AN851 Datasheet, PDF (23/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
2. Measure the impedance of the antenna using network analyzer (S11 measurement). Write down the
resistive and reactance values.
3. Measure the impedance of the cable.
4. Convert the above circuit into a simplified parallel LC circuit (shown in previous section).
5. Choose and fix the shunt indicator (L2 value) and then calculate the capacitance required using resonance
frequency equation F=1/2LC at the lowest frequency.
6. Use the antenna graph function of Si46xx EVB GUI to measure the capacitance value required to form
resonance by engaging internal varactor.
a. Verify the capacitance calculated in step 5 (circuit analysis) approximately equals the capacitance
calculated by GUI minus the parasitic capacitance (PCB + Si46xx front end).
7. Ideally, to form resonance across the entire band, the varactor capacitance required at low frequency
should be close to its maximum value 32pf (128 varactor counts).
8. Change L2 if the varactor capacitor measured at low frequency is not close to its max value of 32pf (128
counts). The inductor L2 may not have any impact in forming resonance at low frequency within the band
because the inductive reactance in the Norton branch (shown in the previous section) may be dominating.
9. Change C1 or L1 and repeat steps 4 through 8.
10. Verify that with chosen values of C1, L1, and L2 the entire range of varactor capacitor is put in use across
the frequency band.
11. For tuning across a different band, the inductor switch VHFSW option shall be exercised to engage L3
inductor keeping other component values fixed. For forming resonance within this band, choose a value for
L3 and follow steps 2 through 9.
The single optimized network for multiple band (FM and DAB) usage shall be formed after repeating the above
process two to three times.
Silicon Labs Recommended Front End Network
Silicon Labs recommends using the front end network shown in Figure 18. The component values (C1-33pf, L1-
56nh, L2-120nh, and L3-120nh) are optimized to use with fully extended telescopic antenna (24 inches) and 6.5 to
7-inch length of coax cable connecting the antenna and the module.
Figure 18. Silicon Labs Recommended FE Network
Rev. 0.2
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