6

These values are what the RF port pins want to “see” as an antenna load for maximum power transfer.

Antennas ideally suited for this would be a Dipole, Folded Dipole, and Loop. For all transmit antenna

applications a bias or “choke” inductor must be included since the RF outputs are open-collector type.

The TXC101 may also drive a single ended 50 Ohm load, such as a monopole antenna, using the

matching circuit as shown in Figure 1. Use of a balun would provide an optimum power transfer, but the

matching circuit of Figure 1 has been optimized for use with discrete components, reducing the cost

associated with use of a balun.

The matching component values for a 50 Ohm load are given in Table 2.

Table 2.

Ref Des

315

433

868

916

C1

3.9pF

2.7 pF

1.8 pF

1.8 pF

C2

2.2 pF

1.5 pF

1 pF

1 pF

L1

72 nH

43 nH

10 nH

10 nH

L2

390nH

390nH 100nH 100nH

L3

110 nH

82 nH

27 nH

27 nH

Antenna Design Considerations

The TXC101 was designed to drive a differential output such as a Dipole or a Loop antenna. A loop

antenna is recommended for applications where size is critical. The dipole is typically not an attractive

option for compact designs based on its inherent size at resonance and distance needed away from a

ground plane to be an efficient radiating antenna. A monopole is possible with addition of a balun or

using the matching circuit in Figure 1.

PCB Layout Considerations

PCB layout is very critical. For optimal transmit and receive performance, the trace lengths at the RF pins

must be kept as short as possible. Using small, surface mount components, like 0402, will yield the best

performance and also keep the RF port compact. It is recommended that all RF connections are made

short and direct. A good rule of thumb to adhere to is to add 1nH of series inductance to the path for

every 0.1” of trace length. The crystal oscillator is also affected by additional trace length as it adds

parasitic capacitance to the overall load of the crystal. To minimize this effect the crystal must be placed

as close as possible to the chip and all connections must be made short and direct. This will minimize the

effects of “frequency pulling” , that stray capacitance may introduce and allow the internal load

capacitance of the chip to be more effective in properly loading the crystal oscillator circuit.

When an external processor is used, the TXC101 provides an on-chip clock. Even though this is an

integrated function, long runs of the clock signal may radiate and cause interference. This can degrade

receiver performance as well as add harmonics or unwanted modulation to the transmitter. Keep clock

connections as short as possible and surround the clock trace with an adjacent ground plane pour where

needed. This will help in reducing any radiation or crosstalk due to long runs of the clock signal.

Good power supply bypassing is also essential. Large decoupling capacitors should be placed at the

point where power is applied to the PCB. Smaller value decoupling capacitors should then be placed at

each power point of the chip as well as bias nodes for the RF port. Poor bypassing lends itself to

conducted interference which can cause noise and spurious signals to couple into the RF sections, thus

significantly reducing performance.