Air tests with Hammerhead ver C

[Carl-NC]

The HH board has room to add an R-C LPF after the preamp, unfortunately it appears that the noise has about the same frequency as the desired tau of the circuit. Still, you could try it. Also look at the supply pins of the opamp, make sure they are exceptionally clean, use good quality low-esr caps.

- Carl

[nick_f]

Hi Carl,
I replaced the standard electrolytical caps with Tantalum capacitors and I got a noticeable reduction in noise at TP4. Without the coil, when I shield the circuit, I get a very significant reduction in noise. When the coil is connected, shielding doesn't make any difference.
I checked the supply rails of IC6 and they are clean (I can see some very small voltage differences like a saw tooth on them though, but I think that's normal).

Regards,
Nicolae

[Carl-NC]

So you are picking up EMI from something nearby. Could even be test equipment. Generally, indoor environments are very noisy, and when you get outside away from it all the detector will settle down. For indoor testing, try rotating/moving the coil around for minimal interference. You can also use a figure-8 coil for EMI cancellation.

- Carl

[Tinkerer]

Noise problems

Nicolae,

There are a few areas where you can improve:

Adding the 100k resistor across R13, reduced your gain from about 1000 to less than 100. Therefore you have less noise, but also less signal amplitude at TP 4.

One of your scope pix, shows the sampling point too early. When you sample on the slope, you get instability. Look at TP5. You need to be able to adjust the output of IC6, such that you get 0V at TP5 when there is no target.

How do you connect the shield of the coil to the shield of the coax? I can not tell from the picture what the shield is made off. Aluminum foil? How do you solder the ground wire to aluminum? It is possible but difficult. If you do not have a good connection between the shield and the ground, the shield is useless.

When you are ready to wind another coil, I can show you how I do it. Mind, BBsailors coils are just about perfect, however it is often difficult to find cable with Teflon insulation.
I live in a country where it is very difficult to get any parts, not even hook-up wire. So I developed a method using magnet wire and polypropylene spacers. for the shield I use graphite. I can show you how to make a graphite shield.

I believe the GQ5 is Eric Fosters best design that has become Whites TDI.

As for my own tinkering? I can sample at 2uS after switch OFF, where I get a very high signal amplitude. I also do FE discrimination, but this is with an IB coil not a mono.
Speaking of mono coils: Once you get the coil building perfected, I suggest you make a coil with separate windings for TX and RX. You can do this with minimal changes on the HH design. Actually I think the HH2 design has this possibility.

All the best

Tinkerer

[nick_f]

Hi Carl,
I agree with you, I have plenty of interferers around me. The economy light bulbs generate heaps of interference, as they operate at a few tens of kilohertz. I turned them off, including soldering iron, regulated power supply, etc. I used a 9V battery to check if the noise has been reduced (and it hasn't). I suspect the interferer is actually the flyback pulse, because when I touch the metal part (drain) of IRF740, or even the insulated part of the damping resistor, the same noise increases a few times. On the other side, I think the circuit has a tendency to self-oscillate exactly on that frequency, because even without the coil connected, I can still see some very small oscillations with the same frequency.
I will do some more testing, maybe one of my observations is not accurate and puts me on the wrong track. Thanks for help.

Regards,
Nicolae

[nick_f]

Tinkerer,
You are right, I know that with 100k in parallel, the gain is less than 100. I actually tried to increase the value, and I ended up with a 330k resistor instead of 1Meg. After following advice from Carl, I installed tantalum caps in that part of circuit (they are bloody expensive little buggers ) and I reverted back to 1Meg resistor. I have to find out what's causing the oscillations.
Thanks for pointing out that my sample was taken too early. I've done that because I noticed the circuit is more sensitive to the aluminium foil. I normally set the delay at the end of the slope. I wasn't aware about instability problems (my circuit is quite unstable anyway ).
You just said you can sample at 2us??? That is, when most of us are happy with a sample point of anything less than 10us?
So, you say to place two windings in the same coil, one for RX and one for TX? What would be the advantage of that?
I was lucky to find a blob of terribly messed-up teflon insulated wire-wrap in my wire box. (at a previous workplace, there was a big bobbin, with a few kg of teflon wire - nobody was using it, because it is hard to remove the teflon for wire-wrapping and I think it got thrown away later). Fifteen minutes later, I was able to wind my coil.
All my coils are shielded with aluminium foil at the moment. Please let me know about how to use graphite for shielding (and where to get it from), it sounds like a very interesting idea (I know graphite is conductive and it would have the advantage of not being a metal). I am surprised about your question for a better connection at the shielding. I just wound up a few cm of wire-wrap around the aluminium shield and put some electrical tape on it, and there is my shield connection! Isn't that good enough, should I go to such lengths as even soldering the wire onto the aluminium? I know it can be soldered by removing the oxide under a bit of mineral oil.
I live in Australia, I can get just anything I want from buying it from the internet. The only problem is that my wife is very tight with the money I spend on electronics

Best regards,
Nicolae

[Tinkerer]

Noise problems

Yes, I sample at 2uS. I am keen to see your circuit working to its best, so that I can compare my signal output with yours.
Separate TX and RX windings:
If you make a TX coil of, lets say 28cm diameter and 300uH inductance you have a certain amount of flyback decay time due to the inductance and capacitance of the coil.
Added to that is the decay time caused by the Mosfet Drain capacitance.
Now, if you look at the inductance divided by R 900.
However, the TC for the capacitance is pf times R. So the capacitance of the Mosfet, coax cable and shield added up slow down the decay curve a lot. Specially since the flyback voltage of several hundred V charges all these capacitances up to that voltage.
If you place a fast diode in series with the drain, you can block the drain capacitance and speed up the decay.
If you place most of the damping resistor at the coil, you reduce the flyback voltage on the coax and the Mosfet drain, so again you gain some decay time. Then you add a CERMET POT to do the rest of the damping on the board. This allows you to make fine adjustments and gain more time.
A separate RX coil, lets say with 14cm diameter, 300uH, will be much better for pinpointing. It will show a much reduced flyback voltage. It also needs to be damped but will decay faster. There are quite a few other advantages more.
Now, back to the coil shield:
Aluminum oxidizes instantly when exposed to the air. Aluminum oxide is transparent and characterized by very high insulation capacity. In fact into the thousands of volts. So simply wrapping a copper wire around the alu foil might work for the moment, but depends on constant friction to maintain conductivity.
Alu foil is also a good target, that means that the alu shield on the coil is detected by your circuit. It loads the input.

Make sure you use a spacer between the coil winding and the shield. About 5mm should be OK. You can wrap some of the cheap plastic sting around the coil up to that thickness. The cheapest string is usually made of Polypropylene which is an excellent dielectric. But if you look at the thickness of the string that you build up, you see that it is made up of mostly air. Air is the best dielectric. So then cover the string with painters paper tape and apply the graphite shield to that.

Before applying the shield, add a thin copper drain wire on top of the paper tape. (Later you solder the loose end to the coax shield.) Glue it at a few spots so it stays put while you apply the graphite coating.

Here is my recipe for the graphite shielding:
1cc low viscosity epoxy resin
5cc graphite powder
1cc coloidal or fumed silica
7cc epoxy solvent or thinner.
First mix epoxy, the add graphite and mix again then add the rest and mix well. apply a thin coat. Apply a double coat on the drain wire.
Leave a 1 mm gap in the shield.
After drying, paint pure Epoxy over the shield to keep the moisture out.

If you cant find graphite to buy?

Pick the old graphite brushes out of an old drill or some DC motor like a starter motor. Take some very fine sand paper and sand the graphite brush down until you have enough powder.

Well, this is enough for today, but there is still much more to be done.

Good luck
Tinkerer

[hobbes_lives]

Lots of good info there Tinkerer. Thanks for posting all of that. Between your ideas and bbsailor's article, we've got some great coil info here on Geotech.

Here's a product that may work well as a spray-on shielding material. I haven't tried it myself, and it's a little pricey, but I think it would work quite well. What do you think?

http://www.mouser.com/search/Product...252bdK6A%3D%3D

[nick_f]

Hi Thinkerer,
Thank you very much for all the explanation provided. What MOS-FET are you using? I use IRF740 and I already have a fast diode in series with its drain. What is the capacity of your coax cable? The one I use (50 ohm coax, 1.5m) has capacitance of 150 pF. I measured a 75 ohm coax cable, same length and this one only has about 70pF. The braid of this is not solder-friendly though.
I tried it with the 75 ohm / 70 pF cable and, surprize, surprise! I can't get a pulse width lower than 75us! I split the damping resistors, on the board I have a 3k, and on the coil I have another 3.3k in parallel with a semiadjustable large 10k carbon pot.
But I know how to investigate this strange matter: I will add a capacitor of around 80pF to simulate the first cable, and if it works, then reduce the capacitance to see what's going on.
With my initial cable, back to a gain of 1000 from 330, the pulse width has increased from 6us to 10us. The sensitivity to 1" sq Al foil is very poor again (hardly detectable at 1cm).

Regards,
Nicolae

[nick_f]

I put in parallel with the cable some caps, 50pF and 100pF and the delay is still very wide (many tens of us). It is not the capacitance of the cable that causes this huge delay. What kind of coax cables are you people using?

Regards,
Nicolae

[Tinkerer]

Lots of good info there Tinkerer. Thanks for posting all of that. Between your ideas and bbsailor's article, we've got some great coil info here on Geotech.

Here's a product that may work well as a spray-on shielding material. I haven't tried it myself, and it's a little pricey, but I think it would work quite well. What do you think?

http://www.mouser.com/search/Product...252bdK6A%3D%3D

Hobbes_lives,

The Nickel spray looks good. The crucial part will be to connect the shield drain wire.
I mentioned graphite brushes above. These have a pigtail that is made of very fine bare copper wire. This is very good stuff for the shield drain. Making a small blob of graphite-epoxy to stick the shield drain on the shield spacer should work. Then spray the Nickel paint.

If you have access to the 3M shielding material that BBsailor recommends, it is probably the best solution.

Tinkerer

[nick_f]

I found something similar on the web site www.farnell.com (they sell in Australia as well). http://au.farnell.com/electrolube/en...und/dp/1675347.
It is not very cheap, AUD65 and at the moment, they don't have it in stock.
At the moment I am thinking at a possible "poor man" material instead of the 3M shielding tape. I don't know if it will work though. The video cable of CRT computer monitors has some internal mesh for shielding. It has a silver colour and if I can manage to remove it from the cable, I will give it a try.

Regards,
Nicolae

[nick_f]

Tinkerer, thanks a lot for sharing all the info about Rx-Tx coils, I can see the advantage of having a separate Rx coil and I will build one as well. If the Rx would have the same diameter as Tx coil, would there still be any advantage (like a faster decay rate)?
Where did you get that recipe from? What's the role of colloidal silica in the mixture?
I am not sure I can obtain that stuff from somewhere.
About the graphite: I know the electrical brushes have high conductivity, but I don't have any. Maybe the thick graphite pencils used by artists could also work (they might not have such a good conductivity, because perhaps they mix graphite with other materials).
I don't have any resin either - buying all that stuff could exceed the price for the 3M shielding tape I actually just searched on ebay and I can buy 5 rolls of Scotch 24 tape, 1 inch x 15 feet for US$20 + US$14 freight. Not that expensive actually.
Also, on ebay there is teflon spiral wrap, for US$75. I should start making some extra money, to buy all these

Regards,
Nicolae

[Tinkerer]

Hi Thinkerer,
Thank you very much for all the explanation provided. What MOS-FET are you using? I use IRF740 and I already have a fast diode in series with its drain. What is the capacity of your coax cable? The one I use (50 ohm coax, 1.5m) has capacitance of 150 pF. I measured a 75 ohm coax cable, same length and this one only has about 70pF. The braid of this is not solder-friendly though.
I tried it with the 75 ohm / 70 pF cable and, surprize, surprise! I can't get a pulse width lower than 75us! I split the damping resistors, on the board I have a 3k, and on the coil I have another 3.3k in parallel with a semiadjustable large 10k carbon pot.
But I know how to investigate this strange matter: I will add a capacitor of around 80pF to simulate the first cable, and if it works, then reduce the capacitance to see what's going on.
With my initial cable, back to a gain of 1000 from 330, the pulse width has increased from 6us to 10us. The sensitivity to 1" sq Al foil is very poor again (hardly detectable at 1cm).

Regards,
Nicolae

Not all 50 Ohm coax cables are the same. Only trying out will show what works. I got my best results by using a shielded, twisted pair.
Also watch out for attenuation on the cable.
The best Mosfet for your application will be one with low COSS. A lot also depends on how much power you want to use on your TX. For small nuggets, high PPS and low power works best. Pick the Mosfet accordingly.

For the OPamp, I like to use the OP37 or the LT1037. The spec-sheets look very similar, but there are notable differences in behavior. The 5534 adds at least 1uS to the decay time because of saturation and slow recovery.

Tinkerer

[nick_f]

Tinkerer,
Your shielded twisted pair was for audio or RF applications?
About the OP37, it sounds good. I can get them from www.futurlec.com (.au) for AU$2.09
About the MOS-FET, I think somebody else found this ones: FQD4N50 (COSS = 70pF), 500V, 2.6A. They can be purchased from Mouser Electronics.

Regards,
Nicolae