Blue Max 600

[mikebg]

not. ideal nulling you will get of if you have IDEAL transformer, ie RX coil turns number=TX coil turns number, using one wire for both. also you have to use geometrically ideal DD coil form.

Anatoly, I also prefer the DD form, but all other is not true. The term "ideal transformer" has other meaning, which is opposite to induction balance. It means 100% coupling coefficient between windings and zero energy loss (windings without resistance).
The design of each windings in the search head should observe its efficiency at given weight limit of metal.

The efficiency of TX winding has dimension ampere turns per kilogram wire. This function increases with capacitance of TX tank. We should use large TX capacitance and reduce turn number in order to operate with high ampere-turns. High efficiency of TX means both low weight of battery and low weight of search head.

The efficiency of RX winding has dimension S/N per kilogram wire. Unfortunately the sensitivity of conventional metal detectors is not limited by thermal noise. Despite this, it is better the preamp to operate with more large TGT signal in input. That means maximal turns number of RX winding which is limited by resonance frequency. To increase turns number, we should use minimal capacitance for RX tank. Note that in above design there is a resistor 51 ohm, which increases both the thermal noise in input and the weight of RX winding. The noise increasing is not significant, but we can reduce the weight of RX winding if instead resistor 51 ohm, we use wire with more small diameter.

[Geo]

Sorry for late answer.
I must thanks Anatoly (ok.. not E.foster) and Mike for answers.
I am far from home now and i have not any translator so i can't reply with the right terms.

Regards

[Geo]

The attached below simulation shows voltage transfer functions (frequency response) of RX tank circuit without load. Operating without load is realised in most preamp circuits - RX tank is connected to noninverting input of operational amplifier.

The marker shows TX frequency about 6500 Hz used in most White's machines.
The RX tank operates as second order low pass filter. Resonance frequency divides frequency spectrum in two regions - passed region in left and stopped region in right. Low pass filter means that we can receive larger signal if TX frequency appears below resonance frequency. In this region TX tank will add too small phase shift.

However the designer of White's search head decided to operate in the beginning of stopped region where RX tank attenuates signal and adds phase shift near to 180 deg.

Now let we see how operates NEXUS. The TX frequency appears at resonance of RX tank. At this coil we will receive a near 9 times more large amplitude, more sensitivity at AIR TEST. But look how steep is phase response in this region. We will have strong angle and amplitude modulations when search head changes height or when the soil has different electromagnetic properties. That means variation of GND setting and change of discrimination level.

CONCLUSION: If I was designer of search heads, I would use minimal capacitance for RX tank. This allows to increase turn number, ie the RX coil will receive more strong signal. In addition, when TX frequency appears in passed band of the LP filter, the RX coil will receive more strong TGT signal than at White's machines. Unfortunately the improvement is insignificant because in practice the sensitivity of RX is not limited by thermal noise.

Hi Mike.
Can you run on the stimulator the follow????
Tx frequency 7100Hz. Tx coil = 540uH
Rx coil =35mH. At who frequency i must tune the Rx tank so to have the same frequency shift (near to 180 degrees) as the prototype Blue Max who is tuned at 6590Hz.

Regards

[mikebg]

Geo, to calculate RX resonance frequency simply multiply TX frequency to 0.75. Most important is to tune capacitances of RX tank for proper operating of GND BAL and TID. However this is difficult because your TX frequency is different. For best results add capacitance to TX tank for oscillation at original frequency 6590Hz, and then tune RX tank for proper GND BAL and TID.

[Geo]

Thanks Mike.
I had calculate the Rx frequency resonanse as you write here. I run it to Multisim 11.0 simulator but i do not take the same waveform (sure i have a mistake!!!) with you, this is the reason that i wanted to run it again with difference frequency.

Regards

[Jose]

For some reason file did not attach?

Hello Altra
By chance would have you the coil data, quantity of turns TX and RX
Thank you.
Jose

[salih]

Is it possible that any one tell me what is the diameter TX and RX mm=?????
thanks.