Thursday, December 28, 2017

Logic Oscillator options.

The chief designer said he would like to use some logic in the radio design. The second detector oscillator will be crystal controlled and he wants to use a logic gate to make it. This is and interesting approach. What are the requirements for an oscillator?
1. an amplifier overall loop gain greater than 1
2. a feedback circuit providing positive feedback

So the question is can a logic gate be used as an amp? Let's take a quick look at a couple of gate circuits.

For our oscillator the amp does not have to be high gain. It simply has to replace circuit losses.
The CMOS inverter has possibilities.
It seems we have some possible circuits let's look at some oscillators using them.
This one looks good as far as simplicity and component number count. Notice the RC circuit adjust the frequency so it would need the variable capacitor. They are still available so this is a good option.
This one might be better because the two gates provide the phase shift required for oscillation. In the first circuit the RC circuit had to be set to provide the phase shift. In this one the capacitor is trimming or pulling the crystal frequency.
This one is possibly better. It will oscillate without the crystal. The crystal establishes the frequency. (R2 and C2 shunt spurious frequency) Let's look at another version of this one.
The best one I could find. As it says will oscillate without the crystal and has a buffer. If I could just remember where I put those chips I would have to try one.
I made this one and it oscillates with 1-1/2 volt to 4-1/2 volt. It locks to the crystal frequency. I can adjust the duty cycle and shape the wave a little. It seems the pulse train is locked to the crystal frequency. The duty cycles swings from 37/63 to 48/52 but the frequency counter holds steady?
I tried 2.0, 4.0, 6.0, 8.0, 12.0 mhz crystals and it worked with them all. I could adjust the duty cycle but not the frequency with them all. The Texas Instrument App Notes shows the circuit and gives info on adjusting the operation. They show sine waves being produced. I am seeing square waves. The chip I'm working with is a 2 input NOR gate,single 5 pin device. Maybe I'll try again with the proper IC. It will be next year before I get some.

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