TGSL Tuning & troubleshooting

[simonbaker]

Examples with positive phase shifts (Tx J1-2 vs LF353 pin 7):

a) http://www.geotech1.com/forums/attac...1&d=1261343207

Source: http://www.geotech1.com/forums/showthread.php?t=16122

Timebase: 20us; Phase shift: positive 57 degrees.
[calculated: 12us /76us x 360 ]

b) http://www.geotech1.com/forums/attac...1&d=1192949609

Timebase: 50us; Phase shift: positive 102 degrees. (calculated: 20us / 70us x 360 ]


Source: http://www.geotech1.com/forums/showt...hase#post61260


c) My phase shift negative 17%; its means 0.17 x 360 = - 61 degrees negative.
Timebase: 50us.
Attached picture, it looks very closely i got with pc soundcard scope.

Ok, here's the thing with RX null signal phase -- it can really vary a lot, depending on how you null your coils, and what kind of shield you have. And people have different theories on what is best. However, if you follow a procedure to null your coil using ferrite, and if your RX circuit has correct resonant frequency relative to the TX frequency, then you probably don't need to worry at all about what phase your null signal is.

Are your coils fixed (glued/potted)? If not, you will see when you null your coils by shifting them, you can change the phase of the RX signal quite a lot when near the minimum voltage. It depends on where you choose your position for "best" null. So it is difficult to judge your RX null signal without seeing how you nulled your coils, etc.

It may be true that you chose the null point on the opposite side of the minimum voltage than most people -- so your phase would be different.

Using a DC voltmeter, can you tell us what are the voltages on capacitors C15 and C12 (connected to Source pins of JFets)?

-SB

[johnsmith77]

Ok, here's the thing with RX null signal phase -- it can really vary a lot, depending on how you null your coils, and what kind of shield you have. And people have different theories on what is best. However, if you follow a procedure to null your coil using ferrite, and if your RX circuit has correct resonant frequency relative to the TX frequency, then you probably don't need to worry at all about what phase your null signal is.

Are your coils fixed (glued/potted)? If not, you will see when you null your coils by shifting them, you can change the phase of the RX signal quite a lot when near the minimum voltage. It depends on where you choose your position for "best" null. So it is difficult to judge your RX null signal without seeing how you nulled your coils, etc.

It may be true that you chose the null point on the opposite side of the minimum voltage than most people -- so your phase would be different.

Using a DC voltmeter, can you tell us what are the voltages on capacitors C15 and C12 (connected to Source pins of JFets)?

-SB

Coils are not glued. I will measure voltages on C15, C12.

But everything started because i have positive polarity on Gb channel Tr drain.
Others guys has negative.
Example here: http://www.youtube.com/watch?v=htUUla-vk-E

Source here: http://www.geotech1.com/forums/showp...postcount=4026

Dennis the Mennis can't get opposite polarity when phasing are changed. He can't achieve half-sinus in positive.
The same behaviour in mine TGSL, but i achieve positive, he - negative.

I want know why my polarities are different I have different phase shift also. Phase shift should be enough big to change polarities on Tr 4, 5.
And mine are negative 61 degrees. Others has - positive 50-100 degrees. Difference: 111-161 degrees. How this big difference achieved? I don't think different shielding can change phase shift so much. And different coil nulling make phase shift 161 degrees?

Other reason should be here

[dfbowers]

Examples with positive phase shifts (Tx J1-2 vs LF353 pin 7):

a) http://www.geotech1.com/forums/attac...1&d=1261343207

Source: http://www.geotech1.com/forums/showthread.php?t=16122

Timebase: 20us; Phase shift: positive 57 degrees.
[calculated: 12us /76us x 360 ]

b) http://www.geotech1.com/forums/attac...1&d=1192949609

Timebase: 50us; Phase shift: positive 102 degrees. (calculated: 20us / 70us x 360 ]


Source: http://www.geotech1.com/forums/showt...hase#post61260


c) My phase shift negative 17%; its means 0.17 x 360 = - 61 degrees negative.
Timebase: 50us.
Attached picture, it looks very closely i got with pc soundcard scope.

How are you measuring phase? Are you really using a dual trace scope? Both signals must be triggered at the same time to get a phase relationship.
To start with, I would just look for the deepest null using a DVM. As long as you can eliminate ferrite within the range of the GB trimmer it should be fine. And assuming your frequencies are correct things "should" work right. That would simplify any troubleshooting for sure.

The whole point of the null phase is to minimize the effects of amplitude change due to the "X" or magnetic component of soil. But after we go through all that trouble we have to adjust it differently anyway because ferrite and soil are not purely magnetic. So, like Simon was saying you probably don't need to worry about your null phase too much as long as your frequencies are right.

[johnsmith77]

How are you measuring phase? Are you really using a dual trace scope? Both signals must be triggered at the same time to get a phase relationship.
To start with, I would just look for the deepest null using a DVM. As long as you can eliminate ferrite within the range of the GB trimmer it should be fine. And assuming your frequencies are correct things "should" work right. That would simplify any troubleshooting for sure.

The whole point of the null phase is to minimize the effects of amplitude change due to the "X" or magnetic component of soil. But after we go through all that trouble we have to adjust it differently anyway because ferrite and soil are not purely magnetic. So, like Simon was saying you probably don't need to worry about your null phase too much as long as your frequencies are right.

I don't worry. But i am very interested why my phase shift is different even by 160 degrees comparing to others like Max... If it would be 5-10 degrees difference then ok, but now so big
I use pc soundcard scope. Tested by sweepgen. When sweepgen L/R channels in phase, pc scope shows 0 phase shift. There option on sweepgen - 180 phase, and scope shows phase shift 180 degrees. Can this testing confirm pc scope is working good?

[Jerry]

I am surprised at how well the sound card deals with the 14 khz signal. Are the top and bottom traces fed into Left and Right channels of your sound card?

It looks like you have two separate pictures merged as one. The background grids do not line up. Makes it confusing to read.

Jerry

[simonbaker]

I don't worry. But i am very interested why my phase shift is different even by 160 degrees comparing to others like Max... If it would be 5-10 degrees difference then ok, but now so big
I use pc soundcard scope. Tested by sweepgen. When sweepgen L/R channels in phase, pc scope shows 0 phase shift. There option on sweepgen - 180 phase, and scope shows phase shift 180 degrees. Can this testing confirm pc scope is working good?

I am like you -- I want to know why something is different. So even though it may not be important to your MD working well, I don't mind trying to analyze this phase question.

Your pc scope may be adequate for these phase comparisons based on your test with sweepgen -- but I still would have some doubts without a good scope to confirm with.

Let me ask you this -- when you null your coils, what is the range of RX phases (relative to TX) you can get as you move the coils past the minimum point? Tell us which value is for greater overlap and which for less overlap.

Regards,

-SB

[johnsmith77]

Let me ask you this -- when you null your coils, what is the range of RX phases (relative to TX) you can get as you move the coils past the minimum point? Tell us which value is for greater overlap and which for less overlap.

Regards,

-SB[/QUOTE]
1) Maybye you mean from overnull till overlap? It is very likely here: http://www.youtube.com/watch?v=htUUla-vk-E

2) Does ferrite should be rejected in Disc mode? Stupid question? Please assume it is first detector in my life
I think yes, cause we should reject ground in both modes. Now i can reject ferrite only in am.

[johnsmith77]

I am surprised at how well the sound card deals with the 14 khz signal. Are the top and bottom traces fed into Left and Right channels of your sound card?

It looks like you have two separate pictures merged as one. The background grids do not line up. Makes it confusing to read.

Jerry

Will help 12us shift / period 68us = 60 degrees.
The picture is made by paint from others two and uploaded just for understanding. I measured by soundcard and did not take photo, because of a little sweeping by trigger. That sweeping still let get correct values.

[simonbaker]

Let me ask you this -- when you null your coils, what is the range of RX phases (relative to TX) you can get as you move the coils past the minimum point? Tell us which value is for greater overlap and which for less overlap.

Regards,

-SB

1) Maybye you mean from overnull till overlap? It is very likely here: http://www.youtube.com/watch?v=htUUla-vk-E

2) Does ferrite should be rejected in Disc mode? Stupid question? Please assume it is first detector in my life
I think yes, cause we should reject ground in both modes. Now i can reject ferrite only in am.[/quote]

I think that video is different signals, not TX and RX.

I just want to know, what is phase diff at "overnull", at "minimum null", and at "under null". You need to move coils far enough in each direction to observe maximum range.

Also, what are voltages on C12, C15 at those positions.

Regards,

-SB

[simonbaker]

Let me ask you this -- when you null your coils, what is the range of RX phases (relative to TX) you can get as you move the coils past the minimum point? Tell us which value is for greater overlap and which for less overlap.

Regards,

-SB

1) Maybye you mean from overnull till overlap? It is very likely here: http://www.youtube.com/watch?v=htUUla-vk-E

2) Does ferrite should be rejected in Disc mode? Stupid question? Please assume it is first detector in my life
I think yes, cause we should reject ground in both modes. Now i can reject ferrite only in am.[/quote]

Yes, when you set your GB pot correctly, ferrite is rejected in both modes. Some crackling is OK.

-SB

[Jerry]

Will help 12us shift / period 68us = 60 degrees.
The picture is made by paint from others two and uploaded just for understanding. I measured by soundcard and did not take photo, because of a little sweeping by trigger. That sweeping still let get correct values.

Ok.... The pictures are a representation rather than from your sound card scope. If this scope is not stable enough to get a good picture of both traces you are taking a lot on faith that the 12Us shift is accurate.

This whole discussion may not matter in the long run if it is working ok.

Here is a graph that I made last year showing the relationship of the coil phase shift to the null voltage. It may help shed some light on things.

Jerry

[johnsmith77]

Ok.... The pictures are a representation rather than from your sound card scope. If this scope is not stable enough to get a good picture of both traces you are taking a lot on faith that the 12Us shift is accurate.

This whole discussion may not matter in the long run if it is working ok.

Here is a graph that I made last year showing the relationship of the coil phase shift to the null voltage. It may help shed some light on things.

Jerry

Nice graph. Md is not working good 100%. The ferrite is not rejected in disc mode.

[johnsmith77]

I just want to know, what is phase diff at "overnull", at "minimum null", and at "under null". You need to move coils far enough in each direction to observe maximum range.

Also, what are voltages on C12, C15 at those positions.

Regards,

-SB

My results.

Phase diff tx J2-1 / lf353 pin 7:

a) undernulled: ~ 160 degrees, negative shift
b) nulled: - 60 degrees, negative
c) overnulled: ~ 40 degrees, positive.

C12, C15 voltages.

a) undernulled:

c12, -0,7v
c15, -0,7v

b) nulled:

C12, -0.2v
c15, +0,2v

c) overnulled:

c12, -0,7v
c15, -1,4v

Best Regards

[johnsmith77]

Some news. In am mode i hear single beeps always. In disc mode when metal is discriminated i hear double beep. It even could be used, if one beep than "dig"; if two beeps - "trash". But but there is two beeps for ferrite also

[simonbaker]

Ok.... The pictures are a representation rather than from your sound card scope. If this scope is not stable enough to get a good picture of both traces you are taking a lot on faith that the 12Us shift is accurate.

This whole discussion may not matter in the long run if it is working ok.

Here is a graph that I made last year showing the relationship of the coil phase shift to the null voltage. It may help shed some light on things.

Jerry

Remember this type of graph, as I hope to return to it with an underlying model that would help make it more widely applicable.