HF Telemetry Modules

A guide to the NRX0, the NTX0 and the multichannel LMR0 and LMT0

A slightly unfamiliar band, 27MHz

The most well-known low power wireless ISM bands are in the VHF and UHF (400 and 870MHz) regions, but there is an often overlooked allocation at 27MHz. Unlike the more familiar bands, which vary as to availability in various countries and areas, almost all regions have some form of 27MHz allocation. This 27MHz band was previously known for it’s use as a toy remote control allocation; however good results can be achieved in this band when high quality radios are used well.

Radiometrix manufactures 27MHz band versions of some of our most popular modules. These include the SIL footprint single channel (crystal controlled) NRX0 receiver and NTX0 transmitter, and the multichannel LMT0 transmitter and LMR0 receiver. These are pinout and (broadly) functional equivalents to the corresponding VHF and UHF module types. Both transmitters output 10mW as standard.

With a suitable antenna, these HF units can easily rival the performance of the higher frequency modules, as the path loss at HF is much lower, giving a more favourable “miles per milliwatt” figure.

Key features  

High sensitivity narrowband operation (10KHz channel spacing, 4kbit/sec data rate)
Easily understood single channel, or flexible (parallel or serial port select) multichannel operation
Best-in-class RF performance.
Consistent easy-to-use SIL and DIL footprint (47x17mm or 43x23mm RX, 43x15mm or 33x23mm TX)
Available on a number of popular spot frequencies in the band (27.095MHz as standard)
Conform to EN 300 220-3 and EN 301 489-3 (and other regulations world-wide)
Various test and evaluation Boards available

Receivers and transmitters

The HF module family is conveniently divided into two broad groups: SIL package and LM family


The NTX0 and NRX0 correspond directly to the NTX1 and NRX1. The transmitter is a simple crystal controlled 10mW design, while the receiver is a sensitive single channel crystal-controlled superheterodyne. Units are supplied on 27.095MHz as standard, but other frequencies can be ordered (subject to crystal MOQ and lead times)

The LMT0 and LMR0 similarly correspond to the LMT1 and LMR1. Both use a low noise VCO controlled by a PLL locked in turn to a stable TCXO. A small on-board microcontroller programs the PLL and provides multichannel operation, with the channel frequency controlled via either a parallel or a serial port. The default power output for the LMT0 is also 10mW, but for applications where more power is allowed (such as model control in the UK) versions capable of up to 100mW can be supplied.

All these modules have feature rich user interfaces. The transmitters provide separate enable (TXE) pins and have a digital (or analogue with external parts) baseband mod input. The receivers all provide both analogue AF and squared-up logic compatible digital (RXD) outputs, along with 60dB (min.) dynamic range RSSI outputs.

APPLICATIONS

This series of radios are suited to a variety of medium range commercial applications such as EPOS and meter reading systems, simple data loggers, in-building environmental monitoring and control, and basic security/alarm systems

Agricultural, marine and forestry applications can typically make good use of the specific advantages of these lower frequency allocations.
Finally, it is worth noting that the 27MHz ISM allocation is adjacent to the 10m (28MHz) amateur band, and that the LMT0 and LMR0 ought to be usable (possibly with some re-tuning, and only by licensed amateurs) in that allocation.

Beyond the basic family

To further simplify the implementation of user designs, a number of application/demo boards are offered, with necessary connectors and support and interface circuitry. Effectively: if one of the conventional VHF or UHF modules has a test board or an application/demo board, then it can be used with the HF modules too.

Both SIL and LM series carrier boards can be supplied for the NBEK test station. The full range of the Radiometrix control and modem chips (CTA88, M48, Small controller and i1200 to name a few) can be used with these easily used modules, while the single-relay DX control boards are designed around SIL modules.

Evaluation & Application Boards

To further simplify the implementation of user designs, a number of application/demo boards are offered, with necessary connectors and support and interface circuitry.
The TDL family has the TDi2 evaluation board (adding an SMA RF connector, an RS232 buffer with D9 connector, and power supply regulators) and the TDL eval board (which takes the TDi2 and adds sockets for the module and provision for an (optional) PP3 battery)

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