AN303
Si55
X
VCXO T
E M P E R A T U R E
S
TA B I L I T Y
D
E F I N I T I O N
1. Introduction
The Si55x VCXO utilizes a unique architecture to provide VCXO functionality and performance in an industry
standard 5x7 mm CLCC package. This unique architecture provides nearly any clock frequency from 10 MHz to
1.4 GHz and voltage control for closed-loop applications (phase-locked loops).
The basis for frequency synthesis is a single-frequency bulk-acoustic wave (BAW) AT-cut crystal resonator that
offers tight frequency stability over temperature and well understood aging behavior. A CMOS IC based on Silicon
Labs’ proprietary DSPLL™ technology provides the frequency translation from the crystal to the desired output
frequency.
Voltage control is achieved via a ground-referenced analog-to-digital converter (VCADC) that dynamically adjusts
the frequency synthesis.
2. Temperature Effects
Both traditional VCXOs and Si55x VCXOs have specifications that are dependent on temperature. Temperature
affects two areas of VCXOs: the crystal’s absolute frequency across temperature and the control voltage circuitry’s
response to temperature.
2.1. Crystal Temperature Behavior
BAW AT-cut crystals have absolute frequency that is dependent on the cut angle and the temperature as shown in
SAW oscillators have a different behavior. The Si55x VCXO data sheet specifies this parameter as “temperature
stability” indicating the base oscillator’s frequency stability across temperature. The specification is given as a
symmetric minimum and maximum.
25
20
15
10
5
R
8
’
7
’
6
’
AT-cut
r
m
R
Z
o
35¼
49
o
BT-cut
R
r
m
Y
R -1
’
0
Z
5
’
4
’
Y- bar quartz
1
’
2
’
3
’
4
’
5
’
6
’
7
’
8
’
ppm
f/f)
0
-5
-10
-15
3
’
2
’
1
’
0
’
-1
-20
-25
’
-45 -40 -35 -30 -25 -20 -15 -10 -5
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Temperature (
o
C)
Figure 1. Relative Frequency Across Temperature for AT-Cut Bulk-Acoustic-Wave Crystal
Rev. 0.1 7/06
Copyright © 2006 by Silicon Laboratories
AN303
SILABS [ SILICON LABORATORIES ]
