Tone/Frequency Detector (Decoder)

We can use tone/frequency detector (decoder) to detect the presence of a signal with a certain tone. The output of the circuit will be active if the signal is the same tone with the tone of a series of internal oscillator. LM567 is one of decoder that we can use.

The LM567 and LM567C are general purpose tone decoders. Both are designed to provide a saturated transistor switch to ground when an input signal is present within the pass band. The circuit consists of an I and Q detector. Both, I and Q detector are driven by a voltage controlled oscillator which determines the center frequency of the decoder. To independently set center frequency, bandwidth and output delay, external components are used.

Oscillator internal frequency of the circuit is:

f = 1/(1.1xR1xC1)

The output will be low level logic if the input signal has a minimum voltage of about 25 mV and the frequency around the frequency of internal oscillator.