My Homemade VLF Detector

[ivconic]

Ah! I got it. In case coupling between those two coils is 1 it would be true broadband balanced contraption, and in reality it is well balanced only within some frequency span. Actually it makes coil nulling easy. Even I could do it I guess the trimmer is not accessible. Otherwise you could fine null your coil wherever you want.

Not exactly. Not "wherever you want" - but in closer ranges.
Usually internal trimmers are placed there to finally null coil to least possible residual voltage for given frequency.
That's because of imperfections in wire, jitters in constraining the windings...etc...etc..

[ivconic]

White's, for example, used several parallel capacitances in some of their coils. (I opened dozen Spectrum coils so far)
So you can meet combinations like: 1uF+82nF+1nF in some of those, while a bit different at some others.

[Skippy]

ACE250 frequency measured as 6.69KHz, but bear in mind it is a plain LC oscillator, so could easily be 6.6 or 6.8 on another machine.
James, as your coil is not a Genuine Garrett one, it may have different internal nulling methods. If you do want a more broadband coil, I suggest getting a 11 x 7 inch DD Teknetics Greek-series/Fisher F5 coil, as these are also used on the G2 and Goldbug machines, meaning they are good at 7.8KHZ and 18KHz.

[satdaveuk]

White's, for example, used several parallel capacitances in some of their coils. (I opened dozen Spectrum coils so far)
So you can meet combinations like: 1uF+82nF+1nF in some of those, while a bit different at some others.

now that is interesting considering the hi tech kit they have to make these coils you would think by now with modern tools they could keep a tight specification with L.
Can see why MD coils are so exspensive, not really mass produced like putting beans in tins so to speak.

[ivconic]

ACE250 frequency measured as 6.69KHz, but bear in mind it is a plain LC oscillator, so could easily be 6.6 or 6.8 on another machine.
James, as your coil is not a Genuine Garrett one, it may have different internal nulling methods. If you do want a more broadband coil, I suggest getting a 11 x 7 inch DD Teknetics Greek-series/Fisher F5 coil, as these are also used on the G2 and Goldbug machines, meaning they are good at 7.8KHZ and 18KHz.


Thanks!
I agree; frequency will most probably vary in small ranges.

[ivconic]

now that is interesting considering the hi tech kit they have to make these coils you would think by now with modern tools they could keep a tight specification with L.
Can see why MD coils are so exspensive, not really mass produced like putting beans in tins so to speak.

I think "modern philosophy" has changed from what was the case in the recent past.
Nowdays i see quite different approach at modern coils.
Nowdays high tech electronics included inside detector is able to compensate those minor jitters and imperfections which are still present in coils.
I had chance to open and see few of Explorer coils, for example. ()

[ivconic]

And we forgot to mention that there are tuned and untuned types of coils.
Obviously ACE coil is TUNED.
Not so appreciative for experimenting in wider frequency ranges.
So, i guess; JamesPicard would have much easier job if he picked some other, untuned, coil to work with it on new design. (as Qiaozhi suggested)

[JamesPicard]

If you do want a more broadband coil, I suggest getting a 11 x 7 inch DD Teknetics Greek-series/Fisher F5 coil, as these are also used on the G2 and Goldbug machines, meaning they are good at 7.8KHZ and 18KHz.

Well, I will stick to this coil. Somehow I will find the secret to making this thing work. Since I am building the detector from scratch, I will just build around the coil. Many people on the web are excited about this coil....so I believe it is good.

Unfortunately, the hardware lab didn't have an inductance meter. Amazing! What kind of lab doesn't have a inductance meter.

So now, I quess I will do this....put in a 10KHz 1 Vp-p sine wave in series with a known resistor and use some math to determine the inductance.

[Skippy]

Better to put a known capacitance in parallel with the coil, drive it with a sig-gen through a high-ish value resistor, eg. 470R, tune until you hit resonance, viewed on a 'scope. I suggest 220nF, that should give you F in the 5KHz to 10KHz region.

[Qiaozhi]

So now, I quess I will do this....put in a 10KHz 1 Vp-p sine wave in series with a known resistor and use some math to determine the inductance.

Since you don't have immediate access to an inductance meter, I would suggest building a Colpitts oscillator and connect this to the coil. As you will then know the value of the tuning caps, you can calculate the inductance from the frequency measurement

[JamesPicard]

ACE250 frequency measured as 6.69KHz, but bear in mind it is a plain LC oscillator, so could easily be 6.6 or 6.8 on another machine.

Thanks for making the measurement Skippy. That seems to coincide with the reports on the web that the ACE250 uses coils that operate at 6.5KHz. Since KelloCo Detectors said this coil is made for the ACE250, I quess there is really no need for me to measure it. Did you say what value of capacitor you see in parallel with the Rx coil when you looked at the ACE250? I'll look through this thread again in case I missed it.

So if the magical frequency is somewhere between 6.5KHz and 6.8KHz, I should be able to play around in that area with my Tx generator circuit.

[JamesPicard]

I needed a refresher on resonance with a LC circuit. I found this video so I'm posting it here for others who are learning like me and need a good short video to help them.

This video confirms what I remember long ago...that is when you see the peak as you change the frequency, that is your resonant frequency with the value of capacitor that you have.

www.youtube.com/watch?v=G_RCyDdt2rM

[JamesPicard]

I made some changes to the circuit. I switched out the 220K resistors to 100K. This cleaned up much of the terrible noise that was being amplified. I also have a 115nF cap now in parallel. The resonant frequency with this cap is 12.8KHz. With these conditions, I can move my little flashlight within about 5 inches and I see an increase of 15mV in amplitude and a phase shift of 0.4 micro seconds.

I just don't get a good sensitivity when I try to operate at 6.7KHz. I had to use a 0.288 uF cap to get resonance at that frequency. Seems like this circuit or coil does not like operating that low. The sensitivity is not very good.

Also, I wonder if a JFET would help with the sensitivity. It can operate with several hundred times less bias current...10 - 50 picoAmps compared to my opamp which needs about 50 - 70 nanoAmps. What do you guys think?

Also, on the web it says the a VLF detector uses phase shift to discriminate between metals and amplitude to detector metal. So seems like I need to worry about amplitude now, not phase shift. Is that right?

[Qiaozhi]

Also, on the web it says the a VLF detector uses phase shift to discriminate between metals and amplitude to detector metal. So seems like I need to worry about amplitude now, not phase shift. Is that right?

To prevent you from jumping in at the deep end, so to speak, I would advise focusing on amplitude changes for the present time. This will enable you to experiment with a T/R type of detector that can still provide elementary discrimination capability. When the coil is balanced correctly, non-ferrous targets will give a change in amplitude in one direction, and ferrous will move the other way. There are still detectors available today that work on that exact principle. Once you have an understanding of the T/R method, you should move on to using synchronous demodulation to extract the in-phase and quadrature components of the receive signal, and hence use the phase-related information for better discrimination.

[JamesPicard]

To prevent you from jumping in at the deep end, so to speak, I would advise focusing on amplitude changes for the present time.

Yes, good advice. I worked today on just studying the amplitude. So with my differential preamp, I studied the output on the oscilloscope. Using the O-scope "PeaK V" capability, and my little metal flashlight, I could move the flashlight to about 5 inches from the coil and the scope could distinctly measure a 2mV increase every time. So, correct me if I am wrong, but I'm thinking at this point I need to work on 2 things.

1. Work on the preamp circuit to increase this 2mV to as much as possible without amplifying to much noise.
2. Determine a method to measure microvolt changes.

For #2, this would most likely be solved with a 24bit ADC...I think.

Since I could actually see the scope increase by 2mV, maybe that is where I was wrong about this. I was expecting I would see an increase of 2mV from a small coin about 20-30cm away. But I never saw it, so I thought my circuit is bad. Maybe the preamp is almost complete, and the small coin IS IN FACT changing the amplitude...it's just that I never see it because instead of 2mV, maybe it only increase by several micro volts.

Am I correct?