# Tag Info

## Hot answers tagged signal-integrity

31

Try not to fixate too much on the fact that these are vias in a PCB. The point is that they represent impedance changes in the signal's path. These effects are not unique to vias, they can be caused by many different signal path geometries. The titles under the graphs (on page 18, slide 36 of your linked PDF) tell which impedance change is most dominant for ...

28

With electrical transmission lines, it all has to do with the speed of light being finite, thus so is the speed of EM propagation in a wire. You can think of a wire as a long series of infinitesimal capacitors (connected by infinitesimal inductors). If you start charging the capacitors at one end, you have to keep pumping charge into the wire to charge ...

22

There are quite a number of things that will do this to you. You have not stated the length of the interface. I do direct chip to chip PCIe frequently and you really need to take this into account as you will get attenuation of roughly 0.18dB per inch due to skin effect losses and about 0.5dB per inch due to dielectric absorption on 'ordinary' FR4. I think ...

22

The maximum frequency is mostly related to the frequency-dependent loss characteristics of the cable. Eventually you get to a frequency where you simply don't get enough signal at the other end to use. Resistive losses in the conductors (including skin effect) Dielectric losses in the insulating materials Radiation losses if the cable is not fully shielded ...

21

The impedance of a transmission line, in ohms, is the ratio of voltage wave and current wave that travels down the line. For a 100 ohm line for instance, a 1 volt wave will always be accompanied by a 10mA wave. Intuitively, the current wave delivers charge to the parts of the line that have to 'charge up' to the voltage of the voltage wave. If the 100 ohm ...

21

The only way that a signal that is "buried in the noise" can be detected is if you can run the signal + noise through some filter that attenuates the noise more than it attenuates the signal. At which point the signal is no longer buried in the noise, so "buried in the noise" was just a hasty assumption. In radio carrying an audio (or Morse code) signal in ...

18

The larger AC coupling value passes the low-speed 'ping' pulses used for sensing the presence of a link partner. The TX PHY sends the pulses, then watches the signal using the known R-C time constant to detect an open or terminated line. Here's an excerpt from an article describing this: "Receiver detection: PCIe uses an ingenious means to recognize ...

17

The HD-15 (aka DE-15) connector for VGA connection was compatible with hand-crimping of pins to conductors of coaxial elements of the multiwire connecting cable. That practically requires a two-pin set for each of R, G, B video signals, to accommodate a signal pin and ground (coaxial shield) pin. Those signals were not logic level, and lacked the noise ...

16

The eye you show is firmly in the category of "good" eyes. There's nothing there that should cause an error any more often than once every 100,000 years or so. Some things to look for: Are you sending real data while measuring the eye? An idle pattern or similar behavior could create an eye that looks much cleaner than the eye would be with real data. Is ...

16

No. The differential signalling is performing as intended. No. As long as the "common mode voltage" does not exceed the ratings of the transceiver. And the maximum DC ratings, which for a CAN transceiver like the MCP2551 is -42 V to +42 V. The twisted pair cable and the differential signalling mode make the single-end signals look like gibberish ...

16

Noise in oscilloscope front ends is quite high, maybe 1mVp-p. Using the 1:1 probe lowers the input-referred noise floor by an order of magnitude. Still pretty crappy, but opens a few doors.

14

Thanks for everyone's help. I believe Bruce Abbott has given the correct answer. I'm posting from my bed and I cannot test it yet until tomorrow, but The analysis below is confirmed, when he mentioned the word "refresh", I think the problem is already solved. I knew how Z80 refreshes the memory, but completely forgot about it in the previous days. ...

14

Best would be to shorten the cables. A shorter cable means less loss of signal in the cable, and a neater installation. Doing so requires proper tools, new connectors, and some practice in attaching the connectors to the cables. That's too much to expect pilots working on their own planes, or even mechanics who do the work for many pilots. There are ...

14

Both patterns are necessary. AA/55 causes no net bus current in the transitions, pairs of lines are effectively differential, so there's no ground current in either driver or driven ICs. Alternating FF/00 causes maximum ground pin current, so is needed to test ground bounce. If the tracks run parallel, then AA/55 excites them in odd mode, with capacitance to ...

13

Do not confuse power transfer with voltage transfer. If the source is indeed 50 ohms, and your scope is set to 50 ohms, it becomes a voltage divider and what you see is half the voltage that you will see when the scope is set to high impedance. simulate this circuit – Schematic created using CircuitLab Power transfer has to do with the relationship ...

13

Even if ambient temperature is well controlled, air currents can cause noise if there is significant voltage across the capacitor. Capacitance degradation with time is different for different dielectrics. And the big one, voltage coefficient of capacitance. If you can use them, NP0 caps suffer from such effects much less than X5R X7R etc. parts, but they ...

13

While the document talks about more wavelengths, usually the threshold where large impedance mismatches start to matter is regarded as 1/10th of a wavelength for digital systems. Below that the practical effects of impedance mismatches are small, and at 1/4th wavelength they are already quite large. For analog and RF electronics the threshold can be even ...

12

The point of this probe is that the scope has its 50Ω termination enabled. Since the scope input impedance is matched to the cable, there is no reflection from the scope. Once the load side is matched, we don't need to worry about any source-side impedance mismatch; reflections have already been suppressed. The point of the 950Ω resistor on the input side ...

12

You can stuff more identical sticks end-to-end into a pipe if the pipe is longer. Then if you whack one end of the pipe it disturbs more sticks since there are more of them in the pipe.

11

When TX switches low-to-high, the current flows like this: Power supply Vcc -> PCB Vcc plane -> U1.Vcc pin -> U1.TX pin -> U2.RX pin -> U2.Gnd pin -> "return path" -> PCB Gnd plane -> Power supply Gnd It's great that you understand that what we call the "return path" will be the nearest plane (in this case the Vcc plane). This makes sense as the fields can'...

11

The best thing to do is leave the cable the original length, and to gather the excess in whatever is the physically tidiest way. Few people are equipped to shorten the cables and remake a connector properly, and it needs to be done properly. If you have to ask, don't do it. There are two aspects to 'cable inductance', differential and common mode. ...

10

You are confusing the DVI connector with the cable. That "single" analog ground is a big honking ground, but what goes into a cable? The simplest is a DVI-A cable, which usually has a VGA connector on the other end. Internally, it will carry 3 color channels (red, green and blue) and 2 sync: vertical and horizontal. And guess what? A DVI-A cable will ...

10

Convenience. A 1:1 probe (or the x1 setting on a switchable x10 probe) will probably have slightly lower capacitance than a 50ohm coax of the same length, and also handy clips on signal and ground. It's therefore a convenient tool for small signals where noise makes a 10:1 probe unusable, and for low frequencies where the relatively long ground lead doesn't ...

9

This is my understanding, based on very little knowledge: The the return current for the high speed part of the signal will take the path of least inductance. This means travelling along whichever plane is closest to the track. (In fact, it will travel on the side of that plane closest to the track). When your signal track moves from the top layer to the ...

9

Your first circuit is perfect... If you use a mosfet instead of a bjt. A mosfet does not pull anything low when it is on, it indeed is a capacitive load but you are going at 20kHz so that should not be an issue at all. Just be careful: your mosfet should support a quite high $V_{GS}$, i.e. 12V, that's not something all MOS are happy to do. Your circuit ...

9

It depends what you mean by 'capacitance'. If you mean two conductors separated by dielectric, then of course it doesn't. However, if you mean 'behaves like capacitor', then yes it does. a) If you take (for instance) a high value resistor from a power supply, and connect it to a capacitor, then the voltage will start at 0v and slowly ramp up. b) If you ...

9

This is a typical powered headphone splitter/amplifier. It is a single gain amp to multiple non-amplifying buffers. The 5.1kΩ Resistors represent the load. As it will be in parallel with your actual load, they won't affect much. Notice, no pull-down resistors needed between the Gain Amp and Buffer Amps. This is as generic as it gets.

9

In general, adding an inductor in series with a chip's power feed and not adding decoupling capacitors to ground (at the pins) is really bad. Just don't do this whether you split it into three or just have a common track. Even if you add capacitance you have to ensure that any changes in the current taken by the chip do not suddenly produce resonance ...

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