PSN-L Email List Message

Subject: Re: spike problem
From: Mauro Mariotti mariotti@.........
Date: Tue, 06 Jun 2006 19:41:52 +0200

Hi Jon,

i saw that many things has been discussed to help you
for the spikes problems.

Please be sure to have applied the following rules.

1) When use a CAT5 shielded cable be SURE to connect to
with the shield ALL the UNUSED wires of the cable.

2) ALL MARK L15 geophones has their case (metal) connected to
one of the signal poles. To identify what is it try it
with a ohmmeter (before this measure is better you disconnect
your premamplifier).
Touch the ohmmeter in the case (be sure the probe really touch
the metal case, metal oxide sometime doesn't allow good contact).
L15 has been designed to be used in plastic case.
If you use them direct in ground or in a metal case
then your metal case and the ground itself become in contact
with one of the 2 poles of the preamplifier input.
This will makeup a GROUND LOOP capable of pick almost all
consistent spikes.
To solve this be sure the L15 are insulated from each other
and from ground.
when the L15 geophones are insulated you can connect their
poles to the CAT5 twisted couples. Each geophone on a twisted
The CAT5 cable shield now can be connected safely to the
(eventual) metal box of your sensor box.
Then go to the larry board.

3) For the Larry's amp board (that if I am not wrong is a
single-ended input, Larry correct me if I am wrong)
you have to apply the so called

pseudo-differential connection.

The cable with the sensor's signal willbring the signal
in the differential mode. This will increase the capacity
of the system to reject the commom mode noise.
The pseudo-differential connection then is accomplished
connecting the hot-pole of the sensor in the
signal input of the larry's preamplifier and
connecting together the shield and the cold-pole of the
sensor to the signal ground of each channel.


4) BE SURE to keep separates the analogue power supply and
the digital power supply. So use a coil transformer
for the Larrys board and DO NOT USE THAT POWER FOR ANY OTHER

These four rules usually solve ALL spikes problems.

Ask for explanation if i forgot to specify something!

hoping this helps
receive all bests



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