mstechca

Is there anyway I can use 1 or 2 555's to make a PLL circuit?

I want to take the LED output of my receiver, and compare it with a set of pulses coming from one 555 timer at a certain frequency. Probably 1Hz - 1Khz. I want to be able to obtain a high or low output based on whether or not the pulses are in sync or not.

I think 555's will be helpful because it has a reset pin which can reset the pulses.

can anyone help me out on this?

thanks.

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Russlk

You can use the 555 as a VCO using pin 5 as the control input. Compare the signals in an XOR, filter and apply to pin 5. I will try to come up with a circuit. Hmmm - 1 Hz to 1kHz is too wide a range, why do you need that?

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audioguru

At Digikey.ca, a CD4046 PLL with detectors IC costs $.71 and does everthing you want and more.
LM555 ICs cost $.63 and you need other stuff with them to make a frequency detector.
An LM565 frequency detector IC costs a small fortune but will also do what you want.

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Russlk

The 565 VCO is non-linear, so it will not cover a wide range. The 4046 VCO is linear and will tune a range of 20:1. You will need a different circuit to tune a wider range.

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audioguru

Hi Russ,
I think MStechca wants to decode frequency-shift keying that has different audio frequencies of no more than 3:1.

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Russlk

That is a different story. The 565 was made for that.

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mstechca

I just want to detect when two pulses are completely in sync. I would rather use 555 timers, or even transistors, just because I want the smallest PLL. A digital PLL is ok. I specified a range because the input frequencies can be between 1Hz and 1Khz.

For example, Let's say I have a squarewave signal of 1Hz into one input, and a squarewave signal of 1Khz into another input. I want to create an output that indicates that these signals are not equal.

Also, Let's say I have a squarewave signal of 1Hz into one input, and a squarewave signal of 1Hz into another input. I want to create an output that indicates that these signals are equal.

The output could be as simple as an LED.

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audioguru

Why? Two pulses will never be exactly the same unless they come from the same oscillator, or if the oscillator in a PLL locks onto the input. Two separate oscillators will be in sync only for a moment then there could be a fairly long wait for them to be in sync again as their frequencies and phasing vary.

1Hz is ridiculous. It would take a very long time to detect it and for a PLL to lock onto it.

A PLL can quickly detect that the 1kHz signal is within its lock range of frequencies but the 1Hz signal will take a very long time to detect and for another PLL to determine if it is within its lock range.

They won't be equal. One will be high when the other is low and vice-versa. Their frequencies and phasing wil be slightly different. Only for a moment they will be in sync.

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Uncle $crooge

akg

just thought of a method .
assuming a sq: signal

differentiate the i/p signal on one side say low-to-high transition, so u get a spike for each transition , charge a cap: using this spike (thru a buffer),
after a fixed time , compare the voltages of the two caps , same freq , same voltage , (high precision needed for all componets)
this is a very crude method . but if there is a very large variation in freq, this could work .

But definitely and FFT would work

mstechca

Wouldn't making use of a reset function of the 555 somehow override this and make the signals more in sync?

Let's think about it.

The two inputs are the signal input and the local oscillator (LO) input.
the signal input cannot be changed and must be detected.
The LO input is the signal I am providing from the 555. I think that making use of the reset function will make the signals more in sync when the frequencies are the same.

I think I almost found the answer. I'll continue to research.

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audioguru

Using the reset function of the 555 will reset it with any input frequency, even noise.
The 1st cycle from a reset 555 oscillator is longer (lower frequency) than the rest.

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Uncle $crooge

Russlk

Since you have the input signal, why do you need another to be the same? Why not use the input signal?

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mstechca

Because one signal is remote, and the other is local. I want to check to see if they are the same. You know how they make those lost airplane model detectors.

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audioguru

The lost airplane transmitters aren't crystal controlled so aren't the same.
Some transmit slightly different beeps and others transmit an unmodulated carrier which is heard as pulses of silence on an FM radio.

I don't know why you want to see if two unrelated signals are the same. They will be slightly different.

With frequency-shift-keying you don't need good frequency accuracy of the tones. The high-pitched tone is a logic high and the low-pitched tone is a logic low. The phase detector in a PLL will be high with the high-pitched tone and will be low with the low-pitched tone if both tones are within its lockin range. The phase detector normally drives the VCO so that it locks on the input frequency. You might need to amplify the output of the phase detector so that it is high and low enough for logic.

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Russlk

If you are looking for a lost airplane, a directional antenna will be more useful.

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