Learning RF

[BusyBee]

I was at the point of replying when I noticed where the post was as I couldn't readily repond.  In looking at the content (without seeingor needing to see the diagram) I felt there were a number of basic questions that could be asked of someone apparently without great experience.

Tracy

Tracy,
I attached the project data. He made the one on the board first and then the one on the coffee can lid. The second does function but breaks into oscillation when tuned across the band. He has put in the time to learn the math as you can see. Needs help with the physical layout ( I think)?
Greg

The things I was going to ask have probably been fairly well answered by the previous replies. My main concern was that someone was trying to produce a working circuit at VHF when having little previous experience. There are many pitfalls, and at higher frequencies people have their own experiences to fall back on. That is what I was going to ask about. I know from experience that it is possible to get something working at VHF on veroboard (stripboard) as I copied a VHF bug to that years ago, but that was a very simple (effectively) oscillator circuit. Personally I have been challenged more than once over using veroboard (and have successfully produced a backplane based microprocessor system on veroboard although that was at low MHz) but it is not the recommended way. I have also used veroboard upside down (even this week). It is really experience and applying theory into practice. Close conductors are capacitors and long (even not very) ones are inductors. A combination is a tuned circuit. I think, from what I see, that this is understood, but it is easy to overlook the obvious. Really I cannot stress too highly that it is understanding and have good feeling for what is happening.

Ok for my own (personal) view. The first thing I notice from working on veroboard is that the components are in close proximety. Bearing in mind this is my view and not from any specific professional training, effectively all the components are in close proximety and do behave as aerials at the frequencies we are talking about so inter-react greatly. I prefer to keep things as two dimensional as I can even if that means a bit of spread in the circuit. I do actively think of the inverse square law as to relates to electric field strength (rather nerdy :/).  If then keeping input and output circuits apart there is likely to be minimal feedback between the two. The board looks like it has been sorted, but with veroboard I also remove tracks that may have coupling to unwanted tracks.

Without going into fine detail of the circuit or the maths, the circuits, in principle, looks like it should work. One thing I would think about is that (from what I understand) transistor emitter followers are susceptible to parasitic oscillation, although I am not sure about this configuration. Also coupled with the fact that there is very little tuning in the circuit means that there is plenty of scope for undesired feedback to be amplified, rather than tuned out. Another thing to understand is that with a few changes this circuit should work at lower frequencies. My approach would be to try at around 50MHz with a more open layout (keeping I/P, O/P and power apart) with perhaps parasitic damping if required, then when fully working as required at the lower frequency, progressing to the intended frequency. I would not suggest going too low as the design parameters may be too different to be relevant. Another point to think about is that, for fault finding, the divide and conquer approach is recommended. In that way each stage could be tested in isolation before being put fully into circuit. To do this a signal generator and oscilloscope are ideal, but it is possible to use a signal injector and radio. This sort of circuit is perhaps ideal for learning, but a problem when biting too much off at once. Another of my personal views is not to take too much time trying to sort a pig. I prefer to re-design, but obviously that would not be everyone's view.

Tracy