Power, cooling, host connection, antennas and LEDs


Setting up your UmTRX

This page covers:

  1. Power supply
  2. Environment
  3. Host connection
  4. Antennas
  5. Status LEDs

The UmTRX Lab Package includes a power supply, GPS antenna and antennas for GSM use, but you have to take care of assembly and cooling, and a 3-pin mains lead is required.

Fairwaves also provide packaged and turnkey solutions:

  • UmTRAY is a UmTRX in an enclosure and with appropriate cooling
  • UmDESK and UmSITE are complete base stations

The only hardware setup required with UmDESK and UmSITE is to connect suitable antennas.

1. Power

UmTRXv2 requires a stabilised supply of 8-28V DC.

UmTRXv1 requires a stabilised supply of 7.5-15V DC.

A supply rated at 15W constant should be sufficient for a UmTRX without expansion. Adding the UmSEL daughterboard increases the power consumption to around 20W.

For detailed measurements and for details of how to measure power consumption, please see the page on the developer wiki.

Power connector


5.5 x 2.1 x 10mm center positive barrel connector.


Molex Mini-Fit 39-30-3022 (socket), 39-01-3022 (plug).

Power-over-Ethernet (PoE)

Current UmTRX hardware does not have PoE support. Where this is required, e.g. with indoor pico-BTS installations, an external splitter can be used. Since around 20W power is required this means using IEEE 802.3at-2009, a.k.a. POE+, compliant equipment.

2. Environment

This section deals with mechanical, cooling and temperature sensing.


For board dimensions and the size and position of mounting holes and connectors, please see the diagram below.

UmTRX v2.1 mechanical

UmTRX v2.1 mechanical (click for a larger version)

The diagram is also available for download as a PDF.

For an example of a DIY laser cut acrylic enclosure see the UmTRX Open Case.


Cooling is necessary and the heat generated must be dissipated in a stable manner in order to avoid permanent damage of the UmTRX.

An optimal configuration will use thermal stabilisation and/or repeat calibration when there is a significant temperature change.

Passive cooling

A heatsink may be used without a fan and this is mostly suited to outdoor use in a waterproof enclosure. A thermal gap filler of 3-5mm thickness should be used to provide good thermal conduction from UmTRX to the heatsink.

Forced air cooling

Cooling using fans is easier to implement but is mostly suited to indoor use.

Two fans may be connected via the connectors X17 (FAN1) and X18 (FAN2). FAN1 is constantly powered and FAN2 is automatically switched on/off by U35 (MAX6665ASA45) when the temperature is around 45°C.

The maximum DC current of FAN2 must be less than 400mA.

As of UmTRXv2.1 the connectors are powered separately and from either the 6V DC/DC converter (default) or from 8-28V input voltage (re-solder the 0603 jumpers near the connectors for this).

With UmTRX v2 both connectors together take their power from either the 8-28V input voltage (default) or from the 6V DC/DC converter (re-solder the 0603 jumper near the connectors for this).

Temperature sensing

As of UmTRXv2.1 there are thermal sensors U38 (TMP102) that measure the temperature of the board at the point where it gets the hottest.

UmTRXv2 should be used with external thermal sensors with an SPI or I2C interface that is connected to X12 (AUX RF). This will make it easier to ascertain when re-calibration is required, or to switch to using a different look-up table of constants to compensate for drift.

Overheating and shutdown

U35 (MAX6665ASA45) monitors the temperature and the following action is taken at:

  • 45°C: FAN_ON (FPGA_pin_AB14)
  • >60°C: OVERHEAT (FPGA_pin_AB13)
  • >75°C: SHUTDOWN transceiver.

3. Host

UmTRX does not support 100M Ethernet at present and it will not be recognised if your computer does not support gigabit Ethernet. Also it will not work with the stock Ettus version of UHD and the Fairwaves version must be used.

  • Connect UmTRX to a gigabit Ethernet port on the computer which will be running OpenBTS.
  • By default UmTRX has a static IP address, so it is recommended to set your computer’s IP address to

4. Antennas


TX and RX antennas should be selected according to the application.

Some turnkey solutions include a built-in duplexer, enabling use with a single antenna.

Care should be taken when when connecting U_FL pigtails to the TX and RX sockets, as these can be damaged if excessive force is applied while incorrectly aligned. It is recommended that the number of connection/disconnection cycles be minimised and pigtails to more robust connectors such as SMA or N type should always be used.

Damage may occur if UmTRX is set to transmit without a suitable 50 ohm load connected!



With indoor use the GPS antenna should be attached to a window with a view of the sky. Alternatively an external clock source may be used to provide a reference.

Status LEDs

There are 12 LEDs in total:

Three groups of three LEDs (9) at the front of the board.


Two LEDs to the left of the FPGA and one above it.



LED1ATx1 (Channel A)OFF: not transmitting

GREEN: RF output
CRx1 (Channel A)OFF: not receiving

GREEN: receiving
ETx2 (Channel B)OFF: not transmitting

GREEN: RF output
LED2BRx2 (Channel B) OFF: not receiving

GREEN: receiving
DZPU firmwareOFF: not loaded

GREEN: successfully loaded
FFPGA firmwareOFF: not loaded

GREEN: successfully loaded
LED3HUnusedThis LED is connected to the FPGA but is not currently used
IClock distributionOFF: loss of clock signal

GREEN: clock signal is present
GGPS moduleOFF: powered off

BLINKING: position fix

GREEN: operational
N/ALED4FPGA memoryOFF: configuration memory not loaded

RED: configuration memory loaded (normal)
LED5Ethernet PHY TXOFF: not transmitting data

RED: transmitting data
LED6Ethernet PHY modeOFF: 10/100 Base-T

RED: 1000Base-T