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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 ±50 ppm to ±1500 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
·Remote-control
·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 = ±50 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 10-6/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
± 25ppm ~ ± 100ppm
TEMPERATURE
RANGE
Available
-45℃ to 85℃
Frequency Pullability
Available
±50ppm ~ ±200ppm
INPUT
Voltage
+5.0VDC±10%, +3.3VDC±10%
OUTPUT (SQUAREWAVE)
Symmetry
40~60% @1/2 Vdd
Tr and Tf
(Max.)
15ns
"0" Level
(Max.)
10% Vdd
"1" Level
(Max.)
90% Vdd
FANOUT
15pF
LOGIC FAMILY
TTL/HCMOS
  
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