Data formats and range extension
The TX1 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 1.2V 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 TX1 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 RX1 AF output is
used to drive an external decoder directly.
Although the modulation bandwidth of the TX1 extends down to DC, as does the AF output of the RX1,
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
RX1 audio output.
The RX1 in standard form incorporates a low pass filter with a 6kHz nominal bandwidth. In
conjunction with similar filtering in the TX1 an overall system bandwidth of 5kHz is obtained. This is
suitable for transmission of data at raw bit rates up to 10kb/s. A lower rolloff frequency of around 50Hz
has been chosen for the internal filter and data slicer in order to keep receiver settling times reasonably
fast. This results in a lowest usable data speed of about 1kb/s for the standard module.
In applications such as long range fixed links where data speed is not of prime concern, a considerable
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. The internal data slicer is not suitable for
data having longer than 1.8ms between transitions and in such circumstances the RX1 audio output can
be utilised to drive an external filter and data slicer.
The RX1 produces an audio output of approximately 400mVp-p at pin 8, but due to the internal filtering
this exhibits a rolloff at low frequencies giving a reduced output of some 100mVp-p as DC is approached.
This rolloff can be eliminated and a flat response to DC obtained by using an external RC compensation
network, as follows:
Input from
RX1 pin 8
Output to low pass filter, etc.
100mV p-p, high impedance
100k
33k
100nF
Fig.9: Audio compensation network
The output of the network will be flat from DC to 5kHz+. It will have a standing DC bias of 1V approx.
and should not be significantly loaded (input impedance of following stage should ideally be ≥1MΩ).
Radiometrix Ltd., TX1 & RX1 Data Sheet
page 9
RADIOMETRIX [ RADIOMETRIX LTD ]
