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Accuracy of frequency deviation – Rainbow Electronics ATA5757 User Manual

Page 8

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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