power line treat as transmission line? - Page 1
Forum section: electronics / beginners

author: 3roomlab
author_rank: Frequent Contributor
timestamp: November 16, 2015, 09:49:20 am
content: having seen the thread about making high speed rise time circuit for testing scope bandwidth.if we look at power lines on PCB like transmission lines, having rise time (due to current draw etc) etc, what is the approach or method to calculate capacitor "filters" to make such a transmission power line stable? or do such considerations not exist?

author: Simon
author_rank: Unknown
timestamp: November 16, 2015, 12:35:16 pm
content: how long is a peice of string

author: tron9000
author_rank: Frequent Contributor
timestamp: November 16, 2015, 01:13:28 pm
content: Well if you know about transmission lines then you should be able to work it.your Tx line has x-F/m, y-H/m & z-ohms/m giving a characteristic impedance. Its should be a case of match loading shouldn't it?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: November 16, 2015, 07:06:58 pm
content: Quote from: tron9000 on November 16, 2015, 01:13:28 pmi understood bits and pieces, i think what you said is similar to the advice said by another regarding the DC-ish loading, like imp matching. hmmm chicken or egg problem. i dunno the load yikes

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2015, 12:59:29 am
content: Yes, precisely!If you go further and consider that a "piece of string" has inductance and capacitance per unit length, then it has an impedance; and if it's loaded periodically (more or less) with bypass capacitors, all the more so.In essence, you're building a lumped equivalent transmission line (of relatively low impedance), especially if you route the power trace(s) in a point-to-point chain fashion.How do you keep a transmission line from bouncing? Simple, put a terminator on it! A few-ohms load resistor would be rather inconvenient for DC, of course, so you'd usually add a capacitor in series with said resistor. The capacitor needs to be big enough that, at the characteristic frequency(ies) of the line, the resistance is dominant. In other words, C > 2.5 * Cbyp is pretty typical (take Cbyp = total value of all small bypasses chained on the supply).If you have a star routing topology, then at the nodes (where the multiple traces join up or branch), the impedance will be especially low, and resonant currents can arise (and additional filtering or R+C damping will have little effect, or make things worse!). If you like, these can be dampened by adding series resistors or inductors to the traces (mind that ferrite beads don't have much impedance under DC load, so only use them for low current supplies). Likewise, at the ends of traces, the impedances will be relatively high (and able to resonate against the low bulk impedance of the node), so R+C dampers can be used at the ends.Note that adding explicit inductors acts to increase the impedance at some frequency, which is how you get the filtering effect; but you get resonances, too. The impedance is proportional to sqrt(L/C), and damping resistors are always proportional to this value.The bulk damping capacitor can be incidental as part of the supply filtering network (usually you'll have a bulk cap to handle ripple or supply filtering), or added intentionally for this purpose.Mind that very low ESR capacitors (large ceramics, aluminum polymers) can make thingsmuch worse, because with such low impedance, resonances tend to be enhanced.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2015, 01:01:56 am
content: Oh, and you can -- at least vaguely -- estimate the magnitude of a power supply disturbance, based on current draw. How? Impedance is impedance -- the ratio of voltage to current. Now, if you have peaks in the impedance at certain frequencies, you can expect much more than V_ripple = I_disturbance * sqrt(L/C), but with damping (to bring the Q of those resonances down below 1), it won't be worse than this amount.Tim

author: 3roomlab
author_rank: Frequent Contributor
timestamp: November 17, 2015, 05:11:54 am
content: thanks for all the explainations, i got lost at string and Cbyp ...what should i google for to read in detail? (or what kind of calculation are these called? time to download some youtube tutorials)*edit - ok i found some vids i thinkhttps://www.youtube.com/results?search_query=transmission+line+calculationahhhhh i see why you said string simon, analogy to voltage sag etc

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2015, 05:53:17 pm
content: One of the downsides to being me, I mostly didn't learn from any set of textbooks, but pieced much together from intuition and higher math.Ultimately, you're down in the nitty-gritty of passive network theory and filters. If you've ever done AC steady state analysis of an RLC circuit, you've probably noticed that it's a serious pain passing around all the variables, all the time. Give this a try: factor out everything to make it dimensionless ratios. Instead of L and C, use Z_0 = sqrt(L/C) and w_0 = 1/sqrt(L*C) (that's lowercase omega = 2*pi*F_0). Instead of R, use R / Z_0. And instead of w, use w / w_0. Everything is much easier as ratios, and seems to interact that way. When you're tuning a filter, often many components have to adjust at the same time (e.g., all the L's and C's proportionally larger to reduce the cutoff frequency, holding impedance and filter envelope constant), so it helps to have these stand-ins while working the equations.And for particularly beneficial solutions (such as the named filter types), the resulting equations -- polynomials in w -- are adjusted to specific coefficients. Ultimately, filter synthesis is solving the problem of polynomial roots, which you might've thought was a problem from mathematics that was utterly useless. But it's a difficult problem indeed in high order filters (no closed form solution for N >= 5). Hence calculated tables of filter coefficients..Now, a power distribution network doesn't need that kind of accuracy, and would be rather inconvenient to build as a chain of finely tuned filter sections. But using the general theory, understanding that when inductances and capacitances get together, you have characteristic impedances and frequencies: that's the most important part.Tim

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 17, 2015, 07:00:23 pm
content: This reminds me of a solution I've been thinking about. I call it "POZ" or Power over Zip-cord.If I have several cameras up to 500ft away and I am trying to minimize the cabling required to support them, I can feed 18~24VDC through some ordinary ~16ga "zip" cord (lamp cord) which can be regulated down to the 7.6V required by the camera, and the 12V required by either the robotic pan/tilt head or the viewfinder for a human camera operator.But I need to send a digital stream to the far (camera) end, for camera control (zoom, focus, iris, etc.) and for pan/tilt head control.I don't really need return data, so basic simplex.Why can't I simply use my power zip cord as a differential pair for sending digital signals?I can optimize the the data transmitter and receiver circuits for the characteristics of the "transmission line".I figure I can make my own transformers at each end by hand-winding ferrite torroid cores.Certainly I can use "primary" wire gauge sufficient for the current load.But where do I find information about core material, number of turns, etc. to make the transformers?

author: SeanB
author_rank: Super Contributor
timestamp: November 17, 2015, 07:14:39 pm
content: Just use a capacitor to inject the digital data, modulating either using AM or FM modulation on a high frequency carrier ( around 10 times the shortest bit period so decoding is easier), and at the camera end a simple RF choke to block it and a capacitor to transfer to a demodulator. If you only need standard remote controls use a 38kHz modulation and a IR receiver chip ( not the 3 pin ones, the older ones with a separate photodiode) to get the data across.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 18, 2015, 03:51:58 am
content: What you're doing is a combination of bias tees, splitters, filters, etc.Remember that you need to make very certain the common mode is small (like << -40dB below differential small), otherwise you'll be transmitting as much right into the air! So the splitter should be very well balanced.To get DC out, obviously, that can just be straight through, as shown. Not shown is the local filtering, which should present a low impedance on the device side of the coupling inductors.The signal can be coupled in via capacitors, or magnetically (as shown). Either way, balance needs to be good. It's not usually enough to throw a CT winding at it, because the windings won't be exactly matched. One will be closer to the AC driving coil, having lower leakage; the other one will be far, and inverting, so has extra phase delay. Plus propagation through the windings.A combination of common mode choking and differential matching is sometimes used. This is used in some 1000BASE-T transformers: see Circuit Bhttp://www.haloelectronics.com/pdf/discrete-single-1gbaset.pdfMaking a single CT winding, versus a full transformer, is a bit easier, and even if it's not perfect, the combination of CT'd source, plus CMC, plus CT'd balancing transformer, makes it that much cleaner.As for winding size, bandwidth and all that, you need to figure a system impedance. For zipcord, it'll be around 100 ohms (and quite lossy above maybe 100MHz..). You'll want the impedance matched from the source end, and terminated at the load end, to minimize reflections and ensure proper function even if the cable's impedance is really awful.The cutoff frequency of the CT'd winding is given by F = Z_0 / (2*pi*L), where L is the total inductance of that winding. For pulsed communications, you'll probably want the cutoff frequency >5 times below the baud rate. (Preferably, you'll be using a zero-DC encoding, like Manchester coding. Otherwise the pulse detector circuitry needs to handle the loss of DC bias, which is complicated.)For a driver, you might use a logic gate cap-coupled to the splitter transformer. The gate should be a few in parallel (to get an output resistance around, say, 20-30 ohms, using 74HC family), plus a series resistor which serves as main termination -- bringing the output resistance up to the system impedance, say 100 ohms. The impedance at this point is an open variable, because you can use any ratio for the splitter. A 2:1+1 ratio is easy enough to wind, and will provide 100 ohm input impedance for a 100 ohm differential, a reasonable load for logic.You want bandwidth limiting also, because any bandwidth beyond what you need to communicate clearly is just more risk of transmitting switching noise. A simple CLC filter, following the gate and proceeding the splitter, is probably fine.Back to winding. Once you know the required cutoff frequency and winding inductance, select a core. This is a pulse transformer application, so you'll probably be looking at high permeability ferrite materials.For a given core, you need to satisfy three things:1. Inductance: number of turns N >= sqrt(Lmin / A_L)2. Flux (to avoid saturation): N >= Vpk / (4*F*Bmax*Ae)3. Current capacity, or attenuation or efficiency: you need to get all the turns in the winding window, of large enough wire for the application. Typically, for power applications, you need the available winding area more than twice the area required for all the windings and turns, and 5-10x for toroids (you don't want a fully packed toroid, it's less efficient).There's also the consideration that, the wires in the winding itself act as transmission lines (indeed, a transformer is best viewed as a transmission line, as transformers exhibit inductance and capacitance, or characteristic impedance and cutoff frequency), and you need those line lengths to be sufficiently shorter than the wavelength of the maximum frequencies you'll be transmitting. So you have some pressure to use small cores to begin with, and to use relatively thick cores to reduce the number of turns, and length, required. Shorter windings also have less loss.Related:http://seventransistorlabs.com/Precompliance/index.htmlI built this ISN for precompliance testing. It basically does everything I've just talked about, except exactly the opposite of it: it's designed to extract the common mode component, rather than differential. Nonetheless, the methods are identical, and you can see the value in keeping windings small -- the bandwidth is good. (The differential performance at higher frequencies wasn't measured, but it does reliably transmit 100BASE-T, which is good enough for me.)Tim

author: German_EE
author_rank: Super Contributor
timestamp: November 18, 2015, 10:22:44 am
content: "But where do I find information about core material, number of turns, etc. to make the transformers?"Speak to Coilcraft, they have an amazing technical support department who know their products and the environment where they are used.www.coilcraft.com