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>= 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 withthe 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 theRF signal and scale one with the other. Anothe problem is that seismic= signalsare mostly below 10 Hz, maybe even down to = 1/100 Hz. Any signal reflected offthe ionosphere will show large variations. More = local signals may show changeswith trees moving in the wind, with rain and with sn= ow and even passingvehicles. <= FONT size=3D2> With FM modulation of an= RF signal, the amplitude is represented by thefrequency deviation from the mean and the os= cillatory component by the rate ofchange of the frequency deviation. These can be detected = absolutely whatever thesignal strength. You would need to convert the ac geophone signal int= o afrequency sig= nal and use this to modulate the RF transmitter. A standard FMdetector would recover the fr= equency signal. I would not expect the frequencystability of an ordinary RF signal oscillat= or to be sufficiently drift free.Geophone networks usually convert the ac output signals= into 12 bit digitalsignals and then transmit the serial pulse sequences.Regards, Chris