4MHz Crystal Oscillator using sn74ls393n

[anad2560]

Hi everyone,

I was wondering if/how it is possible to use the sn74ls393n alongside a crystal oscillator for generating a 4MHz signal? What pins would I attach the crystal? Many thanks.

 

[Reloadron]

If you want a 4 MHz signal with reasonable uncertainty why noy just buy a 4.0 MHz crystal oscillator for under $2.00 USD. Something like this unit maybe? Such units are common from a variety of manufacturers.

Ron

 

[Diver300]

You can't connect a crystal directly to an SN74LS393.

To make a crystal oscillate, you need an amplifier of some sort. There are plenty of designs that use simple logic gates, usually inverters, as amplifiers for crystal oscillators, but the SN74LS393 doesn't have any simple logic gates that can be used.

HCMOS gates work a lot better than TTL gates as oscillators.

I also agree with Reloadron. Just use an oscillator. All you need to do is to connect it to a supply and it works. There is no need to worry about load capacitance and packaged oscillators are not affected by layout nearly as badly as crystals.

 

[anad2560]

Thank you for your replies. I was hoping to use some of the things I already had. I may have some NAND gates I could use.
In terms of frequency stability, would it be better to go for a design such as a Pierce oscillator or one using logic gates? Would anyone be able to point me to some stable oscillator circuits and designs.

Thanks again.

 

[Reloadron]

Just how stable does this need to be? See figure 10 in this link. Component placement of the caps becomes critical.

Ron

 

[jpanhalt]

I suggest Fairchild Application notes AN-118 and AN-340 and National application note AN-400. Also see the pdf attached. You will find that you need to get the capacitor loading right.

John

View attachment PierceGateLoadCap.pdf

 

[Diver300]

Unless you really want to get into making an oscillator, don't. Just buy one.

If you are going to make an oscillator, use an HCMOS gate, preferably an unbuffered one such as a 74HCU04.

The frequency stability of an oscillator is not something that you should worry about. Unless you are looking at temperature compensated or oven stabilised designs, the frequency of the crystal will change so much with temperature that you will never notice the oscillator.

Also, if you don't have any way of checking the initial adjustment of an oscillator, it could be a long way off.

But initial adjustment is not important unless there is some reason for good accuracy.

Let me put some figures on that:-

Initial adjustment if component values are guessed. ±500 ppm or 40 seconds per day
Initial adjustment if component values are selected for the design. ±50 ppm or 4 seconds per day
Initial adjustment if component values are selected for each oscillator, or a trimmer is fitted. ±10 ppm or about 1 second per day
Temperature stability with a cheap crystal. ±50 ppm or 4 seconds per day
Temperature stability with an expensive crystal. ±10 ppm or about 1 second per day
Temperature stability of the oscillator, ignoring crystal stability. ±5 ppm or about 1/2 second per day

As you can see from this, if the oscillator works, and you get it adjusted reasonably well, other things just don't matter. The easiest oscillators to get working are Pierce oscillators using an HCMOS inverter, or a Colpitts oscillator using a bipolar transistor.

With a 4 MHz crystal, it is quite easy, especially for the Pierce oscillator with an HCMOS inverter if you don't use an unbuffered one, for it to oscillate at 3rd Overtone. If that happens, you need a resistor between the output of the gate and the common point of the crystal and the capacitor.

There have been whole books written about crystal oscillators. Just buy the oscillator.

 

[Diver300]

I suggest Fairchild Application notes AN-118 and AN-340 and National application note AN-400. Also see the pdf attached. You will find that you need to get the capacitor loading right.

John

View attachment 58285

Those application notes are very good. I would also add that the delay through the inverter has a significant effect in the initial adjustment values of the capacitors. The variation of time delay with temperature and supply voltage can be significant at high frequencies.

An unbuffered gate usually has shorter transition times than a buffered one. Typical transition times for an unbuffered gate at 5 V is 7 ns which is 10 degrees of phase lag at 4 MHz. The transition time becomes much more significant at higher frequencies. A lot of the calculations of capacitance values assume a phase lag of 180 degrees for an inverter, but in fact it will always be more because of the transition times.