UHF FM Data transmitter and receiver modules

The TX3A & RX3A are miniature UHF radio transmitter & receiver modules designed for PCB mounting. They facilitate the simple implementation of data links at speeds up to 64kbps and distances up to 75m in-building or 300m over open ground.


  • Frequencies available as standard: 869.85MHz, 914.5MHz
  • CE certified by independent Notified Body
  • Verified to comply with Radio standard EN 300 220-3 by accredited Test Laboratory
  • Verified to comply with EMC standard EN 301 489-3 by accredited Test Laboratory
  • North American version conforms to FCC part 15.249
  • Data rates up to 64kbps
  • Fully screened

Available for operation in the 868-870MHz band in Europe and the 902-928MHz band in North America, both modules combine full screening with internal filtering to ensure EMC compliance by minimising spurious radiation and susceptibility. The TX3A & RX3A will suit one-to-one and multi-node wireless links in such applications as car and building security, EPOS and inventory tracking, remote industrial process monitoring and data networks. Because of their small size and low power requirements, both modules are ideal for use in portable, battery-powered applications such as hand-held terminals.


  • Handheld terminals
  • EPOS and inventory tracking
  • Remote industrial process monitoring
  • Data loggers
  • Industrial telemetry and telecommand
  • In-building environmental monitoring and control
  • Vehicle and building security and fire alarms
  • Vehicle data up/download

Technical summery

Transmitter – TX3A

  • Crystal-locked PLL, FM modulated at up to 64 kb/s
  • Operation from 2.2V to 16V @ 7.5mA
  • Built-in regulator for improved stability and supply noise rejection
  • 0dBm (1mW) nominal RF output
  • Enable facility
  • Update of the original TX3A with enhanced performance

Receiver – RX3A

  • Single conversion FM superhet with SAW front end filter
  • Operation from 2.7V to 16V @ 11mA
  • Built-in regulator for improved stability and supply noise rejection
  • -100dBm sensitivity @ 1ppm BER, 64kb/s version
  • -107dBm sensitivity @ 1ppm BER, 10kb/s version
  • RSSI output with 60dB range
  • Enable facility
  • Extremely low LO leakage, -120dBm typical

Evaluation Platform: Universal Evaluation kit or Narrow Band Evaluation Kit

Functional description

The TX3A transmitter module uses a frequency modulated crystal-locked PLL and operates between 2.2V and 16V at a current of 7.5mA nominal. At 3V supply it delivers nominally 0dBm (1mW) RF output. The SIL style TX3A measures 32 x 12 x 3.8 mm excluding pins.

The RX3A module is a single conversion FM superhet receiver capable of handling data rates of up to 64kb/s. It will operate from a supply of 2.7V to 16V and draws 11mA when receiving. The RX3A features a fast power-up time for effective duty cycle power saving and a signal strength (RSSI) output with 60dB of range. Full screening and a SAW front-end filter give good immunity to interference. The SIL style RX3A measures 48 x 17.5 x 4.5 mm excluding the pins.

TX3A transmitter

Fig. 1: TX3A block diagram

Fig. 2: TX3A physical dimension

Pin description

RF GND  (pins 1&3)
RF ground, internally connected to the module screen and pin 6 (0V). These pins should be directly connected to the RF return path - e.g. coax braid, main PCB ground plane etc.
RF OUT  (pin 2)
50Ω RF output to the antenna. Internally DC-isolated. See antenna section of apps notes for details of suitable antennas.
En  (pin 4)
Tx enable. <0.15V shuts down module (current <1mA). >1.7V enables the transmitter. Impedance ~1MW. Observe slew rate requirements (see apps notes).
Vcc (pin 5)
+2.2V to +16V DC supply. Max ripple content 0.1Vp-p. Decoupling is not generally required.
0V (pin 6)
DC supply ground. Internally connected to pins 1 & 3 and module screen.
TXD (pin 7)
DC-coupled modulation input. Accepts serial digital data at 0V to 2.5V levels. See applications notes for suggested drive methods. Input is high impedance (>100kΩ).

RX3A receiver

Fig.3: RX3A block diagram

Fig.4: RX3A physical dimensions

Pin description

RF IN (pin 1)
50Ω RF input from antenna. Internally DC-isolated. See ante0nna section of applications notes for suggested antennas and feeds.
RF GND (pins 2 & 3)
RF ground, internally connected to the module screen and pin 6 (0V). These pins should be connected to the RF return path - e.g. coax braid, main PCB ground plane etc.
En (pin 4)
Rx enable. <0.15V shuts down module (current <1mA). >1.7V enables the receiver. Impedance ~1MΩ. Observe slew rate requirements (see apps notes).
module (current <1mA). >2V enables receiver. Impedance 2MWnominal.    
RSSI (pin 5)
Received signal strength indicator with >60dB range. See applications notes for typical characteristics.
0V  (pin 6)
DC supply ground. Internally connected to pins 2 & 3 and module screen.
Vcc (pin 7)
+2.7V to +16V DC supply. Max ripple content 0.1V>p-p. Decoupling is not generally required.
AF out (pin 8)
Buffered and filtered analogue output from the FM demodulator. Standing DC bias 1V approx.
External load should be >10kΩ // <100pF.
RXD (pin 9)
Digital output from the internal data slicer. The data is squred version of the signal on pin 8 (AF out) and is true data, i.e. as fed to the transmitter.
Output is "open-collector" format with internal 10kΩ pull-up to Vcc (pin 7).

Absolute maximum ratings

Exceeding the values given below may cause permanent damage to the module.

Operating temperature
Storage temperature
-20°C to +70°C
-40°C to +100°C
Vcc, (pin 5)
TXD (pin 7)
En (pin 4) 
RF OUT (pin 2)
-0.3V to +16.0V
-0.3V to +16V
±50V DC, +10dBm RF
Vcc, RXD (pins 7,9)
En (pin 4)
RSSI, AF (pins 5,8) 
RF IN (pin 1)       
-0.3V to +16V
-0.3V to +Vcc V
-0.3V to +3V
±50V DC, +10dBm RF

Performance specifications: TX3A transmitter

(Vcc = 3.0V / temperature = 20°C unless stated)







DC supply
Supply voltage 5 2.2 3.0 16 V 1, 6
Supply current 5 7.5 9.5 mA 2
RF power output @ Vcc = 2.2V 2 -1 dBm 2
RF power output @ Vcc 2.8V 2 0 dBm 2
Harmonics / spurious emissions
-55 -45
Initial frequency accuracy -25 0 +25 kHz
FM deviation (peak) ±30 kHz 4
Modulation bandwidth @ -3dB 0 35 kHz
Modulation distortion (THD)
10 % 6
TXD input level (logic low)
0 +0.2 V 5, 6
TXD input level (logic high) 7 +2.3 +2.5 +3 V 5, 6
Dynamic timing
Power-up time (En —> full RF) 1.0 1.5 ms















1. RF output is automatically disabled below 2.2V supply voltage.
2. RF output terminated with 50W resistive load.
3. Meets or exceeds EN/FCC requirements at all frequencies.
4. With 0V - 2.5V modulation input.
5. To achieve specified FM deviation.
6. See applications information for further details


Performance specifications: RX3A receiver

(Vcc = 3.0V / temperature = 20°C unless stated)







DC supply
Supply voltage 7 2.7 5.0 16.0 V
Supply current 7
11 16 mA
RF sensitivity@10dB (S+N)/N 1, 8 -114 dBm
10kbps version
RF sensitivity@10dB (S+N)/N 1, 8 -107 dBm
64kbps version
RF sensitivity@ 1ppm BER 1, 9 -107 dBm
10kbps version
RF sensitivity@ 1ppm BER 1, 9 -100 dBm
64kbps version
RSSI range 1, 5 60 dBm
IF bandwidth 180 kHz
Image rejection 1 40 50 dB
IF rejection (10.7MHz) 1
- - dB
±1MHz spurious rejection 1 67 dB
LO leakage, conducted 1 -120 -110 dBm
Baseband bandwidth @ -3dB 8 0 7.8 kHz
10kbps version
Baseband bandwidth @ -3dB 8
64kbps version
AF level 8
DC offset on AF out 8 0.3 1.0 1.75 V
Distortion on recovered AF 8 1 10 %
Load capacitance, AFout / RXD 8,9 100 pF
Dynamic timing
Power up with signal present
Power up to valid RSSI 4, 5 1 ms
Power up to stable data 4, 9 10 30 ms
3, 10kbps version
Power up to stable data
4, 9
3, 64kbps version
signal applied with supply on
RSSI response time (rise/fall) 1, 5 100 ms
Signal to stable data 1, 9 5 30 ms
3, 10kbps version
Signal to stable data
1, 9
3, 64kbps version
Time between data transitions 9 0.1 15 ms
4, 10kbps version
Time between data transitions 9 15.6 1500 ms
4, 64kbps version
Mark : space ratio 9 20 50 80 % 5

1. Current increases at higher RF input levels (-20dBm and above).
2. For received signal with ±30kHz FM deviation.
3. Typical figures are for signal at centre frequency, max. figures are for ±50kHz offset.
4. For 50:50 mark to space ratio (i.e. square wave).
5. Average over 30ms (10kbps version) or 3ms (64kbps version) at maximum data rate.

Module test circuits

Fig.5: TX3A test circuit

Fig.6: RX3A test circuit

Applications information

Power supply requirements

Both modules incorporate a built-in regulator which delivers a constant 2.8V to the module circuitry when the external supply voltage is 2.85V or greater, with 40dB or more of supply ripple rejection. This ensures constant performance up to the maximum permitted supply rail and removes the need for external supply decoupling except in cases where the supply rail is extremely poor (ripple/noise content >0.1Vp-p).

Note, however, that for supply voltages lower than 2.85V the regulator is effectively inoperative and supply ripple rejection is considerably reduced. Under these conditions the ripple/noise on the supply rail should be below 10mVp-p to avoid problems. If the quality of the supply is in doubt, it is recommended that a 10mF low-ESR tantalum or similar capacitor be added between the module supply pin (Vcc) and ground, together with a 10Ω series feed resistor between the Vcc pin and the supply rail.
The Enable pin allows the module to be turned on or off under logic control with a constant DC supply to the Vcc pin. The module current in power-down mode is less than 1mA.

NOTE: If this facility is used, the logic control signal must have a slew rate of 40mV/ms or more. Slew rates less than this value may cause erratic operation of the on-board regulator and therefore the module itself.

The TX3A incorporates a low voltage shutoff circuit which prevents any possibility of erratic operation by disabling the RF output if the supply voltage drops below 2.2V (±5%). This feature is self-resetting, i.e. restoring the supply to greater than 2.2V will immediately restore full RF output from the module.

TX3A modulation requirements

The module will produce the specified FM deviation with a TXD input to pin 7 of 2.5V amplitude, i.e. 0V "low", 2.5V "high". Reducing the amplitude of the data input from this value (usually as a result of reducing the supply voltage) reduces the transmitted FM deviation to typically ±25kHz at the lower extreme of 2.2V. The receiver will cope with this quite happily and no significant degradation of link performance should be observed as a result.

Where standard 2-level digital data is employed with a logic "low" level of 0V ±0.2V, the logic "high" level applied to TXD may be any value between +2.5V and +3V for correct operation. However, if using multi-level or analogue signalling the maximum positive excursion of the modulation applied to TXD must not exceed +2.5V or waveform distortion will result. If the input waveform exceeds this level a resistive potential divider should be used at the TXD input to reduce the waveform amplitude accordingly. This input is high impedance (>100kΩ) and can usually be ignored when calculating required resistor values.

Data formats and range extension

The TX3A data input is normally driven directly by logic levels but will also accept analogue drive (e.g. 2-tone signalling). In this case it is recommended that TXD (pin 7) be DC-biased to 1.25V with the modulation ac-coupled and limited to a maximum of 2.5Vp-p to minimise distortion over the link. The varactor modulator in the TX3A introduces some 2nd harmonic distortion which may be reduced if necessary by predistortion of the analogue waveform. At the other end of the link the RX3A AF output is used to drive an external decoder directly.

Both the AF output on pin 8 and the RXD output on pin 9 of the RX3A are "true" sense, i.e. as originally fed to the transmitter.

Although the modulation bandwidth of the TX3A extends down to DC, as does the AF output of the RX3A, it is not advisable to use data containing a DC component. This is because frequency errors and drifts between the transmitter and receiver occur in normal operation, resulting in DC offset errors on the RX3A audio output.

The RX3A incorporates a low pass filter which works in conjunction with similar filtering in the TX3A to obtain an overall system bandwidth of 32kHz. This is suitable for transmission of data at raw bit rates up to 10kbps and 64kbps, depending on the receiver version. To keep settling times within reasonable limits for the data speed in use, the adaptive data slicer in the RX3A is subject to a maximum time limit between data transitions (see page 5). This limitation must be taken into account when choosing a code format. It is strongly recommended that a reasonably balanced code containing no long 1s or 0s (such as Manchester or similar) is employed.

In applications such as longer range fixed links where data speed is not of primary importance, a significant increase in range can be obtained by using the slowest possible data rate together with filtering to reduce the receiver bandwidth to the minimum necessary. In these circumstances, because of the limitations of the internal data slicer it is better to use the RX3A audio output to drive an external filter and data slicer.

RX3A Received Signal Strength Indicator (RSSI)

The RX3A receiver incorporates a 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 5 of the module has a standing DC bias of typically 0.25V with no signal, rising to 1.1V at maximum indication. The RSSI output source impedance is high (~50kΩ) and external loading should therefore be kept to a minimum.
Typical RSSI characteristic is as shown below:

Fig.7: RX3A RSSI response curve

To ensure a reasonably fast response the RSSI has limited internal decoupling of 1nF to ground. This may result in a small amount of ripple on the DC output at pin 5 of the module. If this is a problem further decoupling may be added, in the form of a capacitor from pin 5 to ground, at the expense of response speed. For example, adding e will increase RSSI response time from 100µs to around 1ms. The value of this 10nF hercapacitor may be increased without limit.

Expected range

Predicting the range obtainable in any given situation is notoriously difficult since there are many factors involved. The main ones to consider are as follows:

  • Type and location of antennas in use (see below)
  • Type of terrain and degree of obstruction of the link path
  • Sources of interference affecting the receiver
  • “Dead” spots caused by signal reflections from nearby conductive objects
  • Data rate and degree of filtering employed (see page 7)

Assuming the maximum 64kb/s data rate and ¼-wave whip antennas on both transmitter and receiver, the following ranges may be used as a rough guide only:
1. Cluttered/obstructed environment, e.g. inside a building: 25-75m
2. Open, relatively unobstructed environment: 100-300m

It must be stressed that range obtained in practice may lie outside these figures. Range tests should always be performed before assuming that a particular range can be achieved in any given application.

Antenna considerations and options

The choice and positioning of transmitter and receiver antennas is of the utmost importance and is the single most significant factor in determining system range. The following notes apply particularly to integral antennas and are intended to assist the user in choosing the most effective arrangement for a given application.

Nearby conducting objects such as a PCB or battery can cause detuning or screening of the antenna which severely reduces efficiency. Ideally the antenna should stick out from the top of the product and be entirely in the clear, however this is often not desirable for practical/ergonomic reasons and a compromise may need to be reached. If an internal antenna must be used try to keep it away from other metal components and pay particular attention to the “hot” end (i.e. the far end) as this is generally the most susceptible to detuning. The space around the antenna is as important as the antenna itself.

Microprocessors and microcontrollers tend to radiate significant amounts of radio frequency hash, which can cause desensitization of the receiver if its antenna is in close proximity. 900MHz is generally less prone to this effect than lower frequencies, but problems can still arise. Things become worse as logic speeds increase, because fast logic edges are capable of generating harmonics across the UHF range which are then radiated effectively by the PCB tracking. In extreme cases system range can be reduced by a factor of 3 or more. To minimize any adverse effects, situate the antenna and module as far as possible from any such circuitry and keep PCB track lengths to the minimum possible. A ground plane can be highly effective in cutting radiated interference and its use is strongly recommended.

A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same regardless of whether the microcontroller or other logic circuitry is running or in reset.

Depending on the application and bearing in mind applicable legal requirements (see p.11), a variety of antenna types may be used with the TX3A and RX3A.

Integral antennas generally do not perform as well as externally mounted types, however they result in physically compact equipment and are the preferred choice for portable applications. The following can be recommended:

Whip (¼-wave): This consists simply of a piece of wire or rod connected to the module at one end. The lengths given below are from module pin to antenna tip including any interconnecting wire or tracking. This antenna is quite simple and performs well, especially if used in conjunction with a ground plane. This will often be provided by the PCB on which the module is mounted, or by a metal case.

Base-loaded whip: This is a shortened whip, tuned by means of a coil inserted at the base. This coil may be air-wound for maximum efficiency, or a small SMT inductor can be used if space is at a premium. The value must be carefully chosen to tune the particular length of whip in use, making this antenna more difficult to set up than a ¼-wave whip.

Helical: This is a more compact but slightly less effective antenna formed from a coil of wire. It is very efficient for its size, but because of its high Q it suffers badly from detuning caused by proximity to nearby conductive objects and needs to be carefully trimmed for best performance in a given situation. It can, however, provide an extremely compact solution.

Loop: A loop of PCB track, tuned and matched with 2 capacitors. Loops are relatively inefficient but have good immunity to proximity detuning, so may be preferred in shorter range applications where very high component packing density is necessary.

Fig.8: Integral antenna configurations

Integral antenna summary:

whip loaded whip helical loop
Ultimate performance *** ** ** *
Ease of design set-up *** ** * *
Size * *** *** **
Immunity to proximity effects ** * * ***

External antennas have several advantages if portability is not an issue. They can be epitomized for individual circumstances and may be mounted in relatively good RF locations away from sources of interference, being connected to the equipment by coax feeder. Apart from the usual whips, helicals etc, low-profile types such as microstrip patches can be very effective at these frequencies. Suitable antennas are available from many different sources and are generally supplied pre-tuned to the required frequency.

Type Approval requirements: Europe

The modules are verified to comply with European harmonised standard EN 300 220-3 and EMC standard EN 301 489-3 by United Kingdom Accreditation Service (UKAS) accredited Test Laboratory. The modules are CE Certified by independent Notified Body. The following provisos apply:

  1. The modules must not be modified or used outside their specification limits.
  2. The modules may only be used to transfer digital or digitised data. Analogue speech and/or music are not permitted.
  3. The TX3A must not be used with gain antennas such as multi-element Yagi arrays, since this may result in allowed ERP or spurious emission levels being exceeded.
  4. Final product incorporating the TX3A/RX3A should itself meet the essential requirement of the R&TTE Directive and a CE marking should be affixed on the final product.

Type Approval requirements: USA

Radiometrix TX3A and RX3A modules are sold as component devices which require external components and connections to function. They are designed to comply with FCC Part 15.249 regulations, however they are not approved by the FCC. The purchaser understands that FCC approval will be required prior to the sale or operation of any device containing these modules.

  1. Antennas must be either permanently attached (i.e. non-removable) or must use a connector which is unique or not commonly available to the public.
  2. The user must ensure that the TX3A/antenna combination does not radiate more than the maximum permitted level of 50mV/m at 3m distance (FCC Part 15.249).
  3. The appropriate FCC identifying mark and/or part 15 compliance statement must be clearly visible on the outside of the equipment containing the module(s).

Module mounting considerations

The modules may be mounted vertically or bent horizontal to the motherboard. Good RF layout practice should be observed - in particular, any ground return required by the antenna or feed should be connected directly to the RF GND pins at the antenna end of the module, and not to the OV pin which is intended as a DC ground only. All connecting tracks should be kept as short as possible to avoid any problems with stray RF pickup.

If the connection between module and antenna does not form part of the antenna itself, it should be made using 50Ω microstrip line or coax or a combination of both. It is desirable (but not essential) to fill all unused PCB area around the module with ground plane.
The module may be potted, provided that precautions are taken to ensure that no compound can enter the screening can during the potting process.

Warning: DO NOT wash the module. It is not hermetically sealed.

Variants and ordering information

The TX3A transmitter and RX3A receiver modules are manufactured in the following variants as standard:
For European applications in the 868-870MHz band:

Frequency = 869.85MH
TX3A-869-64     Transmitter
RX3A-869-64     Receiver, 64kbps
RX3A-869-10     Receiver, 10kbps

For USA applications in the 902-928MHz band:

Frequency = 914.5MHz
TX3A-914-64     Transmitter
RX3A-914-64     Receiver, 64kbps
RX3A-914-10     Receiver, 10kbps

Other variants can be supplied to customer requirements, at different frequencies and/or optimised for specific data speeds and formats. Please consult the Sales Department for further 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:
Licensing policy manual