VCO Design Using SpectreRF
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August 2010 Product Version 10.1 5
■ γ = junction gradient coefficient
The varactor diode for this VCO has the following values
■
0j
C =8 pF
■ f = 0.75 V
■ γ = 0.4
Because
var
C is inversely proportional to V, and Fo is inversely proportional to
var
C , the
oscillation frequency is proportional to V. In other words, as you increase V,
var
C
decreases and Fo increases.
Example Measurements Using SpectreRF
To achieve optimal circuit performance, you should measure and evaluate several VCO
characteristics or parameters under varying conditions. As an example, one fundamental
measurement is the plot of VCO output frequency versus tuning voltage. An extension of
this parameter is tuning sensitivity (expressed in Hz/V), which is the differential of the
output frequency versus tuning voltage curve. The slope change (as a function of
frequency) is a critical design parameter.
In practice, both of these parameters should be evaluated under different supply (Vcc)
conditions because the output frequency may shift with Vcc changes. This DC power
sensitivity is called frequency pushing.
The RF power output is a function of both Vcc and output frequency. You should
evaluate the RF power output because an output power level that is too low results in
excessive noise and an output power level that is too high creates distortion and
consumes excess DC power. Moreover, the DC power has the chance to translate Vcc
noise into oscillator output modulation and noise.
Many of the parameters you must consider for a complete evaluation are not covered
here. In this workshop, you begin examining the flow by bringing up Cadence Design
Framework II environment for a full view of the reference design:
Action P-1: Move into the ./rfworkshop directory.
Action P-2: Run the virtuoso& tool.
Action P-3: In the CIW window, select Tools — Library Manager….
