All About Circuits Forum Frequency sweep for a high-pass filter???
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#1
05-25-2008, 12:48 AM
 Lainya New Member Join Date: Apr 2008 Posts: 4
Frequency sweep for a high-pass filter???

We had a basic resistive-capacitive high-pass filter. In the manual frequency sweep the output voltage across the resistor increased with frequency as it was supposed to until we reached 1MHz, then the voltage dropped. What could cause this? Would the capacitor beaking down cause this? We are working with really old capacitors.
#2
05-25-2008, 01:05 AM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

Quote:
 Originally Posted by Lainya We had a basic resistive-capacitive high-pass filter. In the manual frequency sweep the output voltage across the resistor increased with frequency as it was supposed to until we reached 1MHz, then the voltage dropped. What could cause this? Would the capacitor beaking down cause this? We are working with really old capacitors.
Capacitors are not perfect as well as resistors and wires. Your capacitors may be broken but what i suspect is that after 1 MHz the stray inductance of the capacitors leads and the inductance of your wires (if they are long) cause the voltage to drop because the stray inductance in series with the capacitor form a band-pass filter.
#3
05-25-2008, 08:14 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

Can you post a schematic of the circuit so that we can study it? It is always easier to discuss a circuit when a schematic of the circuit in question is present.

hgmjr
#4
05-25-2008, 09:39 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,039

"Ideal" capacitors consist of only capacitance - but they exist only in simulations.
"Real" capacitors, along with capacitance, have relatively small parasitic values of resistance and inductance. The higher the frequency, the more apparent these parasitics become. See the attached.

All electronic components have parasitic values besides their main attribute. Even resistors have parasitic inductance and capacitance.
Attached Images
 Capacitor-IdealVsReal.PNG (7.2 KB, 5 views)
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Last edited by SgtWookie; 05-25-2008 at 11:42 PM.

 Tags filter, frequency, highpass, sweep

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