LM567CN tone decoder

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mikeb
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LM567CN tone decoder

Post by mikeb » Sat Apr 03, 2004 8:34 pm

I'm building a unit that recieves preset frequencys that are to be decoded by the lm567cn tone decoder, I.m using a function generator to create the decoder frequencys at the decoder input, that the transmitter will send to the reciever/decoder unit. I can get the tone decoder to work but,it appears that instead of 1 band of frequencies the decoder decodes at different frequencies.. For instance one decoder is set to detect frequencys centered around 400hz the unit responds at 400 hz and rejects frequency at about 500hz which is pretty consisent with normal operation, but when the frequency is increased to about 600hz the decoder thats set to detect at 400hz will detect this band as well, then again at about 700hz the decoder rejects everything from 700hz and above. any help with a possible cause will be apreciated.

mikeb
Posts: 85
Joined: Fri Jul 12, 2002 1:01 am
Location: calif.
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Re: LM567CN tone decoder

Post by mikeb » Sat Apr 03, 2004 8:53 pm

I'm also wondering if maybe the passband is not set properly, which from the data sheet is set with the ext. caps at pin1 and pin2.<p>At pin1 I have a 2.2uf cap, at pin 2 I have a 1uf cap, pin3 is the input, pin4 is vcc at +5volts, pin5 goes to pin6 thru a 2.7k resistor with a 1uf cap from pin6 to gnd, pin7 is at gnd and pin8 is pulled high thru a led and 1k resistor. the cathode of the led connected to pin 8.

rshayes
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Re: LM567CN tone decoder

Post by rshayes » Sat Apr 03, 2004 10:46 pm

The LM567 operates by phase locking a variable oscillator to the input signal using a phase detector (the Q phase detector) of the balanced modulator type. The oscillator frequency is varied until the DC level of the Q phase detector is zero. At this point, the oscillator frequency is equal to the input signal frequency and locked at a 90 degree angle with respect to the input.<p>The oscillator is shifted 90 degrees in phase and applied to the I balanced modulator. This is in phase with the input signal, resulting in maximum output from the I balanced modulator. This maximum output condition triggers the output signal.<p>With sine wave signals and linear balanced modulators, there would only be a response at one frequency. The oscillator in the LM567 probably generates square waves, and the balanced modulators are arranged to clip on three out of four ports. In effect the balanced modulators are using square waves. This means that they will respond at a lower level to signals that are 3, 5, 7, or higher times the oscillator frequency. If the input is also a square wave, responses are also possible at frequencies such as 3/5, 3/7, 5/3, 7/3, and so forth. Some of these responses may be strong enough to create an output signal. If the input signal does not have a 50% duty cycle, responses are also possible at 3/2, 2/3, 3/4, 4/3, and so forth.<p>Using a low level sine wave as an input signal may reduce these responses enough to enable reliable discrimination of a single frequency.

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