Optimizing VCO/PLL Evaluations & PLL Synthesizer Designs

Todayís mobile communications systems demand higher
communication quality, higher data rates, higher frequency
operation, and more channels per unit bandwidth. As much
of this equipment is portable, low power consumptionand
small size are also required. All of these constraints combine
to make the whole design including component selection
and evaluation quite challenging. One portion of this design
that is very critical with regard to all of the requirements
mentioned above is the synthesized oscillator. Typical
synthesized oscillators combine a Voltage Controlled
Oscillator (VCO) with a Phase-Locked Loop IC (PLL),
frequency reference (e.g. Crystal / TCXO) and a loop filter.
The VCO is used to generate the RF output frequency. 
The PLL (which is of the "analog type"; i.e. different from a
pure digital PLL) is used to stabilize and control the frequency
The loop filter design must integrate all of the components
to establish, among other things, a tradeoff between noise
and transient response (Figure 1). Within this paper we will
describe the evaluationof the PLL and VCO and relate thos
evaluations toinformation that will allow the circuit designer
to optimize the whole oscillator design including the loop filter

 VCO characteristic parameters and problems
with the conventional evaluation method

Shown below are many of the common VCO evaluation
parameters. To perform these evaluations, many
instruments and set-ups are required even
including special DC sources for both the
power supply and tuning voltage.



To achieve optimal circuit performance, many VCO
characteristics should be evaluated under varying
conditions.For example, a very fundamental parameter
is the VCO output frequency versus tuning voltage (F-V).
One extension of this parameter is tuning sensitivity (Hz/V),
which is the differential of the F-V curve. Ideally this is a
constant, but it is not. The slope change, as a function of
frequency, is very important to know since this is a critical
design parameter for the loop filter. But both of these
parameters should also be evaluated under different power
supply (Vcc) conditions since the output frequency may
shift with Vcc changes. This DC power sensitivity is called
frequency pushing. The RF power output will also be a
function of both Vcc and output frequency. This should be
evaluated since too low of an output power will result in
excessive noise and too high of a level will create distortion
and consume excess DC power. In addition, the DC power
sensitivity has the opportunity to translate Vcc noise into
modulation/noise of the oscillator output.

Many other parameters must also be evaluated; not the
least of which is phase noise. Phase noise is a crucial
parameter.  It must be known for proper loop design and
it will impact many critical operating characteristics of the
synthesized oscillator including adjacent channel power.



pagina fuente: http://www.minicircuits.com/pages/pdfs/an95006.pdf

Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia

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