RF42/43
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 240–930 MHz is rovided
on-chip. Using a ΣΔ synthesizer has many advantages; it provides large amounts of flexibility in choosing data rate,
deviation, channel frequency, and channel spacing. The transmit modulation is applied directly to the loop in the digital
domain through the fractional divider which results in very precise accuracy and control over the transmit deviation.
The PLL and Δ-Σ modulator scheme is designed to support any desired frequency and channel spacing in the range
from 240–930 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 1–128 kbps, and the frequency deviation can be programmed between ±1–160 kHz.
These parameters may be adjusted via registers as shown in "3.6. Frequency Control".
Figure 9. 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 spiral
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 Δ-Σ
modulator. The tuning resolution of the Δ-Σ modulator is determined largely by the over-sampling rate and the number
of bits carried internally. The tuning resolution is sufficient to tune to the commanded frequency with a maximum
accuracy of 312.5 Hz anywhere in the range between 240–930 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". A 2X VCO is utilized to help avoid problems due to frequency
pulling, especially when turning on the integrated Power Amplifier. In receive mode, the LO frequency is automatically
shifted downwards (without reprogramming) by the IF frequency of 937.5 kHz, allowing transmit operation on the same
frequency. The VCO integrates the resonator inductor, tuning varactor, so no external VCO components are required.
The VCO uses 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.
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