AN851 Datasheet, PDF (2/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
3. EMI Mitigation
Due to a high frequency (2880–3840 MHz) on-chip VCO, there is some conductive and magnetic coupling from the
VCO to the adjacent traces. The VCO fundamental spur level can be reduced by adding external filtering and using
the proper layout.
3.1. External filtering
Add filter network comprising of L4 and C15 on the RF input trace (VHFI) for filtering the VCO spur
(2880–3840 MHz). The capacitance C15 provides a low impedance path to ground, and inductor L4
provides high impedance to the VCO spur. The inductor L4 also helps in protecting the chip from an
external ESD event by providing high impedance path to the ESD event. The two component network
attenuates the VCO spur from reaching external antenna port and radiating out. The values of these
components are selected to achieve the balance between the sensitivity and the emission levels.
Add low pass network C13 & FB5 and C12 & FB6 on the audio lines Right and Left respectively for
attenuating the VCO coupling
Add high Self Resonant Frequency (>3 GHz) capacitors C2, C6, C7 and C8 on the supply lines to decouple
the VCO leakage currents.
3.2. Front-End Matching
The components (C1, L1, L2, and L4) are used for maximizing the voltage gain on the VHFI pin.
For the FM and DAB band, the matching components C1, L1, L2, and L4 will require optimization to maximize the
voltage gain on the VHFI pin. The inductor L2 can be engaged or disengaged using a VHFSW switch. Refer to
property “0x1712” of AN649 for information on how to handle the VHFSW switch. The voltage gain is maximized by
forming a high Q parallel LC resonant tank circuit. The inductor of the tank circuit is the parallel combination of L1
and L2. And the tank capacitance includes the antenna capacitance, capacitance of the external front end network,
PCB parasitic, internal chip parasitic and internal variable capacitance provided by on-chip varactor tuning.
With a given antenna source impedance and the parasitics (pcb and chip internal), the resonant peak of the LC
tank circuit across the FM and DAB band can be maximized by finding the right combination of C1, L1, L2, and L4,
and the internal varactor capacitance. Refer to Appendix A for additional details on using internal varactor tuning.
Note that the procedure outlined in Appendix A for internal varactor tuning is considering the signal generator
source impedance (50 ) but the same procedure can be used with different antenna source impedance. Refer to
Appendix B on how to select component values for optimizing the front end.
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Rev. 0.2