XI. Theory of Operation
Introduction
The TXC100 is a crystal-referenced transmitter designed to operate in the 315/433 MHz frequency spectrum. The carrier and crystal reference
relation is given by:
fC = fXTAL * 32
It is capable of supporting OOK/ ASK and FSK data transmissions at 100kbps and 20kbps, respectively. The output power is adjustable from
-10dBm to +10dBm thru a resistor at the ESOUT (pin 7). The FSK frequency deviation is programmable with up to eight different deviation values.
The IC also provides a buffered clock output of the reference crystal for use by an external processor. The clock output is also programmable.
Frequency Synthesizer
The frequency synthesizer is simply a Phase Locked Loop circuit with a loop bandwidth of 300 kHz. The PLL contains a phase detector, charge
pump, VCO, integrated loop filter, ÷32 clock divider, and crystal oscillator drive circuit. The internal PLL is self contained and requires no external
components for filtering or dividing. Only a reference crystal is needed.
50W Output Matching
When properly matched, the TXC100 can output up to +12 dBm into a 50W load. The output is an open-drain configuration which requires a pull-up
inductor for proper internal biasing. The pull-up inductance serves to provide biasing for the power amplifier and is a high frequency choke to reduce
unwanted coupling back into the power supply. Maximum power transfer occurs when the output is closely matched to 250W. For best performance
use wirewound inductors instead of chip inductors. Wirewound inductors provide lower insertion loss as opposed to chip inductors. See Typical
Application Circuit (section IV) for topology and matching component values.
PCB Layout Considerations
PCB layout is critical to proper and consistent operation. Always use controlled impedance lines from the PAOUT (pin 8). For a .062” thick FR4 board
a 50W impedance line is approximately .110” wide. Component spacing is critical as well. Keep all output matching components as close together
as possible to minimize stray inductance and capacitance that can detune the matching network. Keep ground planes at least a board thickness
away from the signal output leading to the antenna or RF connector.
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RFM [ RF MONOLITHICS, INC ]
