Rainbow Electronics ATA5757 - Manual (Page 8)

8
ATA5756/ATA5757 [Preliminary]
4702D–RKE–02/04
Figure 7. Timing ASK Mode with ENABLE Connected to the Microcontroller
Accuracy of Frequency
Deviation The accuracy of the frequency deviation using the XTAL pulling method is about
±
20% if
the following tolerances are considered. One important aspect is that the values of C
0
and C
M
of typical crystals are strongly correlated which reduces the tolerance of the fre-
quency deviation.
Figure 8. Tolerances of Frequency Modulation
Using a crystal with a motional capacitance of C
M
= 4.37 fF ±15%, a nominal load
capacitance of C
LNOM
= 18 pF and a parallel capacitance of C
0
= 1.30 pF correlated with
C
M
results in C
0
= 297
×
C
M
(
the correlation has a tolerance of 10%, so C
0
= 267 to
326
×
C
M
). If using the internal FSK switch with C
Switch
= 0.9 pF ±20% and estimated
parasites of C
Stray
= 0.7 pF ±10%, the resulting C
4
and C
5
values are C
4
= 10 pF ±1%
and C
5
= 15 pF ±1% for a nominal frequency deviation of ±19.3 kHz with worst case tol-
erances of ±15.8 kHz to ±23.2 kHz.
FSK
CLK
Power-up,
PA off
ASK
∆
T
XTO
Power-down
Power-up,
PA on
(High)
> 250 µs
Power-down
Power-up,
PA off
(Low)
ENABLE
~
~
V
S
XTAL
C
M
L
M
R
S
C
0
C
Stray
C
4
C
5
Crystal equivalent circuit
C
Switch