Design a Filter

This tutorial walks through a complete filter design workflow using mcltspice tools. You will create a circuit from a template, simulate it, measure the bandwidth, and adjust component values to hit a target specification.

Goal

Design a 1st-order RC lowpass filter with a -3dB cutoff at 10 kHz.

Workflow

1. Create the circuit

   Use create_from_template to generate an RC lowpass filter netlist:

   {
     "template_name": "rc_lowpass",
     "params": {
       "r": "1k",
       "c": "15n"
     }
   }

   The theoretical cutoff is f_c = 1 / (2 * pi * R * C) = 1 / (2 * pi * 1000 * 15e-9) = 10,610 Hz. Close to 10 kHz.

2. Simulate

   Run AC analysis with simulate_netlist on the generated .cir file. The template includes an .ac dec 100 1 1meg directive by default.

3. Measure bandwidth

   Use measure_bandwidth on the .raw file with signal V(out):

   {
     "raw_file_path": "/tmp/rc_lowpass.raw",
     "signal_name": "V(out)"
   }

   This returns the -3dB frequency, which should be near 10.6 kHz.

4. Iterate if needed

   If the cutoff is off-target, use tune_circuit to get suggestions:

   {
     "template": "rc_lowpass",
     "params": {"r": "1k", "c": "15n"},
     "targets": {"bandwidth_hz": "~10000"},
     "signal": "V(out)"
   }

   The tool will measure the actual bandwidth and suggest adjusted component values.

5. Visualize

   Generate a Bode plot with plot_waveform:

   {
     "raw_file": "/tmp/rc_lowpass.raw",
     "signal": "V(out)",
     "plot_type": "bode"
   }

   This creates an SVG showing magnitude and phase vs. frequency.

Tip

The design_filter prompt automates this entire workflow. Try it as a conversation starter and mcltspice will walk you through each step interactively.

Key takeaway

The RC lowpass cutoff frequency is f_c = 1 / (2 * pi * R * C). To lower the cutoff, increase R or C. To raise it, decrease them. The tune_circuit tool does this math for you and suggests standard component values from E12/E24 series.