APN1005 Datasheet, PDF (1/11 Pages) Skyworks Solutions Inc. – A Balanced Wideband VCO for Set-Top TV Tuner Applications

APPLICATION NOTE
APN1005: A Balanced Wideband VCO
for Set-Top TV Tuner Applications
Introduction
Modern set-top TV DBS tuner systems require more channel
coverage, while maintaining competitive prices. This situation
creates tough design goals: to improve performance and simplify
design.
Balanced VCO configuration could be a competitive circuit
solution, since it provides the widest tuning range with practical
circuitry and layout. However, tuning margins would be further
improved by optimizing the varactor manufacturing process.
Skyworks has developed such a process to satisfy the most
ambitious wideband design goals.
In this publication, we will address the design of the balanced-
type voltage control oscillator (VCO) based on the newly
developed varactor SMV1265-011 with the unique set of
capacitance tuning ratios and Q-quality.
VCO Model
Figure 1 shows the VCO model built for open loop analysis in Libra
Series IV including the SMV1265-011 varactor model.
The circuit schematic in Figure 2 shows a pair of transistors in a
single feedback loop, connected so that collector currents would
be 180° shifted (ideally). A pair of back-to-back connected
SMV1265-011 varactors is used, rather than a single one. This
allows lower capacitance at the high-voltage range, without
changing the tuning ratio. The reason is that, apart from package
capacitance, certain mounting fringing capacitances, though
small, may strongly affect higher frequency margins. The effects
of parasitic capacitances were summarized in the model as C₄
and C₃, valued 0.4 pF each. These values may vary depending on
the layout of the board. Varactor DC biasing is provided through
resistors R₆ and R₈, both 1 k, which may affect the phase noise,
but eliminate the need for inductive chokes. This minimizes
overall costs and the possibility of parasitic resonances — the
usual cause for frequency instability and spurs.
The phase corrector DC chokes, SRL₁ and SRL₂, were modeled
as lossless inductors at 33 nH since their losses are dominated
by the 30 Ω emitter biasing resistors. DC blocking series capaci-
tances (C_SER1 and C_SER2) are modeled as an SRC network,
including associated parasitics. Their values were optimized to
10 pF providing smooth tuning over the design band.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005
1