VCXO
are crystal oscillators whose output frequency is changed or modulated by direct
application of an external control voltage. Depending on the nominal frequency,
the pulling voltage and the case dimensions, they can be typically adjusted from
¡¾50 ppm to ¡¾1500 ppm. This type of oscillators can offer ingenious solutions to
a large number of applications including:
¡¤Frequency modulation
¡¤Phase locked loops (PLL)
¡¤Telemetry ¡¤Transmission of command signals
¡¤SONET/SDH/ATM
¡¤Remotecontrol
¡¤Telecommunications, ¡¦
A VCXO consists of a crystal and a network including
inductors and a varactor diode. The whole assembly is designed to give a linear
voltage / frequency response. The limit for tuning range is an inverse function
of the Q of the resonator. Special techniques are available to extend the pulling
range, but those techniques will also degrade the phase noise. Because the crystal
resonator is much more stable than the LC circuit, even a simple VCXO has better
stability than a VCO. The phase noise near the carrier of a VCXO is much lower
than a VCO, because he is determined by the Q factor, The floor phase noise is
determined by the signal level and the oscillator's instabilities. For a VCO,
the floor phase noise can be lower than a VCXO and a VCO are shown below.
TECHNICAL
TERMS
Control voltage: The
minimum, nominal and maximum voltage to be applied to the control voltage input.
The nominal control voltage is the voltage that sets the oscillator to nominal
frequency.
Frequency deviation : The frequency
deviation is the frequency difference relative to the nominal frequency obtained
by applying the control voltage from nominal to the minimum or maximum voltage.
It is expressed in units of ppm:
Delta
F/Nominal frequency = ¡¾50 ppm
Or in absolute values:
Delta
F/Nominal frequency < 100 ppm
Linearity of the voltage / frequency :
response Linearity is generally expressed by the maximum relative error between
the frequency measurement and the theoretical frequency (absolute linearity).
It is expressed in %. SOOJUNG VCXOs have a linearity of 20%.
Slope : The slope is expressed
in 106/V relatively to the nominal frequency. The slope of the voltage versus
frequency curve can be either positive or negative. An increase in frequency caused
by an increase in control voltage is a "positive slope". The standard VCXOs have
a positive slope.
Deviation sensitivity :
The slope of the voltage versus frequency curve, which is expressed in ppm /Volt
or Hz/Volt.
Voltage
Controlled Crystal Oscillator(VCXO)
MODEL 
VC4000  VC8000

PACKAGE 
 
FREQUENCY RANGE  1 MHz
to 45.0 MHz  
FREQUENCY
STABILITY  Available
 ¡¾ 25ppm ~ ¡¾ 100ppm

TEMPERATURE
RANGE  Available
 45¡É to 85¡É 
Frequency Pullability 
Available  ¡¾50ppm
~ ¡¾200ppm 
INPUT  Voltage
 +5.0VDC¡¾10%, +3.3VDC¡¾10%

OUTPUT (SQUAREWAVE)  Symmetry
 40~60% @1/2 V_{dd}

Tr
and Tf (Max.)  15ns

"0"
Level (Max.)  10%
V_{dd} 
"1" Level (Max.) 
90% V_{dd} 
FANOUT  15pF 
LOGIC FAMILY 
TTL/HCMOS 