PSN-L Email List Message

Subject: Re:How much gain
From: Roger Sparks rsparks@..........
Date: Tue, 04 Dec 2007 05:33:55 -0800

In a message dated 04/12/2007, tchannel1@............ writes:

"I am using Larry's board, AmaSeis and  Winquake.   I just made a new coil and 
magnet assembly,
I am just looking for some advise as to setting  the gain.  
The best one I heard, was to use a quiet night  and set the gain to show none 
or just a little back ground  noise."

Hi Ted,

The investigator has two challenges here.   The first is the detect every ground motion, however small.  This requires the highest possible gain from the initial amplifier.  This is comparable to setting the RF (initial) gain in a radio receiver.

The second challenge is to display the trace in a pleasing manner.  This requires control of the the display gain, and is comparable to setting the audio gain in a radio receiver.  

Can you get too much gain? Yes, the background noise can overdrive some component into a "clipped" mode, or saturate the display.

What is "background noise"?  The first background noise is from the electronics.   There is no such thing as a perfect amplifier, they all have some minimum noise level.  Investigate this noise level by locking your seismometer beam and observe the readings from the A/D device.

The second background noise level is from the microsceims caused by ocean waves(?).  Atmospherics,
human activity and machines can also cause background noise but you will soon learn to recognize these sources.  At my station, I often have the background noise from ocean sources reading 100 counts with peaks to 300 or more.

Here in Washington State, we have local quakes at frequencies up to 10 hz and more.   I can only detect to 5 hz right now (due to sample rate) but I often see traces from the local quakes "riding" on the much slower undulating trace from the ocean waves.  Any weak readings from more distance quakes also "ride" on the ocean wave trace.  

Best wishes,  Roger


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