Using scope terminations - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: April 04, 2024, 12:10:19 pm
content: I have this doubt.I am using a scope that has two termination inputs. 1Mohm and 50ohms internal termination on the probe inputs.My question is :1. Suppose I connect the probe to a USB differentia signal. I connect only 1 probe to D+. The ground of the probe is connected to the ground. Which termination scope mode should I use? 1Mohm or 50ohms?I am not aware on what would be the single ended impedance of D+ line with respect to ground. So, I am not sure which termination setting I need to keep. So, I assume I need to keep 50ohms mode.2. Suppose, I use a differential probe, I need to connect the differential probe terminals to both ends of the USB lines. I need to make sure that the impedance of the differential probe should be 90ohms matching the USB differential impedance. Correct?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 04, 2024, 01:04:18 pm
content: Question 1: 1M in conection with 10:1 probe works sure for USB with 12MBitwith 50Ohm Termination at scope USB will no longer work, except you use something special like a 5kOhm at tip probefor higher data rate 480MBit and above you need special equipment anyway - scope of enough Bandwith and special probes.Question 2: no, connect 1 tip to D+ the other to D-. Ground to ground.And Input Impedance of probe needs to be high compared to the USB differential impedance.

author: tggzzz
author_rank: Super Contributor
timestamp: April 04, 2024, 01:18:36 pm
content: Please don't double post.Your signals should be terminated in the relevant impedance.You want your probes to disturb your signals as little as possible. For voltages, that means as high an impedance as possible. For currents, as low as possible.How the probe is connected to the scope is a different question. Some probes (especially high frequency probes) require 50ohms, some don't. Terminate at the scope as specified by your probe.

author: CaptDon
author_rank: Super Contributor
timestamp: April 04, 2024, 01:22:58 pm
content: Think about it. 99.99% chance your probe was designed to see a 1meg termination and that is what you would select. With a 10:1 probe if you did select 50 ohms the scope would see almost no signal as the output of the probe would appear to be nearly dead shorted. The probe tip would still represent a very high impedance to the USB data line and no harm would come. Typically the 50 ohm termination selection is used with current probe amplifiers and with certain R.F. sources usually connected directly by a 50 ohm coax and expecting a 50 ohm termination. BE AWARE that with the 50 ohm termination activated you will be very limited to the input voltage to not burn up the terminator. Something like 10 volts peak to peak.

author: xvr
author_rank: Frequent Contributor
timestamp: April 04, 2024, 01:55:56 pm
content: 1M and 50 Ohm terminations in oscilloscope intended for very different mode of operation. 1M mode used to 'peek' signals on line without disturbance them. 1M considered to be very high resistance value, so probe will be 'invisible' to circuit it connected. (Of course some circuits can be affected even 10M load and/or capacitance of probe, but this is another story).50 Ohm mode used to connect oscilloscope as part of transmission line. In this mode it replace part of circuit - receiver. (Once again - there is some probes exists that connected to 50 Ohm input, there is another story also). So, when you use 50 Ohm mode oscilloscope by never means will be 'invisible'.So, if your oscilloscope can provide USB Host functionality (from its probe) to your device - you can use 50 Ohm termination. But I suspect it can't.So turn on 1M mode, connect D+/D- to real USB Host and probe whatever line toy want with probe. And pray that 1M/15pF of oscilloscope probe will be 'invisible' enough to USB circuit. For USB 1.1 it will be, for USB 2.0 probably not, for USB 3.x - definitely not

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 04, 2024, 11:47:58 pm
content: Here is soothing to elucidate XVR's pointSay you wish to evaluate the output of a signal generator, here specifically the square wave.... If you "terminate" the signal into a 1 megohm scope input there will be reflections and the square wave will be distorted, If you directly connect the 50 ohm square wave output of the signal generator into the 50 Ohm input of your scope, there will be theoretically no reflection and the square wave will appear nice and squareYou do this with a 50 ohm coax jumper with BNC connectors and attach it directly to the scope without a probe.Your applied signal should be obviously small in this case so as to not hurt your scope input or attenuators....you will get a nice view of the generator's output both in form and in amplitude, as limited by your scopes accuracy.The scope PROBE is specifically made to "match" the 1 megohm input of your scope, the probe and its lead are made for this. But the use of it is not to "terminate" or act as a good load for the signal, it "picks off" a small portion of the signal to avoid changing the signal in the process, but the signal needs to be "terminated" into a load for the circuit to perform effectively. This means connected to another USB device.USB lines are around 100 ohms (Google this) and will not give you clean signals terminated into a 50 ohm scope

author: jonpaul
author_rank: Super Contributor
timestamp: April 05, 2024, 12:53:38 am
content: https://w140.com/tekwiki/wiki/Concepts_Serieshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfhttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfRead and learn!Enjoy!Jon

author: donlisms
author_rank: Frequent Contributor
timestamp: April 05, 2024, 03:10:37 am
content: I think it's good to think in terms of a voltage divider, made of the source impedance and the input impedance. The source voltage feeds the divider, and the oscilloscope sees the voltage at the tap.If the source impedance is very high compared to the scope input impedance, most of the voltage will appear across source and very little across the scope input. This is what a 10x probe does, and that's often good; this is like two dividers chained together. If the high/low ratio is not intended, it might be bad, because the scope never sees much voltage.If the scope impedance is high, two things happen: much of the voltage is dropped across the input impedance, and the current required to do that is low. Those are usually both good things.

author: NY2KW
author_rank: Regular Contributor
timestamp: April 05, 2024, 12:44:38 pm
content: With a typical 1Meg input scope, I always thought you can 'cheat' and use a BNC T-connector with a 50ohm load on one end and 50ohm coax to your DUT and set scope to 1:1. Isn't that how higher-end scopes work that offer either 50 or 1Meg by switching in a 50ohm resistor internally?Jerry NY2KW

author: wasedadoc
author_rank: Super Contributor
timestamp: April 05, 2024, 12:50:36 pm
content: Quote from: NY2KW on April 05, 2024, 12:44:38 pmYes you can do that. Not 'cheating' unless you need high precision and/or high frequencies. Not all scopes just switch in a 50 Ohm resistor. Some have two input buffer stages and switch between them.

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2024, 12:57:31 pm
content: Quote from: NY2KW on April 05, 2024, 12:44:38 pmYes, you can do that - but you end up with 50ohm//15pF (or 20pF, depending on your scope). That mismatch causes a VSWR, and as a radio amateur I'm sure you understand the consequences of that. Adding a 3dB pad attenuator reduces the consequences, or course.The Tek 485 has two input attenuators, one the traditional 1KMohm, the other 50ohm. A relay switches between them, and disconnects the 50ohm attenuator if the power is too high.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 05, 2024, 01:12:17 pm
content: Quote from: tggzzz on April 05, 2024, 12:57:31 pmTypo.

author: tggzzz
author_rank: Super Contributor
timestamp: April 05, 2024, 01:32:12 pm
content: Quote from: wasedadoc on April 05, 2024, 01:12:17 pmCorrectedMind you, 1kohm is still an order of magnitude higher than the tip impedance of a "10Mohm" probe