RF Chain Construction

While testing the 1rst LO/VCO, I noticed that above a certain number of turns on the main tuning pot, the VCO control voltage went too high and the VCO cut off. While the cut off was supposed to be at 200MHz, my device actually cut off about 193MHz (in my MFJ frequency counter). No matter how I set R24 on the Timing Base (the center frequency calibration trimmer), I could not get around it cutting off.

Note that to find out what the center frequency is for a particular setting on the main tuning pot, set the span to minimum or zero span. This makes the sweep VERY tight right around the center frequency.

I decided that what I needed to do was widen the action of the pot by adding a resistor between the +15V and the pot and the pot and ground. This is effectively a voltage divider, and reduces the actual range that the pot covers in 100% of it's turning. In this photo, I'm experimenting with adding these resistors. I never got it exactly right, and gave up, deciding to fix it later. As a practical matter, it really doesn't make all that much difference (unless of course you are using a center frequency very close to 70MHz). I may or may not go back and fix it later.