Radiometrix Ltd., TX1 & RX1 Data Sheet

page 9

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

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.

This rolloff can be eliminated and a flat response to DC obtained by using an external RC compensation

network, as follows:

100k

33k

100nF

Input from

RX1 pin 8

Output to low pass filter, etc.

100mV p-p, high impedance

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).