Correct Phases?

[Silver Dollar]

Hi Guys,

I just got my TGSL working but it seems to be a bit weak. I can only get 150cm on a big silver target.
I have a fancy scope and can check the phases for GEB and Disc and I have pictures that someone posted
but they don't show clearly what the ranges for phase are.

Does any one know the correct values?

[joop]

Can you explain for each picture what the top and bottom traces are? I have my TGSL on the bench and can post some pictures this evening...

[Davor]

The top picture would be a near perfect phase for GEB, but only if the sinewave presented was Rx, and it is not - a small hump suggests it is Tx signal. So it should be additionally shifted for Tx to Rx phase shift. In case there is no such shift, your GEB is OK.If your Rx signal is too weak to be shown on scope, you can enhance it without spoiling phase differences by approaching a coil with a piece of non-ring ferrite.

Disc phase should ideally be anything in vicinity of 90° from GEB, say 60° to 120°, or 80° to 140°, but in any case encompassing 90° from GEB.

[Silver Dollar]

Hi thanks for helping me out! I'd prefer all these in degrees if you can but if not in microseconds is ok.

First the TX width for one cycle - T1 (or TX frequency) as they vary board to board.

Second the width of the Disc pulse - T2 and width of the GEB pulse - T3

Next with Disc and GEB fully CCW measure from a zero of the TX to an edge of
the Disc and GEB - Disc Min and GEB min.

My Disc and GEB pulses are 120 degrees wide, that seems wrong to me. My Disc
seems to go from 90 degrees +35 degrees to 125. I forgot to move the GEB to
both ends to see what it does. I'll do that tonight.

Finally with Disc and GEB fully CW measure from a zero to an edge of the Disc
and GEB - Disc Max and GEB max

[Silver Dollar]

I don't have my pictures with me but I measured the Disc to be 70 - 104 degrees from the GEB.

Then I got kind of confused and realized I didn't know what ranges to expect!

I measured off the TX as that is the master for all timing. Sometimes the RX isn't 20 degrees.

I understand the data is shifted by the amount of RX shift but would expect to reference the TX
to measure Disc and GEB for correctness then verify the RX is 20 degrees for proper demodulation...

[Silver Dollar]

Here are some new pics. It looks like the RX is sort of 20 degrees from TX.

Disc goes from 0 to 75 or 240 to 305

GEB goes 318 to 370 or 92 to 140

[Davor]

This is very useful on several accounts. Yes, this is the way it should work. Disc regulation span is somewhat narrow, but should be OK for most of the uses.

Now, the part most of the people will miss here is the pulse width asymmetry of the comparators output. I often mentioned the problem of 2nd harmonic as a device of detecting false targets through PWM modulation of the switchers, and here you have it more obvious than elsewhere. Due to the 2nd harmonic created in Tx the waveform becomes distorted. Typically one half wave becomes narrower than the other, and thus the comparators are switching at duty cycle different from 50%. These CRO shots show precisely that problem happening, mostly due to the obviously distorted Tx signal.

With loading (say, proximity of soil, etc.) 2nd harmonic changes twice as lively than the carrier, 3rd is even more lively etc.

This problem is partly alleviated with full wave mixers.

[joop]

How the heck did you get so much signal output from the RX channel? Here's my TX signal and RX output from the first opamp (inverted). I get about 130 mV peak signal. This is with a standard concentric tesoro coil. Phase difference is about 40 degrees.

[Silver Dollar]

Wel I don't know. I haven't fully nulled a coil set yet. I have the standard 105 turn coil and
this 175 turn RX coil (minelab style). I grounded the 4.7K resistor on the negative input of
the LF353 and moved it around to get a 20 degree offset...

[Davor]

You'll have the 20° offset this way or another, but for seeking deep stuff you'll need to null your coil properly. And try to convince your Tx to provide a somewhat cleaner signal.
As for measurements of this kind, and with properly nulled coils, you can always spoil the null with a piece of ferrite.

[Silver Dollar]

The general idea is to make sure it is working properly then make the mods in the other topic (H bridge + RX preamp)

I found some wrong caps that was reducing my air depth to 6" now I get 12" but it has stopped working and now just
makes random beeps! I found with the 6" mode I could see ferrite at 20"!!

I figured to start from the beginning and verify all signals but I am still unclear what my Disc and Geb should look like.
Here's some pics from the Bandito topic. Mine look a lot different from these!

[Davor]

Phases were OK, at least they were very close, so don't worry.
Could you be more specific about the capacitor? Wrong value or something else?

[Silver Dollar]

Well thinking I was using some better caps I swapped some ceramic .47's to some silver mica ones marked 470
Later realizing they were actually 470 pf not 470n. After that it worked very good for a few minutes getting 12"
in air but then it started "falsing". Today I found pin 8 shorted to pin 7 on one LM308 and changed out the metal
case transistors for plastic ones but it still falses a lot...

[Silver Dollar]

So as far as I can tell we need the GEB to start at the zero crossing going positive and
move say 20 degrees to ignore the ground.

The Disc should start at tthe same place and be able to move 90 degrees to ignore steel and
iron similar to this picture.

Am I on the right track?

My confusion comes from some of the patents they switch the zero and direction on some of the
drawings. I'm trying to visualize where on the sinewave these valuse are located....

[Davor]

Your pictures are about 90° off, but your reasoning is correct. So GEB is supposed to produce 0V for a signal in phase with ground, which in easy conditions happen to be almost equal to Tx phase (adjusted for phase shift in Rx front end). GEB pulse is supposed to span equally left and right from the zero crossing. In electrical terms it would be about +90° against a Tx phase.
You noted correctly that disc is supposed to span from there to 0° but also past 0° because you often want to remove aluminium foil as well.

I made a phase shifting solution that I think will be just right for your TGSL upgrade. It would be nice if you zip your .sch file so that I can put in my suggested solution. I'll be off for the weekend, but I'll be able to fix it on Monday. It will have GEB span of over 90°to accommodate for beaches, including coarse and fine tune to avoid multiturn pots and speed up tuning for salty beaches. Disc tuning span can go 180°, but for practical reasons it can be reduced to a smaller span. Tuning is almost linear.