Question about signal generator and 50ohm - Page 1
Forum section: electronics / beginners

author: asgard20032
author_rank: Regular Contributor
timestamp: June 07, 2016, 05:52:22 pm
content: I just got a signal generator, and I know that I need to match impedance. I know the basic of impedance matching, but what confuse me is not the concept of impedence matching, but wich cable match or not, and which cable termination match or not the impedance. I found out the cable/terminator thing confusing. To better understand, I will create few scenario, and maybe some people will sort out those scenario :Scenario 1 : Generator to Oscilloscope- Signal generator (50 ohm)- BNC cable- Oscilloscope either in 50 ohm or with a 50ohm adapter*Matched right? Everything fine thereScenario 2 : Generator to some breadboard/customboard- Signal generator (50 ohm)- BNC cable to alligator- Alligator to circuit*What happen here? Do the alligator do the 50 ohm matching? Do i need to match my feed point on my circuit?Scenario 3 : Same as precious, but instead of breadboard, alligator touch the scope probe*What happen here?Scenario 4: Generator to some breadboard/customboard V2.- Signal generator (50 ohm)- BNC cable- 50 ohm adapter bnc- BNC to alligator- Alligator to circuit*What happen here?Also, does the generator mode (TTL VS cmos VS other) affect the output impedance?Generator : BK precision 4040aOscilloscope : DS1074z (no 50ohm input, so i need an 50 ohm termination adapter)About to buy banana cable and bnc cable to equip myself, and need to clarify this before i buy anything.

author: rstofer
author_rank: Super Contributor
timestamp: June 07, 2016, 06:30:59 pm
content: Impedance matching allows maximum power transfer and to some extent, limits reflections. Then you turn around and drive an 8 Ohm speaker with a 1 Ohm amplifier output, deliberately! When matching is actually required is a bit of a question.So, for breadboarding at modest frequencies, I don't worry about it. If my square wave leading edges are so sharp they result in reflections (FPGA driving a Compact Flash device), I add a 330 Ohm resistor in series with the signal. I tend to do that on all FPGA and uC pins as a matter of practice. It limits "oopsie" current should something go sideways.Here's an article (there are others):http://electronicdesign.com/communications/back-basics-impedance-matching-part-1Bottom line, I have never really worried about matching a signal source for a breadboard.What did you gain by turning your scope impedance down to 50 Ohms besides cutting the voltage in half? A better trace? Well, maybe, but does it matter? How many times do you need to stuff a signal generator output into a scope?For my purposes, and those don't include RF or long transmission lines, I just haven't spent a lot of time on impedance matching. Maybe I should...

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2016, 06:33:25 pm
content: For *most* low frequency use quite ordinary connections will work just fine however if the load doesn't match the generators output impedance it may be subjected to higher voltages than intended.Units with Hi Z and 50 Ohm outputs will adjust the *seen* amplitude to match the *set* load.As yours seems to be only 50 Ohm a BNC Tee on the generator output can be useful to monitor amplitudes with your scope until you know you can trust what you set is what you get.The BK precision 4040a datasheet lists a 0.8V to 2.4V range for TTL and simple checks with a HiZ scope input can quickly check that you aren't subjecting IC inputs to greater than they can withstand, normally IC VDD + 0.5V max, remember most logic IC's inputs need only pA's.

author: asgard20032
author_rank: Regular Contributor
timestamp: June 07, 2016, 07:03:50 pm
content: Quote from: tautech on June 07, 2016, 06:33:25 pmI know what happen if i don't match impedance, one side will probably see highter voltage and there can be signal reflection. Like I said, what I don't understand is not the impedance matching concept, but if the bnc to alligator, and bnc to whatever else. Generator = 50ohm, cable =? 50 ohm, alligator end ?=?. Do I need to match my input circuit? If i probe the alligator? I just can't find the datasheet of a bnc to alligator, what arcane magic happen inside those.Already know what is impedence matching, I just don't understand the different bnc to whatever else cable thing, if they do or not the matching in the cable or if i need to match at the input point (probe tip, circuit input feed, whatever else here). All I know is that a normal bnc to bnc require a matching at the other end. It is much easier to simply say which scenario is good or wrong than explaining me what is impedance and why I need to match (i already know). I don't want to be harsh, I am just trying to be sure we don't lost time and we go straight to what I don't understand, thus not the impedance matching theory but different type of bnc to (bnc, alligator, banana, tweezer, whatever else) and their termination(which impedance they got aka if they will do the impedance matching).

author: w2aew
author_rank: Super Contributor
timestamp: June 07, 2016, 07:23:06 pm
content: In the scenarios where you want to drive a breadboard or other (2, 3, and 4), you can certainly get away with using a 50ohm through terminator at the end of the cable, and attach the short alligator adapter to the DUT, circuit, etc. The thru termination will do two things - it will make the output amplitude correct (assuming the generator is expecting a 50 ohm load), and it will help to prevent reflections. Since the 4040A is a low frequency generator, the only application where the reflections would be an issue is for waveforms that have a steep/fast rising or falling edge (pulses, square waves, sawtooths, etc.), because these fast edges are what contain high frequency content. Reflections on these edges can cause logic glitches if they are too large, so the termination is a good idea. The 4040A rise/fall speed is spec'd at <30ns, and I suspect it isn't too much faster than that. Thus, the impedance discontinuity of a few inches of wire between the 50ohm termination and your circuit shouldn't really affect things at all - so don't worry about it.

author: asgard20032
author_rank: Regular Contributor
timestamp: June 07, 2016, 07:31:40 pm
content: Quote from: w2aew on June 07, 2016, 07:23:06 pmIn other word, you are saying that bnc to alligator still kepp the 50ohm source impedance (so dosen't match it) and that either the circuit feed point should be 50ohm matched or a bnc 50ohm adapter should be used somewhere between the bnc to alligator and the generator.By the way, I am one of your subscriber on youtube, I liked your video about modulation and the diode ringmixer.

author: rstofer
author_rank: Super Contributor
timestamp: June 07, 2016, 07:52:30 pm
content: A typical BNC to alligator adapter has no internal impedance matching. Here is the Pomona 3788 datasheet and it says nothing about matching.http://www.pomonaelectronics.com/pdf/3788_001.pdf

author: asgard20032
author_rank: Regular Contributor
timestamp: June 07, 2016, 08:31:35 pm
content: So I think I shall get 1-2 bnc <- > bnc + 1-2 50 ohm termination (to plug my scope to some instrument), then get a T bnc with 50 ohm match.But if i put a 50 adapter between the gen and the whatever I want to plug it, I should put the 50 ohm adapter at the end of the cable, and get a bnc to alligator with no coaxial between the alligator and the bnc socket, else I would put a non 50 ohm signal into the coaxial between the bnc socket and the alligator, thus impedance mismatch. Plugging the 50 ohm adapter on the scope is correct, since no coaxial between the 50 ohm adapter and the scope input. It also mean a t bnc connector with integrated 50 ohm adapter would do the same thing, if I put the t on the source.And yes, those thing all matter to me since I want to get into high-speed digital with FPGA and to radio frequency (although my FPGA activity is unrelated with my radio-frequency experimentation)Also, my 4040a datasheet talk about the output labelled "output" as having an output impedance of 50 ohm, but it dosen't say the impedance of the output terminal labelled "TTL/CMOS". I don't know if they are also 50 ohm or if they are low impedance.And lastly, what happen with a T bnc, with those, load are put in parallel, so one side impedance will also affect the other side impedance right? How to work with those T bnc adapter?EDIT : Ok about the T bnc, I think i found out, I don't need an adapter, but a 50 ohm termination. Putting the termination on one side put a 50 ohm resistor to the ground. Thus, the other side of the T is in parallel with 50 ohm to the ground. So if the load is a high impedance : high impedance // 50 ohm = almost 50 ohm. But if the load is not high impedance, but like 400 ohm, it will do 50 ohm // 400 ohm, and thus not matched. So the T with 50 ohm termination only work on high impedance load. A T on the generator is maybe not the best thing, because between the T 50 ohm matched (because of the 50 ohm termination) and the load, there will be a coaxial cable. And the impedance matching is done before the coaxial cable, thus feeding a non 50 ohm signal into the coaxial cable.

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2016, 08:59:14 pm
content: While I can relate to your concerns if you are dealing with high frequencies, in this case you are not.Just use it, you have sufficient understanding of impedance matching implications to diagnose any unexplained results.As far as TTL levels for this FG are concerned the stated risetime is much slower on TTL setting and unlikely to cause problems from any fast rising edge.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 08, 2016, 02:31:50 am
content: I think you are asking good questions.These videos might help address some of your questions:Your 4040A is a pretty nice signal/function generator and your scope is good too. I think it might be worth getting a couple of 50 Ohm terminators like the ones shown in the first video above. Sometimes you can find nice Tektronix terminators on eBay for a reasonable price but there are plenty of sources for 50 Ohm terminators. Just having a couple on hand will be helpful when doing various tests with the 1 MOhm inputs on your scope.Something to keep in mind is that when working with DC Ohms Law is pretty straightforward because impedance is strictly resistive. When you start working with AC you get to complex impedance which has reactive capacitive and inductive components. This starts to show up in various places including RF projects such as antenna and feed line designs where Voltage Standing Wave Ratios are a consideration. An interesting thing is that when radio operators want to have a 1:1 VSWR (or as close to it as possible) to avoid reflection collisions between forward and reflected waves (for example between the output of a radio transmitter and an antenna) they also consider the impedance matching process that deals with the fact that with capacitance current leads voltage by 90 degrees and with inductance voltage leads current by 90 degrees (in purely resistive circuits voltage is aligned with current). So signal phases become a consideration and what better place to observe signal phase than on our oscilloscope! I'm just learning this stuff myself so take it all with a grain of salt but the interesting thing is that AC impedance has these reactive components that are frequency dependent and that are not present with DC resistance. It's kind of a beautiful set of relationships. So, in any event, since 50 Ohm impedance is such a popular baseline for many electronics designs I think having a couple 50 Ohm terminators (and using 50 Ohm coax cables) might help you more easily observe and think through and learn more about these things as you use your scope and generator to do various experiments and projects.One other thing: there is more than one kind of T terminator. This thread has a discussion:https://www.eevblog.com/forum/beginners/50-ohm-bnc-adapter-needed/Here is another video you might enjoy:

author: CJay
author_rank: Super Contributor
timestamp: June 08, 2016, 07:23:18 am
content: I can't add anything to the knowledge imparted by some of the giants who have already commented but to say I've found that impedance matching at the sort of frequencies I'd use on a breadboard is largely irrelevant unless you're dealing with very high impedance sources.Which you're not.Transmisson line theory can wait a while until you've nailed the basics I thinkAs an aside, I have a nicely useful collection of 10Base2 terminators and connectors (T-Pieces) which are perfect for my needs and seem to work just fine up to (and possibly beyond) VHF, they were salvaged from the various network installs and refurbs I've done over the years so it might be worth digging around in corners of drawers and dusty storage rooms if you've any connection to IT.

author: w2aew
author_rank: Super Contributor
timestamp: June 08, 2016, 01:33:59 pm
content: Quote from: asgard20032 on June 07, 2016, 07:31:40 pmThe alligator clips do not keep the 50 ohm impedance. What I am saying is that even though the wire+alligator clip connection between the thru terminator and your circuit is not 50 ohms - it doesn't matter at the frequencies and distances that you're working with with your 4040A generator. Simply connect your 50 ohm coax to the generator, attach a 50ohm thru terminator at the other end of the coax, and then attach your BNC-Alligator-clip adaptor to that, and clip it to your circuit. In many cases you wouldn't even need the 50 ohm thru terminator, but it does help to keep the rise/fall edges from ringing and will keep your amplitude indication on the generator correct.

author: rstofer
author_rank: Super Contributor
timestamp: June 08, 2016, 01:51:57 pm
content: Quote from: w2aew on June 08, 2016, 01:33:59 pmI'm not clear on the amplitude indication. In my mind, the voltage would be measured at the output BNC after the 50 Ohm series resistor (or however they build up the output impedance). It makes no sense to do it any other place and not account for the voltage drop across that impedance. It would just be a guess.So, if they measure the voltage at the output, why would it be wrong if the output was unterminated? It will certainly be different when the 50 Ohm load (terminator) is added/removed but I can's see it being wrong either way.

author: JPortici
author_rank: Super Contributor
timestamp: June 08, 2016, 02:03:38 pm
content: well because the cable is not a wire but a transmission line. If it's not terminated with the correct impedance there will be reflections and standing wave oscillations at the end of the cable, as already said.Like sothis is a random photo found on google, but this could very well be a (floating) output pin of a cortex m3 mcu

author: w2aew
author_rank: Super Contributor
timestamp: June 08, 2016, 05:15:30 pm
content: Quote from: rstofer on June 08, 2016, 01:51:57 pmMost function generator outputs have a 50 ohm output impedance. Thus, when they are terminated into a 50 ohm load, the voltage appearing at the load will be 1/2 of voltage that would appear in the open-circuit (high-Z) load case. For function generators that give you a display of output voltage amplitude, there is usually some assumption (or setting) for what the load impedance is so that the proper voltage scaling is included in the displayed value. Maybe this video will help:

author: rstofer
author_rank: Super Contributor
timestamp: June 08, 2016, 08:08:10 pm
content: Quote from: w2aew on June 08, 2016, 05:15:30 pmIt is worth noting that the author's function generator can be instructed to assume a Hi-Z input impedance for the load. The generator itself doesn't care what the load impedance is and, if so desired, it could be specified to some arbitrary value (within some limits, I suppose).The quesion is simply this: Is it NECESSARY to always match the impedance? No, it isn't. Whether it is necessary depends on a lot of factors (frequency, waveform, length of cable, type of cable, etc) and for most low frequency applications, it simply doesn't matter. Oddly, some generators only specify the output impedance for the SINE function. No mention for the square wave. Probably because it is just the output of some buffer chip. Yet it's the square wave that generates all the harmonics and potential reflections.There is certainly nothing wrong with installing an in-line terminator at the end of the cable. That will probably always be the best answer. But it probably isn't the only answer. Particularly when breadboarding...