TDA5251F1 Datasheet, PDF (54/88 Pages) Infineon Technologies AG – ASK/FSK 315MHz Wireless Transceiver

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3.2.4 Calculation of the external capacitors
1. Determination of necessary crystal frequency using formula [3-19].
e.g. f_FSK- = f_{COSC LOW}
2. Determine corresponding C_Load applying formula [3-18].
e.g. C_{L FSK}- = C_{L ±}
3. Necessary C_V using formula [3-17].
e.g.
1
( ) ^{CV -}
=
1
C_{L,FSK -}
+
2pf _{FSK -}
2 _{* LOSC}
TDA5251 F1
Version 1.0
Application
1. When the necessary C_v for the 3 frequencies (C_v- for FSK LOW, C_v+ for FSK HIGH and C_vm for
FSK-receive) are known the external capacitors and the internal tuning caps can be calculated
using the following formulas:
-FSK:
+FSK:
FSK_RX:
C_v1 + C_tune1 = C_v- – C₂₁
C_v2 + C_tune2 = ⁽-₍--CC----^v-_v--¹¹----++-----CC----^tt-^u-_u--ⁿ-_n--^ee--¹-₁--⁾⁾---–-^×--₍-⁽-C-C----_vv--₊-⁺----––----C-C---₂-²--₁₁---₎₎- – C₂₀
C_v3 + C_tune2 = ₍-⁽--C-C---_v-^v--₁-¹---++-----CC----_tt-_u-^u--_n-ⁿ--_e-^e-₁-¹--₎-⁾---–^×---⁽-₍--CC----^v-_v--^mm-----––----C-C----²²--¹-₁--⁾-₎ – C₂₀ – C_v2 – C₂₂
[3 – 27]
[3 – 28]
[3 – 29]
To compensate frequency errors due to crystal and component tolerance C_v1, C_v2 and C_v3 have to
be varied. To enable this correction, half of the necessary capacitance variation has to be realized
with the internal C-banks.
If no finetuning is intended it is recommended to leave XIN (Pin 21) open. So the parasitic
capacitance of Pin 21 has no effect.
Note: Please keep in mind also to include the Pad parasitics of the circuit board.
In the suitable range for the serial capacitor, either capacitors with a tolerance of 0.1pF or 1% are
available.
A spreadsheet, which can be used to predict the total frequency error by simply entering the crystal
specification, may be obtained from Infineon.
3.2.5 FSK-switch modes
The FSK-switch can be used either in a bipolar or in a FET mode. The mode of this switch is
controlled by bit D0 of the XTAL_CONFIG register (subaddress 0EH).
In the bipolar mode the FSK-switch can be controlled by a ramp function. This ramp function is set
by the bits D1 and D2 of the XTAL_CONFIG register (subadress 0EH). With these modes of the
FSK-switch the bandwidth of the FSK spectrum can be influenced.
When working in the FET mode the power consumption can be reduced by about 200 mA.
Preliminary Specification
54
2003-02-18