The KD2BD ATV AM Video Modulator


Posted on Nov 27, 2012

A 2N3904 in the base circuit sets the `Q' point of the amplifier. FM subcarrier audio at 4.5 MHz is also injected at this point through a 3300 ohm resistor and a DC blocking capacitor. A high frequency 5-watt RF power transistor (2SC1909) serves as the last stage of the modulator. An NTE-235, NTE-236, or equivalent is suitable in this application. Clamped video is also fed into an LM311 voltage comparator that serves as a video sync detector. Some builders have used an LM1881 sync separator chip for this stage. Either way, sync pulses detected by this circuit trigger a CD4016B silicon bilateral switch that lowers the negative feedback around the video amplifier and voltage follower at sync time to compensate for high power gain compression in any solid-state linear power amplifiers that may follow the video modulated stages.


The KD2BD ATV AM Video Modulator
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Building an in-band, duplexed ATV repeater system is not a trivial task. Just ask anyone who's tried. The output spectrum of the repeater's transmitter must be extremely clean, and both the transmitter's output as well as the receiver's input must be tightly filtered to achieve adequate isolation and reject interference from strong signals on neighboring frequencies. When the Brookdale ATV Repeater System was first constructed, a PC Electronics model RTX-70 exciter was used in the transmitter portion of the repeater. However, the intermodulation distortion products generated by the exciter when amplified by a Mirage D100ATV-R amplifier were so severe, they completely overwhelmed the repeater's receiver, despite the extensive combline and interdigital filtering used in the system. After much study and research, the cause of the exciter's strong IMD products was found to be the result of distortion and non-linearity in the exciter's video modulator, and not the modulated RF stages themselves. Once the original video modulator was replaced with the one described on this page, the harmful effects of IMD were reduced to imperceptible levels. An added bonus was a noticeable increase in the exciter's RF power output. This method of sync expansion is unique in that it also compensates for audio subcarrier compression at sync time, resulting in a clean output signal with no sync buzz in the transmitted audio. The first...




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