Logic gate prop delay - Electronic Circuits Projects Diagrams Free

JasonL220 · 9th November 2006, 05:38 PM

i was thinking of making a oscillator similar to this,

with a 50MHz crystal, and was looking at not gates, and found these but they have a propagation delay of 8ns seconds and the clock will be oscillating witha period of 2ns. does it matter that the prop delay is longer than the period of oscillation?

thanks

**Nigel Goodwin** · 9th November 2006, 06:03 PM

I don't see any particular connection between propagation delay and the frequency of oscillation - but from a practical point, can you easily source a 50MHz fundamental crystal?.

audioguru · 9th November 2006, 07:18 PM

The crystal's datasheet has the oscillator circuit they use to test their crystals.

JasonL220 · 10th November 2006, 05:39 PM

well what i want is and oscillator that has one output line ( instead of 2 ) those diagrams looked like they would work but if the prop delay of the not gate is greater than the oscillator period would that mean that it would not oscillate properly.

**Nigel Goodwin** · 10th November 2006, 05:41 PM

Feel free to completely ignore my previous post!

evandude · 10th November 2006, 06:11 PM

Why don't you just use a 'canned' oscillator that has built-in drive circuitry? these seem to be more common for frequencies that high, and it's the simplest solution, especially considering it sounds like you are trying to reinvent the wheel and build exactly the same thing with an inverter anyway... digikey has some for under $2 apiece, like part no. 535-9183-5-ND , for example.

Sceadwian · 11th November 2006, 03:46 AM

I think the hardest part of getting an oscillator started at higher frequencies is finding a noise source for the initial feedback to start the oscillation. A long propogation delay simply means the intial noise pulse that starts the oscillator has to be longer than the delay, or the instrinsic noise at the input has to be at least that high. A simple tuned high gain amplifier stage just before the input to the logic gate would pretty well solve any difficuties with starting the oscillator.

The logic gate driven Crystal amplifier as described is going to have severe stability issues, especially with stray capacitance (a human finger is more than effective at disturbing oscillations at those frequencies) and the severe phase difference (What 700+ degree's out of phase?) that are generated.

Evandude is right, just use a canned oscillator.

hjames · 11th November 2006, 11:02 PM

These oscillator circuits are *ANALOG* and "propagation delays" don't matter. Be sure to get the unbuffered versions inverter chips that the crystal manufacturer recommends, otherwise the oscillator will be erratic.

(Normal inverter gates usually have multiple inverters in series. Crystal oscillators want 2 mosfets biased in their analog region, hence the feedback resistor. This doesn't work out too well with the buffered versions of the chips).

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9th November 2006, 05:38 PM	#1
JasonL220	Logic gate prop delay i was thinking of making a oscillator similar to this, with a 50MHz crystal, and was looking at not gates, and found these but they have a propagation delay of 8ns seconds and the clock will be oscillating witha period of 2ns. does it matter that the prop delay is longer than the period of oscillation? thanks

9th November 2006, 06:03 PM	#2
Nigel Goodwin	I don't see any particular connection between propagation delay and the frequency of oscillation - but from a practical point, can you easily source a 50MHz fundamental crystal?. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

9th November 2006, 07:18 PM	#3
audioguru	The crystal's datasheet has the oscillator circuit they use to test their crystals. __________________ Uncle $crooge

10th November 2006, 05:39 PM	#4
JasonL220	well what i want is and oscillator that has one output line ( instead of 2 ) those diagrams looked like they would work but if the prop delay of the not gate is greater than the oscillator period would that mean that it would not oscillate properly.

10th November 2006, 05:41 PM	#5
Nigel Goodwin	Feel free to completely ignore my previous post!

10th November 2006, 06:11 PM	#6
evandude	Why don't you just use a 'canned' oscillator that has built-in drive circuitry? these seem to be more common for frequencies that high, and it's the simplest solution, especially considering it sounds like you are trying to reinvent the wheel and build exactly the same thing with an inverter anyway... digikey has some for under $2 apiece, like part no. 535-9183-5-ND , for example. __________________ EEgeek.net Last edited by evandude; 10th November 2006 at 06:14 PM.

11th November 2006, 03:46 AM	#7
Sceadwian	I think the hardest part of getting an oscillator started at higher frequencies is finding a noise source for the initial feedback to start the oscillation. A long propogation delay simply means the intial noise pulse that starts the oscillator has to be longer than the delay, or the instrinsic noise at the input has to be at least that high. A simple tuned high gain amplifier stage just before the input to the logic gate would pretty well solve any difficuties with starting the oscillator. The logic gate driven Crystal amplifier as described is going to have severe stability issues, especially with stray capacitance (a human finger is more than effective at disturbing oscillations at those frequencies) and the severe phase difference (What 700+ degree's out of phase?) that are generated. Evandude is right, just use a canned oscillator. Last edited by Sceadwian; 11th November 2006 at 03:52 AM.

11th November 2006, 11:02 PM	#8
hjames	These oscillator circuits are ANALOG and "propagation delays" don't matter. Be sure to get the unbuffered versions inverter chips that the crystal manufacturer recommends, otherwise the oscillator will be erratic. (Normal inverter gates usually have multiple inverters in series. Crystal oscillators want 2 mosfets biased in their analog region, hence the feedback resistor. This doesn't work out too well with the buffered versions of the chips).