Let's explain, normally the transmitter (whether it's VLF LF or whatever) produces a carrier frequency that is held accurate. Now when the morse key is pressed the carrier is transmitted, when key not pressed carrier is not transmitted.
Now at the receiver we can receive the carrier but not hear it. To render it into a tone that we can hear, the operator adjusts a local oscillator in the receiver to beat with the carrier. So carrier - beat = tone that's the tone we can hear whether it be a dot or dash.
For VLF specifically SAQ Grimeton another possibility is available to render the carrier into a tone. We simply record what we receive as a sound file ( .wav file) we can do this because the SAQ carrier is in the audio sound spectrum at 17.2 kHz. But we cannot hear such a high frequency sound so we need to process it.
As a sound file we have two main ways to process so as to get a morse tone:- 1. As before beat with another oscillator frequency then:- carrier - beat = tone or:- 2. We can do pitch conversion where simply the frequency or frequencies in the sound file are divided by a number we can choose initially, then we get carrier / divisor = tone we can choose the divisor usually it's integer value, as an example for SAQ:- 17200 / 30 = 573.33 Hz tone.
So for any of these schemes it's us the receiver operator (or sound file processor) that chooses any sensible tone frequency he or she likes, typically morse tone choice is something in the range of 440 to 800 Hz. The question is what is the best Morse code tone for readability, the best tone optimal for demanding situations, the commonly accepted standard Morse tone (if there is one). Even do operators choose tone to be a musical note eg the note A at 440 Hz.
Does the tone frequency matter, probably not much.
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During my time with R&S Sales in Canada we sold their calibrated H-Field Loop Antenna.
Users I recall off the top were labs such as Litton, Garret & CSA.
The H-Field loop was terminated in a current to voltage circuit, almost a short.
The H-Field loop was enclosed in an aluminum E-Field screening tube, the loop broken at the end so that the H-Field was not shorted.
Useful frequencies measured were 9 KHz to 30 MHz.
Users I recall off the top were labs such as Litton, Garret & CSA.
The H-Field loop was terminated in a current to voltage circuit, almost a short.
The H-Field loop was enclosed in an aluminum E-Field screening tube, the loop broken at the end so that the H-Field was not shorted.
Useful frequencies measured were 9 KHz to 30 MHz.
That is very interesting. It's making me think about the antenna (especially for VLF) as a transformer. Suggesting that the H-Field magnetic flux lines intersecting the wire loop are inducing current into the loop according to Faraday's law.
Already in 1990 a tunable VLF ferrite loop antenna wad published, and also mentions that at VLF the VLF loop outperforms an open frame loop up to 100 KHz.
https://www.qsl.net/v/vk5br/LoopAntennas/Ferrite_Loop_Ant.pdf
This article describes a frame loop but also provides calculation.
https://physicsopenlab.org/2020/05/03/loop-antenna-for-very-low-frequency/
https://www.qsl.net/v/vk5br/LoopAntennas/Ferrite_Loop_Ant.pdf
This article describes a frame loop but also provides calculation.
https://physicsopenlab.org/2020/05/03/loop-antenna-for-very-low-frequency/
Well its far-field, not near-field, so the mutual inductance is vanishingly small - not really a transformer by most definitions. Better to go straight to Maxwell's equations for antenna's perhaps?
Surely a Loop or Ferrite Rod antenna can only be reacting to the magnetic field component of the EM wave, and the magnetic component must create magnetic flux that couples into the Loop or Ferrite Rod antenna.
I was not suggesting that the antenna was coupling with lines of flux from the transmitter near field.
Well the flow of energy into an antenna is complex, I'd be wary of making such a simplifying assumption without backing it up with a simulation. Half the power is in the E-field and half in the B-field, and you can't have propagation of energy without both fields being present, so the E-field has to be involved too.
@Mark Tillotson
@jhstewart9 mentioned that "The H-Field loop was enclosed in an Aluminium E-Field screening tube with the tube broken at the end so that the H-Field was not shorted"
That seems to me to be a situation where the antenna has no mechanism for transferring any or the E-Field energy into an electrical input to the receiver.
My understanding was that energy in an EM wave was constantly being transferred from E to H and back rather than 50:50 you are suggesting.
I suppose what I'm saying (without having gone back to the maths) is that for low frequency EM wave propagation - detection of the magnetic component by a coil is more straightforward than for HF or VHF and above and works according to Faraday's law.
We know that the practical design of a loop antenna or Ferrite Rod antenna works just like a coil in a transformer, larger the loop diameter or more wire turns larger the signal, same for the Ferrite Rod.
@jhstewart9 mentioned that "The H-Field loop was enclosed in an Aluminium E-Field screening tube with the tube broken at the end so that the H-Field was not shorted"
That seems to me to be a situation where the antenna has no mechanism for transferring any or the E-Field energy into an electrical input to the receiver.
My understanding was that energy in an EM wave was constantly being transferred from E to H and back rather than 50:50 you are suggesting.
I suppose what I'm saying (without having gone back to the maths) is that for low frequency EM wave propagation - detection of the magnetic component by a coil is more straightforward than for HF or VHF and above and works according to Faraday's law.
We know that the practical design of a loop antenna or Ferrite Rod antenna works just like a coil in a transformer, larger the loop diameter or more wire turns larger the signal, same for the Ferrite Rod.
This looks like a good project .. except for the virtual ground option. Virtual ground means "almost ground" or pseudo ground. It is a trick to use a single supply such as a battery. It is not necessary for op-amps which can be biased at 1/2 VCC. Some people call that a virtual ground because there is no convenient name for it.
Thanks for the good explanation. I am less confused now.
Hmm I'm not sure what you are referring to? If you mean the so called "Virtual ground" of OpAmps in general - well its a fine idea.
Typically if you connect the non-inverting input to ground (assuming +and - supplies) and some sort of feedback network to the inverting input. Then the OpAmp output will behave in such a way as to keep the inverting input at ground potential (assuming it can) hence the inverting input becomes a "virtual ground" And it is amazing how good OpAmps are at doing this. A quality OpAmp will keep the inverting and non inverting inputs at the same voltage to within milliVolts or less. Some people call it a "virtual earth.
In the (usually annoying) situation where someone has used one power rail and split it in half using resistors. For example say they split a 12 volt supply into 2 x 6 volt and connect the mid point (at 6 volt) to the non-inverting input then given feedback the inverting input will become a "virtual 6 volt" and again it will track the 6 volt rail to milliVolts or less.
Overall the concept of an OpAmp virtual ground or virtual earth is correct and is very useful to keep in mind when designing OpAmp circuits.
There are some types of OpAmps that are happy working with a single supply, they are because their common mode input range extends below ground, even can be happy well below ground, (typically JFET OpAmps can do this). For these OpAmps the concept of virtual ground works below ground even with one supply.
@richgwilliams said: Hmm I'm not sure what you are referring to?...
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It seems I have a problem using "virtual" when referring to hardware. It is often used in software because it is abstract and we have things like Virtual Private Network etc. But hardware either has a ground or it has a virtual ground, or it has both. What happens when a virtual ground amp is connected to another hardware module which only has a real ground? Short circuits or ground noise are probable.
There is a tone control amp designed for single supply in TI document SL0A042 which has a TLC074 with the input pins at 1/2 VDD. A TLV2461 provides a low-impedance MID voltage for the quad opamp. They mention virtual ground but there is nothing unreal in the schematic. I have seen such circuits in some VLC amps.
BTW, the TLE2426 precision reference or virtual ground is now obsolete. It is weak for rail splitting but it is fine for providing 1/2 VDD without the resistors.
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It seems I have a problem using "virtual" when referring to hardware. It is often used in software because it is abstract and we have things like Virtual Private Network etc. But hardware either has a ground or it has a virtual ground, or it has both. What happens when a virtual ground amp is connected to another hardware module which only has a real ground? Short circuits or ground noise are probable.
There is a tone control amp designed for single supply in TI document SL0A042 which has a TLC074 with the input pins at 1/2 VDD. A TLV2461 provides a low-impedance MID voltage for the quad opamp. They mention virtual ground but there is nothing unreal in the schematic. I have seen such circuits in some VLC amps.
BTW, the TLE2426 precision reference or virtual ground is now obsolete. It is weak for rail splitting but it is fine for providing 1/2 VDD without the resistors.
They don't really use the term "virtual ground" correctly in SLOA042. All they mean is they have created a mid point supply at 1/2 VDD they call mid. The "virtual ground" is not their created "mid" it is still the four OpAmp inverting inputs. See the link below for a full explanation.
Their mid is low impedance because it is buffered by U1. Now they use mid in place of ground so that the other OpAmps can swing above and below mid.
Lets assume that the external inputs Rin or Lin are referenced to ground (audio ground, earth, the metalwork, 0V) the capacitors C1 and C2 decouple the inputs and the signal swings both sides of mid. Now they make a mistake because the outputs Rout and Lout cannot swing below ground rather they are still swinging around mid so to correct this you need another decoupling capacitor for Rout and Lout.
I would say whatever you do don't do it like this! Much better is do away with U1, use negative and positive supplies and the external audio ground so that you have +Vdd -Vdd and audio ground. That way Rin, Lin, Rout and Lout are all referenced to audio ground. Audio ground also goes to the four non-inverting inputs. This way you don't need any input or output decoupling capacitors. No earth loops are implied.
Here is a link to the proper explanation of how the term virtual ground is usually used in electronics:- Wikipaedia Virtual Ground
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In the SLOA042 circuit you are OK as regards short circuits because of C1 and C2 (and similar decoupling on output needed).
In the midst of a high quality audio amp - its not the place for any fancy level shifting, it can do nothing but add noise.
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Yes this TLE2426 is intended to split a power supply voltage exactly precisely in half. There are situations where a designer has to do this but its best avoided (maybe why the device is obsolete). The TLV2461 used in SLOA042 is an OpAmp so needs R3 R4.
In audio amps suggest you avoid all this virtual ground crap and stick to the definition given by Wiki. In OpAmps the term virtual ground is a mathematical/functional property of the OpAmp inverting input.
I do accept that in the English language or even in electronics you can call mid (or the centre of a split power supply) a virtual earth rail but its confusing as regards OpAmp circuitry, you could also call it a floating ground, centre point, level shifted ground or even a bad ground !
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So I went back to read the pdf of the DIY Front end project and I now understand and agree with your comment. Its the use of "virtual ground" as the name of the middle rail of a split single power supply that confuses me, I agree that there is no convenient name for it.
I'm just confused lol.
I landed upon this geophysical project using VLF signals to detect things many meters underground.
If interested, visit https://github.com/ecabuk/vlf and click ExtendedAbstract.pdf. It uses two 12 inch ferrites and NE5532 preamp.
If interested, visit https://github.com/ecabuk/vlf and click ExtendedAbstract.pdf. It uses two 12 inch ferrites and NE5532 preamp.
Attachments
That's interesting. Twelve inch Ferrite Rods that's a good size, the NE5532 has been around a long time but is a reasonable choice (we have been discussing JFET OAmps). They don't show a circuit diagram but I guess they would have had a lot of input noise masking the VLF. I like the sound card with 192 kiloSamples/sec sampling.
Should expect it to relate to what they get from ground probing radar and likely more sensitive to metals.
Should expect it to relate to what they get from ground probing radar and likely more sensitive to metals.
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The pdf and the website are both useful interesting links. The "loop antenna" (with ferrite core) is the same as the frame antenna (but without ferrite core) at VLF except that the ferrite concentrates the magnetic field component. I think ferrite cores/rods still work well above 100kHz and will easily cover the MW band.