lowpass filter not filtering

Test of low pass filter to filter noise at a much higher frequency than the main signal. Lowpass as I've used it here doesn't seem to filter the data.

t=0:3.13e-8:1e-3;
y=sin(2*pi*200000*t); %noise
y2=sin(2*pi*20000*t); %signal
ysum=y+y2;
plot(t,ysum);

y40khz=lowpass(ysum,40000,1/3.13e-8);
plot(t,y40khz)

A test of this with a smaller ratio of filter to sample frequency works as expected.

tslow=0:1e-5:1e-3;
y=sin(2*pi*20000*tslow);
y2=sin(2*pi*4000*tslow);
ysum=y+y2;
y5khz=lowpass(ysum,5000,1/1e-5);
plot(tslow,ysum)
hold
plot(tslow,y5khz)

Why does the first filter not seem to work while the second example does?

Answer 1

[Jan]

As expected Matlab does filter the data, of course:

t=0:3.13e-8:1e-3;
y=sin(2*pi*200000*t); %noise
y2=sin(2*pi*20000*t); %signal
ysum=y+y2;
y40khz=lowpass(ysum,40000,1/3.13e-8);
plot(t, y40khz - ysum)   % Show the difference
ylim([-1.5e-3, 1.5e-3])

According to the chosen filter parameters, the difference is small.

[Jay]

Ok, thank you. Maybe I should have asked what operations should I do to extract the signal from the noisy data in this example? I was thinking that what I had tried would pass frequencies less than 40 kHz and so remove the 200 kHz 'noise'.