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Crystal Oscillator Circuits

A well-designed crystal oscillator will provide good performance with TTL gates.
Two types of crystal oscillator circuits can be used:
One with series resonance, or one with parallel resonance.
Parallel X-tal osc

The Figure above shows implementation of a parallel resonant oscillator circuit.
The circuit is designed to use the fundamental frequency of the crystal.
The 74AS04 inverter performs the 180-degree phase shift that a parallel oscillator requires.
The 4K7 resistor provides the negative feedback for stability.
The 10K potentiometer biases the 74AS04 in the linear region. This could be used for external oscillator designs.

Series X-Tal osc

The Figure above shows implementation of a series resonant oscillator circuit.
The circuit is also designed to use the fundamental frequency of the crystal.
The inverter performs a 180-degree phase shift in a series resonant oscillator circuit.
The 330 Ohm resistors provide the negative feedback to bias the inverter in their linear region.

Series X-Tal osc

The Figure above shows implementation of a series resonant oscillator circuit, using TTL NAND GATES.
This circuit is tested by me and found working with the following IC types:
7400 Total current consumption is 21 mA.
74LS00 Total current consumption is 15 mA.
74HCT00 Don't use this type, it will NOT work, because the HCT type is too fast.

7400 PIN 6 SCOPE PICTURE
7400 PIN 11 SCOPE PICTURE
74LS00 PIN 6 SCOPE PICTURE
74LS00 PIN 11 SCOPE PICTURE

The parallel resonator above doesnt work with 74HCT04. Well, in fact it doesnt.
But it's not because the HCT technology is faster than LS, or, too fast.
In fact the propagation delay is similar in HCT and LS.
What really happens is that the HCT technology has a higher gain, that is,
the inverter has a mutch sharper characteristic than LS devices.
So, with that configuration, it gets very dificult do biase the inverter in the liner or high gain region.
Nevertheless, because the HCT is a low power technology, and nowadays preferable,
there's a very simple circuit to build clocks with HCT. I think that you know the configuration,
yet I leave it here just in case you dont. Note that this configuration
doesnt work with LS because of the gain being to low to biase the inverter.

Thanks to J.A.Serralheiro for this information and the good picture

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