Rainbow Electronics RF12B - Manual (Page 3)

RF12B
DETAILED FEATURE-LEVEL DESCRIPTION
The RF12B FSK transceiver is designed to cover the unlicensed frequency bands at 433, 868 and
915 MHz. The device facilitates 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 RF12B 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.
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 approximately 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 in four steps (between 0 and -20dB relative to the highest gain)
according to RF signal strength. It can be useful in an environment with strong interferers.
Baseband Filters
The receiver bandwidth is selectable by
programming the bandwidth (BW) of 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.
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