lobo improved

[maxbiri]

Hi Porkluvr

Very interesting stuf!
You are perfectly right phase is more important respect simple gain but as i said i'm not skill with pspice, i need more time for remember how set spice for phase schift.
I notice what i think is a error, not sure, but i think that 1.3 mH must be tx coil and 10 mH the rx coil.
I'm working using a about 1.3 mH tx, maybe this is the problem i have around distorsion in tx stage?

Byr Max

[aft_72005]

Hi maxbiri
Another software which be able simulate circuit is proteus 7.
Also working whit it is simple.
Best regards.

[porkluvr]

see coil data chart

I notice what i think is a error, not sure, but i think that 1.3 mH must be tx coil and 10 mH the rx coil.
I'm working using a about 1.3 mH tx, maybe this is the problem i have around distorsion in tx stage?
Byr Max

That is an interesting possibility but, the 10mH TX and 1.3mH RX information agrees with the coil data chart.

http://geotech.thunting.com/cgi-bin/...oils/index.dat

I have no way to know either way. What value capacitor do you use in the TX tank to resonate with 1.3mH?

I reversed the coils (to have 1.33mH + 47nF in the TX tank, 10mH+22nF RX) and adjusted the pots (giving me about 15VP-P on L1) and was able to get about 8x more output from the receiver amplifier (this is only a simulation). I have nice clean signals all around, but, there is a substantial phase offset... all other things being the same - the received signal lags an additional 10 degrees compared to using 10mH in TX.

This makes me to believe that the coil data chart is correct because otherwise the receiver section components would be different for "proper" phase alignment (as I understand "proper"). The GCX (and also DISC) would lose 10 degrees of adjustment range with a 10mH receiver coil!!!

I don't know why the TX tank capacitor's value has never been entered into any of the schematics posted on Geotech... that would be very good to know!! But I'm not complaining because there is enough to work with so that I think I can fit a LOBO into my Bounty Hunter Quick Draw II (wadda piece of junk) cabinet.

I calculate that 6.2nF resonates with 9.93mH at about 20.26kHz. (I am approximating the data for the 5X10 coil... the 8x15 coil has slightly higher inductance and resonates at about 19.7kHz). Reversing the TX and RX coils; we could use 47nF to resonate with 1.35mH (this is the medium value according to the coil chart) at about 19.96kHz.

You could use 10mH in the receiver side, but I think some RX phasing component values would best be changed in order to eliminate the 10 degrees of lag and it should not to be ignored.

[maxbiri]

Hi

I'm now working a little with ps and i began having same interesting results.
I can only simulate the input stage using mc33178, for tx stage i'm not so skilled and i cant have results.
From simulations results the best rx coil is 1.3 mH with 47nf and not 10mH so i do a big mistake thinking that values are revesed.
Tomorrow i will post plot of gain and phase with the two options.
I think that with the simulation plots we can do the right choice.
It seem that rx stage have a phase shift about -200 -220 degree.

Bye Max

[maxbiri]

Hi

Here are same plot about rx stage, not sure about the right way for input singnal.

[porkluvr]

RX is not tuned

I think that the LOBO RX section is NOT a tuned circuit. I want to repeat one of my previous statements: phase response is a critical issue,
as is circuit gain. The receive circuit components are selected to rotate the phase so that GCX and DISC work correctly.

So, although there is a higher amplitude signal with a 47nF RX tank capacitor, it throws the RX signal completely out of whack
in relation to the TX phase reference.

You don't say which version of PS you use, but LTspice is FREE, and there is
no restriction on the number of components that you can use.

My screen shot shows your circuit (1), and the original (2). (Actually, I have modified the original circuit in #2.
There is more gain, but my modification gives almost exactly the same phase response as the original circuit
and uses components that I have on hand. More gain, but same phase response... the MC33178 @5MhZ GBW
should be able to handle it. Circuit #2 is how I will go.)

I used .0001 as coefficent of coupling between the TX and RX coils. This might represent NO target, but a small
residual backround signal - such as a slightly imperfect null or maybe a nuetral ground return.... ummh?
This "k" selection is only a guess, but:

Notice the wide variation in phase response between the two circuits. The GCX pot is set at 100% in both circuits
and (besides my modification that maintains the same phase response) the only difference is the additional 47nF
tank capacitor in circuit #1. (Not shown, there is series 51r and 11r in the TX and RX coils.)

[maxbiri]

Hi Porkluvr

You are perfectly right, phase is more important respect total gain.
Because i'm not skilled about PS, i asked a friend and he suggested me to put a resistor in serie with generator, only for simulation not in real circuit, and now i have result in good agrement with yours.
I have a difference about 78 degree with and without 47nf capacitor.
If i have well understood yours plots you have about 90 degree of difference.
I think that the better solution is to leave space for capacitor on pcb and after see what happen in "real word".
The gain of rx stage is about 30 around 20kH.
I need study Lt spice because seem a very powerfull soft.

Bye Max

[maxbiri]

Hi all

This is the new version for Newlobo, the board is populated about 95%.
After testing the tx + power supply i preferred design a single board and not a splitted board.
After test if all is ok i will post pcb design.

Bye Maxbiri

[Max]

Hi all

This is the new version for Newlobo, the board is populated about 95%.
After testing the tx + power supply i preferred design a single board and not a splitted board.
After test if all is ok i will post pcb design.

Bye Maxbiri

Hi,
seems good work.

Have you made the coil ?

Kind regards,
Max

[maxbiri]

Hi Max

No i havent still built coil, i need copper wire and need found my old template for ellitical coil's shell for cheking exat dimension.
It look nice but usually when a board look too nice dont work !
When a board look terrible usually work in a beatifull way!
I hope this time theory will be not confirmed!
All component are recicled, only LS204 are new.
I need even change ceramic NP0 with polistirene type, very difficult find value < 500 pf.

Bye Maxbiri

[Eugene52]

lobo improoved

Maxbiri , how is your progress on this project ??? Which schematic are you using ??? Thanks.................Eugene

[maxbiri]

Hi Eugene

I started from second schematic but i do same changes.
Project at moment is stopped, I'm very busy and i have no time for hobby.
I'm waiting copper wire for coils.

Bye Max

[Eugene52]

lobo improoved

Maxbiri , when this project finally gets going , it will become one of the top projects to build on this forum . Here are the Top Projects to build from Geotech , and each are almost ready for that final schematic posting !! And also the "Engineering Members" who highly contributed :

1. Tesoro Golden Sabre Light from "Ivconic"
2. Tesoro Lobo "1989 VLF version"
3. Minelab Relic Hawk from "Max & Fred"
4. Hammerhead P.I. latest version from "KingJL"
5. Delta Pulse P.I. latest version from "KT315"

Regards.............Eugene

[maxbiri]

I' m thinking change lm 358 with ls204 in first filter's stage.
What do you think about?

Bye Max

[Eugene52]

lobo improoved

Yes Maxbiri , LS204 is the correct opamp to use . W. Lahr did excellent reverse engineering work , Always !!!! He was very correct and accurate . Use his LOBO schematic on your first prototype , then test your new Lobo circuit. And if it works great , make a case for the circuit board , and seal the search coil and put it away !!! And then if you want to improove or modify the lobo circuit design , build a second lobo circuit board for experimenting !!!! And your first Lobo will always be working for test points , and a reference for comparison ..........Regards.........Eugene