Need Help! Question about cutoff frequency measurement - Page 1
Forum section: electronics / beginners

author: easywanner
author_rank: Newbie
timestamp: April 12, 2013, 01:07:51 am
content: Dear all,I'm a physics major student working on material science. Basically we have a material which may behave as frequency filter, so I build a simple circuit and want to do some measurement to test between 100Hz and 1MHz.At the beginning, I used a function generator (less than 10V output) as output source and connect it to a standard resistor, so one end of the resistor is connected to source and the other end of the resistor is connected to an oscillascope to measure the voltage. In theory, the AC signal should travel through a pure resistor. However, since our material has a relatively high resistance in DC case, around 1M to 10M ohm, the oscillascope has an input imdepence of 1Mohm, so basically I got the wrong voltage signal from the oscillascope due to this impedance issue.So to solve that problem, I use an NI-DAQ data acquisitiion device to replace the oscillascope because it's input impedence is >10Gohm as in specifications (it says input impedence is ">10Gohm in parallel with 100pF", which I'm not sure what this exactly means). By doing that, it works very well at low frequency range (less than 1kHz), which means if the source voltage is 2V, I will get a 2V reading with the DAQ device. However, the voltage reading decays very quickly as source frequency is larger than 1kHz, seems to have the same impedence matching issue again. So my experiments can not be continued.I'm really a beginner to these impedence knowledges, so I want to ask: Is there a way to overcome this? Can I get a "true" voltage reading for a AC signal passing through a sample of serveral Mohm resistance? Is there anyway to do this measurement with my current hardware, or is there some available device in market to accomplish that?Thank you all very much.

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 01:13:35 am
content: Quote from: easywanner on April 12, 2013, 01:07:51 amWell there's your problem! A capacitance (the 100pF) is an impedance that changes with frequency - 1/(2*pi*f*C). Your input impedance is >10Gohm at DC, but at 1kHz, it's down to 1.6 Meg, and up at 1 MHz it's only 1.6 kilohm! Use an oscilloscope with a 10x attenuator probe.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 12, 2013, 01:19:59 am
content: The "10 G in parallel with 100 pf" is the problem. Capacitors have a frequency dependent impedance of 1j/(2*pi*f*C). At 1 kHz, that is 1.6 megaohm. At 1 MHz, that is 1.6 kiloohm. Also, the 100 pF is probably optimistic. That may be the input impedance of the card, but any wiring or breakout boxes between the card input and your device under test will add capacitance, making the problem worse.To start with, you would be much better off using a 10x scope probe. They typically have an impedance of 10 meagohm in parallel with 12 pF. The 10 megaohm still loads your circuit, but you just use the voltage divider formula to find out what the resistance of your sample is.A function generator + scope is a good way to do a rough measurement. If you want better accuracy, noise immunity, and frequency resolution, a lock-in amplifier will be better.

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 01:21:10 am
content: Copycat!

author: easywanner
author_rank: Newbie
timestamp: April 12, 2013, 01:23:10 am
content: Well there's your problem! A capacitance (the 100pF) is an impedance that changes with frequency - 1/(2*pi*f*C). Your input impedance is >10Gohm at DC, but at 1kHz, it's down to 1.6 Meg, and up at 1 MHz it's only 1.6 kilohm! Use an oscilloscope with a 10x attenuator probe.Thanks for the reply! Now I get why.Would you please explained more about "oscilloscope with a 10x attenuator probe" and how it might going to work out? The oscilloscope I used is TDS420 (old one...). Thanks!

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 01:28:45 am
content: Quote from: easywanner on April 12, 2013, 01:23:10 amA 10x probe isn't specialized equipment - there's a good chance that the probes you're using are already 10x. It will be labeled somewhere in that case, and probably have a little adjustment screw as well. You'll have toadjust thisfor best results. Some (usually cheaper) probes are switchable, just switch it into 10x mode. If you don't have a 10x probe, they're really easy to find.It has a series resistance that isolates the circuit from the loading effect of the input capacitance, and on top of that just has low input capacitance in general. Even the cheap ones can handle 1 MHz with no problem. It divides the signal by ten - if it's an official Tektronix one, the scope may automatically compensate for that, or it might not - you'll have to test this. I've never used the TDS420 but I imagine there's an option in a menu somewhere to tell the scope you're using a 10x probe (with an analog scope you usually have to just multiply by ten by yourself).

author: easywanner
author_rank: Newbie
timestamp: April 12, 2013, 01:41:41 am
content: A 10x probe isn't specialized equipment - there's a good chance that the probes you're using are already 10x. It will be labeled somewhere in that case, and probably have a little adjustment screw as well. You'll have toadjust thisfor best results. Some (usually cheaper) probes are switchable, just switch it into 10x mode. If you don't have a 10x probe, they're really easy to find.It has a series resistance that isolates the circuit from the loading effect of the input capacitance, and on top of that just has low input capacitance in general. Even the cheap ones can handle 1 MHz with no problem. It divides the signal by ten - if it's an official Tektronix one, the scope may automatically compensate for that, or it might not - you'll have to test this. I've never used the TDS420 but I imagine there's an option in a menu somewhere to tell the scope you're using a 10x probe (with an analog scope you usually have to just multiply by ten by yourself).Thank a lot! I will try that out.

author: easywanner
author_rank: Newbie
timestamp: April 12, 2013, 01:43:26 am
content: A function generator + scope is a good way to do a rough measurement. If you want better accuracy, noise immunity, and frequency resolution, a lock-in amplifier will be better.Thanks for the reply! I will try 10x first, if that does not work, I might ask for the remaining ways! Thanks again.

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 01:44:00 am
content: Read this too.

author: c4757p
author_rank: Super Contributor
timestamp: April 12, 2013, 01:45:32 am
content: Why do you keep removing the opening [ quote ] tag?

author: mrflibble
author_rank: Super Contributor
timestamp: April 12, 2013, 02:09:10 am
content: Why do you keep removing the opening [ quote ] tag?I'm pretty sure he has a totally reasonable explanation for that.

author: easywanner
author_rank: Newbie
timestamp: April 12, 2013, 03:15:50 am
content: Quote from: c4757p on April 12, 2013, 01:45:32 amSry. My mouse is broken...wont happen again

author: ejeffrey
author_rank: Super Contributor
timestamp: April 12, 2013, 06:15:32 am
content: Quote from: c4757p on April 12, 2013, 01:28:45 amHe said he was a physics student. Chances are he is connecting directly with BNC cables and the probes that came with the scope very well may be in the original packaging on a shelf somewhere next to the manual and software installer CD. I rarely use probes myself, and I know plenty of people who don't really know what that 'bag of wires' that came with their scope is for.