Help with VCO Design

[sylvia24]

Hi,

I have a project to design a VCO which i have difficulties with. Could
someone please help me out? I have the following specifications:
band: 1430 to 2230 MHz
comparison frequency: 7.8125 kHz
VCO output power: 10 mW

I don't know how to find the value of the needed varactors and the s-parameters.

I would be eternally grateful for the help or a guide! Thanks!

 

[BradtheRad]

Your diagram shows an LC tank with tuning varactors.
I have not used varactors, but I believe more components will need to be added to make this into an oscillator.

As for the values, it often works to use an inductor which is 100 to 1000 times the capacitor value.

Your desired frequency range suggests the varactors will be as large a value, or larger, than Cp.

 

[betwixt]

It certainly isn't an oscillator but it is a tuned circuit which could be part of an oscillator.

To calculate the varactor capacitance, treat it just like a normal LC parallel tuned circuit. The capacitance is Cp in parallel with half the varactor capacitance (because there are two in series = half the capcitance of each). Find the additional capacitance needed in parallel with Cp to achieve the tuning range you want and choose varactors capable of twice that capacitance at Uc. Use varactors with a range not much larger than you need so that the V/F ratio is fairly small, this will give best precision of tuning and minimum modulation by ripple on the tuning voltage.

The comparison frequency is irrelevant to the tuned circuit, I'm guessing that is the step frequency resolution of a PLL being used to produce Uc. The PLL reference and divider circuits along with the loop filter will decide the best values for your needs.

Brian.

 

[kenambo]

as Betwixt said.. it is just a tuning circuit to be a part of an oscillator..

and you need to have a active element such as an amplifier to make this circuit to oscillate..
and you mentioned about comparison frequency.. i hope it is your reference frequency...

so you have have to generate with respect to this frequency...
you will need a whole PLL to achieve this result.. since you need variable voltages to build variable frequencies...

and for your information... if you use some delay cell concept means.. it is better to get frequencies without this tuning circuits..

 

[sylvia24]

Thank you very much Brian, BradtheRad and kenambo !