Design of Equivalent Sampling Oscilloscope Based on Single Chip Microcomputer

　　In digital oscilloscope technology, there are two commonly used sampling methods: real-time sampling and equivalent sampling. Real-time sampling is usually performed at equal time intervals, and its highest sampling frequency is the Nyquist limit frequency. Equivalent sampling refers to the reproduction of a signal waveform by continuously sampling multiple signal cycles. The sampling system can reproduce the signal waveform with a frequency much higher than the Nyquist limit frequency in an extended manner.

　　1 Overall Design

　　Since the designed oscilloscope has a wide input frequency range,   

　　this system uses two sampling methods: equivalent and real-time. If the input frequency is less than 1.25MHz, real-time sampling is used; otherwise, equivalent sampling is used. The output and frequency division number of the clock chip are determined according to the input frequency. When the input frequency is higher than 1kHz, the programmable frequency synthesis chip SY89429V is used to generate the reference clock; when the input frequency is less than 1kHz, the single-chip microcomputer provides a 40kHz reference clock. Then, different frequency division numbers are used for the reference clock according to the size of the input frequency to generate the sampling clock.

　　2 Hardware Design

　　2.1 Overall Design

　　The hardware of this system can be divided into five parts: controller, frequency measurement and keyboard control, waveform sampling, program-controlled clock and LCD module. The system function is shown in Figure 1. 

(1) Controller The controller part has a heavier task and is completed by two 89C52 chips (MCU1 and MCU2). Among them, MCU1 is responsible for sampling, data processing, and control of program clock and LCD; MCU2 completes frequency measurement, DAC output and keyboard interface processing functions. The two microcontrollers communicate through the serial port.

(2) Frequency measurement module The frequency range of the input signal of this system is relatively wide (1Hz～80MHz), and the interval range is relatively large (0.1V～2.2V). The shaping circuit is implemented by the high-speed comparator TL3016, and its reference voltage is obtained by MCU1 sampling the input signal. The shaped signal is sent to MCU2 for frequency measurement through the programmable divider, and the accuracy can reach four effective digits.   　　
(3) Waveform sampling module This module consists of a 40MSPS analog-to-digital converter TLC5540, a static memory CY7C128A-20, and a programmable logic device ispLSI1016E-80. Under the control of the programmable clock and the programmable divider, CLPD generates a memory address and writes the sampling data of the high-speed ADC into the static RAM at the programmable frequency. After writing 256 points, the control of the static RAM is transferred to MCU1 through the data selector, and MCU1 processes the data and sends it to the LCD display. 

(4) Programmable clock circuit The programmable clock circuit is the key to achieving equivalent sampling in this system. Its core is the programmable frequency synthesis chip SY89429V. The output clock range of SY89429V is 25MHz~400MHz, with a step value of 0.125MHz~1MHz. It uses a high-frequency phase-locked loop structure inside and is very sensitive to interference. Certain anti-interference measures are taken in hardware to ensure its stable operation. 

(5) Liquid crystal display The liquid crystal part consists of a dot matrix liquid crystal display EDM160160, a liquid crystal controller SED1335, a SRAMHM62256, a negative pressure generator and a backlight AC drive circuit generator. 2.2 Implementation of equivalent sampling Equivalent sampling is the key and innovation of this system. It mainly uses a precision clock generation circuit with the chip SY89429V as the core to control the high-speed ADC to perform cyclic intermittent sampling of high-frequency signals. The system block diagram for implementing equivalent sampling is shown in Figure 2.

 

　　The input frequency of equivalent sampling is 1.25MHz～80MHz. In order to make the reproduced waveform as accurate as possible, the system is designed to sample one point per cycle in the signal range of 1.25MHz～40MHz, and one point per cycle in the signal range of 40MHz～80MHz to reproduce the waveform. That is, the sampling frequency range should be between 1MHz～40MHz, and there should be a controllable small step value. The output frequency of SY89429V is 25MHz～400MHz, and an external frequency division circuit is required to expand the low-frequency part. At the same time, the ADC selected by this system is TLC5540, and its conversion rate is 5MSPS～40MSPS. The low-end sampling rate will be limited, and the sampling rate can be controlled by controlling the RAM write speed. After comprehensive consideration, the CPLD device is used to pass the FOUT frequency output by the SY89429V chip TEST through a program-controlled binary divider. On the one hand, it is provided to the ADC as the CLK operating frequency, and on the other hand, it is controlled by a program-controlled binary divider to control the RAM write speed as the sampling frequency of the low-end signal.

　　3 Software Design

　　3.1 Overall software process

　　The overall software flow is shown in Figure 3. MCU1 receives the frequency measurement result from MCU2 and passes the amplitude information to MCU2. MCU1 determines the sampling mode according to the input frequency and controls the precision clock generation circuit to provide the sampling clock for the ADC. After one sampling is completed, MCU1 processes the sampled data and sends it to the LCD for display.

　　3.2 Implementation of real-time sampling

　　In real-time sampling, in order to make the sampled waveform as accurate as possible, the system divides the signal below 1.25MHz into three frequency ranges. In each frequency range, a fixed reference clock is generated by the programmable clock device, and sampling is performed in combination with the corresponding frequency division number. The specific settings are as follows: when the input frequency is 1Hz~1kHz, the microcontroller provides a 40kHz sampling clock; when the input frequency is 1kHz~1MHz, the chip SY89429V provides a 40MHz sampling clock; when the input frequency is 1MHz~1.25MHz, the chip SY89429V provides a 50MHz sampling clock.

　　3.3 Implementation of Equivalent Sampling

　　Since the programmable frequency synthesis chip SY89429V is within the frequency range of 25MHz to 50MHz used in this system, the step value is always 0.125MHz. To facilitate data processing, all actual frequencies can be converted into values ​​in units of 0.125MHz during the software design process, that is, divided by 0.125MHz. The conversion rate of the analog-to-digital converter TLC5540 used in the system is 5MHz to 40MHz. When the input frequency is less than 40MHz, the method of sampling one point per signal cycle is adopted; when the input frequency exceeds 40MHz, the method of sampling one point between two signal cycles is adopted.

　　The following takes the case where the input frequency is between 1.25MHz and 40MHz as an example, and the case where the input frequency is greater than 40MHz can be deduced by analogy. The converted value of the input frequency fin (hereinafter referred to as the value) is recorded as dfin, and the frequency word SY of the chip SY89429V is set according to the input frequency (only integers are taken to control the frequency of the output signal), and the sampling frequency is generated after n division. According to the principle of equivalent sampling, the sampling frequency is close to the input frequency (for the case where the input frequency is 40MHz to 80MHz, the sampling frequency is close to the input frequency divided by two), and the difference between the two frequency values ​​is recorded as da, then SY can be expressed as: SY=(dfin-da)%26;#215;n

(1) The number of sampling points required to reproduce a waveform is: d = (dfin-da)/da

(2) The LCD used in this system is a 160%160 dot matrix. The time represented by 40 points on the x-axis is defined as a grid time base, denoted as A. The number of cycles displayed on the LCD screen is: N=160%da/(dfin-da) (3) Therefore, the time base can be expressed as: A=N/(4%fin)=40%da/[fin%(dfin-da)]

(4) The number of sampling points of a waveform can also be expressed by the time base and input frequency: d = 40/(A%26;#215;fin)

(5) The author uses the algorithm introduced in this article to achieve equivalent sampling, which can reproduce the waveform of periodic signals from 1MHz to 80MHz with satisfactory results. This simple oscilloscope provides users with two working modes, manual and automatic, on the human-machine interface. It has high cost performance and has a broad market space.