lmk04000b National Semiconductor Corporation, lmk04000b Datasheet - Page 46

• Manufacturer Part Number: lmk04000b
• Description: Low-noise Clock Jitter Cleaner With Cascaded Plls
• Manufacturer: National Semiconductor Corporation

www.national.com
Example crystal specifications are presented in Table 33.
See Figure 7 for a representative tuning curve.
The tuning curve achieved in the user's application may differ
from the curve shown above due to differences in PCB layout
and component selection.
This data is measured on the bench with the crystal integrated
with the LMK04000 family. Using a voltmeter to monitor the
V
frequency is swept in frequency and the resulting tuning volt-
age generated by PLL1 is measured at each frequency. At
each value of the reference clock frequency, the lock state of
PLL1 should be monitored to ensure that the tuning voltage
applied to the crystal is valid.
FIGURE 7. Example Tuning Curve, 12.288 MHz Crystal
TUNE
node for the crystal, the PLL1 reference clock input
Frequency Stability, -40 °C to +85 °C
Frequency Stability, T = 25 °C
Equivalent Series Resistance
Operating temperature range
Nominal Frequency (MHz)
Motional Capacitance (C
Shunt Capacitance (C
Load Capacitance
Parameter
C
Drive level
0
/C
1
ratio
TABLE 33. Example Crystal Specifications
0
)
1
)
30027168
46
The curve shows over the tuning voltage range of 0.17 VDC
to 3.0 VDC, the frequency range is ± 163 ppm; or equivalently,
a crystal frequency range of ± 2000 Hz. The measured tuning
voltage at the nominal crystal frequency (12.288 MHz) is 1.4
V. Using the diode data sheet tuning characteristics, this volt-
age results in a tuning capacitance of approximately 6.5 pF.
The tuning curve data can be used to calculate the gain of the
oscillator (K
from the most linear portion of the curve, a region centered
on the crossover point at the nominal frequency (12.288
MHz). For a well designed circuit, this is the most likely oper-
ating range. In this case, the tuning range used for the calcu-
lations is ± 1000 Hz (± 0.001 MHz), or ± 81.4 ppm. The
simplest method is to calculate the ratio:
ΔF2 and ΔF1 are in units of MHz. Using data from the curve
this becomes:
A second method uses the tuning data in units of ppm:
VCO
225 typical, 250 Maximum
). The data used in the calculations is taken
2 mWatts Maximum
-40 °C to +85 °C
25 Ω Maximum
5 pF Maximum
20 fF ± 30%
± 10 ppm
± 15 ppm
12.288
Value
14 pF
30027135
30027136
30027169