Here's the equivalent in matlab:

dt = 100e-6;    %time interval
t = 0:dt:250e-3;         %sample steps
freq1 = 697; freq2 = 770; freq3 = 853;   %frequencies

%below we generate the input signal in five steps
sig(1:length(t)) = 0;
sig(1:50e-3 / dt) = sin( 2*pi/freq1 * t(1:50e-3 / dt) );
sig(50e-3/dt : 100e-3/dt) = 0;
sig(100e-3/dt : 150e-3/dt) = sin( 2*pi/freq2 * t(100e-3/dt : 150e-3/dt));
sig(150e-3/dt : 200e-3/dt) = 0;
sig(200e-3/dt : 250e-3/dt) = sin( 2*pi/freq3 * t(200e-3/dt : 250e-3/dt));

k = 1/128;   % filter constant
f_a = [1 -2*(k-1) 1 ]; f_b = [ k^2 ];  %standardized filter
coefficients for 2-pole filter

mix_sin_in = sin(2*pi*freq2*t);  %mixer input signals
mix_cos_in = cos(2*pi*freq2*t);

mixed_sin = mix_sin_in .* sig;    %mixed signals
mixed_cos = mix_cos_in .* sig;

ff_sin = filter(f_b, f_a, mixed_sin);     %do the filtering
ff_cos = filter(f_b, f_a, mixed_cos);

magsq = 4* ( ff_cos .^ 2 + ff_sin .^ 2; )
hold on
plot(t, sig, 'r-');
plot(t, magsq, 'b-');
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