GMVSAT

[Rick S]

Carl,
Can you help me on the best way to construct a bigger coil for a White's gold master series. I have opened up the stock concentric coil and have done alot of measurements. I have measured the inductance,capacitance, ohms and hopefully figured out the impedance. White's sales two different size concentric coils and a DD coil for this series of metal detector. I would like to build a 16" to 20" DD coil. Some where in that range. I have found that the coil has a resistor and two parallel caps in parallel with the two Tx coils. The coils are in series with each other. Also the Rx coil has a big difference in inductance and resistance because of the windings. It has no passive electronic parts in series or parallel. If any it is in the control box. What is the best way to attack the solution to contruct such a coil? Any help would be appreicated. Rick S

[Carl]

Re: GMVSAT

Does the DD coil have 2 TX coils? Concentric coils usually have 2 TX coils but not sure why DD would.

Basically you need to make sure the TX coil(s) has the same reactance (L-C) as the stock one. That's because most detectors use a Colpitts oscillator and the coil reactance sets the frequency. Then, you need to do the same on the RX coil because most receivers (on single frequency detectors) use a bandpass input tuned to the same frequency as the TX.

It's kind of tough to calculate the inductance and capacitance of scramble-wound coils. There's some approximate equations out there (inc. one somewhere on my web site) but I don't know how good they are. So you may end up doing trial-and-error on making the individual coils. Once you get them about right, then you have to put them on a substrate and get the overlap just right.

- Carl

[Rick S]

Re: GMVSAT

The DD coil I have from White's is a friend of mine and I can't open it. I wish I could. It would take about 135 dollars to find out(I would have to buy one). All three coils have an inductance range of 285 to 293uh for the Tx coil. I was thinking of reverse engineering some what because all the coils have to be within the same measurements. I have tried about three different forumlas to calculate the numbers of turns to achieve the inductance and resistance needed. They are all close, so trial-and-error will be most likely be the way to find the correct measurements.

[Rick S]

Re: GMVSAT

Carl,
Sorry, I was thinking of only one formula. I have use reactance formula for capacitance and inductance. Is it true that the coils have to be resonance balance Tx and Rx to the same resonance or is impedance more important? I have measured the Tx frequency from the circuit going to the coil at 50khZ. Would I have to find the receiver amp to see if the coils are within parameters on my scope? I am still learning about this detectors. I don't want to fry anything because it is a pain to replace chips on a double sided pcb. Thanks for your help Carl. Rick S

[Carl]

Re: GMVSAT

I would guess that both impedance and resonance are important, with the resonant freq. more so, esp. on matching the RX to the TX.

Ferinstance, let's say the GM has a Colpitts osc. and you design the TX coil a little bit off, so the TX freq. is 45kHz and not 50kHz. No big deal. Also, your resonant impedance is also off some, which means you may not get as much power transfer to the coil. Again, no big deal, TX power is just a little lower.

OK, now what's critical is to get the RX resonant freq. close to 45kHz. If you err to the high side, say 55kHz, then the received signal will be greatly attenuated, depending on the Q of the RX coil. But let's say you nail the RX resonance at 45kHz, but the resonant impedance is off. Like the TX coil, this might cause a little degradation in power transfer but as long as it's not WAY off you should be OK.

[Rick S]

Re: GMVSAT

Thanks Carl for your time to answer some of my questions. Rick S