Taming the GTX 900 MHz

Posted on Feb 5, 2014

Many of us who use GTX 900 MHz radios to work amateur repeaters are well aware of the erratic output power problem. You go through the complete board replacement procedure, adjusting the deviation and output power, and everything looks great on the test frequencies of 896 through 902 MHz, and 935 through 941 MHz. After all, these are the bands that the radio was designed

Taming the GTX 900 MHz
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for and specified to work in. Then you program in and transmit on some amateur frequencies and you find various output power levels of 0. 8 watts, 12 watts (where you aligned the radio`s output power), or 19 watts. On one radio I have, I would get 12 watts on 902. 4125 MHz and 902. 4875 MHz (two repeater input frequencies), 19 watts on 927. 6000 MHz (a simplex frequency), 0. 8 watts on 927. 5000 MHz (another simplex frequency) and 19 watts on 927. 4125 MHz and 927. 4875 MHz (two repeater output frequencies). Depending on the radio, the power remains controlled on some channels, while others are always erratic. Naturally, the radio still works just fine on the commercial frequencies it was designed for. The GTX radios (as well as some other Motorola products) have 16 alignment points (sub-bands or frequencies) for power and deviation that are adjusted during board replacement or calibration procedures. In the case of the 900 MHz GTX mobile, there are eight of these points, in 1 MHz increments, at frequencies between 895. 5 and 902. 5 MHz, and between 934. 5 and 941. 5 MHz. When the radio transmits in these bands, the appropriate values for that frequency are pulled from the table and used to program the output power and deviation circuits. A problem occurs when an out-of-band frequency is used (902 - 935 MHz); the radio (or RSS) has no idea what value to use, because the sub-bands aren`t calibrated for that frequency range....

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