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Design >> RF Design >> Wide tuning range VCO design problem
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Message started by gggould on Apr 9^th, 2004, 8:41pm

Title: Wide tuning range VCO design problem
Post by gggould on Apr 9^th, 2004, 8:41pm

Hi all,

I am designing a VCO with varactor and switching cap to cover 800MHz wide. But I find that the VCO gain and amplitude variations are big problems. Both Kvco and output amplitude increase with the operating frequency...

Any tips or advices?

Thank you very much...

Title: Re: Wide tuning range VCO design problem
Post by wizz on Apr 12^th, 2004, 8:51pm

To minimize Kvco variation, you could use a lot of switching bands. You'll find up to 8bits worth of bands in published VCO's if I'm not mistaken (which means you need to put in 256 swiched caps). Could be it was 6 bits, which would mean 64 switched capacitors.

And 800MHz tuning: doesn't say anything about difficulty; it's the percentage that counts. If you're trying to do 50-850MHz with one LC VCO I wish you good luck :). If it is around 17GHz, it shouldn't be that hard....

wizz

Title: Re: Wide tuning range VCO design problem
Post by Fei on Apr 19^th, 2004, 11:46am

The variation is caused by the "ON" resistance of the switch transistor. There are a few things you can do to mitigate the effect:

1). Using a larger size transistor;

2). Using an additional cap in series to the switched cap tanks.

Hope it helps

Fei