Table 6. Varactor Parameters
Test Condition Minimum Typical Maximum Unit
C
V_LOW
Low Varactor Capacitance V
C
= 0V 15.4 pF
C
V_HIGH
High Varactor Capacitance V
C
= 3.3V 29.6 pF
REVISION B 7/29/16 11 VCXO-TO-6 LVCMOS OUTPUTS
81006 DATA SHEET
Formulas
•C
Low
is the effective capacitance due to the low varactor capacitance, load capacitance and stray capacitance.
C
Low
determines the high frequency component on the TPR.
•C
High
is the effective capacitance due to the high varactor capacitance, load capacitance and stray capacitance.
C
High
determines the low frequency component on the TPR.
Absolute Pull Range (APR) = Total Pull Range – (Frequency Tolerance + Frequency Stability + Aging)
Example Calculations
Using the tables and figures above, we can now calculate the TPR
and APR of the VCXO using the example crystal parameters. For the
numerical example below there were some assumptions made. First,
the stray capacitance (C
S1
, C
S2
), which is all the excess capacitance
due to board parasitic, is 4pF. Second, the expected lifetime of the
project is 5 years; hence the inaccuracy due to aging is ±15ppm.
Third, though many boards will not require load tuning capacitors
(C
L1
, C
L2
), it is recommended for long-term consistent performance
of the system that two tuning capacitor pads be placed into every
design. Typical values for the load tuning capacitors will range from 0
to 4 pF.
pf
pfpfpfpf
pfpfpfpf
C
Low
7.9
4.15404.1540
4.15404.1540
pf
pfpfpfpf
pfpfpfpf
C
High
8.16
6.29406.2940
6.29406.2940
ppm
pF
pF
pF
pF
TPR 5.22610
4
8.16
12202
1
4
7.9
12202
1
6
TPR = ±113.25ppm
APR = 113.25ppm – (20ppm + 20ppm + 15ppm) = ±58.25ppm
The example above will ensure a total pull range of ±113.25 ppm with
an APR o
f ±58.25ppm. Many times, board designers may select their
own crystal based on their application. If the application requires a
tighter APR, a crystal with better pullability (C0/C1 ratio) can be used.
Also, with the equations above, one can vary the frequency tolerance,
temperature stability, and aging or shunt capacitance to achieve the
required pullability.
Symbol Parameter
LowVSLLowVSL
LowVSLLowVSL
Low
CCCCCC
CCCCCC
C
_22_11
_22_11
HighVSLHighVSL
HighVSLHighVSL
High
CCCCCC
CCCCCC
C
_22_11
_22_11
6
01
0
01
0
10
12
1
12
1
)(
C
C
C
C
C
C
C
C
TPRRangePullTotal
HighLow
