a storage accessible by the phase error controller <NUM>. Next, at block <NUM>, a determination is made whether the current phase code corresponds to the last phase code to be used. If the current phase code is not the last phase code for this iteration, then the process proceeds back to block <NUM>, where the next phase code is selected to be applied to the phase shifter <NUM> of the second channel transmitter module TX2, while the IF signal is applied to the phase shifter <NUM> of the first channel transmitter module TX1, to compute another resultant phase error due to the phase errors in the outputs of the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 for the selected phase code. In an embodiment, the phase codes correspond to <NUM> to <NUM> degrees in fixed steps, e.g., <NUM> degrees. Thus, in this embodiment, there will be eight phase codes that need to be used. If the current phase code is the last phase code for this iteration, then the process proceeds to block <NUM>.

At block <NUM>, a determination is made whether the current LO-to-RF leakage compensation value is the last compensation value to be used. If the current LO-to-RF leakage compensation value is not the last value, then the process proceeds back to block <NUM>, where the next LO-to-RF leakage compensation value is selected to be used. In an embodiment, the LO-to-RF leakage compensation values correspond to <NUM> dB to <NUM> dB in predefined increments, e.g., <NUM> or <NUM> dB, which can be achieved by incrementing the least significant bit (LSB) of the DACs <NUM>, <NUM>, <NUM> and <NUM>. However, if the current LO-to-RF leakage compensation value is the last value, then the process proceeds to block <NUM>.

At block <NUM>, the phase error controller <NUM> sums the resultant phase errors associated with the different phase codes for each of the LO-to-RF leakage compensation values. Next, at block <NUM>, the phase error controller <NUM> selects the LO-to-RF leakage compensation value with the minimum summed resultant phase error value to be used as the LO-to-RF leakage compensation value for the other calibrations. Thus, the optimal LO-to-RF leakage compensation value has been selected for the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2.

As shown in <FIG>, the gain imbalance calibration of the three-step calibration process begins at block <NUM>, where a first gain imbalance compensation value is selected. Next, at block <NUM>, a phase code corresponding to a first phase is selected. Next, at block <NUM>, an IF signal with the optimal LO-to-RF leakage compensation value and the selected gain imbalance compensation value is applied to the phase shifter <NUM> of the first channel transmitter module TX1. Next, at block <NUM>, the phase code with the optimal LO-to-RF leakage compensation value and the same selected gain imbalance compensation value is applied to the phase shifter <NUM> of the second channel transmitter module TX2. Next, at block <NUM>, the resultant phase error due to the phase errors in the outputs of the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 is computed using the FFT results from the FFT module <NUM> and stored in a storage accessible by the phase error controller <NUM>. Next, at block <NUM>, a determination is made whether the current phase code corresponds to the last phase code to be used. If the current phase code is not the last phase code for this iteration, then the process proceeds back to block <NUM>, where the next phase code is selected to be applied to the phase shifter <NUM> of the second channel transmitter module TX2, while the IF signal is applied to the phase shifter <NUM> of the first channel transmitter module TX1, to compute another resultant phase error due to the phase errors in the outputs of the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 for the selected phase code. In an embodiment, the phase codes correspond to <NUM> to <NUM> degrees in fixed steps, e.g., <NUM> degrees. Thus, in this embodiment, there will be eight phase codes that need to be used. If the current phase code is the last phase code for this iteration, then the process proceeds to block <NUM>.

At block <NUM>, a determination is made whether the current gain imbalance compensation value is the last value to be used. If the current gain imbalance compensation value is not the last value, then the process proceeds back to block <NUM>, where the next gain imbalance compensation value is selected to be used. In an embodiment, the gain imbalance compensation values correspond to -<NUM> dB to +<NUM> dB in predefined increments, e.g., <NUM> dB. However, if the current gain imbalance compensation value is the last value, then the process proceeds to block <NUM>.

At block <NUM>, the phase error controller <NUM> sums the resultant phase errors associated with different phase codes for each of the gain imbalance compensation values. Next, at block <NUM>, the phase error controller <NUM> selects the gain compensation value with the minimum summed resultant phase error value to be used as the gain imbalance compensation value for the remaining calibration. Thus, the optimal gain imbalance compensation value has been selected for the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2, in addition to the optimal LO-to-RF leakage compensation value.

As shown in <FIG>, the phase imbalance calibration of the three-step calibration process begins at block <NUM>, where a first phase imbalance compensation value is selected. Next, at block <NUM>, a phase code corresponding to a first phase is selected. Next, at block <NUM>, an IF signal with the optimal LO-to-RF leakage compensation value, the optimal gain imbalance compensation value and the selected phase imbalance compensation value is applied to the phase shifter <NUM> of the first channel transmitter module TX1. Next, at block <NUM>, the phase code with the optimal LO-to-RF leakage compensation value, the optimal gain imbalance compensation value and the selected phase imbalance compensation value is applied to the phase shifter <NUM> of the second channel transmitter module TX2. Next, at block <NUM>, the resultant phase error due to the phase errors in the outputs of the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 is computed using the FFT results from the FFT module <NUM> and stored in a storage accessible by the phase error controller <NUM>. Next, at block <NUM>, a determination is made whether the current phase code corresponds to the last phase code to be used. If the current phase code is not the last phase code for this iteration, then the process proceeds back to block <NUM>, where the next phase code is selected to be applied to the phase shifter <NUM> of the second channel transmitter module TX2, while the IF signal is applied to the phase shifter <NUM> of the first channel transmitter module TX1, to compute the phase error in the outputs of the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 for the selected phase code.

In an embodiment, the phase codes correspond to <NUM> to <NUM> degrees in fixed steps, e.g., <NUM> degrees. Thus, in this embodiment, there will be eight phase codes that need to be used. If the current phase code is the last phase code for this iteration, then the process proceeds to block <NUM>.

At block <NUM>, a determination is made whether the current phase imbalance compensation value is the last value to be used. If the current phase imbalance compensation value is not the last value, then the process proceeds back to block <NUM>, where the next phase imbalance compensation value is selected to be used. In an embodiment, the phase compensation values correspond to - <NUM> degrees to + <NUM> degrees in predefined increments, e.g., <NUM> degrees. However, if the current phase imbalance compensation value is the last value, then the process proceeds to block <NUM>.

At block <NUM>, the phase error controller <NUM> sums the resultant phase errors associated with different phase codes for each of the phase imbalance compensation values. Next, at block <NUM>, the phase error controller <NUM> selects the phase imbalance compensation value with the minimum summed resultant phase error value. Thus, the optimal phase imbalance compensation value has been selected for the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2, in addition to the optimal LO-to-RF leakage and gain imbalance compensation values. These compensation values can then be used to modify the signals from the DDS <NUM> used on the phase shifters <NUM> and <NUM> of the first and second channel transmitter modules TX1 and TX2 to ensure that their outputs are the desired signals with respect to phase and amplitude.

The different phase errors that are captured by the phase error controller <NUM> for the different compensation values during each of the three calibrations are illustrated in <FIG>, which is a graph showing phase errors for different phase compensation values during the phase imbalance calibration. In this graph, each line represents the phase errors for a particular phase compensation value.

In simulations, for gain imbalance = <NUM> dB, phase imbalance = <NUM> degrees and LO-to-RF leakage = <NUM> dB, the phase error is +/- <NUM> degrees before the three-step calibration process depicted in <FIG>. After the three-step calibration process, the phase error is less than or equal to <NUM> degrees.

The results of the three-step calibration process can depend on the resolution of the DACs <NUM>. <NUM>, <NUM> and <NUM>, and thus, can be reduced by using different types of DACs. This is illustrated in <FIG>, <FIG> and <FIG>. <FIG> shows the phase errors due to gain imbalance for no calibration, for calibration using <NUM>-bit DACs and for calibration using <NUM>-bit DACs. <FIG> shows the phase errors due to phase imbalance for no calibration, for calibration using <NUM>-bit DACs and for calibration using <NUM>-bit DACs. <FIG> shows the phase errors due to LO-to-RF leakage for no calibration, for calibration using <NUM>-bit DACs and for calibration using <NUM>-bit DACs.

The described three-step calibration process calibrates both the first and second transistor modules TX1 and TX2. The same three-step calibration process can be applied with the first and second transistor modules TX1 and TX2 reversed. The results would be similar since the first and second transistor modules TX1 and TX2 are similar. Although the three-step calibration process has been described using two channel transmitter modules, the described three-step calibration process may be extended for three channel transmitter modules, which may simply involve different connections between the three channel transmitter modules, the DDS <NUM> and the mixer <NUM> so that only two of the three transmitter modules are involved for each three-step calibration process.

After the three-step calibration process, a phase alignment process should be performed to phase align between the first and second channel transmitter modules TX1 and TX2. This phase alignment process will be described with reference to <FIG>. In this phase alignment process, the same phase code (phase code <NUM>) with the same predistortion or compensation is applied on both the first and second channel transmitter modules TX1 and TX2 from the DDS <NUM>. In addition, a complex (I/Q) RF signal consisting of a first (I) signal component cos(ωLO * t) and a second (Q) signal component sin(ωLO * t) is applied to both the first and second channel transmitter modules TX1 and TX2 from the I/Q coupler device <NUM>. The outputs from the first and second channel transmitter modules TX1 and TX2 will be cos(LO + Ø<NUM>) and cos(LO + Ø<NUM>), respectively. Thus, the output of the ADC <NUM> will be cos(Ø<NUM> - Ø<NUM>). The phase error controller <NUM> receives the signal from the ADC <NUM> without FFT and extracts (Ø<NUM> - Ø<NUM>) using the arccosine function to determine the phase misalignment between the first and second channel transmitter modules TX1 and TX2. In an alternative embodiment, the phase difference (Ø<NUM> - Ø<NUM>) can be extracted by applying FFT to the received signal from the ADC <NUM> and using the arctangent function. Using this information, the phase error controller <NUM> provides a phase offset to the DDS <NUM> to compensate for the phase misalignment so that the DDS modifies signals to the first and second transmitter modules TX1 and TX2 so that Ø<NUM> - Ø<NUM> is zero or close to zero (e.g., < <NUM> degrees) to phase align the first and second channel transmitter modules TX1 and TX2.

As the multi-channel RF transmitter <NUM> operates, one or both of the first and second channel transmitter modules TX1 and TX2 may experience phase error drift due to various factors, which may pose a safety threat for certain applications, such as autonomous driving or collision avoidance. Thus, the multi-channel RF transmitter may periodically perform a safety check on each of the first and second channel transmitters.

In an embodiment, a safety check of the multi-channel RF transmitter <NUM> involves checking periodically, e.g., daily, weekly, monthly or other appropriate predefined intervals, the phase error level when compensation has been applied. After compensation for the first time for a particular phase code, the phase error is measured, and the value of the phase error for the phase code is loaded in a register with a programmed tolerance to be used as a reference phase error. In case of a problem in one or both of the first and second channel transmitter modules TX1 and TX2, the phase errors for different phase codes will start to drift, which can be measured as phase differences, or delta phases, between the current phase errors and the reference phase errors. If any of the phase differences due to the drift is higher than a first programmable threshold, then a first flag is generated. If the sum of the phase differences is greater than a second programmable threshold, a second flag is generated. The flags are sent to a microcontroller unit, so that an appropriate action can be initiated in response to the flags, e.g., disabling the multi-channel RF transmitter for safety concerns.

<FIG> is a process flow diagram of a method of calibrating a multi-channel RF transmitter in accordance with an embodiment of the invention. At block <NUM>, an IF signal with different compensation values is applied on a first phase rotator in a first channel transmitter module of the multi-channel RF transmitter. The different compensation values are designed to compensate for a particular phase influencing factor, such as a LO-to-RF leakage, a gain imbalance or a phase imbalance. At block <NUM>, phase codes with the same different compensation values for different phases are applied on a second phase rotator in a second channel transmitter module of the multi-channel RF transmitter. At block <NUM>, resultant phase errors due to phase errors of the first and second channel modules for the different compensation values are measured. At block <NUM>, based on the resultant phase errors, one of the different compensation values is selected to be used as a calibrated compensation value for the first phase rotator in the first channel transmitter module and for the second phase rotator in the second channel transmitter module to compensate for the particular phase influencing factor.

Although the operations of the method herein are shown and described in a particular order, the order of the operations of the method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations.

Claim 1:
A method of calibrating a multi-channel radio frequency, RF, transmitter (<NUM>), the method comprising:
for (<NUM>) each of a set of different compensation values for a particular phase-influencing factor:
a) applying (<NUM>, <NUM>, <NUM>) an intermediate frequency, IF, signal with the compensation value on a first phase rotator (<NUM>) in a first channel transmitter module(TX1) of the multi-channel RF transmitter;
b) applying (<NUM>, <NUM>) phase codes for different phases on a second phase rotator (<NUM>) in a second channel transmitter module (TX2) of the multi-channel RF transmitter; and
c) for each of the phase codes, measuring (<NUM>, <NUM>) a resultant phase error due to phase errors of the first and second channel transmitter modules; and
based on the resultant phase errors, selecting (<NUM>, <NUM>) one compensation value from the set of different compensation values, to be used as a calibrated compensation value for each of the first phase rotator in the first channel transmitter module and the second phase rotator in the second channel transmitter module to compensate for the first phase-influencing factor.