Designing a 10 MHz 1 VPP 50% Duty Cycle Square Wave Oscillator Using LTC6900

Hi folks,

I'm really new to electronic and I have a design challenge I'd like to solve for school:

Design a 10 Mhz square wave oscillator with 50% duty cycle and output of 1 VPP.

I'd like to design a real simple but faily accurate and inexpensive circuit so I started looking online and saw the LTC6900 from Linear Technology as a good option, it only requires one external resistor to set the frequency and it offers 50% duty cycle square wave, I can power it with a standard 5v supply, however, the output will then be a 0 to 4.8 - 4.95, I thought about maybe feeding this output to a non-inverting high-speed op-amp with a gain of 1/4.8 but now I'm really confused because I think there might be a better approach, any suggestions please?

Hi Sam, I think you over-engineered your design a little. You can get a square wave with 50% duty-cycle from the output of a comparator that compares a sine wave (without a DC component) to 0V for example. And a comparator is basically just an opamp. Also to generate the sine wave you can use another opamp as an oscillator - with positive feedback. To make it even more economical, you can use the same opamp for both comparator and oscillator by connecting the inverting input to ground and the non-inverting input to the output through a resistor. All you need is to find an opamp with the correct frequency response.

Here's something really simple and useful opamp application notes document for future analog applications you might want to do yourself, or get as an assignment in school:

http://www.national.com/an/AN/AN-31.pdf

Good luck on your design... and remember if you want a 1 VPP output you need to either power the opamp at 1 V at the power supply pins or use a resistive divider on the output (I'd go with the last one) and you can also move the output in whatever voltage interval you want with another opamp... plenty of ideas in the document above. Again happy designing and good luck!

Hi Dorin, thanks for your suggestion, but unfortunately now I'm more confused than when I started, can I shoot you an email with my questions?

Sure, go ahead. I should have it in my profile