ATR4258 datasheet(6/38 Pages) ATMEL

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4838B–AUDR–05/05

ATR4258

4.2

Test Mode

A special test mode is implemented for final production test only. This mode is activated by set-

ting bit 9 = 1. This mode is not intended to be used in customer applications. For normal

operation, bit 9 has to be set to 0. Bit 22 to bit 30 are deactivated in normal operation mode.

4.3

VCO/Local Oscillator Prescaler

An oscillator circuit is implemented to build a VCO as proposed in the application schematic. The

VCO frequency is used to generate the LO frequency of the 1

st mixer stages. The control voltage

of the VCO is usually generated by the PLL circuit U4256BM. The VCO frequency has a range

of 70 MHZ to 250 MHz to allow the reception of all analog world bands.

A main element of the implemented oscillator circuit is a bipolar NPN transistor. The internally

biased base is connected to pin 13 and the emitter to pin 14. An AGC circuit (bit 30) can be acti-

vated to increase the emitter current until the appropriate oscillation level is reached. The

fundamental emitter current can be changed by bit 52.

In addition (to the AM prescaler) a special VCO prescaler is implemented for all modes (AM, WB

and FM). The divider factor of the prescaler buffer provides the signal of the buffered output (at

pin 16) and the prescaler VCO provides the signal of the 1st FM mixer stage and AM prescaler.

Examples of VCO prescaler settings are described in section “Application Information” on page

29. The divider factor of the VCO and buffer prescaler can be selected according Table 4-5 on

page 7.

Table 4-1.

Operating Mode

AM/FM/Weather Channel

Bit 93

Bit 92

Bit 63

Bit 62

Standby

0100

1011

Weather band

Table 4-2.

Test Mode

Mode

Bit 9

Normal operation

Test mode

Table 4-3.

Local Oscillator AGC

Local Oscillator (VCO)

Bit 30

AGC off (default)

AGC on

Table 4-4.

Local Oscillator Gain

Local Oscillator (VCO)

Bit 52

Low Gain

High Gain

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