Summary of practical skills in high frequency PCB design

1, after the design of PCB, how to choose PCB plate?


The selection of PCB sheet must be balanced between the design requirements and the quality and cost. The design requirements include the two parts of electric and mechanism. This material problem is usually more important when designing a very high speed PCB board (more than GHz). For example, the dielectric loss of commonly used FR-4 material at several GHz frequencies (dielectric loss) will have a great impact on signal attenuation, and may not be applicable. As far as electric is concerned, it is necessary to pay attention to whether the dielectric constant (dielectric constant) and the dielectric loss are combined with the designed frequency.


2. How to avoid high frequency interference?


The basic idea of avoiding high frequency interference is to minimize the interference of high frequency electromagnetic fields, that is, the so-called crosstalk (Crosstalk). The distance between the high speed signal and the analog signal can be used, or the ground guard/shunt traces is added to the analog signal.  It is also necessary to pay attention to the noise interference of the analog ground.


3. In high speed design, how to solve the problem of signal integrity?


Signal integrity is basically a problem of impedance matching. The factors that affect the impedance matching include signal source architecture and output impedance (output impedance), the characteristic impedance of the line, the characteristics of the load side, and the topology of topology. The way to solve the problem is to rely on the termination and the topology of adjusting the line.


4. How is the difference distribution line realized?


There are two points to be noticed in differential pair wiring. One is that the lengths of the two lines should be as long as possible, and the other is the distance between the two lines. There are two kinds of parallel ways: one is two lines on the same line layer (side-by-side), and one is two lines in the upper and lower two layers (over-under). Generally, the former side-by-side (side by side, shoulder to shoulder) realized more.


5. For a clock signal line with only one output terminal, how to achieve a differential distribution line?


It is meaningful to use a differential distribution line to be a signal source and a receiver or a differential signal. So it is impossible to use a differential distribution line for a clock signal with only one output end.


6. Can a matching resistor be added between the receiver difference line pairs?


The matching resistance between the receiver differential line pairs is usually added, and the value of the differential impedance should be equal to the value of the differential impedance. So the quality of the signal will be better.


7. Why should the wiring of the differential pair be close and parallel?


The wiring mode of the difference pair should be properly close and parallel. The so-called appropriate because it is close to the distance affect the differential impedance (differential impedance) value, this value is the important parameters of the differential design. The need to be parallel is also due to the consistency of the differential impedance. If the two line is far and close, the difference impedance will not agree, and the signal integrity (signal integrity) and the time delay (timing delay) will be affected.


8. How to deal with some problems of theoretical conflict in the actual wiring


Basically, it is right to separate the mode / number of isolation. We should pay attention to signal alignment, try not to stride over the segmentation area (moat), and don't let the returning current path of power and signal become too large.


The crystal oscillator is a simulated positive feedback oscillator circuit. If there is a stable oscillation signal, it must satisfy the specification of loop gain and phase. The oscillation specification of analog signal is easily disturbed, even if ground guard traces is added, it may not completely isolate the interference. And too far away, the noise in the ground plane will also affect the positive feedback oscillating circuit, so it is necessary to close the distance of the Jing Zhenhe chip.


Indeed, there is a lot of conflict between high speed wiring and the requirements of EMI. But the basic principle is that the resistance capacitance or ferrite bead added by EMI can not cause some electrical characteristics of the signal to not conform to the specification. Therefore, it is best to solve or reduce the problem of EMI with the technique of arranging the line and PCB overlapping layer, such as the inner layer of high speed signal. Finally, the resistance capacitance or ferrite bead is used to reduce the damage to the signal.


9. How to solve the contradiction between the manual wiring and the automatic wiring of the high-speed signal?


At present, most of the automatic wiring devices of the strong wiring software have set constraints to control the way of winding and the number of holes. Each EDA company's winding engine capability and constraint conditions are sometimes quite different. For example, if there is enough hunting line control constraints (serpentine) winding, the ability to control the differential pair line spacing. This will affect whether the automatic wiring way is in line with the designer's idea. In addition, the difficulty of manually adjusting the wiring has an absolute relationship with the ability of the winding engine. For example, the pushing capacity of the walking line, the pushing capacity of the holes, and even the pushing capacity of the copper applying to the line. Therefore, the choice of a wiring engine with a strong ability of wiring is the solution.


10. About test coupon.


Test coupon is used to measure the characteristic impedance of the PCB board produced by TDR (Time Domain Reflectometer) to meet the design requirements. In general, there are two conditions for the impedance to be controlled by a single line and a differential pair. So, the line width and the line distance on the test coupon are the same as the lines to be controlled. The most important thing is to measure the location of the place at the time of measurement. In order to reduce the grounding wire (ground lead) of the inductance, TDR probe (probe) ground areas are usually very close to the local signal (probe tip), so test coupon measurement signal to ground distance and approach to meet the probe.


 

(Source: internal information)

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