SI4464 datasheet(29/56 Pages) SILABS

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Si4464/63/61/60

Rev 1.2

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 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 maybe directly tied externally for output powers less than +17 dBm, 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, scaled-IF, and

modulated 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 31. For fixed-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.

Figure 9. RX Architecture vs. Data Rate

-120

-115

-110

-105

-100

-95

100

Data rate (kbps)

1% PER sensitivity vs. data rate (h=1)

Fixed IF

Scaled IF

Zero IF

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