In this dissertation, a high power amplifier has been implemented and performed evaluation, which is operating at UHF band (470 ~ 806 MHz) for Digital TV repeater.
The theoretical consideration of 100 Watt power amplifier at UHF band used in Digital TV repeater is studied. After extracting target specification of each part, amplifier is designed and fabricated.
When the driving amplifier stage is flown by bias voltage of 28 V DC and current of 900 mA, it has the gain of more than 53.5 dB, the gain flatness of ±0.5 dB and return loss of less than -12 dB in 470 ~ 806 MHz.
Also, when two signals at 2 MHz frequency interval are input port into the driving amplifier stage with 1 Watt output, it resulted in excellent characteristics to design specification with showing intermodulation distortion characteristics of more than 48 dBc.
The middle amplifier stage using 85 Watt device is fabricated by RO-4003 electric substrate which is 3.38 in dielectric constant and 32 mil-thickness. When the driving amplifier stage and the middle amplifier stage were flown by bias voltage of 28 V DC and current of 4 A, they revealed the gain of more than 66 dB, the gain flatness of ±0.2 dB and return loss of less than -16 dB.
Also, when two signals at 2 MHz frequency interval in using frequency band is input port into the driving amplifier stage with 40 dBm of output power, it showed intermodulation distortion characteristics of more than 38.50 dBc.
The 100 Watt unit power amplifier using temperature independent biasing for Digital TV repeater application is designed and fabricated. The DC operation point of this unit power amplifier at temperature variation from 20 ℃ to 100 ℃ is fixed by active bias circuit. The variation of current consumption in the 100 Watt unit power amplifier has an excellent characteristics of less than 0.6 A. The implemented unit power amplifier has the gain over 12 dB, the gain flatness of less than ±0.5 dB and return loss of less than 15 dB over the DTV repeater frequency range. This unit power amplifier yields intermodulation distortion of more than 32 dBc at 2 MHz offset, which satisfies the IMD at output power of 100 Watt (50 dBm).
The broadband high power 3-way combiner was designed and fabricated for the digital TV repeater. To achieve increase of bandwidth and high power capability, Wilkinson type power divider was adopted in our research. First of all, Wilkinson type power divider of equal-split and unequal-split were combined, the characteristics variation of the same phase four port power combiner was simulated according to the various thicknesses of dielectric substrates. As the results of simulation, the power combiner fabricated by using dielectric substrate of 120 mil-thickness has the characteristics as follows: insertion loss of less than -6 dB, return loss of less than -13 dB, isolation among the output ports of less than -15 dB, and phase difference among the output ports of smaller than 13°. Therefore, this power combiner was possible to improve the limit of microstrip line width due to high impedance, the problems of power loss due to interaction between strip lines in high power combiner and narrow bandwidth simultaneously. Furthermore, since the fabricated 3-way combiner has good characteristics of insertion loss, the return loss, separation between ports and phase difference, making broadband and high power could be achieved.
By using 3-way power divider and power combiner, 100 Watt unit power amplifier stage was designed and fabricated as a type of 3-stage balanced amplifier and to detect the final signal output power with using of 40 dB parallel coupled-line directional coupler. The fabricated 40 dB parallel couled-line directional coupler detects the signal power emitting to the antenna, and then supplies its voltage to the attenuator. In addition, this sets to be changed with a basic attenuation 8 dB, according to the changing of the power amplifier's temperature and 8-VSB modulator's input signal.
Besides, in order to measure the fabricated 100 Watt power amplifier, the estimation technique function which makes equivalent mask by Tektronix RFA-300A was used. As a result of the measurement, the existence of pilot signal is confirmed and the signal transmitted at the rated output power 100 Watt is brought out the flat feature through 6 MHz bandwidth. And it resulted that its value was less than -47 dB at the edge of radiation channel and less than -110 dB at more than 6 MHz position from channel edge. Finally a positive results on the equivalent mask has been complied with related regulations.