U2731B Datasheet, PDF (6/25 Pages) ATMEL Corporation – DAB ONE- CHIP FRONT END

RF Part
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 section “Simple Two-wire Bus Functions” on page 11), 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 com-
ponents stronger than -40 dBc.
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 output pins SAW1, SAW2. The mixer's LO signal is generated
by a balanced voltage-controlled oscillator whose frequency is stabilized by a frac-
tional-N phase-locked loop. An example circuit of the VCO is shown in Figure 12 on
page 23. The oscillator's tank is applied to the pins B1VC, C1VC, B2VC, C2VC as
shown in the application circuit in Figure 8 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 frequency is either
divided by 1 or 2. The setting of this divider is defined by means of the two-wire bus bits
M0 and M1 as indicated in the section “Simple Two-wire Bus Functions” on page 11.
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.
RF AGC
Voltage-generation Block
In this functional block, the output signal of the RF mixer is amplified, weakly bandpass
filtered (transition range: X8 MHz to X80 MHz), rectified and finally lowpass 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 attenua-
tor 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 compared 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).
By means of the input pins WAGC and SLI the current of the RF AGC charge pump can
be selected according to the following table:
Table 1. Current of Charge Pump
WAGC
SLI
HIGH
X
LOW
LOW
LOW
HIGH
Charge-pump Current/µA
off
50 µA (slow mode)
190 µA (fast mode)
The function can be seen in Figure 11 on page 22.
6 U2731B
4671C–DAB–06/04