RC Low-Pass Filter
CIRCUIT
LPFILTER1.CIR Download the SPICE file
Here’s a simple circuit for you to dive into running SPICE simulations and plotting results.
What is the purpose of this circuit? Basically it has two roles: to pass the desired low frequency signals and stop the unwanted high frequency signals.
TIME RESPONSE
Open the netlist file “lpfilter1.cir” with your SPICE simulator. Most simulators display the netlist in a text editor window. You can view, modify and save the netlist from this window.
Run a simulation. (TopSpice users click the traffic light on the toolbar; PSPICE users click the blue square.) View the transient (time) analysis at the input V(1) and output V(2). For R1=1k, C1=0.032uF and sinewave generator at 2kHz, you should see the 2 kHz sinewave (desired signal) pass through to the output V(2) except for a slight decrease in signal and slight shift in time.
Now change the sinewave frequency to 40 kHz by editing the voltage source line to look like
VS 1 0 AC 1 SIN(0 1 40KHZ)
Save the file and run another simulation. Assume this 40 kHz signal represents the undesirable noise in a system. Did the filter reduce the 40 kHz signal?
CUTOFF FREQUENCY
As stated above, the circuit has two roles: to pass the desired low frequency signals and stop the unwanted high frequency signals. But at what frequency does the filter change its behavior from passing the low ones to stopping the high ones. This is called the cut-off frequency.
For R1=1k and C1=0.032uF you get fc = 5kHz. Run a simulation. Plot the AC (frequency) sweep results for the output magnitude VM(2) and phase VP(2). What does the magnitude look like before and after 5kHz?
SPICE FILE
Download the file or copy this netlist into a text file with the *.cir extention.
LPFILTER1.CIR - SIMPLE RC LOW-PASS FILTER * VS 1 0 AC 1 SIN(0 1 2KHZ) * R1 1 2 1K C1 2 0 0.032UF * * ANALYSIS .AC DEC 5 10 10MEG .TRAN 5US 500US * VIEW RESULTS .PRINT AC VM(2) VP(2) .PLOT AC VM(2) VP(2) .PRINT TRAN V(1) V(2) .PLOT TRAN V(1) V(2) .PROBE .END
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