USB circuit disconnects when the circuit is put near another surface

Hello,

I have built a basic circuit for the PIC18f4550 using a 20 MHz crystal
from http://www.create.ucsb.edu/~dano/CUI/ . I have installed the HID
firmware from http://www.lvr.com/usb.htm (USB complete). The circuit
works fine and connects ok then installs the driver as long as it is
suspended in free space.

As soon as the circuit comes near ( 2-3 cm) another surface (paper or
the wooden table) the usb disconnects. When I investigated I found
the voltage on D+ signal line went down from the required 3.1V(approx)
to 1.45V as it neared the surface, thus initiating the disconnection.

Connecting just the ground side of an oscilloscope to the ground on
the circuit also cured the problem. I have no idea how to modify the
circuit to make it more stable. I assume it is interference in the
clock signal that is causing the problem, although this is only a
guess.

Any help would be greatly appreciated.

Best regards

Mike.

.

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