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Design >> Analog Design >> sidebands of VCO
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Message started by sugar on Jun 2^nd, 2006, 6:35am

Title: sidebands of VCO
Post by sugar on Jun 2^nd, 2006, 6:35am

If a VCO is modulated by a control votage Vcont = V1 * cos (w1 * t), then spectrum of the VCO's output consists of three frequency components: w0, w0+w1 and w0-w1, as prof. Razavi explained in his book "Design of Analog Integrated Circuits", page 529.
The problem is that since the VCO's control voltage is continuous, according to w = w0 + Kvco * Vcont, the frequency of its output should be continuous between w0-w1 and w0+w1,
not only three spurs, isn't it?

Title: Re: sidebands of VCO
Post by ywguo on Jun 3^rd, 2006, 8:57am

Hi, Sugar,

I understand that you want to have an intuitive time domain analysis. But your statement is wrong. At any time, the delta phi/delta t varies as the control voltage varies. So it is not a sine wave strictly. Anyway, it is still a periodic signal, then it is combined by 3 components, ω0-ω1, ω0, ω0+ω1. The result can be derived using fouries series or simple maths as that in Razavi's book.

Best regards,
Yawei