SI4463 Datasheet, PDF (28/53 Pages) Silicon Laboratories – HIGH-PERFORMANCE

Si4463/61/60-C
5. Internal Functional Blocks
The following sections provide an overview to the key internal blocks and features.
5.1. RX Chain
The internal low-noise amplifier (LNA) is designed to be a wide-band LNA that can be matched with three or four
external discrete components to cover any common range of frequencies in the sub-GHz band. The LNA has
extremely low noise to suppress the noise of the following stages and achieve optimal sensitivity; so, no external
gain or front-end modules are necessary. The LNA has gain control, which is controlled by the internal automatic
gain control (AGC) algorithm. The LNA is followed by an I-Q mixer, filter, programmable gain amplifier (PGA), and
ADC. The I-Q mixers downconvert the signal to an intermediate frequency. The PGA then boosts the gain to be
within dynamic range of the ADC. The ADC rejects out-of-band blockers and converts the signal to the digital
domain where filtering, demodulation, and processing is performed. Peak detectors are integrated at the output of
the LNA and PGA for use in the AGC algorithm.
The RX and TX pins may be directly tied externally for output powers less than +17 dBm in the higher-frequency
bands and can support +20 dBm in the lower bands, such as 169MHz. This reduces BOM cost by saving the
expense of a switch for single antenna solutions. See the direct-tie reference designs on the Silicon Labs web site
for more details.
5.1.1. RX Chain Architecture
It is possible to operate the RX chain in different architecture configurations: fixed-IF, zero-IF, and scaled-IF. There
are trade-offs between the architectures in terms of sensitivity, selectivity, and image rejection. Fixed-IF is the default
configuration and is recommended for most applications. With 35 dB native image rejection and autonomous image
calibration to achieve 55 dB, the fixed-IF solution gives the best performance for most applications. Fixed-IF obtains
the best sensitivity, but it has the effect of degraded selectivity at the image frequency. An autonomous image
rejection calibration is included in Si446x devices and described in more detail in "5.2.3. Image Rejection and
Calibration" on page 30. For scaled-IF and zero-IF, the sensitivity is degraded for data rates less than 100 kbps or
bandwidths less than 200 kHz. The reduction in sensitivity is caused by increased flicker noise as dc is approached.
The benefit of zero-IF is that there is no image frequency; so, there is no degradation in the selectivity curve, but it
has the worst sensitivity. Scaled-IF is a trade-off between fixed-IF and zero-IF. In the scaled-IF architecture, the
image frequency is placed or hidden in the adjacent channel where it only slightly degrades the typical adjacent
channel selectivity. The scaled-IF approach has better sensitivity than zero-IF but still some degradation in
selectivity due to the image. In scaled-IF mode, the image frequency is directly proportional to the channel
bandwidth selected. Figure 9 demonstrates the trade-off in sensitivity between the different architecture options.
1% PER sensitivity vs. data rate (h=1)
-95
-100
-105
Fixed IF
Scaled IF
-110
Zero IF
-115
-120
1
10
100
Data rate (kbps)
Figure 9. RX Architecture vs. Data Rate
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Rev 1.0