Jim,

You could look at the STM-8, and there are schematics online somewhere for
the STS-2, and Randall Peters' devices. These all work by using a variable
reactance, be it inductive or capacitive, to modulate the amplitude of the
carrier. The differential sensors are effectively double sideband suppressed
carrier (DSB-SC) modulators. If you work out the math, the phase reversal
switches that the above mentioned sensors use can be likened to a product
detector, with the exception that you have all the harmonics from the square
wave in the output. These are far higher in frequency than the signal of
interest, and so easy to filter out.

I am not aware of any devices that use frequency modulation, but there's no
reason it couldn't be done. You would have to look at linearity,
sensitivity, bandwidth, power consumption, sensitivity to other parameters
(temperature, power supply voltage), etc. A differential sensor would
require two separate cores, otherwise the two oscillators would be coupled
together.

Matt

On Wed, Apr 13, 2011 at 1:46 PM, Dave Nelson wrote:

>
> The traffic lights use a buried loop of cable at the intersection.  This
> inductive loop
> changes its inductance value when the large metal body/chassis mass of the
> vehicle is placed  over it.
>  That change of inductance changes the oscillator freq which is detected.
>  We
> (the company I work for) uses the same system for detecting cars passing
> through
>  the lanes of fast food drivethroughs.
>
> not really sure how you could apply it to seismology in an easy way.  Its
> not overly
> compact
>
> Dave
>
>
>
>
> At 02:24 AM 14/04/2011, you wrote:
>
>  Could someone point me in the right direction to view the wiring and
>> schematic diagrams of the FM or AM seismic sensors? I am curious to see how
>> they operate. Keep in mind the following: Automatic traffic signals are
>> using similar concepts to detect presence when a car waits at an
>> intersection. Notice the wires lines on the pavement, they lead to a box at
>> the intersection which times the traffic signals.
>>
>>
>>
>> Jim
>>
>>
>>
> __________________________________________________________
>
> Public Seismic Network Mailing List (PSNLIST)
>
> To leave this list email PSNLIST-REQUEST@.............. with the body of
> the message (first line only): unsubscribe
> See http://www.seismicnet.com/maillist.html for more information.
>
Jim,

You could look at the STM-8, and there are schemati=
cs online somewhere for the STS-2, and Randall Peters' devices. These a=
ll work by using a variable reactance, be it inductive or capacitive, to mo=
dulate the amplitude of the carrier. The differential sensors are effective=
ly double sideband suppressed carrier (DSB-SC) modulators. If you work out =
the math, the phase reversal switches that the above mentioned sensors use =
can be likened to a product detector, with the exception that you have all =
the harmonics from the square wave in the output. These are far higher in f=
requency than the signal of interest, and so easy to filter out.

I am not aware of any devices that use frequency modula=
tion, but there's no reason it couldn't be done.=A0You would have t=
o look at linearity, sensitivity, bandwidth, power consumption, sensitivity=
 to other parameters (temperature, power supply voltage), etc. A differenti=
al sensor would require two=A0separate cores, otherwise the two oscillators=
 would be coupled together.

Matt

On Wed, Apr 13, =
2011 at 1:46 PM, Dave Nelson <dave.nelson@...............> wrote:



The traffic lights use a buried loop of cable at the intersection. =A0This =
inductive loop

changes its inductance value when the large metal body/chassis mass of the<=
br>
vehicle is placed =A0over it.

 =A0That change of inductance changes the oscillator freq which is detected=
.. =A0We

(the company I work for) uses the same system for detecting cars passing th=
rough

=A0the lanes of fast food drivethroughs.



not really sure how you could apply it to seismology in an easy way. =A0Its=
 not overly

compact



Dave









At 02:24 AM 14/04/2011, you wrote:




Could someone point me in the right direction to view the wiring and schema=
tic diagrams of the FM or AM seismic sensors? I am curious to see how they =
operate. Keep in mind the following: Automatic traffic signals are using si=
milar concepts to detect presence when a car waits at an intersection. Noti=
ce the wires lines on the pavement, they lead to a box at the intersection =
which times the traffic signals.








Jim








__________________________________________________________



Public Seismic Network Mailing List (PSNLIST)



To leave this list email PSNLIST-REQUEST@.............. with the body of the m=
essage (first line only): unsubscribe

See h=
ttp://www.seismicnet.com/maillist.html for more information.