I have taken on a hobby project and I am trying to make a circuit which would be able to identify an acoustic signal with a frequency of 40kHz.
Here is the circuit of a band pass filter for this:
Here is the magnitude repsonse curve for it
The issue is:
1) What would be the best way to detect the signal for the input?
2) This circuit will be mounted on a moving object and so it would be connected to a DC power supply (battery). This means that I would need a single supply op amp which has a GBW of 928k Hz but was also recommended 3.98M Hz.
I am new to this and would usually just try to figure this out myself but I cant for the life of me find an op amp of this specification?
I would appreciate any help.
Best Answer
I recommend you do not use narrow band filtering, but a phase locked loop. In 1970's the following IC became popular as a replacement for narrowband filtering + level detector:
http://www.ti.com/product/LM567C#
It's tone decoder LM567 and it's still available.
Some pre-filtering may be needed if there's strong other signals in other frequencies. But that filter needs not to be exactly tuned to the wanted frequency, it only must attenuate enough the unwanted.
ADD due the comments:
Tone decoder doesn't detect more reliably than a high Q filter + output level detector, but tuning and changing the detection bandwidth are radically simpler. In addition this can be a single IC solution and it's designed to handle up to 500kHz.