APN1005 Datasheet, PDF (3/11 Pages) Skyworks Solutions Inc. – A Balanced Wideband VCO for Set-Top TV Tuner Applications

APPLICATION NOTE • APN1005
The pseudo-resonator inductance is formed by microstrip trans-
mission line TL₂, which provides necessary circuit response at
high frequencies. This has little effect at the lower band due to
the resonator’s dominantly capacitive nature.
The function of transmission line TL₁ is both feedback and
phase alignment — providing flat power response over the
tuning range.
Power output is supplied from the collectors of X₁ or X₂ through
the series connected resistance and DC blocking capacitance
SRC₂ and SRC₃.
DC biasing for both of the transistors is supplied through a resis-
tive divider R₁/R₃/R₂ .
The NEC NE68119 bipolar transistors were selected to best fit
performances. Note: The circuit is very sensitive to the transistor
choice (in terms of tuning range and stability) due to wide band-
width design requirements.
For the model of NEC NE68119 we used the Gumel Poon model
of Libra IV with the coefficients provided by CEL RF & Microwave
Semiconductors Catalogue,1997-98.
In the Libra test bench shown in Figure 3 we defined an open
loop gain (Ku = V_OUT/V_IN) as the ratio of voltage phasors at the
input and output ports of an OSCTEST component. Defining the
oscillation point requires the balancing of input (loop) power to
provide zero gain for a zero loop phase shift. Once the oscillation
point is defined, the frequency and output power can be mea-
sured. Use of the OSCTEST2 component for the close loop
analysis is not recommended, since it may fail to converge in
some cases, and doesn’t allow clear insight into the under-
standing of VCO behavior. This property is considered an
advantage of modeling over a purely experimental study.
In the default bench shown in Figure 4 the variables used for
more convenient tuning during performance analysis and opti-
mization are listed in a “variables and equations” component.
Figure 3. Libra Test Bench
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200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005
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