100MHz Schmitt Oscillator

[Hoobbyy_Engineer]

Hello,is it possible to get a 100 MHz schmitt oscillator using 74AHCT14D (http://in.element14.com/nxp/74ahct14d-q100-118/logic-hex-inverter-6-i-p-soic/dp/2445103) ?

The datasheet says it has a propagation delay of max 8ns.So max frequency the chip can handle is 125 MHz. Am i right?
(Datasheet Attached)

Thanks,
Nilesh

 

[Audioguru]

8mHz is a very low frequency. 8MHz is a radio frequency. "m" is thousandth, "M" is Millions. A huge difference.

 

[Alec_t]

The datasheet says it has a propagation delay of max 8ns.So max frequency the chip can handle is 125Mhz. Am i right?

I doubt it. Have you allowed for capacitor charge time and the pulse edge rise and fall times?

 

[(*steve*)]

I would suggest that it is possible.

The propagation delay quoted is for an output capacitance significantly greater than the input capacitance of the gate.

Not so long ago I managed to get 84MHz out of a much lower spec device.

Having said that, there is a difference between getting it to oscillate at this frequency and being able to control the frequency. The capacitance of both the input and the load will contribute to the frequency, different gates within the same package may vary somewhat, and your construction method will have a great input (I'd use a dead bug method).

 

[(*steve*)]

Oh, another thing you need to remember is that propagation delay is for each transition. You will have one delay for high to low and another for low to high.

However, you might find this of interest.

 

[(*steve*)]

At very high frequencies the output is current limited and the edge case has the output swinging between the input hysteresis values. The output also looks a lot like a sine wave. Depending on whether the rise/fall times or the propagation delay dominates, the actual output will tend to look more triangular or more square.

 

[Hoobbyy_Engineer]

Thanks for all the replies. Looks like it is not that easy to make the oscillator.
So to avoid complications I think it would be better to choose LTC6905 oscillator.(http://in.element14.com/linear-tech...pbf/oscillator-17mhz-170mhz-tsot23/dp/1663923)

 

[(*steve*)]

Yep. It's not particularly cheap, but it should deliver a stable and relatively accurate frequency.