A 23cms Preamp.


updated: 06/05/00

Trying to Produce A PCB: This Time A 23cms Preamp.

I suppose it is not surprising that at this time of the year the path between GB3AT in Winchester and me on the South coast seems to deteriorate. I expect it is to some extent due to leaves on trees and the general thickening up of growth in the New Forest. Anyway the net result is that most of the time the pictures are in black and white and very noisy so I thought it was time to see if I could squeeze a little more signal out of the noise. Unfortunately a bigger antenna is not an option; so what about a better preamp?

For some time I have wanted to improve my method of progressing from a circuit design to a finished printed circuit. This seemed like an opportunity to do something about it. Many weeks later I have my first prototype board.
This is it. Luckily the picture is so indistinct that you cannot see the state of the PCB! All the copper was etched to some extent, including the tuned lines. Early replacement of doubtful FETs, and some resistors, led to complete annihilation of some tracks. 23cms preamp

The 'design' is far from good or correct, but it is a start. I have tried it in front of my Solent RX (+ 20dB pad!) with a definite improvement. The picture always locks and is in colour. What the actual noise figure is I have no idea. Sound on the two subcarriers is now useable but not quite fully quietening. I have also noticed almost complete elimination of the breakthrough I used to get of local broadcast TV and data signals on what I suspect to be on an image frequency. Another improvement over the ATF10135 in the Solent converter is that it appears to be stable. Perhaps the signal from 'AT is now almost good enough to try my inter-digital filter in front of the preamp.

I tried the preamp in front of my old BATC TVRO RX. I could just see a hint of GB3AT which, I think, illustrates the advantage of a dedicated ATV RX having a narrow bandwidth.

Press here to see the circuit diagram

(I notice that some detail has been lost. The only 'common' file format in which I can save a circuit diagram is *.DXF. I then converted it to *.GIF file. Next time I will use more 'dpi' in the conversion!)


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