TGSL Experiments

[simonbaker]

I meant that a RC phase shift circuit will have a different phase shift for different frequencies, and thus the shift angle will change when the frequency changes.

Variations in ground response and differences between targets will likely matter more than that, at least it sounds likely.

Units built by different people will likely have different operating frequencies, component values vary with tolerance etc. which won't matter much after tuning the detector. It's the shifting in use after that tuning, or instability, which I thought could present problems.

And yes, the approximate ten picofarad shift in CE capacitance is likely to be insignificant compared to the 22 nanofarad capacitor. My bad.

I agree with your points. My feeling is that the RX coil resonant circuit is the LRC circuitry that would be most sensitive to frequency variations in terms of signal phase, but it seems the synchronous detector is designed to be fairly immune to that.

I have also wondered if frequency "jitter" could be present and possibly cause phase noise. Zero crossing jitter in the comparators could cause phase noise, and the discrim pots have long wires which could pick up stuff. My goal is to try to demonstrate around 28 to 30 cm air depth while on the bench, before packing into a case, then see what the effect of putting in shielded case is. So far it seems that the coil picks up most of the noise I see, but I'm a long way from really being in control of this circuit.

-SB

[ODM]

There are nice articles about phase noise in radio amateur design articles. Some of them are using frequencies in the 100s of kHz range, which would probably mean the same methods of obtaining minimal phase noise would also apply for 10s of kHz as used in VLF detectors.

Phase noise is something that the jfet circuit will definitely cause unless its time constant is small enough. With a diode and pulldown resistor, the time constant for the jfet base voltage is different for the rising and falling edge, and the fast falling edge will cause plenty of phase noise as the bjt bias point jumps at the negative peaks of the TX waveform. Without a good low pass filter it's difficult to keep the phase noise down, but adding more poles into that filter will probably present its own problems, too.

The method of temperature stabilizing the transmitter with a diode avoids this problem, but it doesn't do jack about the amplitude variations due to ground, and needs good thermal coupling with the TX transistor. And a better choice would be to use a similar transistor as a reference, possibly bonded together with thermal paste and tightened with a screw to secure the contact. Whether one uses another transistor or a cheaper diode is not a significant cost issue with an experimenter

[simonbaker]

There are nice articles about phase noise in radio amateur design articles. Some of them are using frequencies in the 100s of kHz range, which would probably mean the same methods of obtaining minimal phase noise would also apply for 10s of kHz as used in VLF detectors.

Phase noise is something that the jfet circuit will definitely cause unless its time constant is small enough. With a diode and pulldown resistor, the time constant for the jfet base voltage is different for the rising and falling edge, and the fast falling edge will cause plenty of phase noise as the bjt bias point jumps at the negative peaks of the TX waveform. Without a good low pass filter it's difficult to keep the phase noise down, but adding more poles into that filter will probably present its own problems, too.

The method of temperature stabilizing the transmitter with a diode avoids this problem, but it doesn't do jack about the amplitude variations due to ground, and needs good thermal coupling with the TX transistor. And a better choice would be to use a similar transistor as a reference, possibly bonded together with thermal paste and tightened with a screw to secure the contact. Whether one uses another transistor or a cheaper diode is not a significant cost issue with an experimenter

Good ideas to test out. If you find some results, feel free to post them here.

Still, a question is whether phase/frequency noise in the oscillator matters much, since the oscillator signal is also used to gate the synchronous detector JFets. In other words, if the oscillator shifts in phase, the target signal will likewise shift in phase, but the synchronous detector driver pulse will also shift the same in phase, and so it all kind of cancels out. I'm more worried about noise in the discriminator and ground balance circuit, such as jitter in the zero crossing detection of the LM393 that makes the synchronous detector driver square waves. But experimenting is the way to find out; maybe we can continue making improvements.

-SB

[lunamay]

how to calculate a resonans freaq in coil, knowing her parameters..i saw some time a go a example somewhere on tgsl thread..don't know where
best regards

[pebe]

how to calculate a resonans freaq in coil, knowing her parameters..i saw some time a go a example somewhere on tgsl thread..don't know where
best regards

It's 1/(6.28 x √LC)

[lunamay]

ad what is the"v" means:P??voltage?

[simonbaker]

ad what is the"v" means:P??voltage?

That's square root sign.

1 / (2 * pi * sqrt(L * C))

6.28 = 2 * pi

http://www.allaboutcircuits.com/vol_2/chpt_6/2.html

-SB

[Dennis the Mennis]

how to calculate a resonans freaq in coil, knowing her parameters..i saw some time a go a example somewhere on tgsl thread..don't know where
best regards

Or this one :

http://www.whatcircuits.com/lc-reson...cy-calculator/

same thing only with build in calculator ... ( for L , C or f )

regards

Dennis the Mennis

[lunamay]

thx :P

[simonbaker]

Bad Parts

Upper trace is TX oscillator.

Lower trace is output (pin 7) of LF353.

I could not understand what was wrong with circuit to make such bad looking Vpin7 in top photo. Thought maybe due to bump in TX sine wave.

Finally, I replaced the LF353 chip. See lower trace in bottom photo. Looks much better.

-SB

[simonbaker]

continuing experiments

I'm continuing basic TGSL experiments using my latest PCB.

This PCB is a modified version of Ivconic's TGSL Final.

Physical
--------
I created a layout for a pre-drilled Radio Shack perfboard, .1 inch spaced holes. Board is approx 5.5 x 4 inches.

The concept was to build a PCB with no chemical etching. I used a perfboard with copper holes and I soldered all connections manually. Yes it was a lot of work to make the connections, but the copper holes helped provide anchor points.

I tried to leave ample space between components for easier testing and construction.

Electrical
--------
I added one small modification which I like but continue to test to convince myself whether it is good or not. I put a voltage divider from TX oscillator to disc and GB circuits to prevent overloading and oscillations in disc circuit. A small capacitor was added to try to minimize any phase shift. It reduces voltage to approx 1/3 of TX oscillator and makes a small 5 degree phase shift (which could be reduced further by trimming).

I also eliminated a couple of resistors at the unused LM358 U107 op amp and used simpler stabilization.

I did not have room for the small bypass capacitors that Ivconic added to the TGSFinal PCB, and so left them out.

Parts
-----
The board accepts back-to-back 10 uf caps if you want for the non-polar 4.7 uf caps on LM358 U103. However, I used single 4.7 uf non-polars. I often wonder if back-to-back electrolytics may not always work as expected, something to think about when comparing our results.

The board also accepts the two BC547 transistors in the speaker circuit. I actually used a single Darlington MPSA13 there myself.

I also used 22K resistors instead of 24K resistors in a couple of places.

In the RX preamp, I used 5K1 resistors, as in the original schematic, not 4k7. 4k7 resistors should give slightly more gain though, something to think about when comparing our results.

I added a small socket for a trim capacitor in parallel with the RX coil. I think it would be useful to add one in the oscillator circuit also to adjust frequency.

Miniature speaker.

Software
---------

PCB Layout and Schematic drawing was done with free PCB Express software. I will post files later.


Construction
------------

I used an iron to transfer a faint outline of the circuit to the PCB to help with connections. See photos.


Results
-------
See later posts.

[simonbaker]

Prelim Results

Using my unshielded test coil, adjustable null, I'm getting with new PCB similar to results of previous TGSL Final PCB:

Good detection of US quarter around 15 cm. Small beep to 20 cm. Sometimes get crack to 25 cm.

I tested to see how critical nulling is. I find nulling is not critical, I get similar depth results in wide range of nulling. This may be because the MD is not too sensitive to begin with. But I have always believed that nulling may not be so critical as we think, but chosen more for other reasons such as stable voltages.

I tested to see how critical nulling was to discrimination. I adjusted disc pot to just knock out a US nickel coin. When I moved null point through a range, it did not affect the discrimination noticeably.

I took MD outside to see if better environment, less noise. I did not see any improvement, which surprised me. But I will repeat that in another location some day.

I am just beginning to experiment with this PCB so have many adjustments and ideas to try. The most important goal is to get a depth of around 28 to 30 cm! I have not achieved that and so far it seems a difficult goal.

-SB

[simonbaker]

beeps

My latest bench TGSL now has a case of the "beeps" -- beeps at about 10/s. Sometimes it settles down.

I'm trying to pin down if it is mechanical or electrical. It seems suspiciously mechanical, since if I move suddenly the oscillations, visible on the scope, get larger and then settle down. But once the oscillations seemed to slow in frequency, which may argue against mechanical, although this building may have many modes.

I tried disconnecting speaker, but oscillations still visible on the scope (I put sweep at about .1 sec / division and monitor pin 6 of LM308's).

So I don't think it is magnetic feedback from speaker.

When I disconnect Rx coil, output is very quiet -- so definitely something picked up or involving the coil.

Using the discrim pot, I can discriminate out the beeps -- they are still visible on the scope, but now the two channels are out of phase and do not trigger the speaker.

Will be interesting to try to hunt this down.

My current feeling is that if you can get rid of these noise signals (maybe go outside), that you can play with increasing the gain to get more depth.

I'm thinking that maybe there is room to boost the gain; for some reason, those of us with only 20 cm depth just aren't amplifying the signal enough. I'll be testing, but the ambient noise and these "mechanical" oscillations are interfering for the time being.

-SB

[Qiaozhi]

My latest bench TGSL now has a case of the "beeps" -- beeps at about 10/s. Sometimes it settles down.

I'm trying to pin down if it is mechanical or electrical. It seems suspiciously mechanical, since if I move suddenly the oscillations, visible on the scope, get larger and then settle down. But once the oscillations seemed to slow in frequency, which may argue against mechanical, although this building may have many modes.

I tried disconnecting speaker, but oscillations still visible on the scope (I put sweep at about .1 sec / division and monitor pin 6 of LM308's).

So I don't think it is magnetic feedback from speaker.

When I disconnect Rx coil, output is very quiet -- so definitely something picked up or involving the coil.

Using the discrim pot, I can discriminate out the beeps -- they are still visible on the scope, but now the two channels are out of phase and do not trigger the speaker.

Will be interesting to try to hunt this down.

My current feeling is that if you can get rid of these noise signals (maybe go outside), that you can play with increasing the gain to get more depth.

I'm thinking that maybe there is room to boost the gain; for some reason, those of us with only 20 cm depth just aren't amplifying the signal enough. I'll be testing, but the ambient noise and these "mechanical" oscillations are interfering for the time being.

-SB

Try putting the PCB in a metal enclosure with the case connected to 0V.

[simonbaker]

Try putting the PCB in a metal enclosure with the case connected to 0V.

Good thought, I will try that and report.

-SB