In a message dated 01/01/2003, meredithlamb@............. writes:

> About the only question of a general nature, I can find to ask is that 
> regarding the shielding of the LVDT unit.  Roughly, what do you use; or 
> would suggest in either the minimum or maximum sense?  Would say, specific 
> shielding surrounding the LVDT board itself might be in itself somewhat 
> adequate; as a lot of amateur seismo's are often rather huge? Take care, 
> Meredith Lamb 

ChrisAtUpw@....... wrote: > 
> >> Hi all seis experimenters, John Lahr has very kindly put my notes on the 
>> LVDT precision seismic sensor Kit on his Website at: 
>> http://jjlahr.com/science/psn/chapman/lvdt/ 

Hi Meredith,

     The circuit is most sensitive to alternating magnetic fields with a 
frequency harmonically related to the oscillator frequency. Screens of Al or 
preferably tinplate are effective. It is preferable to make them with >6 cm 
clearance from the ends of the board. If they are closer and can physically 
move, they can couple to the sensor field and give a false reading. I checked 
the overall frequency response of the sensor by fixing a single 1" square of 
thick copper wire to the shaft of a miniature electric motor. I mounted the 
loop about an inch from the outer end of one sensor coil and varied the 
rotational rate of the motor to modulate the magnetic field.  
     The simplest way to check for interference is to wedge the ferrite rod 
armature and check that the output signal is constant. The only interference 
source that I experienced was due to the computer monitor several feet from 
the sensor. Trimming the oscillator frequency removed the problem, additional 
screening was a bonus. This sensitivity is only a problem if you don't know 
about it.

     Regards,

     Chris Chapman
In a message dated 01/01/2003, meredithlamb@............. writes:



About the only question of a general nature, I can find to ask is that regarding the shielding of the LVDT unit.  Roughly, what do you use; or would suggest in either the minimum or maximum sense?  Would say, specific shielding surrounding the LVDT board itself might be in itself somewhat adequate; as a lot of amateur seismo's are often rather huge? Take care, Meredith Lamb 




ChrisAtUpw@....... wrote: 


Hi all seis experimenters, John Lahr has very kindly put my notes on the LVDT precision seismic sensor Kit on his Website at: 

http://jjlahr.com/science/psn/chapman/lvdt/ 




Hi Meredith,



     The circuit is most sensitive to alternating magnetic fields with a frequency harmonically related to the oscillator frequency. Screens of Al or preferably tinplate are effective. It is preferable to make them with >6 cm clearance from the ends of the board. If they are closer and can physically move, they can couple to the sensor field and give a false reading. I checked the overall frequency response of the sensor by fixing a single 1" square of thick copper wire to the shaft of a miniature electric motor. I mounted the loop about an inch from the outer end of one sensor coil and varied the rotational rate of the motor to modulate the magnetic field.  

     The simplest way to check for interference is to wedge the ferrite rod armature and check that the output signal is constant. The only interference source that I experienced was due to the computer monitor several feet from the sensor. Trimming the oscillator frequency removed the problem, additional screening was a bonus. This sensitivity is only a problem if you don't know about it.



     Regards,



     Chris Chapman