TX1 modulation requirements
The module is factory-set to produce the specified FM deviation with a TXD input to pin 7 of 3V
amplitude, i.e. 0V “low”, 3V “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 ±2.5kHz 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 the module supply is greater than 3V a resistor must be added in series with the TXD input to
limit the modulation amplitude to a maximum of 3V on pin 7. TXD input resistance is 100kΩ to ground,
giving typical required resistor values as follows:
Vcc
Series resistor
-
≤3V
3.3V
5V
10 kΩ
68kΩ
220kΩ
9V
It should be noted that conditions on TXD have a significant effect on the startup time of the module,
i.e. the time between En (or En+Vcc) going high and full RF output being produced. For fastest startup
TXD should either be low or fed with data (preamble etc) for a minimum of 3ms after En has been
asserted. Startup time under these conditions is typically 50-70% of that obtained if TXD is held high
over the same period.
Reducing the output power of the TX1
If the TX1-173.250-10 is to be used for other than industrial/commercial applications its output power
must be reduced to 1mW to comply with type approval requirements. This is done by inserting a 10dB
attenuator network between the module and the antenna or feed, as follows:
antenna
50 ohm microstrip lines
TX1
to antenna
68R
from TX1
4
5
6
7
1
2
3
68R
ground foil/
vias to ground plane
100R
Resistors are SMD (0603/0805)
schematic diagram
physical arrangement
Fig.7: 10dB attenuator for TX1
Keep all tracking around the attenuator network as short as possible, particularly ground paths, and
use matched 50Ω microstrip lines for input and output connections (track width of 2.5mm if using
1.6mm thick FR4 PCB).
Radiometrix Ltd., TX1 & RX1 Data Sheet
page 7
RADIOMETRIX [ RADIOMETRIX LTD ]
