#!/usr/bin/env python3
"""
2nd-order unity-gain Butterworth low-pass (Sallen-Key design) calculator + Bode plots.

- Computes R1=R2=R (user-chosen) and C1,C2 from TI normalized values:
  C1n=1.414, C2n=0.707, with scaling C = Cn/(R*ω0), ω0=2πfc

- Plots magnitude (dB) and phase (deg) of the ideal 2nd-order Butterworth:
  H(s) = ω0^2 / (s^2 + (ω0/Q)s + ω0^2), Q=1/√2

Usage:
  python3 butterworth_sallenkey_bode.py 10000 --R 10000
"""

import argparse
import math
import numpy as np
import matplotlib.pyplot as plt

C1N = 1.414
C2N = 0.707
Q_BUTTERWORTH = 1.0 / math.sqrt(2.0)

def format_si(x, unit=""):
    if x == 0:
        return f"0 {unit}".strip()
    prefixes = [
        (1e-12, "p"), (1e-9, "n"), (1e-6, "u"), (1e-3, "m"),
        (1, ""), (1e3, "k"), (1e6, "M"), (1e9, "G")
    ]
    ax = abs(x)
    # choose prefix so scaled value is ~[1..1000)
    best = min(prefixes, key=lambda p: abs(math.log10(ax/p[0])))
    scale, pref = best
    return f"{x/scale:.4g} {pref}{unit}".strip()

def butterworth_H(jw, w0, Q=Q_BUTTERWORTH):
    # H(jw) = w0^2 / ( (jw)^2 + (w0/Q)*(jw) + w0^2 )
    s = 1j * jw
    return (w0**2) / (s**2 + (w0/Q)*s + w0**2)

def main():
    ap = argparse.ArgumentParser(description="Butterworth Sallen-Key LPF (2nd order) calculator + Bode plotter.")
    ap.add_argument("fc", type=float, help="Cutoff frequency (Hz), e.g. 10000")
    ap.add_argument("--R", type=float, default=10_000.0, help="Choose R1=R2=R in ohms (default 10000)")
    ap.add_argument("--fmin", type=float, default=None, help="Plot min frequency (Hz). Default fc/100")
    ap.add_argument("--fmax", type=float, default=None, help="Plot max frequency (Hz). Default fc*100")
    ap.add_argument("--points", type=int, default=2000, help="Number of frequency points (default 2000)")
    ap.add_argument("--save", type=str, default=None, help="Save plot to PNG path instead of showing it")
    args = ap.parse_args()

    fc = args.fc
    R = args.R
    w0 = 2.0 * math.pi * fc

    # Component calculation (ideal per TI normalized values)
    C1 = C1N / (R * w0)
    C2 = C2N / (R * w0)

    print("2nd-order unity-gain Butterworth Sallen-Key LPF (ideal)")
    print(f"fc   = {format_si(fc, 'Hz')}")
    print(f"R1=R2= {format_si(R, 'ohm')}")
    print(f"C1   = {format_si(C1, 'F')}  (norm 1.414)")
    print(f"C2   = {format_si(C2, 'F')}  (norm 0.707)")
    print(f"Q    = {Q_BUTTERWORTH:.6g}")
    print()

    # Frequency axis for Bode plot
    fmin = args.fmin if args.fmin is not None else fc / 100.0
    fmax = args.fmax if args.fmax is not None else fc * 100.0
    if fmin <= 0:
        fmin = fc / 100.0

    f = np.logspace(np.log10(fmin), np.log10(fmax), args.points)
    w = 2.0 * np.pi * f

    H = butterworth_H(w, w0, Q_BUTTERWORTH)
    mag_db = 20.0 * np.log10(np.abs(H))
    phase_deg = np.unwrap(np.angle(H)) * 180.0 / np.pi

    # Plot
    fig, (ax1, ax2) = plt.subplots(2, 1, figsize=(9, 7), sharex=True)

    ax1.semilogx(f, mag_db)
    ax1.axvline(fc, linestyle="--", linewidth=1)
    ax1.axhline(-3.0, linestyle="--", linewidth=1)
    ax1.set_ylabel("Magnitude (dB)")
    ax1.grid(True, which="both", linestyle=":")

    ax2.semilogx(f, phase_deg)
    ax2.axvline(fc, linestyle="--", linewidth=1)
    ax2.set_ylabel("Phase (deg)")
    ax2.set_xlabel("Frequency (Hz)")
    ax2.grid(True, which="both", linestyle=":")

    fig.suptitle(f"2nd-order Butterworth LPF (fc={fc:g} Hz, R={R:g} Ω)")
    fig.tight_layout()

    if args.save:
        plt.savefig(args.save, dpi=200)
        print(f"Saved plot to: {args.save}")
    else:
        plt.show()

if __name__ == "__main__":
    main()