Barry,

You asked about telemetry for a seismic station. This is either a complex
problem or a simple solution to getting seismic data from yon to hither,
depending on your resources and expectations. 

The most basic is the double FM analogue telemetry. First, the amplified
seismic signal (ground velocity) is used to frequency modulate an audio
frequency (the carrier). The  standard is to use "center frequencies", (or
tones with no data), of 680,1020,1360,1700,2040, 2380, 2720, and 3060 hz.
These eight tones can be multiplexed (resistively added together) to fit
on a single narrow-band FM radio link or a standard telephone line.

The seismic data modulates or changes the frequency of the tones proportional
to the amplifier output voltage. Usually + or - 4 volts swings the 
frequency + or - 125 hz with a VCO (voltage controlled oscillator).
At the receiving or data recording end, a "discriminator" does the 
reverse to recover the seismic data from the FM carrier. This is usually
done with a PLL (phased locked loop) that generates a voltage that keeps
a local VCO tracking the frequency of the incoming FM data. THe "phase
lock" means that the tracking is within a small portion (phase) of a 
single cycle of the FM carrier. So the tracking voltage is the original
seismic signal. If the phase lock is within about 1/20th of a cycle 
(18 degrees) of the 125hz maximum deviation, the dynamic range is 
1:2500, or about 70 db. This is the main limitation of FM telemetry.

I won't go onto the details of a VHF or UHF FM radio link. THere are
some modern ICs that use frequency synthesizers to greatly simplify
the modulation and demodulation, but a commercial low-power link can
still cost $500 and up since they are basically handmade. The parts list
can't amount to more than $50, but VHF/UHF circuits require an experienced
hand. There are probably some ham transceivers that could be used, as well
as some commercial units (I have a lead on one for $180 that can be 
used as either end of a VHF link). But is is too power hungry for 
long-term battery operation.. And there is the problem of which VHF or
UHF frequency to use, and any licensing requirement. Below 0.1 watt
does not require a license in some areas and bands; With proper antennae
and propagation path, 0.1 watt will go 50 km or more.

So if you want to make your own FM telemetry, I can send schematics.
THe VCO is pretty basic, with the main complication being the temperature
compensation circuit. We have only a 90 hz "guard band" between the
highest one carrier can go and the lowest the next up can dip, so
temperature wander of the center frequencies needs to be curtailed.
The circuit uses the VCO section of the cmos 4046 IC.

The discriminator is the complicated circuit. First, it needs to use
a narrow band audio filter to recover its carrier from the multiplexed
group of tones (unless only a single carrier is used). This is usually
a 12-pole tuned bandpass filter consisting of three adjacent narrow 
bandpass filters. A synthesized or digital comb filter can also be used.
Then there is the PLL, usually using the LM565 PLL IC, whose frequency
dependent components need to be carefully selected for stability. Then
the data voltage has to be recovered and properly scaled, and then
at least 4 poles of low pass filtering are needed to clean up the data
(removing any low-frequency carrier components (intermodulations) and
phase jitter of the PLL loop).

I think Larry has a telemetry discriminator that accomplishes most
of this. We made over a 100 of them years ago in order to realize 
the low noise we expected to achieve.

I don't know if an FM telemetry system could be could be called 
inexpensive though. You can make the VCO for $50, but an FM radio
link (synthesized transceivers) will cost $400, and a discriminator will
run $200 per channel, plus considerations for packaging and power.

Regards,
Sean-THomas

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