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Table of Contents
LMT2
Overview
The Radiometrix LMT2 is a more advanced version of the NTX2. Its major advantage over the NTX2 is its frequency agility being able to switch channels betwee 433.900 to 434.650Mhz. Additionally its temperature stability is much better due to the inclusion of a TCXO.
Details
It is possible to linearise the input of the LMT2 by placing a 500k resistor in front of the TXD pin, this has the effect of turning the LMT2's input op amp gain to 1:1 effectively bypassing it. At this point an input voltage of between 0 - 3V on the TXD pin will result in an almost linear increase in output frequency meaning its possible, as with the NTX2, to product RTTY (or other modes).
The frequency change is about 0.000573v per Hz above 1V input. I.e about 0.244v will result in a shift in frequency of 425Hz. Below is the frequency response graph :
LMT2 modules can be powered directly from the batteries having its own internal regulator (3.1V to 15V in). The manufacturers datasheet is here LMT2 Datasheet
Legislative
The regulatory requirements for airborne operation in Europe on this frequency are that the channel bandwidth must be less than 25Khz and less that 10mW ERP. There are no duty cycle requirements in this frequency band. Use of this module in the United States requires operation under an amateur license.
Frequency Stability
The LMT2 temperature stability is very good due to the TCXO. Adequate insulation is still recommended, the manufacturer minimum operation temperature is -10'C. Ensuring the module isn't exposed to rapid temperature changes is essential.
Where to buy the LMT2
Other Notes & Further Reading
Radiometrix are understandably unable to offer technical support on these modules when used in high altitude ballooning therefore please direct all technical support queries regarding the operation of these modules to the UKHAS mailing list or preferably the #highaltitude IRC channel.