This downconverter is a basic RF downconverter system that is designed to convert television or other wideband signals in the 420-440 MHz (70 centimeter) range to a 61.25 or 67.25 MHz IF output frequency (CH3 or CH4) to enable reception of these signals on a standard VHF TV receiver or monitor. It features a low noise RF amplifier feeding a mixer transistor, a tuneable LO, and a feedback pair IF preamp stage. The RF amplifier is a low noise dual gate MESFET that is followed by a mixer stage stage using a UHF bipolar transistor. Three tuned circuits are used in the RF amplifier. The on-board local oscillator (LO) is voltage-tuned by an on board potentiometer or via the coax fed DC tuning and supply voltage. All necessary circuitry for remote tuning is on board for coax DC and IF feed. This enables the downconverter to be mast mounted to avoid feedline losses generally associated with this frequency range. No separate DC feed is necessary since the coax (RG59/U or RG6/U recommended) carries both DC power, tuning voltage and IF signal. A DC block can be used at the receiver for the purpose of separating the DC supply voltage and the tuning voltage. This allows a cable run of several hundred feet if needed. Input impedance is 50 ohms and a suitable 70 cm. antenna should be used.
For on-board tuning the potentiometer may be used. The LO is voltage tuned and is fed from a voltage regulator IC. The downconverter PC board has the same footprint as our 900 MHz and 1300 MHz models and matches the two watt single and three channel atv transmitters for 440 MHz in physical size. The power supply requirements are 11 to 20 VDC at approximately 30 milliamps, or a variable supply from 11 to 20 volts for combined DC and IF feed on a single coaxial cable for remote tuning. Overall gain is 35 - 40 dB nominal with > 20 MHz RF bandwidth and tuning range when set up for 420 to 440 MHz operation. This downconverter is not intended for reception of narrowband FM or SSB signals since it uses a tuneable LO.
This downconverter can also be combined with our IF44 Video IF which will produce baseband video and line level audio output if this is preferred. Both of these units can be mounted in our NC1575 Plastic case to produce a complete video-audio receiver unit.
By using this downconverter and one of our 2 Watt ATV transmitters, a physically small 70 cm. ATV station can be constructed, since both units are each 2.50 x 4.00 inches x 1.00 high, and can be stacked together. A small PC board camera, battery pack, relay, and a pocket TV receiver can be used with these units to make a very compact portable ATV station. Also, 440 MHz antennas are physically small and lightweight. Construction is rather simple and the downconverter is aligned simply by peaking for maximum received signal with no tricky alignment procedures or specialized RF test gear necessary for good results and performance.
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NORTH COUNTRY RADIO
PO Box 200, Hartford NY 12838-0200