TXC101 RFM, TXC101 Datasheet - Page 6

• Manufacturer Part Number: TXC101
• Description: IC TX 300MHZ-1GHZ 16-TSSOP
• Manufacturer: RFM
• Series: TXr

Specifications of TXC101

• Frequency: 300MHz ~ 1GHz
• Applications: General Data Transfer
• Modulation Or Protocol: FSK, OOK
• Data Rate - Maximum: 512kbps
• Power - Output: 3dBm
• Current - Transmitting: 13mA
• Data Interface: PCB, Surface Mount
• Antenna Connector: PCB, Surface Mount
• Voltage - Supply: 2.2 V ~ 5.4 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Features: -
• Memory Size: -
• Other names: 583-1098-2

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.
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.
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Ref Des
C1
C2
C4
C7
L1
L2
L3
110 nH
390nH
2.2 pF
100pF
72 nH
3.9pF
.1uF
315
Table 2.
390nH
2.7 pF
1.5 pF
100pF
43 nH
82 nH
.1uF
433
100nH
1.8 pF
100pF
10 nH
27 nH
1 pF
.1uF
868
100nH
1.8 pF
100pF
10 nH
27 nH
1 pF
.1uF
916
TXC101 - 4/8/08
Page 6 of 27