Looking for VLF Modem

[PlanarMetamaterials]

Hello,

I'm looking for a simple modem (e.g., RS-232 in, modulated analog out; and vice-versa) that operates at VLF (audio) frequencies. Really my only requirements at this point are that it must be able to detect weak signals, i.e., sensitivity lower than -100 dBm, and it should be relatively inexpensive (< $500) - anything else is negotiable, including data rates. I'm having a hard time finding such a device that's not a homemade Arduino/Pi project with questionable sensitivity (e.g.

). Would anyone have other suggestions?

 

[BradtheRad]

Sounds like the modems we used in the 1990's. They plugged into a phone line so we could get on AOL or Genie or other message boards. 9600 baud rate was typical. (Earlier models were 300 & 1200 & 2400 baud.) At first they were shaped like a telephone cradle. You dropped a handset into it and it made buzzing noises back-and-forth. (Featured in the movie War Games.)

Using Xterm or Zterm or Hyperterm I could communicate by keyboard with a buddy in another state. Every parameter had to be set correctly: data rate 7 or 8, baud rate, half or full duplex, parity bit.

When computers had built-in 56k modems, I sold my 9600 type on Ebay. You might find a vintage model.

To amplify weak signals you may need a preamp. The phone line is -50VDC or so. Not sure what it drops to when you lift a handset.

 

[D.A.(Tony)Stewart]

VLF and modems give a mixed message. What freq., modulation, data rate & purpose?

Do you mean something like a DSP controlled US Robotics V.92 modem? or and SDR or a VLF Nav Receiver? These are different animals from what you specified. Or are trying to communicate under-water with megawatt Tx's. That is what they use for VLF with really long antennae.

 

[PlanarMetamaterials]

VLF and modems give a mixed message. What freq., modulation, data rate & purpose?

These days, I suppose. Specs are relatively flexible, but for the sake of having something in writing, let's say 10 kHz, 300 baud, BPSK. Purpose is long-range comms through a challenging medium.

Do you mean something like a DSP controlled US Robotics V.92 modem? or and SDR or a VLF Nav Receiver?

SDR seems attractive, but again I'm worried about sensitivity.

Or are trying to communicate under-water with megawatt Tx's. That is what they use for VLF with really long antennae.

A very lossy medium like water, yes. Unfortunately, I don't have the megawatts at my disposal, hence the Rx sensitivity.

 

[betwixt]

At low frequencies it becomes relatively simple to design high gain amplifiers and filters using standard op-amps. You should be able to achieve enough sensitivity and build a bandpass filter quite easily. How you decode what was sent depends on how it was sent, simple FSK can be detected with a PLL, ASK can be detected by a comparator. You also need something to convert the result to RS232 voltages but that's easy.

To send data, use the output of an RS232 receiver to either shift a tone or key the amplitude or modulating tone.

Brian.

 

[D.A.(Tony)Stewart]

Modems are conceptually simple yet not easy to do. It gets even harder when you don't know what you need.

For example if you needed high gain from -100 dBm @ 10kHz with low noise that is not a problem. But then if you had an adjacent channel signal 40 dB stronger what SNR do you need to get low error rate. How much suppression is needed, what is the stop band attenuation of the frequency that is 1kHz away? Then how much group delay distortion can you handle that causes jitter and what is the required Op Amp Q needed for each stage and thus GBW required. You will be surprised to discover as I have that GBW increases by the square of the Q and now you are searching for 100 MHz to 1GHz GBW Op Amps.



For cave communication lookup mining radio designs.

https://apps.dtic.mil/sti/pdfs/AD0857630.pdf

Make a 40 dB LPF with FET front end then connect to AM antenna to preamp to mic input.

 

[KlausST]

Hi,

amplifiers are available.
Filters need specifications to be designed. Bandwidth, Q, settling time, or: passband frequency and ripple, stop band frequency and attenuation..

also "lock in amplifiers" come to my mind.
Even ADSampling of the filtered signals and digital signal processing using a microcontroller should be possible.

Klaus

 

[PlanarMetamaterials]

Thank you @betwixt @D.A.(Tony)Stewart and @KlausST; I am fully aware I could build one myself (maybe even just code in software!), but I would have preferred not to.
I gather from your responses that there are no such common commercially available modems (besides possibly sound cards, which don't exactly fit the requirements) -- too bad.
I suppose I will need to seriously investigate making my own modem after all! Thanks for the suggestions.

 

[D.A.(Tony)Stewart]

Perhaps 20 years ago there were some but all the links of H/W I found were gone. Keep trying and post the real reason as the SDR solution looks pretty cool.

The demod. schemes are not the same so application specific specs are missing in your search.

 

[BradtheRad]

300 baud, BPSK. Purpose is long-range comms through a challenging medium.

Having acquired a Macintosh in 1990 I wished to transfer my VIC-20 programs to it. Articles told me all I needed was a null modem... 2 wires are sufficient: 1 common ground and 1 for data. (I included a third wire supposing I'd send data from Mac to VIC-20 but the need did not arise.)

The null modem did not include handshaking signals on several other pins. Just have one computer receiving constantly. I could send from the other computer at my leisure.

I distinctly remember my oscilloscope displaying 300 baud. (About 30 characters per second.) My eyes could just about follow what was happening. Bits changed state between high and low. Characters were separated by an idle gap. Occasionally I could recognize a correlation between binary patterns and letters appearing on screen.

I'm pretty sure your own arrangement can be that simple. Does the medium conduct electricity? If so then one wire is sufficient to send a succession of highs and lows. A system like the old-style telegraph wire.