RFM42B datasheet(27/56 Pages) ETC2

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RFM42B/43B

5. Internal Functional Blocks

This section provides an overview some of the key blocks of the internal radio architecture.

5.1. Synthesizer

An integrated Sigma Delta (ΣΔ) Fractional-N PLL synthesizer capable of operating from 240–960 MHz is provided

on-chip. Using a ΣΔ synthesizer has many advantages; it provides flexibility in

channel frequency, and channel spacing. The transmit modulation is applied directly

domain through the fractional divider which results in very precise accuracy and control over the transmit

Depending on the part, the PLL and

- modulator scheme is designed to support any desired frequency and

channel spacing in the range from 240–960 MHz with a frequency resolution of 156.25 Hz (Low band) or 312.5 Hz

(High band). The transmit data rate can be programmed between 0.123–256 kbps, and the frequency deviation

can be programmed between ±1–320 kHz. These parameters may be adjusted via registers as shown in "3.5.

Frequency Control".

Figure 10. PLL Synthesizer Block Diagram

The reference frequency to the PLL is 10 MHz. The PLL utilizes a differential L-C VCO, with integrated on-chip

inductors. The output of the VCO is followed by a configurable divider which will divide down the signal to the

desired output frequency band. The modulus of this divider stage is controlled dynamically by the output from the

- modulator. The tuning resolution is sufficient to tune to the commanded frequency with a maximum accuracy of

312.5 Hz anywhere in the range between 240–960 MHz.

5.1.1. VCO

The output of the VCO is automatically divided down to the correct output frequency depending on the hbsel and

fb[4:0] fields in "Register 75h. Frequency Band Select." The VCO integrates the resonator inductor, tuning varactor,

so no external VCO components are required.

The VCO uses a capacitance bank to cover the wide frequency range specified. The capacitance bank will

automatically be calibrated every time the synthesizer is enabled. In certain fast hopping applications this might not

be desirable so the VCO calibration may be skipped by setting the appropriate register.

5.2. Power Amplifier

The RFM42B contains an internal integrated power amplifier(PA) capable of transmitting at output levels between-1

and +20dBm. The RFM43B contains a PA which is capable of transmitting output levels between –8 to +13 dBm.

The PA can chang the output

efficiency will not be constant. The PA output is ramped up and down to prevent unwanted spectral splatter.

LPF

PFD

Delta-

Sigma

Fref = 10 M

VCO

Modulation

Selectable

Divider

Tel: +86-755-82973805

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choosing data rate, deviation,

to the loop in the digital

deviation.

power by adjusting txpow[2:0] will scale both the output power and current but the

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