I need to block completely a changing signal when the frequency reaches 25Hz... If I use a filter, however, it's not going to be blocked immediately but will be attenuated eventually due to its frequency response.
Is there any other method to block a signal when it immediately reaches a cut-off frequency?
Brick wall filters at that low frequency may be hard to implement, big coils and caps etc.
Use DSP, a brick-wall FIR filter for that frequency could be implemented quite easily. I'd use a dsPIC because I have all the necessary tools including the (somewhat expensive) filter design software.
Depends on what you mean by 'block'. Speaker manufacturers say their speakers have a bandwidth od 20-20K Hz, but they put the "at 10 dB down points" in fine print. Just claim that attenuating the 25Hz signal by 12dB within one octave of 25Hz has blocked it sufficiently for your application, and use a Sallen and Key voltage controlled voltage source topology for the filter. 2 Rs and 2 Cs and one opamp. Done. I can show you how to do it in c also.
Depends what you mean by 'block' and 'changing'.
measure the frequency with a timer, when it reaches 25Hz switch the signal off with an analog switch.
GK
Is there a single signal for which you want an on/off indication based on frequency? Or, do you want the signal to be present if below the critical frequency and absent, if above? Or, is it a mix of signals?
If it is a single frequency, then period/frequency count. Make a logic signal depending on the period/frequency value. If necessary, use this signal to control an analog switch that passes or blocks the signal.
Jim
What is the SNR, what is the duty cycle (if digital)? Is the frequency stable or does it change rapidly?
What is the frequency spectrum of the signal?
