Duty cycle of Schmitt inverter oscillator

[helix1]

I have a single Schmitt inverter with a capacitor (47nF) between the input and ground, and a resistor (6.8K) between input and output. The chip I am using is a 74HCT14 powered by a battery with Vcc at 6V. The oscillator frequency is around 6kHz.

I thought that the duty cycle of this oscillator would be 50%, as the oscillator should be charging and discharging through the same resistor, but I am finding that the output is high for roughly 3 times longer than low. Why could this be happening?

[Marco]

"74HCT14 is a hex inverter with Schmitt-trigger inputs. This device features reduced input threshold levels to allow interfacing to TTL logic levels"



Use a HC.

[helix1]

ok... sorry for being a bit slow, but even if the threshold levels are lower, shouldn't it take the same time to charge up and discharge down between the levels anyway?

[MagicSmoker]

I have a single Schmitt inverter with a capacitor (47nF) between the input and ground, and a resistor (6.8K) between input and output. The chip I am using is a 74HCT14 powered by a battery with Vcc at 6V. The oscillator frequency is around 6kHz.

I thought that the duty cycle of this oscillator would be 50%, as the oscillator should be charging and discharging through the same resistor, but I am finding that the output is high for roughly 3 times longer than low. Why could this be happening?

In addition to not using the TTL compatible version, note that the hysteresis between switching levels in Schmitt trigger logic tends to wander around a bit, which means you can't guarantee 50% duty regardless what family you use (AC, HC, VLC, whatever). If you absolutely need 50% duty - all the time, guaranteed - then use a flip-flop* to divide by 2 (you will need to set the oscillator frequency twice as high, obviously).


* - almost any kind can be used: http://www.electronics-tutorials.ws/counter/count_1.html

[Marco]

ok... sorry for being a bit slow, but even if the threshold levels are lower, shouldn't it take the same time to charge up and discharge down between the levels anyway?

The charging current is around ~(6-1.3)/R and the discharge current is ~1.3/R, so no it shouldn't.

[helix1]

thanks guys, very helpful!

[zapta]

You can use a 555 as a pretty accurate Schmitt trigger with limits at 1/2 and 2/3 VCC.

[Zero999]

I have a single Schmitt inverter with a capacitor (47nF) between the input and ground, and a resistor (6.8K) between input and output. The chip I am using is a 74HCT14 powered by a battery with Vcc at 6V. The oscillator frequency is around 6kHz.

I thought that the duty cycle of this oscillator would be 50%, as the oscillator should be charging and discharging through the same resistor, but I am finding that the output is high for roughly 3 times longer than low. Why could this be happening?

In addition to not using the TTL compatible version, note that the hysteresis between switching levels in Schmitt trigger logic tends to wander around a bit, which means you can't guarantee 50% duty regardless what family you use (AC, HC, VLC, whatever). If you absolutely need 50% duty - all the time, guaranteed - then use a flip-flop* to divide by 2 (you will need to set the oscillator frequency twice as high, obviously).


* - almost any kind can be used: http://www.electronics-tutorials.ws/counter/count_1.html

Yes, a flip-flop is a good idea.

If a dual flip-flop IC is used, such as the CD4013 or 74HC74, then rather than using a 74HC14 as an oscillator, one of the flip-flops could be configured as an astable.