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Write a short note on performance parameters of VCO.

Subject: CMOS VLSI Design

Topic: Mixed Signal Circuits

Difficulty: High

cvd • 1.7k  views
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  • Master application required that oscillator be "tunable" i.e their output frequency be function of a control input voltage.
  • Ideal VCO circuit whose output frequency is a linear function of its control voltage.

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$\omega_{out}=\omega_o+K_{VCO}\,V_{const}=0$

$\omega_o$ -> intercept corr to $V_{const}=0$

$K_{VCO}$ -> denotes gain or sensitivity.

Range $\omega_2-\omega_1$ -> tuning range.


Performance parameters of VCO
(i) Centre Frequency:
a) determined by the environment in which VCO is used.

b) Eg: CLK generation n/w of up, VCO required to run at the clk rate or even twice that.

c) Centre frequency is as big as 10 GHz.

(ii) Tuning Range:
a) Is dictated by 2 parameters:

  • Variation of VCO centre frequency with process and temperature.
  • The frequency range necessary for the application.

b) The centre frequency of some CMOS oscillations may vary by a factor of 2 at the extremes of process and temperature, therefore tuning range $\geq$ 2x (wide).

c) Noise amplitude in the output frequency is proportional to $K_{VCO}$.

(iii) Tuning Linearity:
a) The tuning char. of VCO exhibit non- linearity i.e gain $K_{VCO} \neq $ const.
b) Non-linearities degrades settling behavior of PLLs.
c) Therefore, desirable to minimum variation of $K_{VCO}$ across tuning range.

(iv) Output amplitude:
a) Desirable to achieve large output acsillation amplitude, thus making the w/f less sensitive to noise.

(v) Power dissipation:
a) OSC suffer from trade-off's between speed, power dissipation and noise.
b) Typical oscillation: 1 to 10 mW of power.

(vi) Supply and common mode rejection:
a) OSC- quite sensitive to noise, when realised in single ended form.
b) Even differential oscillation exhibit supply sensitivity.

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