White 3900D

[MartyJ1963]

Sorry DB,,,I'm not very knowledgable when it comes to IC substitutes,,still learning. I'll have a good search though cos' I'm going to be needing these too. I know it's a bit off subject but can you tell me what an ''X2'' diode is? I want to have a go at building an 'Admiral' PI,,there are 2 or 3 X2 diodes in the part list and I don't know what they are? Never come across them in any other of my builds. Wish you luck in finding the CA3140. Regards,,Marty.

[dbanner]

I am gathering parts today, but my junkbox does not have these CA3140 opamps. These are not your average run-of-the-mill opamps. They are BiMOS rubbish. Ha!
Does anyone know of an alternative solution? Otherwise I'll have to order them and I am not going to be pleased when I get fake chips.
Someone can recommend a supplier?
Or maybe some modification to the circuit to accomadate a more conventional opamp? I noticed that these BiMOS opamps are always used in push-tune circuit of various metal detector designs, is there some special feature why this is so? What critical characteristic of these BiMOS opamps is being harnessed for this task?

[MartyJ1963]

Nice one DB!,,Same here, my partner and I came down with the Flu also the day before Xmas eve,,,Enjoy your Xmas though,,,It'll be nice to get my 3900 started so I'm looking forward to your results and progress,,Regards,,Marty.

[dbanner]

My hand is good now, Came down with the flu, lost my voice for two weeks, can you believe it.
But alas I am feeling much better, so I am now going to finish what I started.
I feel some fresh wind in my sails, I feel inspired once again! I cut piece of copper board today for etching! Then I remembered it is Christmas!
So tomorrow I restart in earnest to complete the thing.

[MartyJ1963]

@Dbanner,,,,,How's the hand doing?,,,,Any progress on the 3900D yet?,,Regards,,Marty

[MartyJ1963]

@Dbanner,,,Hiya DB,,,How's the hand?,,,Hope it's healing well,,,,Regards,,Marty

[Artyom]

No, I have put everything on hold for the time being, another freak accident caused a serious laceration to the right hand, it's all a mess.


But I will resume construction shortly, I have everything ready to go, just waiting for the wound to heal sufficiently.
Those night outs are becoming rather dangerous.

Take care of your hands, you have only two! )

[MartyJ1963]

Ok Db,,,,Hope the hand heals for you soon,,,,Looking forward to how your project goes and hope it's successful,,Regards,,Marty

[dbanner]

@DB,,,Any progress on the PCB yet?,,,Just wondering whether to start my build yet,,,Thanks, Marty

No, I have put everything on hold for the time being, another freak accident caused a serious laceration to the right hand, it's all a mess.


But I will resume construction shortly, I have everything ready to go, just waiting for the wound to heal sufficiently.
Those night outs are becoming rather dangerous.

[MartyJ1963]

@DB,,,Any progress on the PCB yet?,,,Just wondering whether to start my build yet,,,Thanks, Marty

[dbanner]

Some observations:

First, the coupling between the Tx and Rx (concentric design) is stronger in the whites design where it is >50% efficiency as opposed to the Tesoro concentric, where it is significantly less. This relates to the relative diameters of the Tx and Rx coils in both designs.
Yet the tesoro Rx inductance is much less (this is counter intuitive since reducing the coupling efficiency should imply a greater turns ratio to bump up transformer action and hence the sensitivity)

Bandwidth, Q, frequency response, temp curves, ground cancelling, sensitivity, coupling efficiency, all seem to play a role in the engineering design of these coils. What a big mess indeed!

Tesoro concentric, with its weak coupling and low inductance of Rx and small difference in relative Tx and Rx inductances, seem to achieve the same depth and cancellation and some would say even better differentiation of targets. The exact opposite of the whites approach. Go figure!

[waltr]

The reason why I say this is because I know of no coil whose Rx resonates above the Tx resonance (tesoro), in which the Rx inductance is several orders greater than the tx inductance. I hope you understand what I'm trying to say.
So in may not make much difference on which side Rx resonates, to the math of phase angles, but it may make a whole lot of difference in other respects like coil parameters.

I also do not know why inductance and frequency differences where chosen.


TGS: TX = 6.0mH || 20nF = 14.5kHz
RX = 6.5mH || 15nF = 16.1kHz
RX is 11% higher frequency, 1.6kHz difference

IDX: TX = 535uH || 1088nF = 6.597kHz
RX = 35.6mH || 32.8nF = 4.655kHz
RX is 29% lower frequency, 1.945kHz difference.

Also interesting that the Whites coils have such a huge inductive difference.
One possibility is the greater number to turns on the RX coil increases the RX signal (transformer ratio).

[dbanner]

The reason why I say this is because I know of no coil whose Rx resonates above the Tx resonance (tesoro), in which the Rx inductance is several orders greater than the tx inductance. I hope you understand what I'm trying to say.
So in may not make much difference on which side Rx resonates, to the math of phase angles, but it may make a whole lot of difference in other respects like coil parameters.

[dbanner]

Yes, I have noticed but don't think it matters. It is the difference of the two resonances that produces a stable circuit with good discrimination.

The other thing I've noticed between Tereso & Whites is the inductance of the RX coil related to the TX coil.
Tereso coils seem to just have about 500uH difference whereas Whites RX coils inductance is many time higher than the TX coil.
Examples: TGS: TX = 6.0mH, RX = 6.5mH
IDX: TX = 535uH, RX = 35.6mH
Then caps are used to resonant each coil to obtain frequency difference.

Yes, I think there must be trade-offs. But a fundamental understanding of all the parameters escapes me. The choices made by whites engineers are based on data that we don't have access to, and there reasoning behind there choices are left to speculation.
There is only broad agreement on some basic coil technical data and some underlying fundamentals, but detailed analysis is sparce.

[waltr]

Anyone notice that the older tesoros Rx resonates above Tx, whereas with the Whites Rx resonates below Tx.

Yes, I have noticed but don't think it matters. It is the difference of the two resonances that produces a stable circuit with good discrimination.

The other thing I've noticed between Tereso & Whites is the inductance of the RX coil related to the TX coil.
Tereso coils seem to just have about 500uH difference whereas Whites RX coils inductance is many time higher than the TX coil.
Examples: TGS: TX = 6.0mH, RX = 6.5mH
IDX: TX = 535uH, RX = 35.6mH
Then caps are used to resonant each coil to obtain frequency difference.