EEVblog #707 Ringing on the rising edges. Is there a way to eliminate? - Page 1
Forum section: electronics / testgear

author: ShockTastic
author_rank: Newbie
timestamp: January 30, 2015, 11:20:29 pm
content: Hi,I've just watched the EEVblog #707 about how good the 100MHz Rigol oscilloscope probes are. Dave showed how both scope probes responded to a step response. Both probes showed a significant overshoot and ringing in the order of 10-20ns. How does someone go about eliminating these from measurements?I want to measure the rising and falling edges of a 300MHz signal but i am finding that the overshoot and ringing is killing my verification.

author: smgvbest
author_rank: Unknown
timestamp: January 31, 2015, 04:56:24 am
content: I am not the expert on this and will only repeat what I've understoodThe source of the ringing is the cable inductance of the ground lead. the longer that lead the worse the overshoot and ringing isSo you have to get that as short as possible. the scope probes allot of time come with what look like little springs these are actually to reduce the inductance of your ground connection. you remove the ground lead and if needed remove the outer sleeve on the probe and place it over the probe tip. it proves a shorter ground. I've also see where someone will take the scope lead not the probe and solder the center strand to the circuit and the ground to as close by a ground as they can get. I believe they also were putting a resistor (50ohm) in series as a termination? There are video's on youtube about high frequency probing that might helpI am also sure someone will come along with a much better explanation than mine

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 31, 2015, 07:50:27 am
content: Quote from: ShockTastic on January 30, 2015, 11:20:29 pmA 300MHz square wave? What are you measuring with?

author: sdg
author_rank: Regular Contributor
timestamp: January 31, 2015, 10:13:51 am
content: Quote from: ShockTastic on January 30, 2015, 11:20:29 pmWith a 100 MHz Rigol scope probe ?Get the "ABCs of Probes" booklet from Tektronix, it probably has the answer to your problem.

author: Bryan
author_rank: Frequent Contributor
timestamp: January 31, 2015, 11:59:42 am
content: This may help.http://youtu.be/g_jxh0Qe_FY

author: smgvbest
author_rank: Unknown
timestamp: January 31, 2015, 02:26:30 pm
content: Quote from: sdg on January 31, 2015, 10:13:51 amHe does not actually way that is what he is using, the OP says that he watched Daves video on the Rigol Probe but does not state what his equipment is.So the first question really should be to the OP, What are you measuring the signal with a DS1104Z with 150Mhz Probe or another scope for clarity to the question?I think my basic answer is still valid on how to measure but then the question becomes can the OP's equipment handle it.The BW indicates is the 3db point to which the signal is attenuated. not the full bandwidth of the probe or scope. If he can get enough signal through the probe and scope to be measured then the question becomes is the sampling rate enough to get an accurate reading.He may have a signal in the mV range that can still be measured. That we don't know

author: joeqsmith
author_rank: Super Contributor
timestamp: January 31, 2015, 05:08:11 pm
content: Quote from: ShockTastic on January 30, 2015, 11:20:29 pmSwitch to a resistive probe. They are cheap, so when you damage it you don't have to take out a bank loan. I have the LeCroy PP066 good for 7GHz. You can make your own, cheap. I have made my own for a few dollars and get well into the GHz with them.I'm a fan of Howard Johnson. You may want to check out his book. The following may be of interest.http://www.sigcon.com/Pubs/news/5_4.htm

author: Rupunzell
author_rank: Frequent Contributor
timestamp: January 31, 2015, 05:48:46 pm
content: The apparently simple act of probing a circuit node can alter circuit performance and behavior is many ways regardless of any probe involved. Knowing what will happen when a specific circuit node is probed is part of how to achieve accurate measurement and not be fooled by what is observed on said instrument. As the frequencies goes up, probing and testing becomes increasingly difficult in a very non-linear way. While scope probe ground lead inductance is one cause of observed "ringing", there will be probe tip loading of the circuit and a LOT more. Probes have varying degrees of cable/instrument related reflections that appear at the probe tip. This is one of the differences between good probes and awful ones.How much all the matters in digital pulse stuff varies, in analog design, probe performance and signal fidelity and test node accuracy is often imperative to understanding and knowing what is actually going on in that circuit.This has been posted before and is worth posting again, Jim Williams app note 47 on high speed circuit stuff which includes good stuff about probes in general.http://cds.linear.com/docs/en/application-note/an47fa.pdfBernice

author: tggzzz
author_rank: Super Contributor
timestamp: February 01, 2015, 09:12:09 am
content: Quote from: ShockTastic on January 30, 2015, 11:20:29 pmThe ground lead inductance is around 140nH, and the tip capacitance 17pF. The resonant frequence is therefore around 100MHz, and if you hit a resonant circuit with a step, it will ring ant the resonant frequency, just like a bell.To increase the resonant frequency, reduce the capactiance or inductance.Reduced capacitance is only available with expensive FET probes or cheap low impedance Z0 probes, as noted elsewhere.Reduced inductance requires a shorter ground lead, either the spring wires supplied with the probe, or, more conveniently, a ground spear/bayonet such as

author: joeqsmith
author_rank: Super Contributor
timestamp: February 01, 2015, 04:24:59 pm
content: Coax alone may be too much of a load.http://koti.kapsi.fi/jahonen/Electronics/DIY%201k%20probe/Below showing one of my home made probes using 1206. A few minutes to build.

author: tggzzz
author_rank: Super Contributor
timestamp: February 01, 2015, 04:47:52 pm
content: Quote from: joeqsmith on February 01, 2015, 04:24:59 pmAs I am sure you are aware, unterminated coax would not work, unless it was very lossy coax!As you indicate, low impedance Z0 probes are very good, especially if the ground lead is short.

author: Rupunzell
author_rank: Frequent Contributor
timestamp: February 01, 2015, 04:49:25 pm
content: 170nH-17pF-x value of R-reflections from the probe cable = complex impedance with resonance. Resonators store energy, it takes energy to excite a resonator and that resonator returns energy back to some where (in this case, what it is connected to) and if the circuit node in question has harmonics related to it, the whole works becomes more complex.As frequencies goes up, that 10 meg or 1 meg DC at the probe tip will be FAR from that resistance, the capacitive and inductive effects at the probe tip becomes the dominate factor and reflections at the probe tip due to probe cable and the networks internal to the probe and instrument input's impedance.How will all this affect the circuit node being probed? This determines the specific type of probe and test set up required to obtain test fidelity and minimal reaction with the circuit under test.As suggested, a coax with physically small series resistor and terminated could do the job. Alternatively, active probes used with proper care can do the job too.Do consider using probe sockets to the test point/node as these can make a significant difference in test fidelity.BerniceQuote from: tggzzz on February 01, 2015, 09:12:09 am