Geoff, GM4ESD, has asked the relevant question "how much shunt capacitance is present in a PCB island" when cut with a core drill?
------------------------- I have found that it's useful capacitance at the right times and very problematic at other times. I've never used "Manhattan" style construction. Years ago I also had one of the drills that produced a round island on pc board copper and, if I wanted, simultaneously drilled a hole in the center for a lead. I used it a lot, but I *never* built a stable, free-running VFO on double sided pc board with that method because the capacitance through the board was sufficient to cause considerable instability in the oscillator's frequency. Circuit board material makes a fairly unstable capacitor dielectric, so the shunt capacitance changed with all sorts of environmental changes. For those applications single-sided pc board was the answer. With other circuits, such as low-level RF amplifiers and even some audio amplifiers, the stray capacitance introduced by the ground plane on the opposite of the board "looking" at the circuit pad through the board material helped ensure stability at very high frequencies. The board then acted like a low-value bypass connected to every point in the circuit, helping to discourage VHF and UHF parasitics. "Manhattan" construction would be FB for amplifiers and other things where the stray capacitance might help, but I'd not use that technique to build a circuit where capacitance had to be stable, such as a VFO. >From forming pads with the special drill bit I subsequently went on to using a Dremel router tool to grind out copper to form traces on my boards. Works FB for simple boards. Most of my "breadboarding" on pc material is done using standoff insulators for all circuit points not at dc ground. Such standoffs are plentiful, cheap and tiny: 1/8 watt 10 megohm resistors. Cut one end short, bend it at right angles and solder it to the board at the right spot. Then the lead at the other end is the insulated terminal for the circuit junction. With very few circuits in solid state gear showing an impedance of more than a few hundred thousand ohms and usually much, much less, a 10 megohm resistor is as good as the best insulator in that application. Indeed, a 1 megohm resistor would be as good in most circuits. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
