Can Scope capacitance Cause low pass effect to the signal - Page 1
Forum section: electronics / projects

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: May 17, 2024, 07:16:49 am
content: I have alternating current flowing through a 100ohm Resistor, I am measuring the voltage drop across this resistor on a Keysight Infinumm Scope. My Scope Capacitance is 14pF at 1M Ohm input impedance. I am using BNC cable to connect to this resistor, and using 1M input.Whenever I increase my signal frequency the amplitude decress on the scope. I want to know if scope capacitance is culprit for this effect.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 17, 2024, 07:23:44 am
content: It could be one of several causes. You have given no clue about what frequency you are observing.

author: wraper
author_rank: Unknown
timestamp: May 17, 2024, 07:30:53 am
content: Signal reflection in coax cable most likely is the culprit. You cannot simply feed high frequency signal into coax with no load on the output and expect it to come out unaffected.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: May 17, 2024, 08:13:45 am
content: It ranges from few Kilohertz to 10 Megahertz,I am also using 20Mhz HW limit of the scope to filter some higher harmonics.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: May 17, 2024, 08:14:42 am
content: So , there nothing to do with the scope capacitance, I should check the BNC cable

author: 2N3055
author_rank: Super Contributor
timestamp: May 17, 2024, 08:26:05 am
content: Coax cable you are using can have 100pf/m of length.So you might have 120pF in parallel with 100ΩWhich is 130ish Ω at 10 MHz... so together cca 55Ω. I would say that makes a difference.But you can be clever.Make shunt 50Ω, put piece of 50Ω coax, and terminate with 50Ω scopeside.That way you have properly terminated transmission line and you keep frequency response.Also search internet for term "coaxial shunt" and "noninductive shunt". Read.Learn to use LTSpice. Then you can model what is going on in it.

author: mikerj
author_rank: Super Contributor
timestamp: May 17, 2024, 10:31:06 am
content: Are you certain the output amplitude of your signal source is flat over this frequency range?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: May 17, 2024, 12:23:24 pm
content: I am oberving slightly better response if I am using a high frequency 18Ghz sma cable with bnc adapters. Earlier I was using a lower frequency generic bnc cable.

author: CaptDon
author_rank: Super Contributor
timestamp: May 17, 2024, 01:37:18 pm
content: All capacitance acting as a shunt to ground will effect high frequency roll off!! That is a standard electronic principle. What is the quality of your signal source?? I would not expect to need a 20MHz bandwidth filter "To get rid of higher harmonics" if the signal source is a quality sinewave to start with? I don't recall if a 20MHz bandwidth filter is 3dB or 6dB down at 20MHz but it does begin to roll off below the 20MHz spec.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 17, 2024, 02:16:06 pm
content: Transmission lines, e.g. coax, require termination to work correctly. 50 ohm line=50 ohm termination at the BNC as an add on terminator, not 1M. Scope probes use a special lossy cable to avoid this problem. For more info on using coax as a probe search for "Z0 probe"These films explain the behavior very well.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: May 17, 2024, 02:20:17 pm
content: Quote from: Arsh ahmad on May 17, 2024, 12:23:24 pmTry turn swich on probe to 10x

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2024, 03:06:12 pm
content: Quote from: Arsh ahmad on May 17, 2024, 12:23:24 pmWelcome to Probe Loading 101!You mention a 'current' through a 100R resistor, but you don't mention the source impedance or amplitude of your signal. If your source is 50R, then ~100pF at 10MHz is going to be noticeable, if it is truly a current source (high impedance) then it will be worse. The effect you are seeing is entirely due to the combined capacitance of the scope and the BNC cable. Making the cable smaller or shorter may reduce this a bit but it is not an appropriate way of dealing with this situtation. The usual method would be to use a properly compensated x10 probe with an input capacitance low enough to not cause significant errors over your bandwidth range. 8-12pF is a common, easily acheivable number for a regular x10 probe and that would give you a pretty small error at 10MHz.Transmission line effects are not important here. 10MHz has a wavelength of ~30 meters in free space and ~20 meters in cable, so for a cable of 2 meters or less you can pretty much ignore the subject entirely.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 17, 2024, 03:17:43 pm
content: OP has made no mention of using a probe.

author: bdunham7
author_rank: Super Contributor
timestamp: May 17, 2024, 03:25:48 pm
content: Quote from: wasedadoc on May 17, 2024, 03:17:43 pmYes, that's the problem!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 19, 2024, 10:37:15 am
content: A quote I've heard often is that an x1 1M Ohm input of a scope has an bandwidth of around 6MHz. That would mean that at your maximum of 10MHz, the signal amplitude on the scope would only be half of what it is in reality, and you may see distortions too.

author: CaptDon
author_rank: Super Contributor
timestamp: May 19, 2024, 01:07:43 pm
content: DoctorP, that is true of many probes, but not of the scope input.