U2741B_05 Datasheet, PDF (5/11 Pages) ATMEL Corporation – UHF ASK/FSK TRANSMITTER

U2741B
5. Application Circuit
The following component values are recommendations for a typical application. C₅, C₆, and C₇
are block capacitors. The values of these capacitors depend on the board layout. C₅ = 1 nF,
C₆ = 1 nF, and C₇ = 22 nF are typically used here. For C₅, the impedance between f = 100 MHz
and f = 1 GHz should be as low as possible.
C₃ is not needed in ASK transmitter applications. In the case of FSK, C₃ is selected in such a
way that the desired transmission frequency deviation is achieved (typical ±30 kHz). The capac-
itance here depends upon the crystal's load capacity (C₄) recommended by the manufacturer of
the crystal. C₂ = 3.9 nF, C₁ = 15 nF, and R₄ = 220Ω.
C_Loop1 and C_Loop2 are selected so that the antenna oscillates in resonance and the adaptation to
the appropriate impedance transformation is possible.
L_Feed is an inductor for the antenna's DC current supply. A typical value is L_Feed = 220 nH. L_Feed
can be either printed on the PC board or be a discrete component.
5.1 Output Power Measurement
The output network [as shown in Figure 5-1] can be used for output power evaluation, the exact
values of L₁₀ and C₁₀ depend on the layout.
L₁₀ and C₁₀ form the transformation network to adopt the output impedance of the IC to 50Ω.
Table 5-1 shows the values for an output power of 2 mW and an R_PWRSET = 1.2 kΩ.
Table 5-1. Transformation Network
f/MHz
315
433.92
C10/pF
2.7
1.8
L10/nH
56
33
Figure 5-1. Measurement Output Network
V_S
PWRVCC
L₁₀
ANT
C₁₀
50 Ω
Z_Load-opt
Z_{Load_opt}/Ω
260 + j330
185 + j268
5
4733B–RKE–09/05