In 2000 I purchased an AV transmitter through Silicon Chip but it has never been much use to me because the range is less than 5 meters. I pulled it out of my box the other day with the hope of improving it. When I first turned it on and tried to tune it I found that the signal was found all over the place from VHFL to the upper UHF and the range hadn't magically improved over its dust collecting years.
It consists of an RF modulator operating on channel 0 or 1. Its output has an 82R pulldown to ground and is then coupled to an uPC1688 MMIC Amplifier via a 1nF which is coupled to a second stage with the same configuration via another 1nF. The input to the second amp also has an 82R to ground connected via a 1nF. The output of this amp then connects to the antenna via yet another 1nF. The modulator has an output of 80dBuV and the Amps have a power gain of 21dB each which should be plenty of power to get better range so the problem must be with the coupling. As far as I can tell the uPC1688 has a Zin of 50R while the Modulator has a Zout of 75R. They have used a 82R on the output so I was thinking the first step might be to replace this with a pi matching network to transform the 75R to 50R and this might also help block some of the harmonics that might be appearing on all the other channels. I could the do the same thing between the output of the second amp and the antenna if it helps. I have been looking into pi matching networks and have found some on line calculators but they all ask for the "Q" and I have no idea as to what that would be in this circuit or how to figure it out, isn't it X/R??? Am I even on the right track with a pi matching network between stages? I was also thinking that if the uPC1688's do have a Zin and Zout of 50R, is there any point to having the 82R to ground via the 1nF on the input to the second amp?
All thoughts and suggestions greatly appreciated.