The series output transistor is a VHF power transistor. It has flat response out past 20 MHz, less than 1 percent total harmonic distortion, and does not generate significant levels of IMD. The results show an emission bandwidth greater than 18 MHz at -38dbc levels. use of a Mirage D24 solid state power amplifier after the transceiver served to increase the bandwidth of the transmitted signal even further.
Circuit DescriptionThe first section of the video 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 to 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.
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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. 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.
R1 is a video gain control. It is used to set the white modulation level. R2 is a video bias control. It is used to set the black modulation level. R3 is adjusted so that the LM311 delivers clean sync pulses to the CD4016B with an input signal present. R4 adjusts the sync modulation level. Up to 9 dB of sync expansion is possible with this circuit.
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