Some dissonance over the question, of the propagation speed of signals along microstrips.

The following frame states an observation which I recently made, about some equations describing transmission lines (that can exist, etched onto PCBs). Most browsers will require that JavaScript from my site, as well as from ‘mathjax.org’ be enabled, to be able to display it correctly…

(Updated 6/11/2021, 23h50: )


 


 

Dirk

 

Simulating Lossy or Custom Transmission Lines, using NG-Spice.

One of the fun things that can be done using the Open-Source circuit-simulator ‘NG-Spice’ is, that transmission lines can be simulated, with the parameters ‘Z0′ and ‘TD’, which stand for the characteristic impedance, and the delay down the length of the transmission line. However, what some people have noticed, and recorded as a bug, elsewhere on the Web, is, that such transmission lines will appear to have ideal behaviour right down to ‘DC frequencies’.

Rather than to think of this as a bug, I’d categorize this as the behaviour, that if the ‘RefDes’ of the transmission line begins with the letter ‘T’, NG-Spice will always simulate a lossless transmission line.

What some people might prefer is, to simulate a lossy transmission line. And When using NG-Spice, this capability is available, but hidden. Basically, NG-Spice uses the same simulation engine, to simulate the Netlists, that other versions of Spice will use. But, the Open-Source version will be lacking in GUI support, as well as in the available libraries of components.

The key to understanding, how to simulate a lossy transmission line, is, that NG-Spice will only process them as being a different type of component, if their reference descriptor begins with the letter ‘O’. This is similar to how NG-Spice will require that the ‘RefDes’ begin with the letter ‘X’, if what is to be simulated, is some sort of sub-circuit. But, because this will stem from a modelcard, the model of the transmission line will actually need to be specified within the Schematic Editor, as the Value of the component, for which there also needs to be a Model entry in the schematic, that points to the file, which will define the model. This Model entry will have a reference descriptor, beginning with the letter ‘A’…

(Updated 6/07/2021, 2h25… )

Continue reading Simulating Lossy or Custom Transmission Lines, using NG-Spice.

Classical, impedance-quadrupling BalUn transformer.

Some readers might ask themselves, ‘What the heck is a Balun transformer?’ And the answer is that, in certain high-frequency applications, this term gets used for a Balanced-to-Unbalanced (impedance-matching) transformer, often implemented as a transmission-line transformer. One common place they did get used in years gone by was, to allow people to connect 300Ω twin-lead TV antenna cable, to the 75Ω coax inputs of more-recent TVs. Actually, what was inside those little adapters was, a toroidal ferrite core, with a piece of sheet-metal (probably aluminum) stamped around it in a clever way, so that this stamped sheet of metal also acted as the ~windings~ of the transformer.

Really, this type of transformer does the same thing that an ‘autotransformer’ does, only, at much higher frequencies. If the reader is picturing a (center-tapped) autotransformer with many windings, then he or she should also picture how many implicit, internal capacitors those have (between the windings), and how capacitors become increasingly conductive, at higher frequencies… Traditional, wound transformers start to become useless well before 100MHz has been reached.

If people look this subject up elsewhere on the Web, They might find diagrams of various types of transmission-line transformers. But, it’s easy to get confused about the way those need to be connected, so that one possible result could be, a transformer that does not work correctly. For that reason, I have just reconstructed how I remember them to have been configured in the past:

 

Peter_Balun_1.svg

 

I suppose that another piece of possibly related trivia could be, that an impedance of, say, 150Ω, connected to a voltage of zero, is equivalent to 300Ω, connected to a relative voltage of (-1). Another related assumption is, that such transmission lines are indeed wound on effective ferrite cores, capable of choking their net current to zero.


 

Now, there’s another, related application of transmission-line transformers, which could be, that a number of transistorized output drivers might only be able to handle some higher (load-) impedance (each), but that the goal is to combine their amperage, so that a divided output-impedance also results, at minimal waste of energy. Additionally, some small mismatch in the outputs could be expected, which should be absorbed, and not result in reflected waves…

(Updated 6/02/2021, 9h15… )

Continue reading Classical, impedance-quadrupling BalUn transformer.

Reducing Induction Effects, Counter-EMF, and Stray Voltages in Low-Voltage Communication Wires.

One of the observations which amaze older people like me, is how high the frequencies have become, at which even household appliances such as USB Cables can communicate. In my youth and young adulthood, such things would not have been considered possible. And the surprise which this progress brings, comes more strongly to older people, who actually did know about Electronics.

There is a basic enemy to allowing communication at high speeds: Plain wire has linear inductance, which becomes significant at the higher speeds.

There is a basic methodology to reducing the unwanted effect: Actual signal-wires are often accompanied by a shield wire, which needs to be grounded or connected to zero, at both ends of a wire bundle.

The concept is quite simple. This shield wire acts as a kind of secondary winding, to a virtual transformer, of which the signal wire would be the primary winding. Whatever counter-EMF the signal wire would produce, would also need to exist along the length of the shield wire. But because the shield wire is grounded at both ends, the counter-EMF which the signal wire can produce is also greatly reduced, in comparison with what one would obtain, if the signal wire existed by itself. When current flows in one direction through the signal-wire, current also flows in the opposite direction through the shield wire. If that current could not flow, then the full linear inductance of the signal wire would seem to exist. Otherwise, not so.

Now I suppose that it would be nice if overhead wires that stretch geographical distances, could also be shielded as easily. But one fact which is highly disappointing is, that shielding / elimination of stray-power problems, is highly lacking in many practical situations. More specifically, power lines may often only seem to have real phase wires, but no neutral wire that runs parallel. Instead, what some Engineers do, is simply to sink a grounding electrode into the earth, at the receiving end of such an arrangement.

The problem with that is the fact, that Earth is not a perfect conductor, and was also never ‘meant to’ participate in Humans’ high-voltage circuitry.

(Updated 3/12/2019, 15h20 … )

Continue reading Reducing Induction Effects, Counter-EMF, and Stray Voltages in Low-Voltage Communication Wires.