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RF250

Rx ASIC for CDMA, AMPS, and PCS Applications

Conexant Systems 

Rx ASIC

RF250

4

CELL_LNA_IN

7

CELL/PCS

23

SLEEP

PCS_LNA_IN

RF SAW(CELL)

AMPS IF SAW

CDMA IF SAW

48

42

FM/CDMA

34

8

28

2

32, 33

2

26, 27

2

11,12

19

I

20

RF250

Rx ASIC

÷ 2,4

21

Q

22

40

35,36

29,30

38

39

2

2

24

13,14

2

16,17

PLL

RF SAW (PCS)

IF SAW (PCS)

C262

Figure 2. RF250 Rx ASIC Block Diagram

Technical Description

Low Noise Amplifier (LNA). The cellular band

LNA is designed with a low noise figure and high

linearity to achieve maximum receiver dynamic

range. Pin 2, the 800 LNA decouple pin, is

required to be grounded through an RF bypass

capacitor with minimum trace length. The input

and output match are external to the chip.

Mixers. The RF250 Rx ASIC has three

independent mixers, one for the PCS band and

two for the cellular band (AMPS and CDMA).

The mixers are designed to operate with very low

LO powers of –10 dBm. The LO ports are matched

internal to the chip.

The cellular band mixers have a high gain and a

low noise figure that allow them to meet the

system noise figure. The cellular CDMA and PCS

mixers have balanced output to drive the IF filters.

The AMPS mixer has a single-ended output to

match the standard IF SAW filters.

Variable Gain Amplifier (VGA). The high dynamic

range required by CDMA handsets is achieved by

the VGA, which is common to all modes. The VGA

has a minimum dynamic range of 90 dB with a

control voltage of 0.2 to 2.7 volts. The appropriate

signal path is switched internal to the device. This

eliminates off-chip switching needed to operate

this common VGA in cellular AMPS, CDMA, and

PCS modes.

I/Q Demodulator. The local oscillator signals are

generated on-chip. The I/Q demodulator is

internally connected to the VGA output. It is

designed to have a very low amplitude and phase

imbalance. The I and Q outputs are differential.

The DC offsets between the differential outputs

and between I and Q channels are designed to be

extremely low to facilitate compatibility with

baseband Interfaces.

VHF Oscillators. There are two on-chip

oscillators, one for the cellular and one for the PCS

bands. These Voltage Controlled Oscillators

(VCOs) work with external tank circuits and

varactor diodes. The outputs of the differential

oscillators are buffered and the output is used to

drive the prescaler of an external Phase Locked

Loop (PLL). The VCOs typically operate at twice

the IF frequency and can operate at up to four

times the IF frequency.

2

Conexant Systems, Inc.

101251A

August 24, 2000

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