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Detailed feature-level description, Rf12, Rf power amplifier (pa) – Rainbow Electronics RF12 User Manual

Page 3: Baseband filters

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RF12

DETAILED FEATURE-LEVEL DESCRIPTION

The RF12 FSK transceiver is designed to cover the unlicensed frequency bands at 315, 433, 868

and 915 MHz. The devices facilitate compliance with FCC and ETSI requirements.

The receiver block employs the Zero-IF approach with I/Q demodulation, allowing the use of a

minimal number of external components in a typical application. The RF12 incorporates a fully integrated

multi-band PLL synthesizer, PA with antenna tuning, an LNA with switch-able gain, I/Q down converter

mixers, baseband filters and amplifiers, and an I/Q demodulator followed by a data filter.

PLL

The programmable PLL synthesizer determines the operating frequency, while preserving accuracy

based on the on-chip crystal-controlled reference oscillator. The PLL’s high resolution allows the usage of

multiple channels in any of the bands.

The RF VCO in the PLL performs automatic calibration, which requires only a few microseconds.

Calibration always occurs when the synthesizer starts. If temperature or supply voltage changes

significantly, VCO recalibration can be invoked easily. Recalibration can be initiated at any time by

switching the synthesizer off and back on again.

RF Power Amplifier (PA)

The power amplifier has an open-collector differential output and can directly drive a loop antenna

with a programmable output power level. An automatic antenna tuning circuit is built in to avoid costly

trimming procedures and the so-called “hand effect.”

LNA

The LNA has 250 Ohm input impedance, which functions well with the proposed antennas. If the

RF input of the chip is connected to 50 Ohm devices, an external matching circuit is required to provide

the correct matching and to minimize the noise figure of the receiver.

The LNA gain can be selected (0, –6, –14, –20 dB relative to the highest gain) according to RF signal

strength. It can be useful in an environment with strong interferers.

Baseband Filters

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electable

BW) of

The receiver bandwidth is s

by programming the bandwidth (

the baseband filters. This allows setting

up the receiver according to the

characteristics of the signal to be

received.

An appropriate bandwidth can be

chosen to accommodate various FSK

deviation, data rate and crystal tolerance

requirements. The filter structure is 7th

order Butterworth low-pass with 40 dB

suppression at 2*BW frequency. Offset cancellation is done by using a high-pass filter with a cut-off

frequency below 7 kHz.