[Moved]Cross-coupled oscillator design

[katayoon_1]

May I ask you to help me? I want to design a cross coupled oscillator in pspice with 50 mhz frequency. but it doesnt work properly. which kind of mosfet should I use ?

Referring to old thread https://www.edaboard.com/threads/28107/

 

[BradtheRad]

I modified previous simulations of similar oscillators. This one worked after I tried a trick or two.

To get oscillations to build, I found I had to make the inductor-to-capacitor ratio very high (11,000). This is to accommodate the low amount of current available to drive the LC tank loop.

When I tried a low L/C ratio, oscillations did not sustain. They faded.

The supply V needs to be high enough as well. The simulation needed 4V minimum. This is just theory, based on the mosfet model in Falstad's simulator.

 

[katayoon_1]

Thank you for your reply, what is the specification of the mosfets you have used. of course their W/L can have effect on the performance. I am using Pspice for the simulation and I used BUZ10 mosfets, But the oscillation just happens in micro seconds and then again gets to zero, I think maybe the mosfets are not the right one.

 

[BradtheRad]

Thank you for your reply, what is the specification of the mosfets you have used.

Falstad's mosfet model has one parameter we can adjust, 'threshold'. I left it at the default value, 1.5V. (Changing it usually has small effect anyway.)

I am using Pspice for the simulation and I used BUZ10 mosfets, But the oscillation just happens in micro seconds and then again gets to zero, I think maybe the mosfets are not the right one.

The oscillator also works with ordinary transistors.

It draws little current. Notice the supply provides less than 1 mA, but the LC loop has several mA traveling back and forth in it.

You may need to create slight non-symmetry in the values, to get oscillations started.

Also, try playing with the timestep.