PCB design tips, teach you to master PCB design easily (1-10)

The selection of PCB sheets must strike a balance between meeting the design requirements and the mass production and cost. The design requirements include the two parts of electric and mechanism. This material problem is usually important when designing very high-speed PCB boards (greater than GHz frequency). For example, the dielectric loss (dielectric loss) of commonly used FR-4 material at several GHz frequencies will have a great impact on signal attenuation and may not be applicable. For electrical purposes, pay attention to the combination of dielectric constant (dielectric constant) and dielectric loss at 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). It can be used to widen the distance between high-speed signal and analog signal, or add ground guard/shunt traces next to the analog signal. Attention should also be paid to the noise interference from the digital ground.

3. How to solve the problem of signal integrity in high speed design?

Signal integrity is basically the problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source (output impedance), the characteristic impedance of the walk line, the characteristics of the load end, the topology of the walk line (topology) and so on. The way to solve the problem is to rely on termination and topology adjustment.

4. How is the difference distribution line realized?

There are two points to pay attention to, one is that the length of the two lines should be as long as possible, and the other is the distance between the two lines (this distance is determined by the differential impedance) to keep the same, that is to keep the parallel. There are two parallel ways. One is to walk on the same line level (side-by-side) for two lines, one for two lines, and the other for two adjacent layers (over-under). In general, there are many ways to achieve side-by-side in the past.

5. How to realize the differential distribution line for a clock line with only one output?

It is meaningful to use differential distribution lines, both source and receiver terminals, and differential signals. Therefore, it is impossible to use the differential distribution line for only one output clock signal.

6. Can a matching resistor be added between the receiver terminals?

The matching resistance between the receiver terminals is usually added, and the value should be equal to the value of 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 of differential pairs should be close and parallel. The proper proximity is due to the influence of the spacing on the differential impedance value, which is an important parameter for designing differential pairs. The need for parallelism is also due to the consistency of the differential impedance. If the two lines are close to each other, the differential impedance will be inconsistent, which will affect the signal integrity (signal integrity) and time delay (timing delay).

8. How to deal with some theoretical conflicts in actual wiring.

• 1>. basically, it is right to divide modules / numbers apart. It is important to note that the signal does not cross the moat as far as possible, and do not make the reflux current path (returning current path) too large for the power and signal.
• 2>. crystal oscillator is a simulated positive feedback oscillating circuit. If there is a stable oscillation signal, it is necessary to meet the specification of loop gain and phase, and the oscillation specification of the analog signal is easily disturbed, even if ground guard traces may not be completely isolated from the interference. And too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be approached.
• 3>. indeed high-speed wiring has many conflicts with 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 be out of specification. Therefore, it is best to use the arrangement line and PCB stacking techniques to solve or reduce the EMI problem, such as high-speed signal inner layer. At last, we use resistance capacitance or ferrite bead to reduce the damage to the signal.

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

At present, most of the automatic wiring devices of strong wiring software have set constraints to control the number of holes and the number of through holes.  The winding engine capability and constraint conditions of EDA companies are sometimes very different. For example, whether there are sufficient constraints to control the way of snake line (serpentine) sinuous, can control the distance between the differential pairs and so on. This will affect whether the way of automatic wiring can meet the designer's idea. In addition, the difficulty of manually adjusting the wiring is absolutely related to the capability of the winding engine. For example, the pushing ability of the thread, the pushing ability of the through hole, and even the pushing ability of the copper wire to the copper and so on. Therefore, it is the solution to choose a router with strong winding engine capability.

10. About test coupon.

Test coupon is used to measure the characteristic impedance of PCB plate produced by TDR (Time Domain Reflectometer) to meet the design requirements. There are two kinds of impedances to be controlled: single line and differential pair. Therefore, the line width and line distance (differential timing) on test coupon must be the same as the line 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 inductance of the grounding lead (ground lead), the place where the TDR probe (probe) is grounded is usually very close to the location of the signal (probe tip), so the distance between the point and point of the measured signal on the test coupon is in accordance with the probe used.