frequency modulation with 555 via pin 5

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Electricman2K5

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when using the 555 in astable mode and frequency modulation induced via a control voltage to pin 5. Does anyone know how to calculate the amount of frequency deviation caused by say 0.5V from a mic input into pin 5. There must be a constant in Hz/V but it isnt on the datasheet- does anyone know what its value is????
 
The constant would be in units of phase per volt. You can solve the problem graphicly by plotting the pin 6 voltage which runs from 2/3VCC to 1/3VCC with no modulation. The modulation varies the upper threshold which is normally internally set at 2/3VCC. The charge and discharge rate can be calculated from the R-C values, which will give the frequency.
 
Trouble is, Russ, that the lower threshold is being modulated too, so both the pin 6 and pin 2 threshold voltages are changing by the same percentage in synchronism. But, while the C is being discharged, the trigger threshold is still being changed and will have a value different from that which it had when the upper threshold was reached.

An interesting question which I had never pondered before.
 
You are right, David, I had not thought that far ahead. The trigger voltage varies 1/2 of the threshold variation and that has to be graphed also.
 
The graph of pulse width variation is interesting. I expected it to be absolutely linear like with an LM331 VCO. In fact, why use a 555 timer with its HUGE supply current spikes as a non-linear VCO when the LM331 is made to do it very much better? I have used it and its performance is excellent.

The LM331 becomes non-linear above about 20kHz and both it and the 555 have a vastly changing duty-cycle over their frequency range. Therefore for a very wide frequency range with a nearly constant duty-cycle, I have used the VCO in a CD4046 PLL IC.
For an extremely wide frequency range, I have used the VCO in a 74HC4046 but they differ greatly depending on the manufacturer. The Philips one is completely different and I think is better than the American ones, but in a PLL (its main function) it doesn't matter.
 
audioguru said:
The graph of pulse width variation is interesting. I expected it to be absolutely linear like with an LM331 VCO.
Click to expand...

Yes, me too. But in my mind the 555 would produce linear pulse width variations when it is triggered externally and not in astable mode.

The graph is strange that it plots a certain fixed percentage (5%,10%...etc) against control voltage instead of the other way round.

Hello Screech, is the graph the result of some calculations or obtained from a real test? Is the 555 in astable mode?

Thanks
 
555 timer--FM?

Im curious here because what im doing as shown by the picture below, is inputting a voltage from a microphone(lets say a sine wave of 0.5v) into pin 5 control voltage, in astable mode- as to produce FM output. Now I was wondering does the control voltage only change the pulse widths- is that frequecny modulation at all???

Anyway i wanted to know how much change (deviation from carrier) is expected from a certain control voltage.. because its not on any datasheet.

Im using a much improved 7555, has more accurate timing and much smaller spikes if any.
 

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Hi Electricman,
You have a nice 'scope. I wish I could feed a squarewave into mine and have it show a nice sinewave like yours. :shock:
He, he. Your squarewave and mic preamp give me the creeps!

National's datasheet shows PWM and Pulse Position Modulator to be about the same where the top width of the output pulses change in width, producing FM.

With a modulation signal that has a p-p value of 3/5 of the supply voltage, it has an FM p-p deviation of a little more than 2. That should be more than enough for your audio application.
 

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ppm

thanks again
Im using your mic amp cct, thats the old one!!
Is it better to describe what im doing as PPM as oppossed to FM?? because FM shifts the frequency either side of the carrier with respect to an input voltage
Here, from what i understand a falling input control voltage moves the pulses further apart and decreases the frequency

I think I understand how it works but I want to do the calculations. What i needed was a method to calculating the frequency shift caused by a voltage- this seems to have been done by Screech (previous post) in his graph but im using about 200mV, do you think his control voltage is not in volts?? It seems strange that this information is not given in the datasheets.

the scope is freeware from the internet!
thanks
 
Hi Electricman,
Signetics invented the NE555 timer. One of their techies or from another company that copies it found out it can oscillate. Another techie must have said, "look what I can do with it, by modulating its control voltage".

So it's not really an FM modulator, but for your application it is. :lol:
 
Hello Screech, is the graph the result of some calculations or obtained from a real test? Is the 555 in astable mode?
Click to expand...

I don't know. It was a graph I downloaded a long time ago.
I have no other info that came with it, thats why I added the Confused smiley with it. :?
sorry.
 
Screech said:
I Did a search for that graph.
**broken link removed**
Click to expand...

Hi Screech, there explains why the non-linearity in the above graph.

Basically, the circuit is NOT modulating the control voltage of 555 but only taken the capacitor charge/discharge voltage and compares it with a user adjustable voltage.

The 555 functions as a astable giving a fixed output frequency. As the capacitor terminal voltage is exponential, that's why the graph is non-linear.

The attached circuit diagram is aslo taken from the same website and I added the red waveform shown at point A.
 

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hi

when using the 555 in astable mode and frequency modulation induced via a control voltage to pin 5. Does anyone know how to calculate the amount of frequency deviation caused by say 0.5V from a mic input into pin 5. There must be a constant in Hz/V but it isnt on the datasheet- does anyone know what its value is????[/QUOTE]
 
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