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Design >> RF Design >> Vco design
https://designers-guide.org/forum/YaBB.pl?num=1315077831

Message started by andr.dimitris on Sep 3^rd, 2011, 12:23pm

Title: Vco design
Post by andr.dimitris on Sep 3^rd, 2011, 12:23pm

I am a senior student in electrical engineering and for my thesis i am designing a pll. I have started with the design of the vco. However i am stuck because i can't get the same frequency when i use varactors (always applying the voltage that gives them the right capacitance) as when i design the oscillator at a steady frequency( 5Ghz). I've tried a lot of things, however i get frequencies much higher than expected. In order to achieve a 5Ghz frequency, i have to set large values to the varactors and the inductance(nH!!). I know that they are large enough as i have a frequeny of 5Ghz with values much lower (pF, pH). I attach a screenshot of the vco (only the varactors and mosfets). I suspect i have done something wrong with the wiring but i am not sure.
Does anyone have any idea?

Title: Re: Vco design
Post by rfidea on Sep 3^rd, 2011, 1:18pm

Strange! Have you measured the capaitance of the varactor with a simulation? Do it with an AC analysis to start with. In the VCO you will get some averaging of the capacitance which will reduce the capacitance. Be sure to measure the "hot" side of the varactor since you have a parasitic on the S/D side.

One more thing. You should swap one of the varactors so you get a symmetrical tank. Swap the right one to not load the tank with the S/D parasitic. (I do not think this is your problem. But if you have included the S/D parasitic in your capacitance estimate, maybe this is it.)

Title: Re: Vco design
Post by andr.dimitris on Sep 3^rd, 2011, 7:39pm

I tried the swapping you mention, but i get no oscillation then, just noise.

Title: Re: Vco design
Post by rfidea on Sep 4^th, 2011, 12:47am

Looks like your Q-value dropped when you did the swap. Have you measured the Q-values for varactors and inductor to see that you have enough gm in the nmos transistors? Maybe you need to increase the core current to get an oscillation. (I assume you "kick" the oscillator with a current pulse or similar to start the oscillation.)

Title: Re: Vco design
Post by raja.cedt on Sep 4^th, 2011, 1:03am

hello,
When you swap also it should oscilate at least at some frequency. may be you forgot to add some intial conditions for start-up, or when you swap the capacitance will change probably it will increase and there is lot of loss at the lower frequency so hence no oscilations.

So one check is swap and increase the current, you will see oscilations.
as rfidea told you should always find the capacitance and resistance by using as AC simulations, add this to tank loss. However many times you will get more frequency in simulation compared to 1/(sqrt(LC)).

Thanks.

Title: Re: Vco design
Post by rfidea on Sep 9^th, 2011, 5:14am

Hi andr! What is happening? Are you still struggling?

Title: Re: Vco design
Post by andr.dimitris on Sep 9^th, 2011, 2:52pm

Actually for the last week no, because i am studying for exams; maybe from next week while studying for the GRE as well ;)

Title: Re: Vco design
Post by loose-electron on Sep 12^th, 2011, 10:00am

rfidea wrote on Sep 4^th, 2011, 12:47am:

(I assume you "kick" the oscillator with a current pulse or similar to start the oscillation.)

A trick here - put an IC current on the inductor. Works great!