what Undesired frequencies are allowed if the filter response is droppy

Hello,

The circuit here https://embedded-lab.com/blog/?p=5508 uses simple filters of highpass and lowpass for limiting the frequencies within the limit of 0.7Hz to 2.4Hz.But can someone explain me the below
1)what are the lower frequencies that are allowed below 0.7Hz, if the filter was designed to have a cutoff frequency of(to allow frequencies below) 0.7Hz.
2) what are the other higher frequencies that are allowed above 2.4Hz, if the filter was designed to have a cutoff frequency of(to allow frequencies below) 2.4Hz.

Thanks.

We talked about those simple filters before.

The highpass filter has two passive RC filters. Each RC filter has a -3dB cutoff at 0.72Hz and the amplitude drops at 6dB (half the input level) per octave.
Then both filters produce a cutoff of -6dB at 0.72Hz with a droopy corner and the amplitude drops at -12dB per octave.
If the input to the filters is 1V then 0.72Hz is 500mV, 0.36Hz is 250mV, 0.18Hz is 62.5mV and 0.09Hz is 15.6mV etc.

The lowpass filter is also simply two simple RC filters, each with a -3dB cutoff at 2.35Hz. Therefore both filters produce -6dB (half the input level) at 2.35Hz.
If the input is 1V then 2.35Hz is 500mV, 4.7Hz is 250mV, 9.4hz is 62.5mV and 18.8hz is 15.6mV etc.

The simple filter frequencies are close together so they affect each other and reduce the maximum output around 1.6Hz.

I really didn't understood of what frequencies are allowed that fall outside limit of 0.7 to 2.34Hz.
Can you pelase explain a bit on about this.

Thanks.

Frequencies are not "allowed". Instead the higher and lower frequencies are attenuated at -12dB per octave.
Simple filters do not "stop" higher and lower frequencies.

At filters we do not speak in terms of allowed/not allowed frequencies but only about more/less atenuation of signal at different frequencies. Only ideal filter can totally reject signal frequencies outside it's pass band but such filter does not exists.

A switched-capacitor lowpass filter IC has many "RC networks" and can attenuate the harmonics of a squarewave so that the resulting sinewave has very low distortion.