• · Frequencies available as standard: 869.85MHz, 914.5MHz
• · CE certified by independent Notified Body
• · Verified to comply with Radio standard EN 300 220-3 by accredited Test Laboratory
• · Verified to comply with EMC standard EN 301 489-3 by accredited Test Laboratory
• · North American version conforms to FCC part 15.249
• · Data rates up to 64kbps
• · Fully screened
  

Available for operation in the 868-870MHz band in Europe and the 902-928MHz band in North America, both modules combine full screening with internal filtering to ensure EMC compliance by minimising spurious radiation and susceptibility. The TX3A & RX3A will suit one-to-one and multi-node wireless links in such applications as car and building security, EPOS and inventory tracking, remote industrial process monitoring and data networks. Because of their small size and low power requirements, both modules are ideal for use in portable, battery-powered applications such as hand-held terminals.

Fig.3: RX3A block diagram

Performance specifications: RX3A receiver
(Vcc = 3.0V / temperature = 20°C unless stated)

Notes:

1. Current increases at higher RF input levels (-20dBm and above).
2. For received signal with ±30kHz FM deviation.
3. Typical figures are for signal at centre frequency, max. figures are for ±50kHz offset.
4. For 50:50 mark to space ratio (i.e. square wave).
5. Average over 30ms (10kbps version) or 3ms (64kbps version) at maximum data rate.

Module test circuits

Fig.5: TX3A test circuit

Power supply requirements

Both modules incorporate a built-in regulator which delivers a constant 2.8V to the module circuitry when the external supply voltage is 2.85V or greater, with 40dB or more of supply ripple rejection. This ensures constant performance up to the maximum permitted supply rail and removes the need for external supply decoupling except in cases where the supply rail is extremely poor (ripple/noise content >0.1Vp-p).
Note, however, that for supply voltages lower than 2.85V the regulator is effectively inoperative and supply ripple rejection is considerably reduced. Under these conditions the ripple/noise on the supply rail should be below 10mVp-p to avoid problems. If the quality of the supply is in doubt, it is recommended that a 10mF low-ESR tantalum or similar capacitor be added between the module supply pin (Vcc) and ground, together with a 10W series feed resistor between the Vcc pin and the supply rail.
The Enable pin allows the module to be turned on or off under logic control with a constant DC supply to the Vcc pin. The module current in power-down mode is less than 1mA.

NOTE: If this facility is used, the logic control signal must have a slew rate of 40mV/ms or more. Slew rates less than this value may cause erratic operation of the on-board regulator and therefore the module itself.

The TX3A incorporates a low voltage shutoff circuit which prevents any possibility of erratic operation by disabling the RF output if the supply voltage drops below 2.2V (±5%). This feature is self-resetting, i.e. restoring the supply to greater than 2.2V will immediately restore full RF output from the module.

TX3A modulation requirements

The module will produce the specified FM deviation with a TXD input to pin 7 of 2.5V amplitude, i.e. 0V "low", 2.5V "high". Reducing the amplitude of the data input from this value (usually as a result of reducing the supply voltage) reduces the transmitted FM deviation to typically ±25kHz at the lower extreme of 2.2V. The receiver will cope with this quite happily and no significant degradation of link performance should be observed as a result.
Where standard 2-level digital data is employed with a logic "low" level of 0V ±0.2V, the logic "high" level applied to TXD may be any value between +2.5V and +3V for correct operation. However, if using multi-level or analogue signalling the maximum positive excursion of the modulation applied to TXD must not exceed +2.5V or waveform distortion will result. If the input waveform exceeds this level a resistive potential divider should be used at the TXD input to reduce the waveform amplitude accordingly. This input is high impedance (>100kW) and can usually be ignored when calculating required resistor values.

Data formats and range extension

The TX3A data input is normally driven directly by logic levels but will also accept analogue drive (e.g. 2-tone signalling). In this case it is recommended that TXD (pin 7) be DC-biased to 1.25V with the modulation ac-coupled and limited to a maximum of 2.5Vp-p to minimise distortion over the link. The varactor modulator in the TX3A introduces some 2nd harmonic distortion which may be reduced if necessary by predistortion of the analogue waveform. At the other end of the link the RX3A AF output is used to drive an external decoder directly.

Both the AF output on pin 8 and the RXD output on pin 9 of the RX3A are "true" sense, i.e. as originally fed to the transmitter.

Although the modulation bandwidth of the TX3A extends down to DC, as does the AF output of the RX3A, it is not advisable to use data containing a DC component. This is because frequency errors and drifts between the transmitter and receiver occur in normal operation, resulting in DC offset errors on the RX3A audio output.

The RX3A incorporates a low pass filter which works in conjunction with similar filtering in the TX3A to obtain an overall system bandwidth of 32kHz. This is suitable for transmission of data at raw bit rates up to 10kbps and 64kbps, depending on the receiver version. To keep settling times within reasonable limits for the data speed in use, the adaptive data slicer in the RX3A is subject to a maximum time limit between data transitions (see page 5). This limitation must be taken into account when choosing a code format. It is strongly recommended that a reasonably balanced code containing no long 1s or 0s (such as Manchester or similar) is employed.

In applications such as longer range fixed links where data speed is not of primary importance, a significant increase in range can be obtained by using the slowest possible data rate together with filtering to reduce the receiver bandwidth to the minimum necessary. In these circumstances, because of the limitations of the internal data slicer it is better to use the RX3A audio output to drive an external filter and data slicer.

RX3A Received Signal Strength Indicator (RSSI)

The RX3A receiver incorporates a wide range RSSI which measures the strength of an incoming signal over a range of 60dB or more. This allows assessment of link quality and available margin and is useful when performing range tests.

The output on pin 5 of the module has a standing DC bias of typically 0.25V with no signal, rising to 1.1V at maximum indication. The RSSI output source impedance is high (~50kW) and external loading should therefore be kept to a minimum.

Typical RSSI characteristic is as shown below:
Fig.7: RX3A RSSI response curve

Integral antennas generally do not perform as well as externally mounted types, however they result in physically compact equipment and are the preferred choice for portable applications. The following can be recommended:

Whip (¼-wave): This consists simply of a piece of wire or rod connected to the module at one end. The lengths given below are from module pin to antenna tip including any interconnecting wire or tracking. This antenna is quite simple and performs well, especially if used in conjunction with a ground plane. This will often be provided by the PCB on which the module is mounted, or by a metal case.

Base-loaded whip: This is a shortened whip, tuned by means of a coil inserted at the base. This coil may be air-wound for maximum efficiency, or a small SMT inductor can be used if space is at a premium. The value must be carefully chosen to tune the particular length of whip in use, making this antenna more difficult to set up than a ¼-wave whip.

Helical: This is a more compact but slightly less effective antenna formed from a coil of wire. It is very efficient for its size, but because of its high Q it suffers badly from detuning caused by proximity to nearby conductive objects and needs to be carefully trimmed for best performance in a given situation. It can, however, provide an extremely compact solution.

Loop: A loop of PCB track, tuned and matched with 2 capacitors. Loops are relatively inefficient but have good immunity to proximity detuning, so may be preferred in shorter range applications where very high component packing density is necessary.