ATR4258 Datasheet, PDF (6/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

Test Mode
VCO/Local Oscillator
Prescaler
Table 1. Operating Mode
AM/FM/Weather Channel
Standby
FM
AM
Weather band
Bit 93
0
0
1
1
Bit 92
0
1
0
1
Bit 63
X
0
1
1
Bit 62
X
0
1
0
A special test mode is implemented for final production test only. This mode is activated
by setting 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.
Table 2. Test Mode
Mode
Normal operation
Test mode
Bit 9
0
1
An oscillator circuit is implemented to build a VCO as proposed in the application sche-
matic. 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 activated to increase the emitter current until the appropriate oscillation
level is reached. The fundamental emitter current can be changed by bit 52.
Table 3. Local Oscillator AGC
Local Oscillator (VCO)
AGC off (default)
AGC on
Bit 30
0
1
Table 4. Local Oscillator Gain
Local Oscillator (VCO)
Low Gain
High Gain
Bit 52
0
1
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 1^st 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 5 on
page 7.
6 ATR4258
4838A–AUDR–10/04