EMRFD Message Archive 10792

Message Date From Subject
10792 2015-03-08 18:56:21 bob_ledoux NE567 Tone Decoder for Receiver Filter
I've noticed this IC has been used in some Morse Code decoders.  Has anyone experimented with the NE567 for other audio filtering functions?

10793 2015-03-09 01:22:28 bubnikv Re: NE567 Tone Decoder for Receiver Filter

NE567 is a PLL tone detector. Its function is different from a narrow band filter. I cannot imagine using it for audio purposes.

73, Vojtech OK1IAK
10794 2015-03-09 09:18:11 Andy Re: NE567 Tone Decoder for Receiver Filter
Many years ago, it seemed everyone was using the 567 for tone decoding.  From touch-tone decoders to RTTY to who knows what else.  I never caught that massive 567 craze.  But I suppose there's no reason why you couldn't still use it.  Not necessarily as a "filter" where output signal = filtered input signal; but as a narrowband tone detector, sure.  My vague recollection is that they could get false triggers so you had to watch out for that.

There's got to be huge collections of 567 circuits for audio-range tone decoders, out there somewhere.  Though most of that craze was pre-Internet.


10796 2015-03-09 11:20:25 bob_ledoux Re: NE567 Tone Decoder for Receiver Filter
The question arose because some CW decoders use the chip to square up the incoming signal, and place it within binary hi/low voltage range for the microprocessor.

Looking at the documentation I see the chip is capable of discriminating signals within 1% of the selected frequency. However, as the discrimination range gets more narrow, the number of input cycles to achieve high state increases.  By setting the discrimination bandwidth to 15% 10 cycles are required to achieve the high state.  So at 1000Hz incoming frequency, the bandwidth is 150Hz and 10 key down cycles have to pass for the output signal to go to high state.

This looks like a filter.  If the signal is tuned to 1000Hz, only frequencies between 925 and 1075 will generate an output.   The bandwidth appears to be altered by signal strength, but I've seen no chart that displays the traditional db attenuation versus frequency, commonly found for active and crystal filters.

The signal to noise ratio impacts the capture ability for the decoder.  This may be an overriding consideration in its utility.

My original questions was simple:  Does anyone have experience in experimenting with tone decoders for filter functions?
10797 2015-03-09 11:56:24 roelofndb Re: NE567 Tone Decoder for Receiver Filter

The only filter application for the NE567 I have seen is a circuit
in which the NE567 keys a tone oscillator. This only works with
strong signals and signal to noise ratio of the audio is perfect of

However, it can not compete in anyway with the human brain ability
to pull out weak signals!

I have also used one triggering a LED as a visual aid in copying

Roelof, pa0rdt
10798 2015-03-09 12:22:06 arfghans Re: NE567 Tone Decoder for Receiver Filter
The 567 is a simple PLL with a lock detector.  Like any PLL, it is not easily characterized as a classical filter at all. Instead, we refer to a couple of key parameters: pull-in range and lock range. Pull-in is a range of frequencies with respect to the center (free-running VCO) frequency where the loop may acquire lock. Once locked, it will maintain lock over a frequency range defined by the lock range, which is normally larger than the pull-in range. Acquisition or lock time is determined by the time constant of the internal lowpass filter. Pull-in range and lock range are inversely related to lock time: A narrow lock range implies a slower lock time. 

Performance in the face of poor SNR is a much more complicated topic, but to the first order, a narrower lock range PLL will generally perform better with a poor SNR.

My first RTTY terminal unit (1974) was a relative of the 567, the NE565, and it was FABULOUS with poor SNR. That's because they got the loop tuning right in the old Signetics app note.

Gary, NA6O
10799 2015-03-09 15:49:26 Alberto I2PHD Re: NE567 Tone Decoder for Receiver Filter
10800 2015-03-09 16:03:52 Dana Myers Re: NE567 Tone Decoder for Receiver Filter