Eight classic Q & A to explain clearly PCB cabling (2)

Eight classic Q & A to explain clearly PCB cabling (2)

5.Q: the grounding plane here is very difficult for the manufacturer, isn't it?

A: 20 years ago, there were some problems in this regard. Today, due to the improvement of solder paste, solder resist and wave soldering technology, the grounding plane has become the routine operation of printed circuit boards.

6.Q: you said that the possibility of a system using a grounding plane to cause it to suffer from ground noise is very small. What remains of the problem of ground noise that cannot be solved?

A: although there is a ground plane, its resistance and inductance are not zero. If the external current source is strong enough, it will affect the precise signal. This problem can be minimized by preventing large currents from flowing to areas that affect the generation of earthing voltages for precise signals. Sometimes breaking or slit on the ground plane can change the direction of large earthing currents from sensitive areas. But changing the ground plane by force can also cause the signal to bypass into the sensitive area, so this technology must be used carefully.

7.Q: how can I know the voltage drop on a ground plane?

A: usually the voltage drop can be measured, but sometimes it can be calculated on the basis of the length of the conductive tape through which the resistance and current of the ground plane material pass. But calculations can be complicated. Voltages in the DC to low frequency range of 50 kHz can be used by instrument amplifiers. If the ground of the amplifier is separated from its power source, the oscilloscope must be connected to the power source of the power circuit used.

The resistance between any two points on the ground plane can be measured by the probe. The amplifier gain and the sensitivity of the oscilloscope can reach 5 μ V / p. The noise of the amplifier will increase the width of the oscilloscope waveform curve. It is about 3 μ V, but it is possible to make the resolution of the measurement up to about 1 μ V, which is enough to distinguish most of the ground noise, and the confidence level can reach 80%.

8.Q: how to measure the high frequency ground noise?

A: it is difficult to measure high frequency earthing noise with an appropriate broadband instrument amplifier, so it is more appropriate to use high frequency and very high frequency passive probes. It consists of a ferrite magnetic ring (external diameter is 6 mm), and there are two coils on the magnetic ring. In order to form a high-frequency isolation transformer, one coil is connected to the input of the spectrum analyzer and the other coil to the probe. The test method is similar to the low-frequency situation. But the spectrum analyzer uses amplitude-frequency characteristic curve to represent noise, which is different from time-domain characteristic. Noise sources can be easily distinguished according to their frequency characteristics. In addition, the sensitivity of using spectrum analyzer is at least 60 dB higher than that of wideband oscilloscope.