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700kHz Square Wave Oscillator

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bawobogo

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Hi. I'm looking to build a 700kHz square wave oscillator in purely discrete parts (transistor level). 0-5 volts.

I've tested a variety of circuit types I've seen scattered across the web, but none of them seem to be able to produce a decent quality square wave with a good accuracy in frequency.

Sadly my best result was a ring oscillator so far. I need some help please. :(
 
For real stability, you need a crystal oscillator. If you use a LC or RC circuit, the frequency will drift more. Using high stability capacitors will help, but not eliminate drift. Is there any reason why only discreet components can be used?
 
This is going to be the clock for part of a school project I'm working on and I can only use discrete components.

I'm not sure how crystal oscillators work. Do you have a circuit by chance that would do 700khz?
 
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Here is a LINK to some crystal oscillator circuits. They will output a sine wave. If you amplify the signal a lot, it'll become close enough to a square wave for practical purposes. Your problem will be finding a 700Khz crystal. I would suggest you find the crystal before continuing further.
 
The clock is for an AD converter and my sampling rate is 700k. I think if worst comes to worst, I'll just change everything to work at 1mhz
 
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The clock is for an AD converter and my sampling rate is 700k. I think if worst comes to worst, I'll just change everything to work at 1mhz

So your A/D is built using discretes? Or is only portions of your project required to be discreet? Perhaps if you showed us your project spec, we could offer better help.
 
Most of the project needs to be discrete. The project is to build an A/D converter with the following requirements:

Successive Approximation type with best guess centering
100khz maximum input freq
8-bit max resolution
Maximum error of 1% at the output

All of the project needs to be discrete except for the D/A return and the Successive Approximation Register.

I found a comparator circuit from this site that works in upwards of 1 Mhz so I'm planning on using that.

I have the sample and hold circuit based off of a Transmission Gate.

The only thing I have left to build is the clock for the S/H circuit.
 
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