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 (>100kΩ)
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.
Radiometrix Ltd, TX3A & RX3A Data Sheet
page 7
RADIOMETRIX [ RADIOMETRIX LTD ]
