ATV Repeater System Exciter

  
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A model RTX-70 exciter that was originally used in the Brookdale ATV Repeater System. This page describes some the design considerations and new features possible with this new exciter, and includes schematics of the circuits designed during the exciter`s development. The ATV exciter described here uses a pair of Hamtronics model TA451 2-watt narrow-band FM voice transmitters
ATV Repeater System Exciter - schematic

to develop video and audio carriers on 439. 250 MHz and 443. 750 MHz. The TA451 is a single board, phase modulated exciter designed for commercial two-way land-mobile as well as amateur radio service. Several circuit modifications were made to each exciter to permit their use in this application. In operation, one TA451 exciter is operated at 439. 250 MHz and is amplitude modulated with video information, while the second operates on 443. 750 MHz and is frequency modulated to produce a broadcast-standard FM aural subcarrier. The RF output of each exciter is combined in a diplexer and further amplified through several linear power amplifier stages. Figure 1 shows the audio circuitry used to produce high-quality frequency modulation from each exciter. Line-level audio from the repeater controller is fed through a 15 kHz active low-pass filter (U1A), a 75 microsecond pre-emphasis network (R8, C5), and an automatic level control circuit (U1B). Automatic level control is used to prevent over modulation should the audio input to the exciter rise above normal levels. This method of level control generates considerably less harmonic distortion than audio clippers typically found in FM voice and some commercial ATV transmitters. Audio signal levels of 20 times above normal can be handled with this circuit without causing overmodulation or excessive distortion. Audio is then fed through a pair of voltage amplifiers having different...



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