MK3771-1 Datasheet, PDF (6/6 Pages) Integrated Circuit Systems – USING MICROCLOCKS INTEGRATED VCXO PRODUCTS

USING MICROCLOCK’S INTEGRATED VCXO PRODUCTS
error_xtal = actual initial accuracy (in ppm) of the crystal
being measured
If the centering error is less than +/- 15 ppm, no
adjustment is needed. If the centering error is more
than 15 ppm negative, the PC board has too much
stray capacitance and will need to be redone with a
new layout to reduce stray capacitance. (The crystal
may be re-specified to a higher load capacitance
instead. Contact ICS MicroClock for details.) If the
centering rror is more than 15 ppm positive, add
identical fixed centering capacitors from each crystal
pin to ground. The value for each of these caps (in pF)
is given by:
External Capacitor
= 2 x (centering error)/(trim sensitivity)
Trim sensitivity is a parameter which can be supplied
by your crystal vendor or calculated using the following
formula:
trim sensitivity = --------¹---⁰----⁶----^×-----^C-----¹--------
2 × (C₀ + C_L)²
If you do not know the value, assume it is 30 ppm/pF.
After any changes, repeat the measurement to verify
that the remaining error is acceptably low (less than
±15ppm).
Example calculation
Using a 19.44 MHz pullable crystal, specified at 14 pF
load capacitance. With its rated 14 pF load, this crystal
is measured to have an output of 19.4405 MHz (26
ppm initial error, positive).
For a MK2058-01, the frequency, using a 0 and 3.3V
input on pin 5, for mode SEL[2:0] = 111, the output is
measured to be 19.4420 and 19.4410, respectively (in
this example, the input frequency and output frequency
happen to be the same), yielding a centering error of:
Centering error
= 10⁶ ⋅ ⁽---¹---⁹----^.-⁴----⁴---²-----^–----¹----⁹---^.--⁴---⁴----⁾---+-----⁽---¹---⁹---^.--⁴---⁴----¹-----^–----¹---⁹----^.-⁴----⁴---⁾- – 26ppm
2 × 19.44
= 51 ppm
Assuming a trim sensitivity of 30 ppm/pF, The external
capacitors to ground should be:
External capacitor = 2 x 51 / 30 = 3.4 pF.
Rounding to the nearest standard value, two 3.3 pF
capacitors should be used.
MAN05 G
6
Revision 050603
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