Nexus Standard SE

[Nexus]

You assure me that the CTX RX, a multi period rectangular waveform, is a nice sine wave?????? You must be joking?

Actually the CTX wave form is not much of a multi period if you look at the square wave sequence. It is a two periods that are redundant. One complete cycle of low frequency followed by eight cycles of high frequency. That is not any king of multi period transmission, but a good marketing name for it.
The simultaneous multi period (28 or more frequencies claimed by Minelab) does not exist, not in the Minelab's or any other brand of detectors.

S-Scope from the UK have taken Minelab twice to court for this claim and twice they won the case. So don't make a fool out of your self on this subject any further.

Are you interested in the technical facts or your fiction?

Just measure the TX signal of the CTX when the search coil is actually connected to the detector. Then you'll see for your self.
I CAN ALSO DO IT FOR YOU. I have CTX 3030 in my collection.

In the same way the F75 have a brilliant pure triangle wave form of oscillation, but after the TX loop is connected to the transmitter the triangle wave becomes a perfect sine wave.

Square waves can not pass a loop unaltered, because of the loops inductance, parasitic capacitance and the resulting parasitic resonance of the loop.

Get some high school book for electricity and read about loops. Then you will understand that the CTX transmitter is not about PI look a like function or measurement, but it is about measurement trough use of harmonic frequencies together with the basic operating frequency. That's why it is called a FBS - full band spectrum.

THE SIMPLE THINGS STILL AND ALWAYS WILL WORK BETTER.

[kt315]

nexus schematic please, layout, part list please. NO BLA-BLA-BLA and all kinda advertizing! MORE TECHNICAL ASPECT HERE!

[Skippy]

I know it's off-topic a bit, but this line is in error:"... the F75 have a brilliant pure triangle wave form of oscillation, but after the TX loop is connected to the transmitter the triangle wave becomes a perfect sine wave."
The raw 13KHz signal is a square-wave generated by the micro-controller, and this simply switches on/off a transistor driving the coil tank. This tank is a complex one, comprising two capacitors, and 2 inductors (the coil, and another inside the control box.) Running the detector without the coil connected will give misleading ideas about how the coil is driven, as only 3 of the 4 components are in circuit. I believe this is done to increase the TX amplitude, from a likely 10V peak-peak using a simpler drive method, to 16V pk-pk using their double-tuned method. The F75's cousin, the Tek T2 has a different arrangement, using a centre-tapped TX coil to get more amplitude.

[kt315]

nexus schematic please. no interest to the loops inductance, parasitic capacitance and the resulting parasitic resonance of the loop brilliant pure triangle wave form harmonic frequencies together with the basic operating frequency

[Nexus]

I know it's off-topic a bit, but this line is in error:"... the F75 have a brilliant pure triangle wave form of oscillation, but after the TX loop is connected to the transmitter the triangle wave becomes a perfect sine wave."
The raw 13KHz signal is a square-wave generated by the micro-controller, and this simply switches on/off a transistor driving the coil tank. This tank is a complex one, comprising two capacitors, and 2 inductors (the coil, and another inside the control box.) Running the detector without the coil connected will give misleading ideas about how the coil is driven, as only 3 of the 4 components are in circuit. I believe this is done to increase the TX amplitude, from a likely 10V peak-peak using a simpler drive method, to 16V pk-pk using their double-tuned method. The F75's cousin, the Tek T2 has a different arrangement, using a centre-tapped TX coil to get more amplitude.

I don't mean to be rude, but I am getting tired explaining small things to smart asses.
The output of F75 before the search coil is a pure triangle wave form.
I HAVE MEASURED IT AND THERE IS NOTHING MISLEADING ABOUT IT.
I have that detector in my collection to.
The primary form of oscillation is a square wave, but it is converted by a switching transistor and a small resonant circuit to a triangle wave, before it is fed to the TX. After the TX is connected it becomes a pure sign wave. The design of the TX output have little to do with the TX loop. It is a triangle wave form transmitter that operates on very high current.
WOULD YOU LIKE A VIDEO OF THIS?

Now the very reason for this TX design is a high current output TX, period.

There are some other delicate specifics about the F75 TX design, but I am not going to disclose them as I do not believe it's right for random folks to get this kind of information just free of charge. Someone have payed a lot of money for this development.

[Skippy]

I have an F75, and am familiar with the 'delicate' specifics of it's TX drive, having tinkered around with its internals quite a bit. But I agree, it's not necessary to analyse a current production machine on this forum, sufficient to say the end result is a sine-wave TX, and the search-coil and receive circuitry (amps, synch detectors etc) is also quite conventional .

I believe on some Russian forum the T2/F75/F70 has been reverse-engineered, should anyone be curious.

[Nexus]

I have an F75, and am familiar with the 'delicate' specifics of it's TX drive, having tinkered around with its internals quite a bit. But I agree, it's not necessary to analyse a current production machine on this forum, sufficient to say the end result is a sine-wave TX, and the search-coil and receive circuitry (amps, synch detectors etc) is also quite conventional .

I believe on some Russian forum the T2/F75/F70 has been reverse-engineered, should anyone be curious.

I probably did not made it clear what I am talking about in specifics.
We consider a transmitter and an oscillator to be to different stages of one block in the schematic.
The primary form /source of oscillation is not so important to us as it is the actual transmitter it self.
We also consider for a valid transmitter any circuit that is capable to deliver any wave form output without the need to have a loop connected to it.
All crystal oscillators with power output can do this, but a basic Colpitts or a symmetrical resonance (like the one in Nexus) oscillators can not work at all if a loop is not connected to their output. When a loop is connected to those simple oscillators only then do they become oscillators and transmitters at the same time. Before that they are just some circuits that can do nothing.

The F75 have a triangle wave output no matter if a loop is connected or not. The way this triangle wave output becomes a sine wave is by connecting the TX loop that forms a series resonance circuit with a large capacitor. This series resonance circuit in conduction with the internal parallel resonance circuit make the sine wave possible. So in F75 the sine wave is produced by two resonance circuits from the switching transistor to GND. First is the parallel and second is the series resonance circuit formed by the TX.

I have to say that the F75 transmitter is one of the most simple, elegant and efficient schematics that I have seen.

[Skippy]

The original circuit, as used in the Teknetics T2, required a centre-tapped TX coil. This is fine for DD coils, but makes producing a concentric coil difficult (both the main TX and the buck coil would have to be wound bifilar, I think). When Fisher's engineers worked on the T2 design in order to create the F75, I guess they wanted to give the option of regular concentric coils as well as DD's, so a single-ended TX is needed. But to create a high amplitude drive from a low supply voltage (5V) they used this double-tuned method. The other obvious alternative would be to generate a -5V supply, and switch the coil across +5V and -5V. But that is more complex.
An interesting circuit, though, I agree. I wonder what the down-sides would be? I feel the extra inductor may introduce problems - temperature drift?, poor initial tolerance of the inductance value?, parasitic capacitance effects? EMI sensitivity?

[mschmahl]

Today a very little coin (3Pfennig 16..), around 1.5 cm diameter came to me. I found it with the nexus and double 9" coil. My settings were:
Disc full clockwise (accept iron also), GB half between 12 and 1 oclock, VCO between 8 and 9 oclock, Treshold 12 ocklock, Sensitivity 1 oclock and Volumen 12 oclock. Searching were done in allmetal, dig only high tones.
The nexus was stable and does not give false tones.

[Carl-NC]

The way CTX is analysing the signals have nothing to do with weather it is a VLF or not.
Some have measured the TX of the CTX to be a square wave. That of course is only when the search coil is not connected. After the search coil is connected this is no longer the case as every loop will filter out the square waves into a sine wave. The reason for use of a square wave in the FBS (full band spectrum) technology is to be able to measure certain harmonics that will not occur in sufficient amplitude if the primary TX is a sine wave.

The one fact that is most important to qualify any metal detector design is the RX loop and what is the measured output signal of that. I can assure you the CTX RX signal is a nice sine wave.

I haven't specifically looked at a CTX receiver, but I do know that BBS & FBS from the Sovereign to the Explorer to the ETrac to the CTX all have the exact same transmit waveform. It is a multi-period voltage square wave that is the same whether you connect a coil or not. With a coil connected, it produces a 2-frequency triangle wave current, at 3.125kHz and 25kHz. You are right, they are sequential, 1 low followed by 8 high. In the Sovereign (which I looked at extensively), the receive signal is rather transient-exponential depending on the target, and is sampled in synchronous integrating demod circuits, with both X and R channels.

BBS/FBS can be considered "time domain" but, really, so can any traditional VLF. I look at BBS/FBS as a mix of "time domain" and "frequency domain," with the sampling done in a shorter "time domain" fashion but -- with 2 frequencies, each with X & R channels that produce the same resulting target info as a traditional VLF -- the end result is basically no different than a VLF, which we like to call "frequency domain" whether it is or not.

The BBS/FBS approach could be done with sine waves, but the TX circuit would be very difficult. The use of a square wave is only for simplicity.

- Carl

[Davor]

Funny thing is that I concluded I can produce a sequential (any) 2 frequency Tx that does perfect sinewaves and conserves energy in tank capacitors switched for the purpose. By supplying the Rx "channels" in counterphase to a LPF of bandwidth « than the frequency sequence repetition ratio, you get a GB mechanism comparable to that in PI, a perfect machine. The only important detail for a GB solution is that the duration of each frequency is roughly the same. Of course, the amplitudes must be tweaked a little, and there are dozens of ways to "skin a cat" but with proper Tx. and a good timing mechanism it can be done.
My simulations say that the subtraction loss becomes favourably small at frequency ratio of 1:6. The other interesting effect is doubling of the phase response. That makes classic discrimination circuits useless.
The relevant physical manifestation here is coil voltage, and square wave does it equally well as the sinewave. The difference is in power efficiency. Square wave = lots of heavy batteries.

[Nexus]

It is a multi-period voltage square wave that is the same whether you connect a coil or not. With a coil connected, it produces a 2-frequency triangle wave current, at 3.125kHz and 25kHz.

- Carl

Since I have not measured the current form of the CTX I wonder how come the voltage is square wave, but the current is a triangle wave?
I know also that the Explorers have at least some part of the TX circuit in the search coil. Very likely the other models mentioned by you would be the same. That would make it impossible to measure the wave form over the loop it self and it is very likely the reason for your measurement to show square waves with or without TX loop.

I am yet to see a loop that does not influence considerably the square wave form, unless it is of a very low Q.

[Carl-NC]

Since I have not measured the current form of the CTX I wonder how come the voltage is square wave, but the current is a triangle wave?

That's the fundamental relationship in an ideal inductor: a step voltage produces a ramp current.

I know also that the Explorers have at least some part of the TX circuit in the search coil. Very likely the other models mentioned by you would be the same. That would make it impossible to measure the wave form over the loop it self and it is very likely the reason for your measurement to show square waves with or without TX loop.

The Sovereign has a preamp in the coil which is powered off the TX magnetic field. I don't think this was done in later FBS designs, although maybe in the CTX which they call "FBS2". I haven't done much with it. In all the BBS/FBS designs, the TX coil is just a coil, typically 500uH. You can directly measure it.

- Carl

[Davor]

...That would make it impossible to measure the wave form over the loop it self ...

Not so. Every inductive coupled coil works as a transformer, so if any coil with little parasitics and terminated with high impedance is introduced to a proximity of another coil, you'll see N2/N1 x k x (N1 coil voltage) [-1 < k < 1] on it that represents voltage on another coil.

[Skippy]

For an inductor: V = -L.dI/dt, so for a coil with low R, a fixed voltage gives a current ramp up/down at a fixed rate, thus a square-wave produces a triangle.

Search the forum for Minelab coil dissections/analysis, the circuit diagrams are there, even PCB layout files if you want to copy them to make your own coils for those machines. The circuit was just a pre-amp, and a bridge rectifier is used to turn the coil-drive square-wave into a DC power supply for the amplifier.
Edit: here's a couple, there are others:
http://www.geotech1.com/forums/showt...inelab-pre-amp
http://www.geotech1.com/forums/showt...Sovereign-Coil