NOTES:

1. EN pin should not be left floating
2. Pinout footprint is as TX1H.
3. Compatible with RX2M, RLC2 and NRX2

Performance specifications: NRX2 receiver
(Vcc = 3V / temperature = 20°C unless stated)

Both modules have built-in regulators which deliver a constant 2.8V to the module circuitry when the external supply voltage is 2.9V or greater, with 40dB or more of supply ripple rejection. This ensures constant performance up to the maximum permitted rail, and removes the need for external supply decoupling except in cases where the supply rail is extremely poor (ripple/noise content >100mVpk-pk).

The Enable pin allows the TX module to be turned on or off under 3Vlogic control with a constant DC supply to the Vcc pin. The module current in power-down mode is less than 1mA.

The Enable pin should be tied directly to the Vcc pin if this facility is not required.

TX modulation requirements

The module is factory-set to produce the specified FM deviation with a TXD input to pin 7of 3V amplitude, i.e. 0V "low", 3V "high

If the data input level is greater than 3V, a resistor must be added in series with the TXD input to limit the modulating input voltage to a maximum of 3V on pin 7. TXD input resistance is 100kW to ground, giving typical required resistor values as follows:

The NRX2 receiver incorporates a wide range RSSI which measures the strength of an incoming signal over a range of 55dB or more. This allows assessment of link quality and available margin and is useful when performing range tests.

The output on pin 3 of the module has a standing DC bias of <0.5V with no signal, rising to 2V at maximum indication..

Typical RSSI characteristic is as shown below:

Predicting the range obtainable in any given situation is notoriously difficult since there are many factors involved. The main ones to consider are as follows:

• Type and location of antennas in use
• Type of terrain and degree of obstruction of the link path
• Sources of interference affecting the receiver
• "Dead" spots caused by signal reflections from nearby conductive objects
• Data rate and degree of filtering employed

The NTX2's TXD 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 1.2V approx. with the modulation ac-coupled and limited to a maximum of 2Vp-p to minimise distortion over the link. The varactor modulator in the NTX2 introduces some 2nd harmonic distortion which may be reduced if necessary by predistortion of the analogue waveform.

Although the modulation bandwidth of the NTX2 extends down to DC as does the NRX2 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 NRX2's audio output.

The NRX2 in standard form incorporates a low pass filter with a 5kHz nominal bandwidth. In conjunction with similar filtering in the NTX2 an overall system bandwidth of 5kHz is obtained. This is suitable for transmission of data at raw bit rates up to 10kbps.

Three types of integral antenna are recommended and approved for use with the module:

A) Whip: This is a wire, rod, PCB track or combination connected directly to RF pin of the module. Optimum total length is 16cm (1/4 wave @ 433MHz). Keep the open circuit (hot) end well away from metal components to prevent serious de-tuning. Whips are ground plane sensitive and will benefit from internal 1/4 wave earthed radial(s) if the product is small and plastic cased

B) Helical: Wire coil, connected directly to RF pin, open circuit at other end. This antenna is very efficient given it's small size (20mm x 4mm dia.). The helical is a high Q antenna, trim the wire length or expand the coil for optimum results. The helical de-tunes badly with proximity to other conductive objects.

C) Loop: A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot' end and fed from RF pin at a point 20% from the ground end. Loops have high immunity to proximity de-tuning.

The modules may be mounted vertically or bent horizontal to the motherboard. Note that the components mounted on the underside of the NTX2 and NRX2 are relatively fragile - avoid direct mechanical contact between these and other parts of the equipment if possible, particularly in situations where extreme mechanical stresses could routinely occur (as a result of equipment being dropped onto the floor, etc).

Good RF layout practice should be observed. If the connection between module and antenna is more than about 20mm long use 50W microstrip line or coax or a combination of both. It is desirable (but not essential) to fill all unused PCB area around the module with ground plane.

Variants and ordering information

The NTX2 transmitters and NRX2 receivers are manufactured in the following variants as standard:

Other frequency variants can be supplied to individual customer requirements in the 433MHz (European) and 458MHz (UK) licence exempt bands