lobo improved

[Davor]

Let we make noise analysis..

I did. It is quite nice for a simple aperiodic frontend. Funny but I was musing about such approach, yet in a fully differential frontend. Low impedance to squeeze juice from coils at low noise conditions.
I replaced generic opamps with NE5532 and gave them proper power, fired noise analysis and got ~6.6nV/sqrt(Hz) when normalised for gain. Not bad at all.

I think the aperiodic approach is much better for amateur builds in case you are building your own coils, and do not care about exact inductances. It just works.

[porkluvr]

[QUOTE=Derx;155768]How to make PSPICE model DD coil and potentiometer?[/QUO

A DD coil would be tough to model but the (linear) potentiometer is pretty simple. Here goes:
*********************************
* This is the potentiometer
**** _____
* 1--|_____|--2
****** |
****** 3
*
.SUBCKT potentiometer 1 2 3
.param w=limit(wiper,1m,.999)
R0 1 3 {Rtot*(1-w)}
R1 3 2 {Rtot*(w)}
.ENDS
********************************

[Derx]

How to make PSPICE model DD coil and potentiometer?

[mikebg]

If you drive the coil by minimum coupling with another voltage source driven coil (instead of the voltage source), you'll see a somewhat different picture, but much closer to your real goal

Hi Davor,
I agree with you, but I think the equivalent circuit of LC tank is enough correct. Because of minimal coupling, the resistance and reactance of voltage source V1 change the coil parameters negligible.

SPICE shows that amplitude and phase characteristic of this section is correct designed according stupid conventional block diagram of metal detectors. That block diagram requires high phase stability of preamp. This is achieved by damping of tuned circuit. Jkdj means increased bandwidth, ie increased interferences and noise.

Note that unlike other Tesoro preamp circuits, the third harmonic of mains frequency is suppressed more than 60dB.
Let we make noise analysis..

[Davor]

If you drive the coil by minimum coupling with another voltage source driven coil (instead of the voltage source), you'll see a somewhat different picture, but much closer to your real goal

[mikebg]

I'm curious about where this analysis will end.

Here is the SPICE analysis:

[Davor]

To make it correct, don't forget the coil inductance at ~11mH and a parallel capacitor at 6n8. I found these values somewhere, but I'm not sure if Rx tank being resonant at Tx frequency makes much sense.

I'm curious about where this analysis will end.

[mikebg]

REVISION AND REDESIGN OF PREAMP

Calculation of cutoff frequencies.

Every RC timeconstant forms a cutoff frequency fc at which the phase is shifted with 45 deg and magnitude is changed with -3dB. The formula for calculation of cutoff frequency is fc=0.16/(RC). Below are shown the results from calculation.
C1 and R1 form a high pass filter with Fc=1.17kHz. That means it will suppress significant a) the audio frequency and
b) the third harmonic of mains frequency. Both frequencies are additional suppressed because R1 and C1 form with L1 a second order bandpass filter.
Timeconstant R2*C2 is not correct calculated because this is high pass filter and TX frequency appears below its cutoff frequency.
Timeconstant R5*C5 is also incorrect because this is low pass filter and TX frequency appears above its cutoff frequency. That means the gain of second stage is below G=R5/R4=48.7/8.45=5.8.
The total gain of the whole preamp is less than 21*5.8=122 or <42dB.
The SPICE analisis will show how the circuit operates.

[Davor]

If the coil allows it, I'd go for a true differential input, and leave filtering to the subsequent opamps. True differential setup has a moderate input impedance that is symmetrically distributed between each input and a ground. The only problem with this design is that resistors values are not easy to pick. However, I calculated values for 6k8 total input impedance that has amplification 48.5 and uses only E6 values. These are 6k8, 330k, 68ohm, 3k3. I use this setup in my IGSL.

[mikebg]

Let we improve the preamp circuit

Different designed preamp.
Lobo is designed somewhere at end of 80-ties. I think the desiner is not Jack Gifford because the preamp circuit of Lobo differs significant from other primitive preamp circuits used by Tesoro. The differences are:
1. The preamp is formed as three stage amplifier. Other Tesoro preamps use only a single stage.
2. The resistors of first stage are designed for low noise. The resistors in other Tesoro preamp circuits have much larger resistances, ie they generate more noise.
3. RX coil is connected to form second order band pass filter. In other Tesoro circuits, RX coil forms second order low pass filter. That means they can not suppress enough the audio frequency (330 - 550Hz) and
third harmonic of mains frequency (150 or 180Hz).
However:
1. Despite its three stages, the preamp operates with low gain. I see something is written for gain in the circuit diagram attached below.
2. Resistors and RX coil have low resistances, but this is useless because opamp generates spectral noise density more than 1kohm resistor. That means the designer can decrease the weight and price of RX coil without increasing input noise..
3. The tuned circuit (band pass filter) is damped with 6kohm resistor. That means increased noise because a) the damping resistor also generates noise and b) damping forms wide band input (noises are proportional to bandwidth)
Let we make analysis, revision and redesign of this circuit.

[tchoci]

Hi all kolegi.Dnes LOBO again went with him to do tests with a colleague with metaldetektor Sovereign.Selishteto was quite dirty from (the Ottoman yoke). If you believe metaldetektora me very dobre.Izvyadih relics as a colleague Sovereign.Razlikata only that a signal of some rusty forged zheleza.Za me this is not a problem because so come and peaks of streli.Prosto am delighted.




Tchoci!

[simonbaker]

The Tesoro website currently has only an HTML version of the Lobo operator's manual. I don't remember when I got this, but here is a PDF.

The file size was over the 2MB maximum upload limit until I found the way to reduce file size in Acrobat. Now it squeaks by. Compatible with Acrobat 6 and up.

Thanks for link. That's a great machine.

-sb

[porkluvr]

PDF operator's manual

The Tesoro website currently has only an HTML version of the Lobo operator's manual. I don't remember when I got this, but here is a PDF.

The file size was over the 2MB maximum upload limit until I found the way to reduce file size in Acrobat. Now it squeaks by. Compatible with Acrobat 6 and up.

[porkluvr]

Hy ALL.
Has Selected the resistor R10.

Thank you, FOMA.

That 470k R10 should give a little bit wider range on the discriminate control, but not too much.

[porkluvr]

If i remember right best conditions, on my pcb, is with a 1.3-1.5 nf rx .
By Max

If i remember right best conditions, on my pcb, is with a 1.3-1.5 nf rx capacitor.
I suspect that all my problems of low deep are a conseguenze of non perfect rx phase.

Bad news you have stopped project. You have 10uH receive coil, correct?
Using +/- 5V could explain some lack of depth, but also rx phase... very important.

The component values in receiver amplifier are extremely sensitive and work hand-in-hand with rx coil components to set demodulator phase.

It would be good to see dual trace scope of transmit signal and receiver amplifier output signal at the same time. Maybe somebody who has more knowledge than I can see from scope presentation if you should make changes. Perhaps you should repost your schematic and show component values you have used. Ever since the first schematic was shown here with many "?" there have been many variations proposed. It's hard to know what is what.

I quit LOBO to work on PI but I've come back and want to finish. PI will wait.
(Your English is OK.)