Re: Yo! RF dudes!

John Larkin wrote:

On Fri, 6 Apr 2007 10:25:28 -0700, "RST Engineering \(jw\)"
<jim@xxxxxxxxxxxxxxxxxx> wrote:


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:dvqc13h0474darn3umfodc6q34ogsdrds1@xxxxxxxxxx


Suppose I have a 1206 ceramic-core inductor on a pc board with nothing
else nearby. What would be the differential effect on inductance of
adding a ground plane on the opposite side of the board? Even more
important, how might it affect the TC?

The center frequency should have gone down. The TC is now a function of the PCB material (G10/FR4?) which is wildly variant with temperature. Get the copper out of there if you can.



RF requires a lot of patience.

...and powdered bat wings, ground toad warts, incantations, and a virgin (if you can find one these days) sacrifice.

Jim



Yes, the frequency shift direction and tc are consistant with extra
board capacitance. I once measured some FR4 capacitance and got +900
ppm per degree C. The oscillator frequency tc apparently shifted from
near zero to -70 ppm when the extra ground was added. I guess I'll
measure the critical node capacitance on a bare pcb and do the math,
just to see if the numbers are close. Luckily, there's a pattern I can
cut out of the backside copper that should seriously reduce C and not
trash any vias. Good thing we only bought 50 boards!

Still, I'm curious about the effect of a ground plane on the inductor.
I can, and will, measure the effect on L, but it's more difficult to
measure the effect on the inductor's TC.


It's most likely miniscule compared to the capacitance (bottom of winding to plane). It would be easy to measure the added capacitance if you can get something like a HP4191 input pod really close to that point. Dremel away the suspect area, measure, place copper tape over it, measure again.

Question: Is there some headroom to just solder silver wire to the pads and then the inductor to those, so it comes 1/4" or so off the board? Those wires should be beefy and stiff to avoid microphonics, else you'd have an earthquake detector.

--
Regards, Joerg

http://www.analogconsultants.com
.

Relevant Pages

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