Introduction

Recently some samples of the Nortel NTAX96PA dual mode modulating amplifiers have appeared on the surplus market. They were used in AMPS/TDMA 800MHz cell sites. In commercial service they are rated at around 50W, due to linearity requirements. They operate from 28V. Inside there are 2 main boards; a modulator board and an RF board. As there is no need for predistortion in amateur service the modulator board will not be needed. All the RF board needs is 28V and a 5V bias voltage

Step by Step Modifications

1. Remove the screws holding the lid in place. Store them  in a safe place. The modulator board will be seen as follows:-

2. Remove the 16 screws holding the board in place and remove the board. There is no need to remove the screws holding the two power FETs down. The RF screen will then be seen:-

3. Remove the screen over the pa board, exposing the RF board:-

4. Unless you happen to have a matching connector to the one on the chassis completely remove it and all the associated wiring. Leave the internal screen in place with the 2 rf connectors and 5 feedthrough capacitors. The SMB connector near the handle provides an approximately  -30dB sample of the output signal

5. Mount a 7805 regulator, with 1uF tantalum decoupling capacitors on input and output on the chassis using one of the tapped holes previously occupied by cable ties.

6. Mount a 28V DPDT relay on the chassis. I used crazy glue

7. Wire the feedthrough capacitors up as per the following photograph. 28V is permanently supplied to the RF board and 28V is applied on transmit by a relay to the 5V regulator providing the bias to the amplifier. A 3.5mm jack was used for the amplifier actuation signal, mounted on the rear panel. A phono connector was wired to provide 28V on transmit for an external antenna changeover relay.

8. As I considered the output cable unable to carry the intended power, I replaced the output cable with a piece of semi rigid and an N type connector. I used a short piece of RG58 Cable with a BNC chassis socket connected directly to the PCB for the input as shown 

Initial testing

 First the amplifier " as provided" was swept with a spectrum analyser and tracking generator at low level. With 1mW of drive the following response was seen, showing around 41dB of gain from the unit at 902MHz. No significant improvement could be made by tweaking any of the variable capacitors on the board.

Next the amplifier was connected to my 902MHz transverter initially attenuated to provide 1mW of drive. An output of around 10W was seen. The power was gradually increased while observing the output power. The amplifier provided 110W output with 10mW of drive while taking 12A at 28V. The amplifier was saturated at 120W output.

Conclusions

Hopefully this article will encourage people to  use these amplifiers as they appear on the surplus market..

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This page last updated 2-Jul-05