PC Electronics model RTX-70 exciter that was originally used in the Brookdale ATV Repeater System was replaced by a new design to improve signal quality and add new system features. 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.
Audio Modulator
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.
Video Modulator
The video modulator used in this exciter is very similar to the video modulator designed by KD2BD in 1996 to improve the performance of several PC Electronics exciters. The first section of the modulator performs a video level clamping function. The high input impedance Darlington amplifier and the small series coupling capacitor combine to achieve an excellent low-frequency response and a clamping function with very rapid response time. A three terminal voltage regulator sets the clamping level at 8 volts.
The clamped video is then level shifted through a zener diode and applied to a single video amplifier stage. A PNP transistor in the collector circuit functions as a constant current source for the video amplifier. The use of a constant current source rather than a collector load resistor in the video amplifier results in very high voltage gain with nearly rail-to-rail output voltage capability. Negative feedback around the video amplifier reduces the gain to the amount required while it increases the bandwidth, flattens the frequency response, increases the linearity, and reduces any distortion products to extremely low levels.
A 2N3904 in the base circuit sets the `Q’ point of the amplifier. A miniature green LED (Mouser ME351-1641) is used as a voltage reference in the emitter lead of video amplifier transistor to prevent the white modulation level from going below approximately 12.5%. An NTE236 RF power transistor serves as the last stage of the modulator. This transistor can develop 13-watts of RF at 50 MHz. A low-cost Toshiba 2SC1969 can also be used in this stage.
Clamped video is also fed into an LM311 voltage comparator that serves as a video sync detector. 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.
Control Circuitry
DC power to the video modulator and each RF exciter is controlled through a controller board that allows both local and remote mode switching, control of the audio subcarrier RF power level, and provides front panel LED indication of the exciter’s status.
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