RFM22
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.
5.6.1. 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 and receive 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.
5.7. Power Amplifier
The RFM22 contains an internal integrated power amplifier (PA) capable of transmitting at output levels between +8 to +17 dBm.
The output power is programmable in 3 dB steps through the txpow[1:0] field in "Register 6Dh. TX Power".
The PA design is single-ended and is implemented as a two stage class CE amplifier with efficiency in the range of
45–50% while transmitting at maximum power. The efficiency drops to approximately 20% when operating at the
lowest power steps. Due to the high efficiency a simple filter is required on the board to filter the harmonics. The PA
output is ramped up and down to prevent unwanted spectral splatter.
5.7.1. Output Power Selection
The output power is configurable in 3 dB steps from +8 dBm to +17 dBm with the txpow[1:0] field in "Register
6Dh. TX Power". Note that Frequency Hopping (FHSS) is required by the FCC when using an output power level of
+17 dBm. See " Analog and Digital Test Bus" for further information on FHSS. The PA output is ramped up and down
to prevent unwanted spectral splatter.
The extra output power can allow use of a cheaper smaller antenna, greatly reducing the overall BOM cost. The
higher power setting of the module achieves maximum possible range, but of course comes at the cost of higher TX
current consumption. However, depending on the duty cycle of the system, the effect on battery life may be
insignificant. Contact HopeRF Support for help in evaluating this tradeoff.
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W Function/Description
R/W
TX Power
txpow[1]
txpow[0]
07h
6D
txpow[1:0]
Output Power
00
+8 dBm
01
10
11
+11 dBm
+14 dBm
+17 dBm
35
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