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NTX2
The Radiometrix NTX2 is the main radio transmitter module in use for UK HABing - it is available in 434.075 and 434.650MHz variants. The NTX2 is a 3V Narrowband FM module and primarily chosen for its data input characteristics - which are analogue rather than the more usual digital. This enables narrow frequency shifts and pulse shaping to be obtained. Output is 10mW. Current draw is 20mA when transmitting and 0.6mA in standby. The input voltage to frequency characteristics of the NTX2 are reasonably linear at around 1.9kHz/V about the 1.5V input centre point - see graph below. The graph shows frequency offset (kHz) against data input voltage over the 0 - 3V range.
There is a guide on linking the NTX2 up to an Arduino here.
The reglatory requirements for airborne operation 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.
The NTX2 temperature stability isn't great - in addition to drift the frequency shift tightens up with decreasing temperature.
NTX2 Adjustments
Removing the sticker from the NTX2 package reveals 3 holes in the metal casing - 1 test point (I haven't checked what it is) & 2 adjustments.
With the module oriented such that the holes are facing you & the pins facing downwards, the upper adjustment screw is frequency adjustment - perhaps adjusting loading on the crystal. Testing with a 434.650MHz NTX2 I was able to push the frequency about 15KHz either way before the oscillator became unstable.
The lower adjustment screw seems to be a pot adjusting the deviation. With the voltage driven FSK implementation we use, this conveniently allows for fine tuning the shift.