Rev. 0.1 10/11 Copyright © 2011 by Silicon Laboratories AN643
AN643
Si446x RX LNA Matching
1. Introduction
The purpose of this application note is to provide a description of the impedance matching of the RX differential low
noise amplifier (LNA) on the Si446x family of RFICs.
It is desired to simultaneously achieve two goals with the matching network:
Match the LNA input to the antenna source impedance (e.g., 50
Provide a single-ended to differential conversion function (i.e., a balun)
The matching procedure outlined in this document provides for achieving the goals listed above.
For those users who are not interested in the theoretical derivation of the match network, but are just concerned
with quickly obtaining matching component values, refer to the Summary Tables shown in "4.1.7. Summary Tables
of 3-Element Match Network Component Values vs. Frequency" on page 12 and "4.2.7. Summary Tables of 4-
Element Match Network Component Values vs. Frequency" on page 19.
Measurements were performed on the Si4461-B0 chip but are applicable to other members of the Si446x family of
chips.
2. Match Network Topology
The LNA on the Si446x family of chips is designed as a differential amplifier and thus has two input pins (RXp and
RXn) on the RFIC. It is necessary to design a network that not only provides a conjugate match to the input
impedance of the LNA but also provides a balanced-to-unbalanced conversion function (i.e., a balun).
Use of two basic matching network topologies will be considered within this application note.
2.1. Three-Element Match Network
The simplest match network that may be fabricated from discrete components is comprised of three discrete
elements. Two forms of the 3-element match network may be constructed: one with a highpass filter (HPF)
response, and one with a lowpass filter (LPF) response. However, the form with a lowpass filter response is not
realizable at all frequencies and input impedances. As a result, only the form with a highpass filter response is
discussed within this document.
A 3-element (CR1-LR1-CR2) HPF matching network is shown in Figure 1. This matching network has the virtue of
requiring a minimum number of components but results in slightly sub-optimal performance. It is not theoretically
possible to achieve a perfectly balanced single-ended-to-differential conversion function with this matching network
for input impedances with finite values of R
LNA
. As will be demonstrated, the waveforms obtained at the RXp and
RXn inputs to the RFIC will not be exactly 180° out of phase; the result is a very slight loss in conversion gain in the
LNA and a small drop in overall sensitivity of the RFIC. The reduction in performance is typically less than 0.5 dB;
many customers may view this as an acceptable trade-off for the reduction in the bill of materials (BOM).
The RXp and RXn inputs of the Si446x RX LNA internally contain high value (~15 k) pull-down resistors to GND.
As a result, supplying a dc voltage to these pins is not recommended; use of external AC-coupling to these pins is
suggested. This is inherently supplied by capacitor CR2 of Figure 1.
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