U2731B Datasheet, PDF (5/25 Pages) ATMEL Corporation – DAB ONE- CHIP FRONT END

U2731B
Pin Description (Continued)
Pin
Symbol Function
41
PLCK Lock-indicating output (open collector)
42
OSCI
Input of reference oscillator/buffer
43
OSCO Output of reference oscillator/buffer
44
ADR
Address selection (simple two-wire bus)
Functional Description
The U2731B represents a monolithically integrated front end IC designed for applica-
tions in DAB receivers. It covers RF and IF signal processing, the PLL section and also
supporting functions such as D/A converters or switching outputs.
Two RF input ports offer the possibility of handling various input signals such as a down-
converted L-band signal or band II and band III RF signals. The high dynamic range of
the RF inputs and the use of a gain-controlled amplifier and a gain-controlled mixer in
the RF section offer the possibility of handling even strong RF input signals. The LO
signal of the first mixer stage is derived from an on-chip VCO. The VCO frequency is
either divided by two or directly fed to the mixer. In this way band II and band III can be
covered easily.
In the IF section, it can be selected if the first IF signal is down-converted to a second,
lower IF or if it is simply amplified to appear at the IF output. If the down-conversion
option is chosen, it can be selected if the LO signal of the IF mixer is directly derived
from the reference signal of the PLL, or if it is generated by doubling its frequency. The
amplifiers in the IF section are gain-controlled in similar fashion to the RF section.
The RF and the IF part also contain AGC functional blocks which generate the AGC
control voltages. The AGC thresholds can be defined by means of three on-chip 4-bit
D/A converters.
The frequency of the VCO is locked to a reference frequency by an on-chip fractional-N
PLL circuit which guarantees a superior phase-noise performance. The reference
frequency is generated by an on-chip crystal oscillator which can also be overdriven by
an external signal. Starting from a minimum value, the reference scaling factor is freely
programmable.
Three switching outputs can be used for various switching tasks on the front end board.
Three 8-bit D/A converters providing an output voltage between 0 and 8.5 V are used to
improve the tuning voltages of the tuned preselectors which are derived from the tuning
voltage of the VCO.
5
4671C–DAB–06/04