Design of automatic measurement circuit for microwave frequency controlled by stepping motor

Microwaves usually refer to decimeter waves, centimeter waves and millimeter waves. Regarding its frequency range, one statement is:

300MHz ~ 300GHz (1MHz = 106Hz, 1GHz = 109) The corresponding wavelength in free space is about 1m~1mm.

The rise and vigorous development of microwave technology has led to the opening of microwave technology courses in most domestic universities. However, there are still the following problems: when measuring, the probe is moved point by point manually and the readings at each point are recorded, and then the experimental results are manually calculated and plotted. The measurement items are single, the accuracy is low, the measurement cycle is long, and the operation is also cumbersome. This paper mainly studies a practical automatic measurement system for microwave frequency of klystron based on Labview.

2. Overall system structure

The overall structure of the system is shown in Figure 2-1. It consists of a lower computer and a host computer. The microprocessor controls the stepper motor through the drive circuit to drive the sleeve of the resonant frequency meter to rotate. The processor samples the detection current and transmits it to the host computer LabVIEW interface for display. It also uses the powerful data processing function of the PC to analyze the minimum current value and calculate the measured frequency.

3. System hardware design

3.1 Design of Microprocessor System Circuit

The microprocessor used in this system is S3C44B0.2.5VARM7TDMI core, with 3.0~3.6V I/O operating voltage range. It can be multiplied up to 66MHz through PLL; 71 general I/O ports; embedded with 8-channel 10-bit ADC, this system selects channel 1 as the crystal detector current input channel.

3.2 Reset Circuit

The system does not use RC circuit as reset circuit, but uses voltage monitoring chip SP708SE, which improves the reliability of the system. The RST end of the reset circuit is connected to the reset pin nRESET of S3C44B0. Because the reset signal of S3C44B0 is low level effective, when the system is powered off or the reset button SW_RST is pressed, the RST pin of the power monitoring chip immediately outputs a reset signal to reset the S3C44B0 chip.

3.3 Design of automatic measurement circuit of resonant frequency meter

3.3.1 Principle of frequency measurement using calibration method

In order to realize the automatic measurement of frequency, this system uses a stepper motor to drive the rotation of the frequency meter. When the cavity rotates to the resonant position, the microwave power reaching the detector drops significantly, and the detection current drops significantly. The frequency corresponding to this position is the measured frequency. The stepper motor drives a non-read-only frequency meter, so the calibration method must be used to fit the corresponding relationship between frequency and scale. Calibration method: Use two frequency meters at the same time, one is read-only, which can read the frequency directly; the other is non-read-only, which only has scales and cannot read the frequency directly. First, manually rotate the non-read-only frequency meter to a resonant position, record the scale at this time, and then rotate the read-only frequency meter to another resonant position and record the corresponding frequency. Repeat this operation to measure as many corresponding points of frequency and scale as possible, and then use the least squares method to fit the corresponding relationship between the two based on the measured data. Finally, a stepper motor is used to drive the non-read-only frequency meter to rotate. When the detection current rotates to a clear "absorption valley", the scale at this time is read. According to the fitted scale and frequency relationship, the measured frequency can be obtained.
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3.3.2 Stepper motor and automatic control circuit

A stepper motor is an actuator that converts electrical pulses into angular displacement. In simple terms, when the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in the set direction. The angular displacement can be controlled by controlling the number of pulses to achieve accurate positioning; at the same time, the speed of the motor can be controlled by controlling the pulse frequency to achieve speed regulation.

This system uses a two-phase stepper motor with the following features: the direction of rotation of the motor can be realized by simply swapping the A+ and A- (or B+ and B-) of the motor and driver wiring; a two-phase four-wire hybrid stepper motor with a step angle of 1.8°, and the number of subdivisions of the subdivision driver is set to 8, and the motor's operating resolution is 0.225° per pulse. In order to effectively drive the motor, this article uses a drive circuit based on the TA8435H chip. The actual application circuit is shown in Figure 3-2 below. The input signals of the chip include enable control, forward and reverse control, and clock input.

The driver chip can be electrically isolated from the input stage through the optocoupler device TLP521, which plays a role in logic level isolation and protection.

M1 and M2 are connected to high level respectively, so it is 1/8 subdivision mode.

Since the REF IN pin is connected to a high level, VNF is 0.8V.

The output stage chopping current is VNF/RNF=0.8/0.8=1A, so R212 and R213 should use resistors with larger power. When using different two-phase stepper motors, appropriate R212 and R213 should be selected according to their current size. R21 and C5 form a reset circuit, and D1~D4 fast recovery diodes can be used to discharge the winding current.

The circuit uses the microprocessor S3C44B0 pins PC0, PC1, PC2 to output enable, forward and reverse, and clock signals to the drive circuit respectively. The motor rotation rate can be controlled by controlling the output pulse interval, and the number of output pulses can control the number of steps of the stepper motor to achieve the purpose of controlling the position of the frequency meter cavity. The circuit output ports A, A, B, B are connected to the corresponding input terminals of the two-phase stepper motor.

3.3.3 Detection current I/V conversion and amplification circuit

The function of the detection crystal is to convert weak microwave signals into DC signals. Therefore, it is possible to observe whether the detection current has an "absorption trough" to determine whether the cavity has reached the resonant position. This system transmits the detection current to the LabVIEW interface of the host computer after processing to observe whether it has reached the resonant position.

Since microwave signals are subject to external interference noise, line noise, component noise, etc. during transmission, a filter circuit is needed to filter out these interference signals. Since the processor has a relatively low acquisition rate for signals, this system uses a low-pass filter composed of R418 and C409 with a relatively large time constant. Its cut-off frequency of fp = 30Hz is conducive to filtering out spike noise in the circuit. The circuit uses a two-stage operational amplifier, the first stage is I/V conversion, and the second stage is voltage inversion amplification. Adjust the variable attenuator, the motor completes the entire process, and the maximum detection current is observed to be 50.9μ A. Therefore, RF4=1K, R416=1K, RF5=45K in the circuit. From Vout1=-RF4*I, it is known that after the first stage I/V conversion, the maximum voltage is 50.9mV. After amplification, the final output voltage is a maximum of 2.291V, which meets the A/D conversion input requirements of S3C44B0.

4. Software Design

4.1 Lower computer software

After the system is powered on and reset, it enters the while(1) infinite loop, constantly checking whether the host computer has sent a frequency measurement command. When the frequency measurement command is received, the frequency measurement module subroutine is called. In the frequency measurement subroutine, the motor needs 1854 steps to complete the whole process. Each step drives the resonant cavity to move 0.005mm, and each step takes 44.44ms. Each time the motor moves one step, the A/D conversion data of 100 detection currents is averaged and sent to the host computer through the serial port for display.

4.2 Host computer software design

In the virtual instrument development platform LabVIEW, the I/O interface functions in the VISA-based instrument driver template can be used to conveniently and quickly develop driver programs. The driver program in this system that realizes data acquisition through RS232 serial communication between the PC and the main control chip S3C44BO uses this method.

As shown in Figure 3-5, the LabVIEW program diagram for frequency measurement. First, use the maximum and minimum functions to find the minimum value of the collected current data, and find its corresponding index value, that is, at which step the stepper motor collects the current value, and then feed this index value back to the frequency array, find its corresponding element, and it will be the measured frequency.
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5. Signal source output frequency measurement experimental results and analysis

In order to obtain the measured microwave signal frequency in the LabVIEW interface of the host computer, the detection current-frequency curve needs to be displayed in the interface, so that the "absorption trough point" of the detection current can be clearly read. It is necessary to manually measure a set of as many data points as possible of frequency-distance (the distance between the current measurement point and the starting point can be calculated by the sleeve scale) through the calibration method, and then use the distance of each step of the stepper motor to convert the distance into the number of steps, and then use Matlab to fit the relationship function of frequency-number of steps. In this way, it can be known which step the stepper motor takes corresponds to which frequency. The motor needs 1854 steps to complete the whole process, so the 1854 frequency values ​​corresponding to the steps are formed into an array as the horizontal coordinate of the curve, and the 1854 current values ​​collected are used as the vertical coordinate.

Limited by the frequency of the signal source and the measuring range of the resonant frequency meter, this system can only measure within the range of 8.48GHz and 9.9GHz. Therefore, from the starting position of the sleeve 9.9mm (corresponding to the frequency 8.48GHz), the end position 0.63mm (corresponding to the frequency 9.9GHz), the total length is 9.9mm-0.63mm=9.27mm. Since the distance of each step of the sleeve driven by the motor is very small, the distance of one step of the stepper motor cannot be directly measured. Taking advantage of the characteristic that the stepper motor has no cumulative error, the stepper motor is used to move 180 steps, and the position difference before and after the sleeve scale is measured. It is concluded that the average distance of each step of the sleeve driven by the stepper motor is 0.005mm. 42 sets of data points of frequency and scale are manually measured, and the curve shown in Figure 5-1 is fitted using MATLAB. The linear relationship function between frequency f and scale L is fitted by MATLAB as f = -0.1456* L + 9.9917 (0.63mm ≤ L ≤ 9.9mm). Since the motor drives the sleeve to move 0.005mm per step, the starting position is 0.63mm, that is, after the stepper motor moves one step, the position of the sleeve is 0.63mm+0.005mm=0.635mm, and the stepper motor needs 1854 steps to complete the whole process, and the end position of the sleeve is 0.63+0.005*1854=9.9mm. Then the relationship function between scale L and step number n is L = 0.005n + 0.63 (0 ≤ n ≤1854).

It can be deduced that the functional relationship between frequency f and step number n is f = -0.000728n + 9.9 (0 ≤ n ≤1854). The 1854 frequency values ​​corresponding to the step number are organized into an array as the horizontal coordinate of the curve, and the 1854 current values ​​collected are used as the vertical coordinate. The waveform diagram drawn by LabVIEW using a PC is shown in Figure 5-2.

Then LabVIEW automatically calculates the frequency value corresponding to the minimum detection current, as shown in Figure 5-4. It can be seen that the output frequency of the signal source is 9.337GHz.

Compare with manual measurement. Replace with a resonant frequency meter that can directly measure the frequency, and the frequency is measured to be 9.357GHz, so the relative error between automatic measurement and manual measurement is:

This system sets the stepper motor to complete the whole process in 82.4 seconds. The reason why it cannot be set too fast is to prevent the stepper motor from "losing steps" (missing pulses and not moving to the specified position). In addition, if it is too fast, it is likely that the "trough point" of the detection current will not be detected. It usually takes more than two minutes to manually measure the output frequency of the signal source. This shows the practicality of the automatic measurement of this system.