LMT1/LMR1

VHF Narrow Band FM Low Cost multi channel radio modules

The LMT1 transmitter and LMR1 receiver modules offer a multi channel, low power, and reliable VHF data link. This makes the LMT1/LMR1 pair ideally suited to those low power applications where existing wideband modules have insufficient range, or where low cost multi-channel operation is needed without compromising on RF specification or regulatory requirement.

Features

• Conforms to EN 300 220-3 and EN 301 489-3
• High performance double superhet, PLL synthesizer with TCXO
• quasi-dc (peak sampling) data recovery circuit (for enhanced performance on unbalanced datastreams)
• Data rates up to 5 kbps for standard module
• Usable range over 1km
• Fully screened. Low profile
• Feature-rich interface (RSSI, analogue and digital baseband)
• Re-programmable via RS232 interface
• Low power requirements

Applications

• Handheld terminals
• EPOS equipment, barcode scanners
• Data loggers
• Industrial telemetry and telecommand
• In-building environmental monitoring and control
• High-end security and fire alarms
• DGPS systems
• Vehicle data up/download
• Heavy vehicle/machinery controls

Technical Summary

• Operating frequency: 173.200 - 173.325MHz (UK band)
  150.825 - 152.450MHz (Australian Band)
• Other VHF frequencies available·
• 4 UK allocation channels (32 maximum)
• 32 channels
• Transmit power: +10dBm (10mW) nominal @ 3.1V (Adjustable 1 - 25mW)
• Supply range: 3.1 - 15V (TX @ 10mW and RX), 4.1 - 15V (TX @ 100mW)
• Current consumption: 34mA @ 10mW, 90mA @ 100mW (transmit) and 20mA (receive)
• Data bit rate: 5kbps max. (standard module)
• Receiver sensitivity -118dBm (for 12dB SINAD)
• Serial configuration by inverted RS232 at 3V CMOS level

Evaluation Platform: Narrow Band Evaluation Kit

Figure 2: LMT1 block diagram

Figure 3: LMT1 Footprint (Top view)

Pin Description - LMT1

NOTES:

1. Serial programming is by an inverted, CMOS logic level, 2400 baud RS232 datastream applied to the P0 pin.
2. Channel select inputs have pullups (50K) to 3v internal rail. Do not exceed 3V logic levels on this port.
3. Channel select inputs are active low
4. TXE has a 100K pullup to Vcc
5. All pins are on an 0.1" grid
6. The pins 5a/b are not present, but are included in footprint for compatibility with other units in this family
7. In the 'off' state a PIN switch open circuits the RF output pin. There are no 'off' state spuri.
8. 10mW unit will operate (with marginally reduced specifications and lower (6-8mW) output power) from a 3.0V rail. This must be well regulated and without noise or ripple, as in this state the unit's internal regulator no longer operates, and provides no supply rejection.
9. 100mW versions should not be run continuously from supply voltages > 9v

Figure 4: LMR1 block diagram

Figure 5: LMR1 Footprint (Top view)

Pin Description - LMR1

NOTES:

1. Serial programming is by an inverted, cmos logic level, 2400 baud RS232 datastream applied to the P0 pin.
2. Channel select inputs have pullups (50kW) to 3v internal rail. Do not exceed 3v logic levels on this port.
3. Channel select inputs are active low
4. All pins are on an 0.1" grid
5. Data recovery circuit used for RXD is not a simple 'average and compare' type. It is a peak sampling quasi-DC coupled design, allowing a greater than usual flexibility in data format.
6. Unit will operate (with maginally reduced specifications) from a 3.0v rail. This must be well regulated and without noise or ripple, as in this state the unit's internal regulator no longer operates, and provides no supply rejection.]

Serial interface commands

LMT1/LMR1 frequency/channel can be serially configured using HyperTerminal or any other terminal program configured with following setup:

2400 baud RS232, 8 bit data, no parity, 1 start bit, 1 or 2 stop bits. No flow control
Serial data is sent to the unit on one of the parallel channel select pins (P0). It is very important that the unit does not 'decode' switch bounce in ordinary operation as a command string, or spurious re-writing of the EEPROM will result. For this reason the user must send the 16 character string ENABLESERIALMODE to fully enable the serial command mode before sending any of the command strings listed below. Command mode is disabled on power down, or on reception of a # character. To successfully program the unit, it must be enabled (RXE high (rx) or TXE low (tx)).

aa = a two digit channel number from 00 to 31
nnnnn = a synthesizer N register value, (up to 65535)
rrrr = the synthesizer R register value, (up to 16383)

For LMT1 transmitter:

For LMR1 receiver:

, So R = 520

Notes:
1. A pause of at least 50ms must be allowed between command strings (EEPROM programming time).
2. SINGLE mode does not store the N value in EEPROM. Therefore the unit is inoperative after a power down until either another valid SINGLE command is received, or mode is changed by a GOCHAN, SETPAR or SETSER command. SINGLE mode is intended for frequency agile applications.

3. For UK version (173.200 - 173.325MHz band) the following channel assignment is supplied: Channel Frequency (MHz)

Condensed specifications

RX Received Signal Strength Indicator (RSSI)

The LMR1 has wide range RSSI which measures the strength of an incoming signal over a range of 60dB or more. This allows assessment of link quality and available margin and is useful when performing range tests.

The output on pin 5b of the module has a standing DC bias of up to 0.5V with no signal, rising to 2.5V at maximum indication (RF input levels of -40dBm and above). DVmin-max is typically 2V and is largely independent of standing bias variations. Output impedance is 40kW. Pin 5b can drive a 100mA meter directly, for simple monitoring.

Typical RSSI characteristic is as shown below:

Figure 6: RSSI level with respect to received RF level at LMR1 antenna pin

Antenna requirements

Three types of integral antenna are recommended and approved for use with the module:

A) Whip: This is a wire, rod, PCB track or combination connected directly to RF pin of the module. Optimum total length is 16cm (1/4 wave @ 433MHz). Keep the open circuit (hot) end well away from metal components to prevent serious de-tuning. Whips are ground plane sensitive and will benefit from internal 1/4 wave earthed radial(s) if the product is small and plastic cased

B) Helical: Wire coil, connected directly to RF pin, open circuit at other end. This antenna is very efficient given it's small size (20mm x 4mm dia.). The helical is a high Q antenna, trim the wire length or expand the coil for optimum results. The helical de-tunes badly with proximity to other conductive objects.

C) Loop: A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot' end and fed from RF pin at a point 20% from the ground end. Loops have high immunity to proximity de-tuning.

The antenna choice and position directly controls the system range. Keep it clear of other metal in the system, particularly the 'hot' end. The best position by far, is sticking out the top of the product. This is often not desirable for practical/ergonomic reasons thus a compromise may need to be reached. If an internal antenna must be used, try to keep it away from other metal components, particularly large ones like transformers, batteries and PCB tracks/earth plane. The space around the antenna is as important as the antenna itself.

Figure 7: Antenna types

Ordering Information

Limitation of liability

The information furnished by Radiometrix Ltd is believed to be accurate and reliable. Radiometrix Ltd reserves the right to make changes or improvements in the design, specification or manufacture of its subassembly products without notice. Radiometrix Ltd does not assume any liability arising from the application or use of any product or circuit described herein, nor for any infringements of patents or other rights of third parties which may result from the use of its products. This data sheet neither states nor implies warranty of any kind, including fitness for any particular application. These radio devices may be subject to radio interference and may not function as intended if interference is present. We do NOT recommend their use for life critical applications.
The Intrastat commodity code for all our modules is: 8542 6000.

R&TTE Directive

After 7 April 2001 the manufacturer can only place finished product on the market under the provisions of the R&TTE Directive. Equipment within the scope of the R&TTE Directive may demonstrate compliance to the essential requirements specified in Article 3 of the Directive, as appropriate to the particular equipment.
Further details are available on The Office of Communications (Ofcom) web site:
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