ULF (5Khz) transmitter and receiver design. - Page 1
Forum section: electronics / projects

author: Hexureus
author_rank: Contributor
timestamp: September 21, 2020, 09:21:11 pm
content: Hello.RF is really very new to me, and I am designing a product that will need to receive and other to transmit 3 bytes of data every few seconds to tens of seconds.The transmitter must be very low power, both transmitter and receiver's antennas should be at about 2 inches maximum size.Any ideas and suggestions would be very appreciated.Thank you.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 21, 2020, 09:37:24 pm
content: 5khz and compact? Your gonna have to either have reciver/transmitter very close and a power amplifier on the transmitter, or use a much bigger antenna!Seems to me like you could just flip an oscillator on and off to create CW in an ism band like 315mhz, if you want compact and low data rate. As for the reciver just use a tuned LC tank and mix it with a tone or measure its power with a shottkey diode.If this isnt about expermentation with rf than just buy those 433mhz ebay modules there like 99cents, less than winding an inductor probably.

author: Hexureus
author_rank: Contributor
timestamp: September 21, 2020, 09:45:38 pm
content: Hi,Thank you for your input.I should have given a more detailed application.This is for underwater communication, and so for that reason the need for very low frequencies. As for distance, a couple of meters would be sufficient.What type of modulation you think would work best, amplitude ou frequency?I really have little knowledge of how electronically you put data (binary) into a EM wave.Thank you

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 22, 2020, 03:19:48 am
content: Oh boy, I dont know anything about underwater rf transmission... if it were me and i needed to communicate at that low of a data rate underwater i would look in to using water proof ultrasonic piezos and then just send the signal CW(Morse code) style by pulsing the transmitter on/off and using the second ultrasonic as a mic listening for the pulse trains of vibration.If you really want to use RF do some research in to submarine VLF communications, I think subs send out very low bandwidth data pretty far utilizing the features of vlf (around 30khz if I'm not mistaken). I would think ultrasonic or laser would be much easier under water though.

author: tkamiya
author_rank: Super Contributor
timestamp: September 22, 2020, 03:43:39 am
content: You'll have to violate a lot of physical laws to make that happen.... Thinking you'll have to be in VLF region for under water application is correct. But rest of it isn't possible or practical.At that kind of frequency, you'll either need a vary large antenna or a magnetic loop of some kind to emit and receive signals. You'll also need a lot of power. Either way, you will not get anything done with 2 inches (5cm) maximum. You may have better luck with sonar technology. It will be the same 5KHz, except propagation mode will be a lot different. Some kind of transducer for both end will need to be used. Also, speed of transmission will be speed of sound, not speed of light.If you look into submarine communication technology, you'll notice they use HUGE power and HUGE antennas. How much distance are you looking to communicate? Will this be in open water where there is a lot of noise or a controlled environment like a tub in a lab?

author: BrianHG
author_rank: Super Contributor
timestamp: September 22, 2020, 04:10:17 am
content: Quote from: Hexureus on September 21, 2020, 09:45:38 pmUse high power Violet 405nm or 450nm blue LEDs. Like 25watt ones with very short bursts in the microsecond range.Verify the transmissability of light at those wavelengths in water. UV LEDs might work even better.This wont work in muddy water.

author: bsdphk
author_rank: Regular Contributor
timestamp: September 22, 2020, 12:09:14 pm
content: Sonar, ie: acoustic, is absolutely the way to go in this case, quartz-transducers are readily availble (think: amateur fisher echo-sounders) and the sounds carries great at that range.

author: Hexureus
author_rank: Contributor
timestamp: September 22, 2020, 01:34:31 pm
content: Hello every one, thank you for your input.I did think about using sound (ultra sound) but I would really like to implement EM wave carrier.I know this is a technology that already exists and is widely used in pressure transmiters for scuba diving computers.This patent describes how its used, but what im looking for is how i could implement it electrically.https://patents.google.com/patent/US20100130123A1/enThank you all for your help

author: CaptDon
author_rank: Super Contributor
timestamp: September 22, 2020, 02:33:56 pm
content: Some of those electronic SPG units are bluetooth and workas well under water as in open air because under water theyonly need to transmit about 1 meter from the tank to the gaugeon your arm. So if you invent something with our help do we geta cut of the profits? The electromagnetic wave is the wrong answerfor under water unless you are trying to go stealth into a placeyou don't belong and even then it would be the wrong answer.Green lasers work good under water and with back scatter fromimpurities in the water the receiver and transmitter don't have to bein direct alignment.

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2020, 03:10:44 pm
content: I'm going to make a wild guess the 5.3 kHz transmission isn't RF at all, it's NFC using magnetic fields.PS. yeah, the transmitting "antenna" is predictablya ferrite electromagnet. The patent also mentions the possible use of two "antennas", this is probably to prevent the receiver from being perpendicular with the transmitter, which would be a problem for NFC.PPS. as for modulation, depends on SNR ... if SNR is high then you want as many bits per symbol (ie. QAM with tons of positions) to keep power consumption down. If SNR is low you can just do binary PSK.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 22, 2020, 05:15:39 pm
content: Hi Hex, I am new here although I've been reading for some time.I had some experience a bit similar to your needs.About 30 years ago I did a home brew project on near field magnetic comms using battery power,which was just to transmit a pulse with jitter of < 5 millisec triggered by an event, across a sports arena.One tx I tried used a tuned circuit consisting of an X pair of ferrite rods as the inductor,with tuned circuit with both rods wound, on rx and tx.That X was necessary to avoid a deep null if the receiving ferrite rods moved through 90 degrees relative to the tx rods.in tx, a power fet was the driver, fed from a gated 555 astable. It was necessary to use a tap on the inductor to "ring up" the tank voltage.The carrier was about 180 kHz amd modulation was about 5 kHz as I recall.The receiver was a similar ferrite X tuned to same ~ 180 kHz carrier and feeding one of the TRF IC AM receivers then available(I think those are scarce these days, some of the amateur radio suppliers still have stock.)I don't recall if I had an RF preamp.I do recall that the recovered 5 kHz from receiver was saturated level over distance of 100 metre or more, in air.The first task I would suggest for your project would be to model it in a 2D FEM,with the properties of the water (rho ) and the needed dimensions etc.(I use QuickField here although I have never considered sea water)That would give an indication of the H field you need along with tank Q factors, drive power, receiver level etc.Good Luck!

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2020, 05:34:19 pm
content: Quote from: mag_therm on September 22, 2020, 05:15:39 pmI guess for simplicity, phase only modulation is the easiest, no need for AGC.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 22, 2020, 06:00:43 pm
content: Quote from: Marco on September 22, 2020, 05:34:19 pmIn my case, there were 5 tx sending to a computer polled I/O board, logged against a clock.The events could be simultaneous.So I tested using both spread carriers and spread AF, with 5 receiversI forgot to mention above that I fed the demod AF to tuned circuits. ( a bit bulky, using toroids)The tuned AF was rectified and fed to a comparator with a high threshold for added cross talk andnoise immunity at the expense of a delay.I know there are a lot of digital refinements that could be added today, but I think the idea of X ferrite rods for tx and rx,might be applicable to Hex's project.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: September 22, 2020, 06:19:36 pm
content: This guy is talking about sending 3 bytes of data, I think cw fits the bill, first it's easy to implement to complex math or circuits required. Second it's easy to recover even with hi signal to noise ratio. Third when pulsed with a micro it still has a ton of headroom to add more data. Lastly if OP realises that RF just isnt going to practical and switches to ultrasonic transducers or lasers as recommended CW still work just as easily. I think BPSK is probably better but judging from OPs posts I think CW is a better idea do to ease of implementation and I would say QAM as someone suggested should not even be looked at for 3 bytes of data in a low and environment not to mention needing vector generation to implement.

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2020, 06:42:02 pm
content: Quote from: rwgast_lowlevellogicdesin on September 22, 2020, 06:19:36 pmThis confused me for a bit, I didn't know HAMs called Morse over RF CW. OOK (on off keying) is a little more descriptive and easier to google.Looking at UWATEC manuals, the way you have to position the devices it does suggest they are using NFC ... it really does seem to work.

author: TimFox
author_rank: Super Contributor
timestamp: September 22, 2020, 06:49:31 pm
content: Historically, Morse telegraphy by spark transmitters was “evanescent” wave (damped oscillation), which was replaced by “continuous” wave transmission by Alexanderson alternators and then by vacuum tubes.

author: BrianHG
author_rank: Super Contributor
timestamp: September 22, 2020, 06:57:51 pm
content: Someone has already done a lot of work on using Blue/Green LEDs here:(And when I say leg-work, I mean the types of ocean water & the math describing the deterioration of the signal over distance with the noise floor, including text fixture with specified LEDs, optical assembly end photo detector with test results.)https://ieeexplore.ieee.org/document/8353342Expect 7 meter distance with really good 2 megabaud bandwidth.IE, if you only need 300 baud, transmit each character at 2 megabaud and keep the LED off for the dead time inbetween making your 5 watt transmitter consume a few ~0.001 watts total. Use a 25 watt led like I said and you would have fairly omni directional distance clearing 10 meters easily.Or, you can add triple redundancy into the source data at the 2 megabaud for guaranteed data and your transmitter will consume 0.003 watts.

author: Hexureus
author_rank: Contributor
timestamp: November 17, 2020, 12:46:19 pm
content: Hello everyone, and thank you all for your input.Continuing my conquest researching the topic, I have gathered some more information and would appreciate some more information.Attached is a photo of an antenna used in one of those devices for communication (120Khz frequency).Can someone explain why you would use an antenna with a center tap? Could it be for PSK?Thank you

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 17, 2020, 12:50:42 pm
content: bear in mind that ferrite rod antennas are very directional - if range is an issue and you have no control over orientation you may need 2 or 3 of them in different orientations

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 17, 2020, 03:55:43 pm
content: That ferrite rod looks like it is an old AM broadcast band antenna tapped for a low impedance output.The large coil would possibly have been in parallel with a variable capacitor to form a high Q tuned circuit.The small low impedance coil would take the output to an amplifier or mixer.They should work at 120 kHz with a suitable capacitor.And use two or three of them in X ( orthogonal) configuration to avoid the null, as Mike mentioned.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 17, 2020, 07:22:52 pm
content: I wonder if infra-red would be an option - you could use normal IR remote receivers to receive a ~36kHz modulated carrier, and they'd even demodulate it for you. High power IR LEDs are cheap and easily available.

author: poorchava
author_rank: Super Contributor
timestamp: November 17, 2020, 08:46:03 pm
content: You could also do something to improve sensitivity. Like using very well tuned frequencies, orthogonal pseudorandom numeric sequences and DSP. You'd be amazed how much improvement can be achieved over standard analog rx circuits. This is not applicable to higher frequencies because of lack of processing power and sufficiently fast ADCs.Unfortunately I'm not at liberty to reveal any specific algorithms.VLF works on magnetic induction and electrical field gradients, rather than waves, since waves are incredibly long. Or 1/4 wave at 5kHz is about 15km, which is why submarine VLF rx antennas are literally large fields or wires on poles and the submarine drags it's antenna behind it (not sure about that). The power levels supposedly go into multiple tens of kW, but who cares when u have a friggin nuclear reactor onboard.The sensitivity of the entenna is directly proportional to its size. So either a large antenna is needed or advanced rx methods to compensate for crappy sensitivity. Transmission range generally depends on how much current you push into the transmit cool, but that's what high-q resonant tanks are for. You can also gather energy slowly into something and then dump it into the LC tank. This can have quite low average power consumption, but probably won't be small. FSK can be achieved by switching reactive components in and out of the resonant circuit as it is oscillating, but this is quite complex circuit-wise and has to be uC-controlled in order to switch at the exact right moment and avoid switching element damage.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 17, 2020, 08:58:35 pm
content: Quote from: Hexureus on September 21, 2020, 09:45:38 pmFor 2m I would definitely look seriously at optical before magnetic, probably IR for this case. Likely to be smaller and lighter, and using narrow pulses, power draw should be pretty good.Standard IR remote receivers ( I like the Vishay ones) will easily pick up a standard IR LED at several meters in air, the 36kHz modulation gives a lot of discrimination against noise sources. They are cheap enough to use several facing in different directions if necessary, ditto LEDs.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 17, 2020, 09:10:14 pm
content: Checkwww.popotomodem.comfor relatively low-cost underwater acoustic modems.Full disclaimer, I do design work with this company.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2020, 07:44:51 pm
content: Some years ago I was at a gathering where people were playing with radio controlled boats.There was also at least one submarine and it worked with a standard hobby level transceiver, probably 27MHz. I think it worked to a depth of around 1meter, maybe 2.

author: TimFox
author_rank: Super Contributor
timestamp: November 18, 2020, 09:10:52 pm
content: From my MRI days, I remember that the skin depth in normal saline solution made it difficult, but not impossible, to penetrate 1 meter at 27 MHz, but fresh water has lower conductivity.This paper suggests using surface waves at the air/water interface to increase the distance:https://arxiv.org/ftp/arxiv/papers/1809/1809.06741.pdf