ATR2731 Datasheet, PDF (7/26 Pages) ATMEL Corporation – DAB One-chip Front End

ATR2731
4. RF Part
4.1 RF Gain-controlled Amplifier
In order to support two different channels, two identical input buffers with balanced inputs
(RFA1, RFA2; RFB1, RFB2) are integrated. By setting the two-wire bus bits M0 and M1 (see
“Simple Two-wire Bus Functions” on page 12), the active buffer can be selected. The buffers are
followed by a gain-controlled amplifier whose output signal is fed to a gain-controlled mixer. The
RF amplifiers are capable of handling input signals up to a typical power of –6 dBm without
causing third-order intermodulation components stronger than –40 dBc.
4.2 RF Gain-controlled Mixer, VCO and LO Divider
The purpose of the RF mixer is to down-convert the incoming signal (band II, band III) to an IF
frequency, which is typically 38.912 MHz. This IF signal is fed to an AGC voltage-generation
block (which is described in the following section) and an output buffer stage. This driver stage
has a low output impedance and is capable of driving a SAW filter directly via its differential out-
put pins SAW1 and SAW2. The mixer's LO signal is generated by a balanced voltage-controlled
oscillator whose frequency is stabilized by a fractional-N phase-locked loop. An example circuit
of the VCO is shown in Figure 13-6 on page 24. The oscillator's tank is applied to the pins B1VC,
C1VC, B2VC and C2VC as shown in the application circuit in Figure 12-1 on page 20. Before
the VCO’s signal is fed to the RF mixer, it has to pass an LO divider block where the VCO fre-
quency is divided by either 1 or 2. The setting of this divider is defined by means of the two-wire
bus bits M0 and M1 as indicated in “Simple Two-wire Bus Functions” on page 12. This feature
offers the possibility of covering both band II and band III by tuning the VCO frequency in the
range between 200 MHz to 300 MHz.
4.3 RF AGC Voltage-generation Block
In this functional block, the output signal of the RF mixer is amplified, weakly band-pass filtered
(transition range: X8 MHz to X80 MHz), rectified and finally low-pass filtered. The voltage
derived in this power-measurement process is compared to a voltage threshold (th1) which can
be digitally controlled by an on-chip 4-bit D/A converter. The setting of this converter is defined
by means of the two-wire bus bits TAi (i = 1, 2, 3, 4). Depending on the result of this comparison,
a charge pump feeds a positive or negative current to pin CPRF in order to charge or discharge
an external capacitor. The voltage of this external capacitor can be used to control the gain of an
external preamplifier or attenuator stage. Furthermore, it is also used to generate the internal
control voltages of an RF amplifier and mixer. For this purpose, the voltage at pin CPRF is com-
pared to a voltage threshold (th2) which is also controlled by an on-chip 4-bit D/A converter
whose setting is fixed by the two-wire bus bits TBi (i =1, 2, 3, 4).
The current of the RF AGC charge pump can be selected using the input pins WAGC and SLI
(Table 4-1):
Table 4-1.
WAGC
High
Low
Low
Current of Charge Pump
SLI
X
Low
High
The function can be seen in Figure 13-5 on page 23.
Charge-pump Current [µA]
Off
50 (slow mode)
190 (fast mode)
7
4904A–DAB–03/06