Added K IIR lpf filter

project.cfg

@@ -23,6 +23,7 @@ files_verilog       = rtl/toplevel/top_generic.v
                       rtl/core/sigmadelta_sampler.v
                       rtl/core/sigmadelta_rcmodel_q15.v
                       rtl/core/mul_const.v
+                      rtl/core/lpf_iir_q15_k.v
                       rtl/arch/spartan-6/lvds_comparator.v
                       rtl/arch/spartan-6/clk_gen.v
 files_con           = boards/mimas_v1/constraints.ucf
@@ -50,6 +51,7 @@ files_verilog       = sim/tb/tb_nco_q15.v
                       rtl/core/lvds_comparator.v
                       rtl/core/sigmadelta_rcmodel_q15.v
                       rtl/core/mul_const.v
+                      rtl/core/lpf_iir_q15_k.v
                       sim/overrides/sigmadelta_sampler.v
                       sim/overrides/clk_gen.v
 files_other         = rtl/util/conv.vh

rtl/core/lpf_iir_q15_k.v

@@ -0,0 +1,64 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// Low-Pass IIR Filter (Q1.15)
+// Simple first-order infinite impulse response filter, equivalent to an
+// exponential moving average. Provides an adjustable smoothing factor based
+// on parameter K.
+//
+// Implements:
+//      Y[n+1] = Y[n] + (X[n] - Y[n]) / 2^K
+//
+// This is a purely digital one-pole low-pass filter whose time constant
+// approximates that of an analog RC filter, where alpha = 1 / 2^K.
+//
+// The larger K is, the slower the filter responds (stronger smoothing).
+// The smaller K is, the faster it reacts to changes.
+//
+// parameters:
+//      -- K : filter shift factor (integer, 4..14 typical)
+//              cutoff frequency ≈ Fs / (2π * 2^K)
+//              larger K → lower cutoff
+//
+// inout:
+//      -- clk   : input clock
+//      -- rst_n : active-low reset
+//      -- x_q15 : signed 16-bit Q1.15 input sample (e.g., 0..0x7FFF)
+//      -- y_q15 : signed 16-bit Q1.15 filtered output
+//
+// Notes:
+//  - The arithmetic right shift implements division by 2^K.
+//  - Internal arithmetic is Q1.15 fixed-point with saturation
+//    to [0, 0x7FFF] (for non-negative signals).
+//  - Useful for smoothing noisy ADC / sigma-delta data streams
+//    or modeling an RC envelope follower.
+// =============================================================================
+module lpf_iir_q15_k #( 
+    parameter integer K = 10 // try 8..12; bigger = more smoothing 
+)( 
+    input wire clk, 
+    input wire rst_n, 
+    input wire signed [15:0] x_q15, // Q1.15 input (e.g., 0..0x7FFF) 
+    output reg signed [15:0] y_q15 // Q1.15 output 
+); 
+    wire signed [15:0] e_q15 = x_q15 - y_q15; 
+    wire signed [15:0] delta_q15 = e_q15 >>> K; // arithmetic shift 
+    wire signed [15:0] y_next = y_q15 + delta_q15; // clamp to [0, 0x7FFF] (handy if your signal is non-negative) 
+    
+    function signed [15:0] clamp01; 
+        input signed [15:0] v; 
+    begin
+        if (v < 16'sd0) 
+            clamp01 = 16'sd0; 
+        else if (v > 16'sh7FFF) 
+            clamp01 = 16'sh7FFF; 
+        else 
+            clamp01 = v; 
+        end 
+    endfunction 
+    
+    always @(posedge clk or negedge rst_n) begin 
+        if (!rst_n) y_q15 <= 16'sd0; 
+        else y_q15 <= clamp01(y_next); 
+    end 
+endmodule

scripts/filter_design/bode_compare_Ks_lpf_iir_q15_k.py

@@ -0,0 +1,42 @@
+import numpy as np
+import matplotlib.pyplot as plt
+
+def bode_mag_phase(Fs, K, n=4096):
+    """Return frequency, magnitude (dB), and phase (deg) for 1-pole IIR."""
+    alpha = 2.0 ** (-K)
+    a = 1.0 - alpha
+    w = np.linspace(0, np.pi, n)
+    ejw = np.exp(-1j * w)
+    H = alpha / (1 - a * ejw)
+    f = (w / (2 * np.pi)) * Fs
+    mag_db = 20 * np.log10(np.abs(H) + 1e-20)
+    phase_deg = np.unwrap(np.angle(H)) * 180 / np.pi
+    return f, mag_db, phase_deg
+
+if __name__ == "__main__":
+    Fs = 15e6
+    Ks = [10, 8, 6, 4]
+
+    fig, (ax_mag, ax_phase) = plt.subplots(2, 1, sharex=True, figsize=(8, 6))
+
+    for K in Ks:
+        f, mag_db, phase_deg = bode_mag_phase(Fs, K, n=32768)
+        ax_mag.semilogx(f, mag_db, label=f"K={K}")
+        ax_phase.semilogx(f, phase_deg, label=f"K={K}")
+
+    # ---- Styling ----
+    f_max = 40e3  # 40 kHz
+    ax_mag.set_xlim(10, f_max)         # start at 10 Hz for nicer log scaling
+    ax_mag.set_ylim(-60, 1)
+    ax_mag.set_title("Ideal Bode Response vs K")
+    ax_mag.set_ylabel("Magnitude [dB]")
+    ax_mag.grid(True, which='both')
+    ax_mag.legend()
+
+    ax_phase.set_xlabel("Frequency [Hz]")
+    ax_phase.set_ylabel("Phase [deg]")
+    ax_phase.grid(True, which='both')
+    ax_phase.set_xlim(10, f_max)
+
+    plt.tight_layout()
+    plt.show()

sim/tb/tb_sigmadelta.v

@@ -27,7 +27,6 @@ module tb_sigmadelta();
         .o(sd_o)
     );
 
-
     wire signed [15:0] sample_q15;
     sigmadelta_rcmodel_q15 rc_model(
         .clk(clk), .resetn(resetn),
@@ -35,4 +34,11 @@ module tb_sigmadelta();
         .sample_q15(sample_q15)
     );
 
+    wire signed [15:0] y_q15;
+    lpf_iir_q15_k #(8) lpf(
+        .clk(clk), .rst_n(resetn),
+        .x_q15(sample_q15),
+        .y_q15(y_q15)
+    );
+
 endmodule