ISM 868 MHz, 433 MHz and 315 MHz FSK Receiver Cell – Preliminary Data Sheet

ASCell3912

Rev. A, February 2000

Page 4 of 14

Austria Mikro Systeme International AG

1.1 Analog Receiver Part

The input signal is a low to moderately high modulation index continuous phase frequency shift

keying modulated RF signal around a carrier Fc. This signal is amplified by the low noise ampli-

fier (LNA) and fed to the In-phase and Quadrature-phase mixers (I/Q mixers). The mixers con-

vert the RF signal directly to base band. The local oscillator signal for the I/Q mixers is generated

by the on-chip PLL.

The two base band signals (signals I and Q) are filtered and further amplified. After DC offset

cancellation to remove the static and quasi-static DC offsets and to ensure fast wake-up of the

receiver, the signals are hard limited. The rectangular signals I’ and Q’ are fed to the digital part

where demodulation and the further signal processing is applied.

1.1.1 RF Synthesizer

Frequency synthesis is performed by a conventional synthesizer consisting of a phase detector,

a charge pump, a voltage controlled oscillator working at 315~868.3 MHz, and a feedback divider

by 16 (315.00MHz); 32 (315, 433.92MHz), or 64 (868.3MHz). A truth table for the different fre-

quencies is given in Table 1.

FB1

FB0

RF-SEL

XO-SEL

19,6875

16

315.000

H

L

L

L

13.5600

32

433.920

L

H

L

H

13.5672

64

868.300

L

L

H

H

Table 1:

Quartz and RF output frequencies.

Note: XO-SEL and RF-SEL are intenal generated Signals from the FB[1:0] bits of the setup information.

1.1.2

LNA

The amplification of the LNA can be switched in two states. The gain can be switched of about

10dB with the LNA bit of the setup command.

Note: LNA is one bit of the setup information.

1.1.3 I/Q Down Converter

The ASCell3912 contains a high performance quadrature down converter with low DC offset and

high isolation of RF- and LO-ports.

1.1.4

Base Band Filter

To achieve optimum blocking performance, the base band filter is realized in two separated cir-

cuit blocks. The first filter block removes high level blocking signals out of receive band, the sec-

ond filter block serves for high selectivity of adjacent interferers.

1.1.5 DC-Cancellation and Adjustment of Lower Cut-Off Frequency

The DC offset is removed by a first order high-pass with switchable limit frequency. In the first

step the frequency offset of transmitter and receiver is not compensated, therefore the lower

band limit is about 10 kHz. In the second step, the receiver frequency is adjusted and the lower

limit frequency of the DC-block is set to about 40 kHz and therefore the total bandwidth of pass

band is reduced. At the output of the DC block is a switch to initialize the DC-offset in the power-

up instant, at the instant of switching, and after appearance of high level interfering signals.