SI4770-77-A20 Datasheet, PDF(26/54 Page) Silicon Laboratories

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SI4770-77-A20 Datasheet, PDF (26/54 Pages) Silicon Laboratories – HIGH-PERFORMANCE CONSUMER ELECTRONICS

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Si4770/77-A20

4.4. FM Receiver Front-End

The Si4770/77-A20 provides a very flexible front-end

interface to accommodate a wide range of applications

from cost-sensitive to high-performance.

An advanced AGC on the Si4770/77-A20 is

implemented with the use of internal RF peak and IF

peak detectors with programmable thresholds (trip

points). The AGC adjusts the resistor values

automatically. Attack and release rates for the AGC are

programmable, providing flexible fast attack and slow

release AGC performance.

For cost-effective performance and superior FM

sensitivity, the antenna output can be received on the

FMI pin (Figure 9). The FM band can be received on the

FMI pin via an input coupling network. This input

coupling network isolates the FM band for best

performance. An internal LNA provides gain for the

signal. The LNA output is routed externally to the FM

mixer input pins. The LNA gain is regulated with an

internal voltage regulator supply via an internal resistor

bank, RL. The AGC circuit automatically controls the

LNA gain, resistor banks FMAGC1, FMAGC2, and RL to

optimize sensitivity and strong signal handling.

50 ï

FMXIP

FMXIN

RFREG

FMO

RF Pkd

Reg

LNA

FMI

RF Pkd

Si477x

FMXIP

RF Pkd

50 ï

FMXIN

RFREG

FMO

Reg

LNA

FMI

RF Pkd

Si477x

Figure 10. Conceptual Illustration of the

Lowest-Cost Configuration

4.4.1. FMI LNA for FM Loop-Through Usage

In dual receiver applications, two receivers (Figure 11)

can be attached to a single antenna. The dual receiver

solution allows for independent radio station listening in

different rooms.

The FMI LNA input impedance is software-configurable

and provides two options: 50 ï and 100 ï. Configuring

the input impedance for 100 ï facilitates a Si4770/77-

A20 receiver 1 and the Si4770/77-A20 receiver 2 to be

interfaced to the antenna output in parallel, providing a

matched 50 ï input impedance. AGC is coordinated

between both receivers whereby the resistor banks,

FMAGC1, FMAGC2, and RL, from one receiver are

used to optimize sensitivity and strong signal handling.

Figure 9. Conceptual Illustration of the Use of

the FMI LNA for Cost-Optimized and Superior

FM Sensitivity Performance

Cost can be further reduced by eliminating the 1:1 balun

and directly interfacing the signal to the FM mixer by

programming the mixer for single-ended input mode

(Figure 10). The trade-off is a drop in linearity of 6 dBÂµV

in IP3.

Rev. 0.9

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