NBFM Multi-channel UHF Transceiver with Programmable RF power

USX2 is small multi-channel half duplex UHF transceiver available for operation on 315MHz, 433MHz and 458MHz bands with the user programmable RF power output of up to 100mW.
USX2 offers up to 128 frequency channels in 25kHz channel spacing. USX2 also features dual VCO which allows transmitter section to be operated on one frequency while receiver on another.


  • Conforms to ETSI EN 300 220-3 and EN 301 489-3
  • High performance double superhet, 128 channel PLL synthesizer
  • 100mW RF power output: adjustable via serial command
  • Data rates up to 5 kbps for standard module
  • Usable range over 500m
  • Fully screened
  • Feature-rich interface (RSSI, analogue and digital baseband)
  • Digital RSSI output
  • Incorporates a 1200baud dumb modem
  • Re-programmable via RS232 interface
  • Low power requirements


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

Technical Summary

  • Operating frequency: 315MHz (USA), 433MHz (European) and 458MHz (UK) bands
  • 128 channels controlled via RS232 interface
  • 16 channels selected by parallel interface
  • Transmit power: 100mW (+20dBm) nominal. Adjustable 0.1 - 100mW
  • Supply range: 3.6V - 15V
  • Current consumption: 100mA (@ 100mW output) transmit, 25mA receive
  • Data bit rate: 5kbps max. (standard module)
  • Receiver sensitivity: -118dBm (for 12 dB SINAD)
  • Size: 50 x 30 x 12mm

Evaluation Platform: Narrow Band Evaluation Kit

Figure 2: USX2 block diagram

Figure 3: USX2 footprint (Top)

Pin Description

Pin Name Function Serial mode Modem mode
1A RF Gnd RF ground
2A RF in/out To the antenna
3A RF Gnd RF ground
1 TXD DC copled input for TTL/COMS logic NC
2 TXE Transmit enable. Low = TX mode NC
3 GND 0V
4 Vcc DC supply input. (3.6 - 15V)
5 P3 Parallel channel select MSB Serial read back BYSY
6 P2 Parallel channel select TBA MOD TXD
7 P1 Parallel channel select TBA MOD RXD
8 P0 Parallel channel select LSB PGM in PGM in
9 RSSI DC level between 0.5V and 2.5V. 60dB dynamic range
10 RXD Open collector output of data slicer. NC
11 AF out 500mVp-p audio. DC coupled, approx. 0.8V bias N

1. No inversion occurs between TXD and RXD. However AF out is inverted relative to TXD.
2. Parallel channel selects (P0-P3) and TXE are active LOW and have 47kW internal pullups to 3V
3. Open collector output RXD has a 47 kW pullup to 3V.
4. The software incorporates a 1200 baud modem, compatible with that implemented in other Radiometrix narrowband units ( i1200 tones and format ). Modem operation is selected by serial command.
5. Main serial port (P0, P3) operates at 9600 baud. (Command set is not the same as TR2M, SMX families). The unit is capable of operating in 'basic' parallel channel select only mode, or in serial controlled 'expanded' mode
6. Transmit power can be set by serial command
7. RSSI can be read back as a digital value. (on P3)
8. If analogue transmit modulation is needed, then connect a series 1mF cap + 500kW trimmer (a multiturn is advised) in the TXD circuit. Adjust trimmer for 90% of peak deviation (+/- 2.7KHz) at mean input level.

USX2 serial interface details (version 4): PRELIMINARY

The USX2 user interface is accessed through the four parallel pins (P0/serial through P3).
It has four operating modes:

1. Parallel

Frequency of operation is selected as one of 16 (chans 0-15), by means of a 4 bit inverted value applied to P0-P3. The P0 pin will also respond to a LOCKSERIALMODE datastream (inverted RS232, 9600 baud), which will cause the radio to enter serial mode. In parallel mode, the IDENT command string is still decoded, but the unit remains in parallel mode.

2. Serial

Frequency of operation, radio set up, power output and various other parameters are controlled by command strings sent to P0. Pin P3 functions as a 'readback' port while P1,2 are not used in this mode.

In this mode, decimal values are used (to simplify direct manual programming via a terminal)

Commands include :

Commands Function Note
: empty command buffer
? or / read back buffer contents
<backspace> delete last character
<cr> process buffer
PARALLEL return to the basic, parallel mode (and turn modem off)
DUMP read back contents of eeprom (output on P3)
MODEM activate 1200 baud modem #
NOMODEM de-activate 1200 baud modem #
DEFCH ccc set default channel (non-volatile) #
DEFPOW pp set default tx power (non-voltatile) #
CHAN ccc jump to channel (volatile)
POWER pp select tx power (volatile)
RSSI single read of RSSI (output on P3)
SHORT enter short command mode #

(The following commands are used to set up the radio operating characteristics, and should be viewed with caution. Especially the CALPOWER command, which initiates a multiple write operation to the power calibration table )

Commands Function Note
NDIV nnnnn force N divider value (volatile)
RDIV rrrrr force R divider value (volatile)
RLOAD rrrrr Set R divider value #
OFFSET ooooo Set RX offset #
LOAD aa nnnnn Set N values for first 16 channels #
START nnnnn Set N value for ch 16 #
STEP i Set increment for table #
LIMIT ccc Set highest permitted channel number #
SETPOW eee Set maximum power output (100mW)
CALPOWER (etc) (see factory procedures for more details)
TEST Generate a 250Hz test modulation (only fuctions in TX mode, cancelled by sending a <cr> byte)
IDENT Readback a single byte, depending on current mode:
Parallel: P
Serial: S
Short: 1
Modem: M

(Channels 0-15 are individually programmed by the LOAD operation. Channels 16-127 are a consecutive table, using the START value as a start point (=channel 16) and increasing the N value by STEP with each increase in channel number)

ccc = a channel number from 00 to 127
aa = a two digit channel number from 00 to 15
nnnnn = synthesizer N register value, (up to 65535)
rrrrr = synthesizer R register value, (up to 16383)
pp = power setting value (0 - 30)
i = table step (increments of N) (0-7)
ooooo = receive mode frequency offset
eee = power calibration figure

N = channel frequency / (13MHz / R)

13MHz / R = 25KHz , so R = 520 (usually)

receive OFFSET = 21.4MHz / (13MHz / R) , so = 856 (usually)

A pause of at least 50mS must be allowed after operations which result in eeprom programming operations (all except NDIV, RDIV, GOTO, PSET and RSSI). This allows the programming cycle to complete.
Instructions marked ' # ' output a three byte ' OK<cr> ' sequence after sucessfully completing their eeprom programming cycles
When first powered up, the unit will operate on channel and output level specified by the last CHAN and POWER instructions, irrespective of previous NDIV, RDIV, CHAN or PSET operations)

3. Short

A limited range of radio functions are controlled by sending a single byte to P0

Commands Function Note
0 - 127 select channel volatile
128 - 159 select power ( = byte -128) volatile
200 single read of RSSI output on P3
201 single byte read, as for IDENT command
222 reception of 16 consecutive 222 bytes returns radio to serial mode

4. Modem

Commands are interpreted as in SERIAL mode (and 'short' mode may be selected.) In this mode the unit operates as a simple 1200 baud packet modem, with TXD (in) on P2 and RXD (out) on P1.

The TXE pin does not control tx switching in this mode, but rather the presence of valid data in the tx buffer initiates a transmit burst. No handshaking is provided, and the unit has a sufficient over-link data rate to transparently 'stream' continuous data. Error correction, re-transmission of corrupt packets and addressing are not provided.

This mode is compatible with other Radiometrix 'i 1200' mode equipment, including the narrow band eval kit.


  • The LOCKSERIALMODE (and FAST) commands are non-volatile. Once a serial mode is selected, the unit will power up in this mode until a PARALLEL command is received.
  • The command interpreter IS case sensitive. Use upper case.
  • Spaces are optional (they are not decoded), provided the command line does not exceed 16 characters.
  • When manually programming this unit we recommend setting your terminal to local echo. The 'backspace' key functions normally.
  • All serial communications use 9600baud 'inverted RS232' 8 bit data, no parity, 1 start bit, 1 or 2 stop bits
  • A simple 'driver' program will be made available to simplify programming of these units, if desired.
  • RSSI read operations only function correctly if the unit is in RX mode (TXE pin is high or floating)

Condensed specifications

Frequency 315 - 315.375MHz (US band)
433.875 - 434.650MHz (433 MHz EU band)
458.525 - 459.1MHz (458MHz UK band)

Frequency stability

+/- 1.5kHz
Channel spacing
25kHz (12.5kHz by special order)
Number of channels
128 channels selected via serial RS232 interface
First 16 are individually programmable Next 112 are a sequential table
16 channels selected via parallel interface
Supply          voltage 3.6V - 15V
Current 45mA transmit (at 10mW output)
100mA transmit (at 100mW output)
25mA receive (or modem 'idle')
Operating temperature -20 to +70 °C (Storage -30 to +70 °C)
Spurious radiations Compliant with ETSI EN 300 220-3 and EN 301 489-3
Size 50 x 30 x 12mm
Interface          user 11pin 0.1" pitch molex


3pin 0.1" pitch molex


5pin 0.1" pitch molexsocket in top case
Recommended PCB hole size 1.2mm
Output power

100mW (+20dBm); Adjustable via serial command 0.1 - 100mW

TX on switching time <50 ms
Modulation type FM, FSK (F1D, F3D)
TX modulation bandwidth DC - 3kHz
Deviation ±3kHz
Adjacent channel TX power -37dBm
TX spurii <-40dBm
Inputs Data (CMOS/TTL compatible)
Sensitivity -118dBm for 12 dB SINAD
image / spurious -60dB
blocking -84dB
adjacent Channel <-60dB (Tested per. ETSI EN 301 489-3)
outputs RSSI, audio, data
Dynamic timing:
RX data recovery settling time 20ms
Power on to stable (50:50 mark / space) RXD 50ms (TXE low/active to stable RX data out)
Maximum time between data transitions 250ms

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:
Licensing policy manual