Subject: SuperHetrodyne FM or AM Seismic Sensor ?


Has anyone ever heard of the following Idea ?
uperHetrodyne FM or AM Seismic Sensor.
Hi Geoff,
    The amplitude you detect with AM modulation on a RF signal varies with =
=20
the signal strength. To use this for a seismometer you would almost certain=
ly=20
need to measure both the amplitude of the modulation and the amplitude of t=
he=20
RF signal and scale one with the other. Anothe problem is that seismic sign=
als=20
are mostly below 10 Hz, maybe even down to 1/100 Hz. Any signal reflected o=
ff=20
the ionosphere will show large variations. More local signals may show chan=
ges=20
with trees moving in the wind, with rain and with snow and even passing=20
vehicles.=20
     With FM modulation of an RF signal, the amplitude is represented by th=
e=20
frequency deviation from the mean and the oscillatory component by the rate=
 of=20
change of the frequency deviation. These can be detected absolutely whateve=
r the
signal strength. You would need to convert the ac geophone signal into a=20
frequency signal and use this to modulate the RF transmitter. A standard FM=
=20
detector would recover the frequency signal. I would not expect the frequen=
cy=20
stability of an ordinary RF signal oscillator to be sufficiently drift free=
.
    Geophone networks usually convert the ac output signals into 12 bit dig=
ital=20
signals and then transmit the serial pulse sequences.=20
    Regards,
    Chris =20








<=
TT>





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Subject: SuperHetrodyne FM or AM Seismic Sensor ?





Has an=
yone ever heard of the following Idea ?
SuperHetrodyne FM or AM Seismic Sensor.

Hi Geoff,
    The amplitude you detect with AM modulation on a RF sig=
nal varies with  
the signal strength. To use this fo=
r a seismometer you would almost certainly <=
/FONT>
need to m=
easure both the amplitude of the modulation =
and the amplitude of the 
RF signal and scale one with the other. Anothe problem is that seismic=
 signals 
are mostly below 10 Hz, maybe even down to =
1/100 Hz. Any signal reflected off 
the ionosphere will show large variations. More =
local signals may show changes 
with trees moving in the wind, with rain and with sn=
ow and even passing 
vehicles. 
<=
FONT size=3D2>     With FM modulation of an=
 RF signal, the amplitude is represented by the 
frequency deviation from the mean and the os=
cillatory component by the rate of 
change of the frequency deviation. These can be detected =
absolutely whatever the
signal strength. You would need to convert the ac geophone signal int=
o a 
frequency sig=
nal and use this to modulate the RF transmitter. A standard FM 
detector would recover the fr=
equency signal. I would not expect the frequency 
stability of an ordinary RF signal oscillat=
or to be sufficiently drift free.
    Geophone networks usually convert the ac output signals=
 into 12 bit digital 
signals and then transmit the serial pulse sequences. 
    Regards,
    Chris