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IC advice to create a regolable oscillator from 0 to 500kHz
Yaroooo:
Hello,
I'm looking to create a circuit that allow me a square clock generator that is regolable. I've found 555, but max. clock is 100kHz, AD654 but seems not suitable.
What I'm looking for is 2 selectable Potenziometers that allow 1 clock output regulation. First, if selected, an output from 0-100Hz and another that goes from 10kHz to 500kHz.
Best regards!
Zero999:
The ordinary 555 can go up to 500kHz, the ICM7555 1MHz and the TLC555 2MHz.
https://www.st.com/resource/en/datasheet/cd00000479.pdf
https://www.renesas.com/eu/en/www/doc/datasheet/icm7555-56.pdf
http://www.ti.com/lit/ds/symlink/tlc555.pdf
Logic gates such as the 74HC14 can work at even higher frequencies, but are less accurate.
What are your tolerance/accuracy requirements?
Yaroooo:
I'd like to generate a 50% duty cycle, any particular tolerance or accuracy. It's just for debug propuses so, if it's not a perfect 50% duty or frequency is not stable, it's not a problem.
That's why I'm looking for a simple solution. I've seen NE555 from ST have a good documentation.
To be able to generate a 500kHz as example, I should look at 4.5 50% duty cycle oscillator, where is written:
t1 = 0.693*RA*C
f=1/(t1*2)
I calculate t1 = 0.693*180ohm*0.01uF = 1.25uS than a freq. of 400kHz?
Seems strange to me since small resistors. Can you kindly check if my logic is correct?
SiliconWizard:
One simple option to get an accurate 50% duty cycle is to put a D-type flip flop (such as the 74HC74) after the oscillator to divide the output frequency by 2, which would give you a 50% duty cycle with no effort. As said above, you can find 555's that can work up to 2MHz, so getting 500kHz would be no problem, or build an oscillator around a Schmitt trigger inverter.
The tricky part will be the wide frequency range you're targetting. Doing that with just one simple RC oscillator will be challenging. If you want your oscillator to be able to switch seemlessly between the two ranges it may not be that simple. Now if you can accept glitches during the switching, that's just a matter of using a switch and switching between two capacitors for instance.
Zero999:
--- Quote from: Yaroooo on March 29, 2019, 02:06:05 pm ---I'd like to generate a 50% duty cycle, any particular tolerance or accuracy. It's just for debug propuses so, if it's not a perfect 50% duty or frequency is not stable, it's not a problem.
That's why I'm looking for a simple solution. I've seen NE555 from ST have a good documentation.
To be able to generate a 500kHz as example, I should look at 4.5 50% duty cycle oscillator, where is written:
t1 = 0.693*RA*C
f=1/(t1*2)
I calculate t1 = 0.693*180ohm*0.01uF = 1.25uS than a freq. of 400kHz?
Seems strange to me since small resistors. Can you kindly check if my logic is correct?
--- End quote ---
Try reducing C by a factor of 10.
That circuit will not give a stable duty cycle, if a variable resistor is used, because the duty cycle is dependant on the ratio of two resistors.
--- Quote from: SiliconWizard on March 29, 2019, 03:38:32 pm ---One simple option to get an accurate 50% duty cycle is to put a D-type flip flop (such as the 74HC74) after the oscillator to divide the output frequency by 2, which would give you a 50% duty cycle with no effort. As said above, you can find 555's that can work up to 2MHz, so getting 500kHz would be no problem, or build an oscillator around a Schmitt trigger inverter.
The tricky part will be the wide frequency range you're targetting. Doing that with just one simple RC oscillator will be challenging. If you want your oscillator to be able to switch seemlessly between the two ranges it may not be that simple. Now if you can accept glitches during the switching, that's just a matter of using a switch and switching between two capacitors for instance.
--- End quote ---
If you're going to do that, you might as well use one half of the 74HC74 as an astable. Here's a circuit I whipped up in another thread. Change the resistor and capacitor values to alter the frequency.
https://www.eevblog.com/forum/beginners/portable-low-frequency-square-wave-generator-circuit/msg2155480/#msg2155480
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