AN383 Datasheet, PDF (60/69 Pages) Silicon Laboratories – Si47XX ANTENNA, SCHEMATIC, LAYOUT, AND DESIGN GUIDELINES

AN383
For sufficiently large Q, the capacitive element is approximately the same for a series and parallel model.
The circuit model shown in Figure 38 can be represented as a parallel resonant RLC circuit by replacing the series
models with equivalent parallel models as shown in Figure 39. Note that only the values of the resistances change
appreciably. The parallel model for each circuit element is indicated with an asterisk.
FM Transmitter Buffer
Tuning Inductor
PCB
Antenna
C_BUFFER
I_BUFFER
R_BUFFER*
L_TUNE
R_TUNE*
C_PCB
C_ANT
R_ANT*
+
V_ANT
-
Figure 39. Equivalent RLC Model of the Electrically Short Vertical Monopole
C_BUFFER = Variable Tuning Capacitance (C _TUNE) + C_{onchip parasitic}
R_BUFFER* = Variable Tuning Capacitor Parallel Resistance
L_TUNE = Tuning Inductance
R_TUNE* = Tuning Inductor Parallel Resistance
C_PCB = PCB Parasitic Capacitance
C_ANT = Antenna Capacitance
R_ANT* = Antenna Parallel Resistance
C_TUNE will range from 0.25 to 47.75 pF.
C_{onchip parasitic} is approximately 5 pF.
Therefore,
C_BUFFER will range from 5 to 53 pF.
R_BUFFER* will range from 1.2 to 1.6 k
R_TUNE* will be approximately than 2.3 k when L_TUNE is 120 nH with a Q > 30
C_PCB should be 4 pF or less to allow sufficient range for C_BUFFER to tune across the FM band.
C_ANT will be approximately 1 pF.
R_ANT* will be approximately 8 M for Q_ANT = 5000 and is approximated as follows:
R_ANT* = R_ANTQ_ANT2 + 1
60
Rev. 0.8