Improving The Communication Concepts Model ATV 3 Downconverter


The Communication Concepts Model ATV3 70-cm ATV downconverter is available in both assembled and kit form and performs very well. Tests alongside a PC Electronics downconverter show the ATV3 yields about 1 "P-unit" better performance when directly fed by an antenna.

The ATV3 uses two RF amplifier stages (MRF-966 + MRF-901), a Mini-Circuits doubly balanced mixer (SBL-1), and an IF amplifier (MPS2222). The MPS2222 is a 2N2222 in a plastic package. It's noise figure is approximately 4 dB, making it a poor choice to be used ahead of a low-noise TV front end. The MPS2222 is used in a common base configuration. Unfortunately, the collector to emitter capacitance of the MPS2222 is fairly high, and in a common base amplifier, this leads to instability and oscillation at VHF frequencies.


old IF amplifier

I.F. Amplifier In The ATV 3 Downconverter


Better performance can been realized by changing the transistor to a low- noise MRF-904, and changing the amplifier to a common emitter configuration.


new IF amplifier

Improved I.F. Amplifier Circuit


A ferrite bead should be placed around the collector lead of the MRF-904 to enhance stability. The noise figure of the MRF-904 is less than 1.5 dB at TV channel 3. The higher input impedance of the common emitter amplifier provides a much better match for the output port of the SBL-1 mixer. The additional gain this amplifier provides is useful in overcoming transmission line losses should the downconverter be mast mounted. It also comes in handy if the output of the downconverter is used to feed several TVs and/or VCRs through a splitter. It also effectively overrides all the noise in your TV's front end, providing identical performance on all receivers. Just be aware that if your TV has poor dynamic range, you could experience some overloading on strong signals, but those are the kind of signal reports ATVers like to hear anyway. :-)



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