EMRFD Message Archive 8655

Message Date From Subject
8655 2013-05-15 20:25:32 mm0gyx Varicap control of 602/386 DC Receiver
Hi folks,

I'm new to the group and although I've added a post to another op's thread this is my first thread.

I'm an enthusiastic beginner in rf design; I've built kits and encountered snags that I've managed to get through, I've built an ugly style rockmite on a piece of board. That was a personal triumph, working from a circuit diagram to produce a functioning rig, albeit at 300mW. I've had qsos with it during sota activations.

So, now I have a copy of EMRFD and I've encountered the first build, I'd like to have a go at it. I have most of the components but no variable capacitors. What I do have though is a pair of MVAM109 varactors which I might be able to use. I've followed the VFO circuit through fig 1.10 and can see the final result and how it's arrived at. The varicaps I have are pretty wide ranging 40pf @ 9v and 500pf @ 1v, I can see from the data sheet that by using a smaller range of reverse bias voltage I can reduce the capacitance swing. So my plan would be to use a range of 1.5 volts to 6 volts, using a couple of diodes between the biasing potentiometer and ground to hold the bottom of the potentiometer up at about 1.4 volts. The 6 volts would come direct from a 6 volt battery supply.

This may still be too much, so I can pad the varicap with a series capacitor to reduce its effect as per the book.

So, I will try this by way of an experiment, I just wanted to share my thoughts and any input is welcomed.