Fisher Gold Bug

[ivconic]

When i made TX the other day; i used alternative TX output (from square on schematic).
This is forced TX, output gives interesting behavior on scope.
When switched On; it shows immediate clean sinus for half a second, than it settles down to bit distorted sinus with a known "notch" on the rising edge.
I guess this is not good and it will need trimming the amplitude. Which is btw ~12vpp.
So the original TX output looks more promising than alternative one.

[ivconic]

I am not simulator freak.
However... Proteus simulator is giving me very interesting results!
When i put 2.5V zener diode there; it will not work, giving me "nan" at voltmeter display.
But if i change 2.5V to 2.7V value; it works.
Giving 3.18V output.
I guess this is due improper choice of Q2, R1 and R2.
Anyway; point is: if there is 2.5V zener there; output will most probably be close to 3.0V.
Actually the right half of the supply.
So it is symmetrical after all. Rails are +/-3V in this case.
Yet i don't like simulators, this must be checked in real world.
In meantime i'll try to trim Q2, R1 and R2 to make it work with 2.5V zener.
Here is what's in simulator:

[dbanner]

I'm pleased that you're working on this circuit. Could you please throw away the battery check going forward, no need for that rubbish.

The original discreet rx front end contained errors, but I corrected those errors and got it to work nicely in ltspice sometime ago.
But I am in agreement that the front end should be re-designed with opamp. This is by far a more practical solution.

As for the power supply, bipolar 5V gives adequate headroom for tweaking with the gain of those stages also more flexibilty with using different opamps, but will have no effect on performance of the circuit as is. The amplifiers/ filters will simply take what it needs from the rails whether it is a 6 V spread or a 10 V spread.

I like the CD4060 approach, since you can also use one of its outputs to drive a voltage inverter for the negative rail.

Have a look at the minelab 15000 schematic, more specifically the tx circuit. I don't recall anybody lifting that tx circuit for implementation on any diy device, maybe it can be used here as an experiment, it is after all a gold detector.

I don't really care about the power supply, you can do whatever solution you think is best, just make it as frugal by way of components so I can build the thing with the parts most diy'ers might have on hand or are easy to obtain. 10Mhz crystal is common enough. I see those quite often.

[dbanner]

LM393 is more commonly known for the quad comparator. TLC374 can only be a recipe for fake chips on the market.

[ivconic]

If Proteus is to be trusted; this is more like it!
Instead BC327 i put now BC558 and instead MPSA there are two diodes.

[dbanner]

Looking good, I'll do the same in ltspice, just for comparison. How about a load on the 3V output.

[dbanner]

Shifting from 2.5 V to 3V leaves less headroom on the positive going side, hopefully this will not matter in the end.
Lm317 regulated at 6.5V will restore the default of the original design.
Notice the non inverting inputs of the 404's after the preamp are referenced to Vdd ie 2.5V
Changing to 3V will level shift the RX signal, decreasing the headroom by 500mV.

[ivconic]

Consider those 3V as ground, GND.
-v than is -3V and +v (+6v) is than +3V.

[dbanner]

But shouldn't make much difference since it applies throughout the circuit. Don't know.

[ivconic]

And yet...
THE HELL with "mumbo-jumbo" solutions from 40 years back in the past.
New technology today allows us much better solution.
This is pretty much straightforward power supply with all the positive sides and none drawbacks.
Two LiPo , 18650 type, with nominal capacity at ~1A will last much longer than two 9V batteries.
Since there are ready balancing chargers for two batteries; we are sure those will be in pretty much the same state when charged.
I've been working a lot lately with those batteries and i notice some tendencies in their behavior.
Fully charged usually are 4.2V. Discharged are 3.7V.
But when fully charged and than used; it drops very fast to ~3.9V and than stays there for looooong period of use!
Hours of working with low power metal detector.
Also now we don't need any voltage regulator to introduce more hum in the rails.
Also now we don't need any virtual ground "generator" to make all this more complicated.
With this kind of setup we have all the benefits you may want from one power supply stage.
Later on is maybe good (or maybe redundant) to add simple voltage monitor on both batteries.
But than again it is redundant.
Such batteries are already including small protection circuit and will immediately cut off when discharged.
With least ~1A per battery... we are sure that this will provide us more than enough working hours.
So... let's cut the crap and focus on more important stages on schematic!

[dbanner]

Consider those 3V as ground, GND.
and +v (+6v) is than +3V.[/FONT]

Yes we are in agreement. But in previous original schematic, ground sits at 2.5 volts, negative rail sits at (-2.5V)and positive rail sits at 6 volts. So there is more headroom on the positive side(6-2.5=3.5volts)

Shifting to 3 volt virtual ground changes this so now it sits at the mid point. Just an observation. So now all signals are level shifted by 500mV and are now closer to to positive rail of 6 Volts.
Not that the signal will clip, but it has 500mv less headroom on the positive transition, that's all I'm saying.

[ivconic]

Yes we are in agreement. But in previous original schematic, ground sits at 2.5 volts, negative rail sits at 0 volts and positive rail sits at 6 volts. So there is more headroom on the positive side(6-2.5=3.5volts)
Shifting to 3 volt virtual ground changes this so now it sits at the mid point. Just an observation. So now all signals are level shifted by 500mV and are now closer to to positive rail of 6 Volts.
Not that the signal will clip, but it has 500mv less headroom on the positive transition, that's all I'm saying.

Actually not. Simulation proves that ground sits at precise 3V, which is exactly half of 6v.
So even in original; it is quite symmetrical power supply.
If simulator is to be trusted.
The similar power supply was on 1265.
I had few of 1265 in very good shape.
I measured the symmetry at rails, +/-4v.

[ivconic]

Post #25 at previous page, "ps.jpg", have you missed it?
With it i simply ended any more brainstorming around power supply.
Period.
...
But i also understand what you want to say.
I don't think that 0.5V difference (if exist in reality) will make any significant differences in detector behavior.
Remember TGSL?
It has even larger difference between the rails! Yet it worked just fine.
Comparing to that; 0.5v is nothing.
Your points are making sense to me.
But with "ps.jpg" all the doubts are ended... at least at my side.

[dbanner]

[ATTACH]53411[/ATTACH]

And yet...
THE HELL with "mumbo-jumbo" solutions from 40 years back in the past.
New technology today allows us much better solution.
This is pretty much straightforward power supply with all the positive sides and none drawbacks.
Two LiPo , 18650 type, with nominal capacity at ~1A will last much longer than two 9V batteries.
Since there are ready balancing chargers for two batteries; we are sure those will be in pretty much the same state when charged.
I've been working a lot lately with those batteries and i notice some tendencies in their behavior.
Fully charged usually are 4.2V. Discharged are 3.7V.
But when fully charged and than used; it drops very fast to ~3.9V and than stays there for looooong period of use!
Hours of working with low power metal detector.
Also now we don't need any voltage regulator to introduce more hum in the rails.
Also now we don't need any virtual ground "generator" to make all this more complicated.
With this kind of setup we have all the benefits you may want from one power supply stage.
Later on is maybe good (or maybe redundant) to add simple voltage monitor on both batteries.
But than again it is redundant.
Such batteries are already including small protection circuit and will immediately cut off when discharged.
With least ~1A per battery... we are sure that this will provide us more than enough working hours.
So... let's cut the crap and focus on more important stages on schematic!

Funny, I never thought of my contributions to your thread as crap.
That being said, I'll leave you to your task.

[ivconic]

Funny, I never thought of my contributions to your thread as crap.
That being said, I'll leave you to your task.

Nooooooooo!
It was not addressed to you and your contribution.
Let's cut the crap again and do talk on important things here.