SX1211I084TRT(2007) Просмотр технического описания (PDF) - Semtech Corporation

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SX1211I084TRT
(Rev.:2007)

Lowest Power Integrated UHF Transceiver Short Range Devices

Semtech Corporation 

SX1211

ADVANCED COMMUNICATIONS & SENSING

or to fill the FIFO buffers with glitch-free data in Buffered mode. The operation of the receiver is now described in

detail.

Note: Receiver image rejection is achieved by the SAW filter.

3.4.2. LNA and First Mixer

In receive mode, the RFIO pin is connected to a fixed gain, common-gate, Low Noise Amplifier (LNA). The

performance of this amplifier is such that the Noise Figure (NF) of the receiver can be estimated to be ≈7 dB.

3.4.3. IF Gain and Second I/Q Mixer

Following the LNA and first down-conversion, there is an IF amplifier whose gain can be programmed from -

13.5 dB to 0 dB in 4.5 dB steps, via the register MCParam_IF_gain. The default setting corresponds to 0 dB gain,

but lower values can be used to increase the RSSI dynamic range. Refer to section 3.4.7 for additional information.

3.4.4. Channel Filters

The second mixer stages are followed by the channel select filters. The channel select filters have a strong

influence on the noise bandwidth and selectivity of the receiver and hence its sensitivity. Each filter comprises a

passive and active section.

3.4.4.1. Passive Filter

Each channel select filter features a passive second-order RC filter, with a bandwidth programmable through the

bits RXParam_PassiveFilt. As the wider of the two filters, its effect on the sensitivity is negligible, but its bandwidth

has to be setup instead to optimize blocking immunity. The value entered into this register sets the single side

bandwidth of this filter. For best performance it must be set to 3 to 4 times the cutoff frequency of the active

Butterworth (or polyphase) filter described in the next section.

3* Fc ButterfFilt ≤ BW passive, filter ≤ 4 * Fc ButterFilt

3.4.4.2. Active Filter

The ’fine’ channel selection is performed by an active, third-order, Butterworth filter, which acts as a low-pass filter

for the zero-IF configuration (FSK), or a complex poly-phase filter for the Low-IF (OOK) configuration. The

RXParam_PolypFilt_on bit enables/disables the polyphase filter.

V3.0 – august 15th, 2007

Page 25 of 73

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