With the higher and higher frequency of pulse signal and the shorter and shorter jump edge (rising edge and falling edge), the analysis of pulse signal transmission integrity becomes more and more important. How to ensure the integrity of pulse signal transmission and reduce the reflection and distortion in the transmission process has become a problem that can not be ignored in the current high-speed circuit design.
For the pulse signal transmission distance of tens of meters or hundreds of meters, the concept of long line transmission is introduced according to the high-speed circuit design theory, and the transmission characteristics of pulse signal are analyzed. In order to ensure the integrity of pulse signal transmission, the RS-485 serial bus standard and the circuit scheme of jumper matching resistance are adopted. The experiments show that the scheme proposed here is feasible and has superior performance. In addition, the scheme has simple structure and low cost, and has great reference and application value for the transmission of short-range pulse signal and high-frequency digital signal.
1 transmission line theory
1.1 definition of long line transmission
Generally, the edge harmonic frequency of the pulse signal is higher than its own frequency, and its rapidly changing rising / falling edge will lead to unexpected results in the transmission process of the signal. According to the definition of high-speed circuit design theory, if the pulse signal transmission length is greater than 1 / 6 of the effective length corresponding to the rising or falling edge time of the signal, the signal transmission can be considered as long-line transmission. In the long line transmission of pulse signal, there will be obvious transmission delay, attenuation and oscillation, which affect the signal integrity, which needs to be analyzed by the transmission line theory.
1.2 transmission characteristics of pulse signal
The transmission line theory is distributed parameter circuit theory. All positions on the line are composed of distributed resistance R, distributed capacitance C, distributed inductance L and distributed conductivity G.
These distribution parameters determine the transmission characteristics of pulse signal long line transmission.
The characteristic impedance of the signal transmission line is expressed by the ratio of incident voltage to incident current on the transmission line. The general expression is:
The transmission delay of the signal is determined by the series inductance and parallel capacitance in the distribution parameters of the transmission line. The transmission delay d t of the unit length transmission line is expressed as:
The overall transmission of pulse signal on the transmission line is shown in Figure 1, where SZ is the source end impedance, 0z is the transmission line impedance, and LZ is the load impedance.
Figure 1 Characteristics of pulse signal on transmission line
(x, h, w) in Fig. 1 represents the attenuation function of the signal through the transmission line; A (W) and 1R (W) represent the input receiving function and reflection function at the source end of the transmission line; T (W) and 2R (W) represent the transmission function and reflection function of the transmission line terminal, and the expressions are as follows:
It can be seen from the above expression that if the characteristic impedance of the transmission line does not match the impedance of the source end and the terminal load, the transmission signal will reflect at the impedance discontinuity, and the reflected signal will go back and forth many times between the terminal and the source end until the attenuation is zero. The superposition of signals causes the distortion and ringing of transmission signals.
1.3 calculation of transmission line distribution parameters
To sum up, the transmission characteristics of the signal are mainly determined by the distribution parameters of the transmission line. As long as the partial parameters of the transmission line and the impedance values of the source and terminal are determined, the transmission characteristic value of the signal can be calculated according to equation (1-7). Taking twisted pair as an example, its characteristic parameters can be calculated according to the following formula:
Where d represents the line diameter of the transmission line conductor, s represents the line spacing between the two lines, and R E represents the effective relative dielectric constant. The distribution parameters of other types of transmission lines can be calculated by referring to reference [5-6].
2 pulse signal transmission scheme and experimental test
When the pulse signal is transmitted over a long distance, in order to ensure the transmission rate and reliability of the signal, RS-485 serial bus standard is used to drive and receive the signal.
2.1 RS-485 bus standard
RS-485 standard is a multipoint and bidirectional communication link based on a single pair of balanced lines, providing a transmission platform with high noise suppression, high transmission rate, long transmission distance, wide common mode range and low cost . In this scheme, MAX485 chip conforming to RS-485 standard is used to build the driving and receiving circuit. One chip is fixed as transmitting and the other chip is fixed as receiving. The corresponding transmitting and receiving ports of the two chips are connected with equal twisted pair.
2.2 transmission characteristic analysis of pulse signal
The transmission characteristics of pulse signal on twisted pair are determined by its transmission line distribution parameters, output impedance of driving chip and input impedance of receiving chip.
The actual measured wire diameter D of the twisted pair is 0.05 cm, s is 0.096 cm, and R E takes the constant 2.5 between the dielectric constant of the line insulator and the air dielectric constant (1.00). The characteristic impedance, distributed inductance and distributed capacitance per inch of twisted pair calculated from equation (9-11) are:
It can also be seen that the drive output resistance of MAX485 chip is about 50 Ω, and the input resistance at the receiving end is greater than 12 K Ω. According to formula (1-7), the unit transmission delay d t, transmission line attenuation function H and the function values a, t, 1R and 2R at the source and terminal of the transmission line can be calculated as follows:
The transmission twisted pair length is 65 m and the pulse signal amplitude is + 4 v. It can be calculated that the delay of the signal passing through the twisted pair is 325.65 ns; The amplitude of the signal reaching the receiving end is 4 × AHT = 5.02 V ； The amplitude of the generated reflected signal reaching the source end through attenuation transmission is 22.4 × AH R = 2.36 V。 The signal will continue to reflect at the source and repeat until the attenuation is zero. Use an oscilloscope to directly measure the waveforms of MAX485 driver chip output and receiver chip input, as shown in Figure 2.
Fig. 2 waveform of port a of transmitting and receiving chip without termination
The time axis gear is 2 US per grid and the signal amplitude gear is 2 V per grid.
It can be read that the amplitude of the input pulse signal is + 4 V, the transmission delay of the signal is about 320 ns, and the positive amplitude of the pulse signal at the receiving end is + 5.3 V and the negative amplitude is – 1.8 v. In conclusion, the results measured by oscilloscope are basically consistent with the theoretical analysis.
When a 100 Ω resistor is connected across the receiving end, the terminal load of the twisted pair is approximately 100 Ω, which matches the characteristic impedance of the transmission twisted pair. At this time, 2 R ≈ 0 and t ≈ 1 can be calculated, and the other parameters are the same as above. Keeping the transmission twisted pair distance and transmission signal amplitude unchanged, it can be calculated that the transmission delay of pulse signal is 325.65 ns, the signal amplitude at the receiving end is 2.668 V, and the terminal does not produce reflected signal. Use an oscilloscope to measure the waveform of the output of the transmitting chip and the input of the receiving chip, as shown in Fig. 3 (a). The input signal amplitude is + 4 V, the signal delay at the receiving end is about 318 ns, and the amplitude is + 2.74 V; Fig. 3 (b) shows the signal waveform of the pulse signal at the transmitting end and receiving end of the circuit. The transmission delay of the signal is 400 ns, in which the delay on the transmission line is about 320 ns, the delay inside the transmitting and receiving chip is about 40 ns respectively, and the amplitude and pulse width of the pulse signal at the transmitting end and receiving end remain the same.
(a) Waveform of a port of transmitting and receiving chip
(b) Waveform of signals at transmitting and receiving ends
Fig. 3 waveform when the twisted pair terminal is connected to 100 Ω resistance
To sum up, it is completely correct to use the transmission line theory to analyze the pulse signal transmitted over a long line. For the distortion and oscillation of pulse signal in long line transmission, the reflection of transmission signal at both ends of transmission line can be eliminated by connecting resistance in series at the source end of transmission line or matching resistance at the terminal in parallel.
When the distance between the transmitting and receiving ends is far or the communication rate is high, it is also necessary to connect bias resistors at both ends of the transmission line to set the level when there is no data on the transmission line to 0 level to reduce the misoperation of the receiving end caused by interference or signal reflection.
For the long line transmission of pulse signal, the transmission characteristics need to be analyzed by transmission line theory, and the integrity of signal transmission can be maintained by changing the transmission line or terminating the matching resistance. Experiments show that the transmission circuit composed of max-485 chip can effectively eliminate the attenuation and interference of signal in transmission, and maintain the stability and integrity of signal transmission by terminating matching resistance. The scheme has simple structure and reliable performance. It has great practical and popularization value in practical application.