# Tag Info

38

Let's look at the formula and equivalent circuit for a transmission line. (1) Impedance rather than reactance. Reactance refers to the opposition to the change in current (of an inductor) or voltage (for a capacitor) - single components. The transmission line has $R,L$ and $C$ components - impedance is the ratio of voltage phasor to current phasor. (...

32

A transmission line has distributed inductance and capacitance along its entire length. We can think of it as infinitely many little inductors and capacitors along the line: simulate this circuit – Schematic created using CircuitLab Each inductor serves to limit the rate at which the capacitor can charge. But, as we divide the line into increasingly ...

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 ...

21

Adding to what Phil said: Now imagine everything starts out at 0 Volts and Amps in this long chain of inductors and capacitors, then you put a voltage step in one end. The way the inductors slow down how the capacitors are charged, a steady current will flow, which will be proportional to the voltage you put in. Since you have a voltage and a current ...

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 ...

16

USB cables do require some precision engineering. There are stringent requirements on value of differential impedance, quality of interconnects, and amount of losses per cable. The high-speed part of USB cable, even at USB 2.0 480 Mbps data rate, is made of a twisted pair of wires, all wrapped into a shield. This makes it a "bi-axial" cable. The cable is ...

13

The short answer is you're going to be fine, route it above VCC and make sure you have some VCC to GND decoupling caps near your chip. Plus your route is pretty short at 600mil, I've seen some people do terrible things to USB routes that still end up working :) I think the best way to understand this is to consider where your return current will flow. ...

12

Jim had a very good answer. To expand on a few, however: 2) 50 Ohms is 50 Ohms (kind of). The dielectric constant of a material IS slightly frequency dependent. Therefore, the trace height and width you choose for 1 GHz will be a slightly different impedance at 10 GHz (if you need to worry about the difference, you probably already know about the difference!...

12

Very few do. If your chip's datasheet does not say it has 50-ohm termination, then it almost certainly does not. Traditional CMOS and TTL logic does not provide matching termination, though a few specialized types (line drivers?) might. Typically drivers are low impedance and receivers are high impedance (with some capacitance). Traditional ECL (emitter-...

11

When to consider a digital signal fast enough to treat the interconnect (PCB trace) as a transmission line depends on two things: Electrical length of the signal Rise/fall-time (which is another way to say frequency content) As a rule-of-thumb(*) whenever the electrical length of the trace is longer than some fraction of the rise/fall time, you will see ...

11

No, the 50-Ohms is not a convention for PCB tracks to carry signals. The 50 Ohms is a standard for coaxial cables and corresponding interconnects - dozens and dozens various SMA/SMB, BNC, type-N, etc. connectors. In fact, typical (thin) PCB traces have 65 - 80 - 100 Ohm characteristic impedance on a typical stack-up (7 mils or 12 mils of FR4 between ground ...

10

You might consider adding an attenuator probe to your PCB consisting of a single series resistor. The 50-ohm coax cable to your oscilloscope can be any length - the oscilloscope input at the far end of this cable must be 50-ohm-terminated, not left to its default 1MEG simulate this circuit – Schematic created using CircuitLab The series resistor Rs ...

9

I've been working on a Gigabit Ethernet project for months, and so far I checked datasheets from Realtek, TI, Microchip, and a few reference designs, here is what I've found out. 0. High-speed routing guidelines are sometimes ignored for RGMII, but big manufacturers recommend them. In many low-cost products, the RGMII signals are routed with no regards of ...

8

Scheme #1 is terminating only the differential mode signal, not the common mode. Scheme #2 is terminating both differential and common mode. Even with a perfectly symmetrical differential output signal you will have what we call "differential to common mode conversion" in the cable. So at the receiver you will have both common mode and differential mode. ...

8

Probe cable is lossy coax. Achieving a matched condition with an oscilloscope probe is virtually impossible because the source impedance of the circuit under test is unknown and generally different from the scope's 1MΩ or 50Ω input impedance. On top of that the input impedance of the oscilloscope has a reactive component as do most circuits under test, ...

7

Yes, a smooth bend would be better for signal integrity than the square corner. All other things being equal, that corner sticking out represents an unnecessary impedance "bump" in the transmission line. Although in this case, the dimensions are so short that you'd probably never be able to measure the difference. The solder fillets on the nearby components ...

6

It appears that I should solve for odd characteristic impedances of 50 Ohms each. Is this true? Yes, the differential-mode impedance is equal to twice the odd-mode impedance (at least for symmetric geometries). My board house can do 5/5 mil trace/space. Is there any reason not to use these values? If you can fit a design with larger gaps and wider ...

6

This seems the simplest mathematical way to derive characteristic impedance. Consider a "lump" of transmission line connected to the continuation of that transmission line ($Z_0$): - R is series resistance of cable for a given length L is series inductance of cable for a given length G is parallel conductance of cable for a given length C is parallel ...

6

What you're asking about is called a transmission line taper. In general, there's no analytical solution to describe the reflections. The link in Chris L's answer (if you follow through to Klopfenstein's paper) gives some examples of specific taper shapes where something close to an analytical answer has been found. The basic way to study it is to imagine ...

6

What is the difference between characteristic impedance and input impedance in transmission lines? Characteristic impedance ($Z_0$) depends on the transmission line and its physical properties. Mathematically it can be shown that if you know the inductance (L), capacitance (C), resistance (R) and conductance (G) per unit length, $Z_0$ is: - $\sqrt{\... 6 I am by no means an expert in this field (there is a lot going to consider), but... Are USB/DVI/Ethernet cables a trivially easier solution for tens of cm? USB, DVI, and Ethernet are all very different, so it's difficult to generalize, but I would say "yes, somewhat". Maybe not for 10-30 cm, but 50 cm+ is definitely getting up there. DVI is definitely ... 6 A rule of thumb is that characteristic impedance becomes seriously important when your trace length gets above a tenth of a wavelength. Below that you can mostly get away with treating your trace like a capacitor. 50 ohm single ended is a standard mostly used by the radio world. Digital signals that are fast enough to need impedance matching usually use ... 6 I like to use this sort of BNC to scope probe adapter, which frequently is included in the accessory kit you get with the probe. You put it on the tip of your oscilloscope probe, then plug it into a perfectly ordinary BNC connector. Since the probe has a high resistance (9 MΩ for a 10x probe) positioned right at the tip, there's minimal loading from this ... 5 There is a simple hand-waving explanation why the effective impedance of a (ideal) transmission line is a constant. Other explanations leave some confusion on how do we "select" Li and Ci in the model of transmission line. What are these Li and Ci exactly? First, once we say "transmission line", we are talking about long wires. How long? Longer than the ... 5 Continuous varying impedances are used all the time for impedance matching. If you have a very capacitive part of a trace (for example, where a large component pad might be), you can have a relatively inductive transition before or after it to "balance" it out. What will end up happening is that the reflections will "stack up" but, instead of being at one ... 5 When we talk about the "impedance" of a pcb trace, we are talking about the characteristic impedance of a uniform transmission line. The characteristic impedance depends mainly on H, W, and$\epsilon_r$in the figure above. To get a 50 Ohm characteristic impedance, you just have to define your trace width in the proper proportion to the H of your ... 5 The velocity that electricity propagates at is related to the dielectric in the cable. In free-space, an EM wave travels at the speed of light but there is only the permittivity of free space ($\epsilon_0$)to hinder it: - So, as the permittivity ($\epsilon_r\\$) rises, velocity decreases. Magnetic permeability is the other factor but this barely changes ...

5

At a risk of collecting few negative points, I will try to answer this question as follows: The "characteristic impedance" has no direct physical meaning. It is just a constant in amplitude coefficients in the solution to the "Telegrapher's Equation", which describes propagation of a sinusoidal electromagnetic wave(s) along a special geometry of uniform ...

5

It seems there's a fairly simple way to get a good approximation of the impedance of a random piece of cable. It requires a fast oscilloscope, a pulse generator with a fast rising edge, and a potentiometer with a range that includes the cable's impedance (cable impedance is usually less than 500 ohms.) The cable you want to test must also be long enough ...

4

This fascinating Bell Labs video beautifully demonstrates wave motion, SWR, and impedance matching sections on a completely mechanical tabletop device without needing mathematics. It's presented in a way that even a layperson can understand these concepts. Reflection of waves from free and clamped ends Superposition Standing waves and resonance Energy loss ...

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