Please help me understand output impedance such as 50 ohm... - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: October 16, 2015, 03:25:57 pm
content: So someone told me yesterday that my DS4014 had a 500ps rise time calibration output on it. I've been waiting to try to get one of those pulse generators I see often around here, but still don't have one so this was a pretty cool discovery for me. I connected this output with a BNC cable to CH1 and sure enough I got a nice fast rise time as expected - but only when I terminated the channel with 50 ohm. When set to 1M it was 5 times larger and the waveform had a larger amplitude and did not look as clean.I am *guessing* that the irregularity in the waveform is because it is not terminated and the signal is bouncing back and forth, I've seen some videos here about that. I did try changing the output type Ext. Impedance to 1M or 50ohms, but it doesn't seem to have any effect except clicking a relay. This takes me to some of the videos I've seen about the 1074Z-S signal generator which also has a 1M or 50ohms option, but from the videos I saw it did not change the output at all, but rather just changed the voltage it was assigning to it. In 50 ohm mode the voltage was exactly half of what it was in 1M mode.I think I have somewhat of a grasp on the idea that when it goes through a cable that there is emf created based on the conductor, insulator, and grounding sleeve, and that you have to balance that with a resistor at the end.I have a short 20" or so BNC cable and if I connect a 50 ohm pass through at the end of it to terminate it, I can use a probe tip/spring clip to also get a good rise time (scope no longer in 50 ohm mode, but 1M).When I try to eliminate the BNC cable and use the scope probe directly on the BNC output, I get the same thing. Not terminated is a messy wave with 5x the rise time, if I add the 50 ohm terminator right there on the bnc output, it is good again.My question is - is the output *designed* to be a 50 ohm output? What type of circuit is used to output this way? Why the wave issues if there is no 50ohm cable? Does the fact that the amplitude is twice as high when connecting to a HiZ load vs 50% for a 50ohm load tell us something about the way the circuit driving it is constructed? Is there an amplifier of some sort setting it up for 50 ohm output? If I could probe it internally with a 10X probe before that amplifer, would the rise time and waveform be clean?Thanks!

author: nbritton
author_rank: Frequent Contributor
timestamp: October 16, 2015, 04:39:22 pm
content: All of your questions are answered in this video:Do you have a screenshot of the 500ps rise time on your DS4014? I'd like to know the rise time is so I can calculate the bandwidth.

author: dom0
author_rank: Super Contributor
timestamp: October 16, 2015, 04:50:45 pm
content: QuoteTo a 500 ps tr pulse a 50 cm run of your typical coax cableis not electrically short(wavelength of TEM wave in a waveguide, from pulse rise time, assuming pulse shape roughly similar to a 1.st order LPF: c * vk * 2.85 * tr — 2.85 is 0.35^-1, which might be familiar from 0.35 / tr = BW (this is where the assumption of 1.st order LPF comes from). vk is the velocity factor of your coax, which is about 2/3).This gives a wavelength of about 30 cm. A cable is considered electrically short if it is shorter than 1/10 of the wavelength in the cable, because in this case no significant reflections are expected.)If you use a very short connection (like a mounted adapter, <3 cm line length), you should not see reflections.However, many fast pulse generatorsrequirea 50 Ohms termination to run their output stage correctly, so you may still see aberrations on the pulse, even when using a short connection.

author: alank2
author_rank: Super Contributor
timestamp: October 16, 2015, 05:19:46 pm
content: Quote from: dom0 on October 16, 2015, 04:50:45 pmAt 22:00 in the video he shows connecting the probe directly to the output bnc on the gen with no termination and says it should work, but my test doesn't reflect that which makes me wonder if you are right, that the 50 ohm termination is required in this case to get the fast edge. Now that I think of it, I may have tried to see how fast the edge was for this "fast" mode and discounted it because I thought it was >3ns or so when I just didn't have it terminated properly.

author: alank2
author_rank: Super Contributor
timestamp: November 03, 2015, 09:27:28 pm
content: A somewhat new question - how do you make something a 50 ohm output? What kind of driver or circuit does that? Any schematic examples?

author: IconicPCB
author_rank: Super Contributor
timestamp: November 03, 2015, 09:38:45 pm
content: A consequence of negative feedback is that the output of the amplifier will have zero ohm output impedance ( notionally).So simply fitting a 50 ohm resistor in series with the output will provide a 50 ohm ( + zero ) source.In case of an existing amplifier or termination ( such as an oscilloscope ) which have high impedance output/input placing a shunt 50 ohm resistor will make the output /input appear as 50 ohm ports.This is the simple response. More complex answer is that non 50 ohm ports can be impedance matched to 50 ohm environment through application of reactive elements.A resistive only solution tends to be more broadband in its performance as opposed to reactive matching.

author: alank2
author_rank: Super Contributor
timestamp: November 03, 2015, 09:45:08 pm
content: Some searching showed paralleling logic to accomplish it. Let's say I have a microcontroller PWM pin and I am trying to produce a stepped sinewave on it and it is 0-5 VDC. Let's say that the output of that pin is spec'd for 20mA max. 5V/50ohm=100mA so we need a way to amplify it. I guess the mystery I am trying to figure out is how is the output at HiZ about twice what it is at 50ohms? Is that because of the amount of amplification?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: November 03, 2015, 10:28:56 pm
content: Quote from: alank2 on November 03, 2015, 09:45:08 pmGenerally, the output has a 50 ohm output impedance regardless (like a 50 ohm resistor in series with whatever is driving the signal, eg: an opamp). When you attach this to a high impedance load, you have a voltage divider, but...not really, because it's say 1M/(1M+50), and your load sees basically the entire signal. When you attach it to a 50 ohm load, however, you now have a real voltage divider, and you only see 50/(50+50) = 1/2 of the signal, the other half gets dropped across the 50 ohm output resistor in the source.

author: smbaker
author_rank: Regular Contributor
timestamp: November 03, 2015, 10:56:55 pm
content: Quote from: alank2 on October 16, 2015, 05:19:46 pmI may have had a similar experience. I was testing a function generator, and following (or mis-following; maybe I failed to grasp a subtlety) Dave's video, I assumed I could plug my 10x probe (with BNC adapter) directly into the RF out jack. That didn't work. There were artifacts in the signal. I needed to use a 50-ohm terminator at the source, and then plug my probe into that terminator. The RF generator did not like no termination. I think it expected a load.

author: Howardlong
author_rank: Super Contributor
timestamp: November 03, 2015, 11:20:56 pm
content: Quote from: smbaker on November 03, 2015, 10:56:55 pmIf it's an RF generator it will expect a 50 ohm load. You can still probe it with your passive 10x probe but the amplitude will not be what you expect. RF generators generally only generate sine waves which may be modulated.If it was a function generator, then a non-sine wave signal will start to look decided ugly at higher frequncies with a 10x passive probe: the probe impedance varies with frequncy due to its parallel capacitance, so that square wave won't be very square any more.

author: w2aew
author_rank: Super Contributor
timestamp: November 04, 2015, 03:46:23 am
content: Quote from: alank2 on November 03, 2015, 09:45:08 pmChanging the output setting on the generator does NOT change the generator's output impedance. It's always 50 ohms. It just changes the generated amplitude to account for the different voltage division that occurs between the output impedance and load impedance. This video should clear this part up.

author: dom0
author_rank: Super Contributor
timestamp: November 04, 2015, 09:26:06 am
content: I get the feeling that the "Hi-Z" and "50 ?" settings on function gens were a huge mistake :-D