EMRFD Message Archive 10061

Message Date From Subject
10061 2014-05-23 21:26:52 Jerry Haigwood VFO PCB
Hi All,
I am laying out a PCB for a VFO. I have read somewhere that VFOs should
be laid out using a singled sided PCB. Outside of the stray capacitance,
why would it matter if I used double sided or single side PCB material?
Anyone have any great words of wisdom?

Jerry W5JH
"building something without experimenting is just solder practice"
10062 2014-05-23 21:52:49 w7zoi Re: VFO PCB
Hi Jerry, and gang,

The problem relates to the board capacitance.  If you do a double sided board layout, there will typically be a few pF of C that is just in the layout, unrelated to the parts added.  The C of the usual board material has a poor Q and may often soak up water vapor.    I suspect the temperature coefficient is poor too.  These problems disappear is there is no ground plane under the runs in the tank part of the oscillator.

I've not done any controlled experiments on this, but have built some VFOs with double sided board that drifted a lot.   A rebuild with the same exact circuit, but without the double sided board layout produced much more stable results.    So do some measurements Jerry and let us know what the difference turns out to be.

73, Wes

10064 2014-05-24 07:55:08 jorschei Re: VFO PCB

Hi, Jerry

If you need a ground plane under a VFO printed circuit to form some screening  you could use a grid

type of ground plane  to reduce the capacity. An example see below.

I develop a ceramic VFO  which has some drift , and  I use a heated white ceramic bead  to bring a spot heating on the components to find out which component has  the most drift in the module.

See in my PE1KTH file for example ground grid.

So you can find out where it is coming from.

73' Joris  PE1KTH


10066 2014-05-24 16:19:30 Tayloe, Dan (NSN ... Re: VFO PCB
I place a square hole in the bottom ground plane sized to fit the torrid inductor and the VFO caps within the hole.  Ground connects to foil surrounding the square hole.  Traces can run on both sides. That approach has worked pretty well for the PCBs I have done.

Of course for dead bug prototypes, it doesn't matter.  I just try to minimize the use of nibble tool created pads for component interconnection to minimize the poor Q stray C. Half a nibbler tool pad is pretty small.

In addition I use mostly #32 enameled wire for dead bug interconnection. That is fine enough to be somewhat limp, which also minizes microphonics.

- Dan, N7VE

Sent from my Windows Phone

10070 2014-05-25 19:18:41 Jerry Haigwood Re: VFO PCB

I take it you did not have a top ground plane either.  Is this correct?

Jerry W5JH
"building something without experimenting is just solder practice"