VLF MD with digital signal processing : Bee-Buzz 1

[Aziz]

Hi all,

unfortunately, I don't have other ring cores to test the dual frequency transmitter. What I need is a larger ring core with less losses up to 100 kHz. I have found two iron powder ring cores (T106-26, Al=93 nH/N²) in my box, which I could test them soon. I can stack them together to a larger ring core (Al doubles to nearly Al=180 nH/N²). Unfortunately, it is still not large enough. Iron powder cores have large losses however. It is difficult to find a good working ring core.

I will get rid of the mixer coke Lm. The capacitive mixer is best and cheap when coupling capacitors are set large enough (min. 1µF, better more, max. 40 V voltage rating is enough). So it doesn't becoming a frequency determining part anymore.
Cs, Cp, C1 are high voltage FKP foil capacitors (400 V). This is very important. C2 is also FKP foil capacitor but it's voltage rate can be lower (40 - 100V).

And if we short the choke L1 (or leave it), we can use the transmitter in single frequency mode. With double power input with left and right output channel of the headphone amp of the usb sound card. Just feed in in-phase signal (same frequency, same phase lag, same level).

I hope I can find a good ring core for the choke L1.
Cheers,
Aziz

[Aziz]

Hi all,

puff - bang! It burned and smoked in a fraction of a second!

No. Just kidding.
This circuit doesn't work in the real world. The ring core material for the choke L1 hasn't been considered into account. So I don't get enough power to the coil. The core material must have immense losses for the operating frequencies.
I have tried two different chokes. Same problem.
Then I have connected a second TX-coil in place of the choke L1 (L1 not inductively coupled to TX). And the power went up! Went up! The TX coil voltage rised immense.
Air core chokes work fine. Ferrite don't. I have to try other core materials some day. The mixer choke isn't much critical I think - but could be too.

Bad day.
Cheers,
Aziz

[Aziz]

Hi all,

my prototype is ready.



No smoke yet. It has not been tested.
The mixer choke has 2.7 mH, the L1 choke has 1.2 mH, the zener diodes are 2.4 V type (what I have found in my box).
Let the smoke out of it. After I find a fitting TX-coil.
Aziz

[Aziz]

Hi all,

this is a weird dual frequency transmitter. It's a trial and error method in the spice simulation to find out the frequency determining values for desired frequency response.

But we have so many options to drive the TX.
- at exact resonant frequencies
- off-resonant frequencies (left or right side nearby the resonant frequencies)
- single frequencies drive
- narrow bandwidth (500 Hz) around resonant frequencies drive
..

But get very useful TX coil (impedance change) + RX coil (impedance change + signal from TX and Target) signal information for better detection. So many parameters makes processing really difficult. But I know a very simple method to get it working.
Aziz

[Aziz]

Oh man!

The capacitive mixer is no good! I'm tending to use the inductive mixer again. And I will leave the coupling capacitors (the output of the headphone amp should already be AC coupled - ain't it?). Frequency determining parts shouldn't be at the input of the transmitter. When I lower the capacitance values for the coupling capacitors, they became frequency determining caps in the last proposal. So damn it .

Back to the stable inductive coupling mixer.
Aziz

PS:
I'm trying to improve the power efficiency of the transmitter. The transmitter should not suck more energy from the external usb sound card. So I have to increase the TX-coils inductance a bit. This forces me to use low resonant capacitor values to get up to 70 kHz for the upper frequency range. And I get a problem with the capacitive mixer.

[Aziz]

Hi all,

I have found my parts, LCR meter, a clean bread board, a ready to solder 2.1 mH choke with iron ring core (for L1), high voltage capacitors (for Cs and Cp). If I find the rest, I'm going to build test board this week end.

[Aziz]

You don't have to calculate the them ...just do a sweep at start up to calibrate

This what I usually do.

But for the sake of science it is good to know, how it is working.
Wolframalpha can solve this. We need only to define all the complex resistors and find the two minimum Z. Then we have the two frequencies.

I don't want to do this. I would much like to search my parts, LCR meter and make nice bread board circuit to start with the coding.
Can't wait to start with.

Aziz

[moodz]

You don't have to calculate the them ...just do a sweep at start up to calibrate

[Aziz]

Hi all,

how to calculate the resonant frequencies f1 and f2 of our dual frequency transmitter?

That's really a good question. I don't know. It is quite complex. Yeah, you need complex math as we have lot's of complex resistors Z there. In series and in parallel. All mixed together.

Best way is tweaking Cs and Cp for given L1 and LTX inductances. In spice simulations using ".step param list" command for instance. Spice simulations are making it really easy for us.
For fine tweaking the frequencies in the real circuit add a small capacitor (470pF- 1nF) parallel to Cs and/or Cp to move the resonant frequency to the center of the bin-frequency. Depending on which is tweaked, it affects one frequency more than the other.
Best seen in the spice simulations.

Note, that the coupling capacitors C1 and C2 and the capacitive voltage divider caps C3, C4 will also have little effect to the resonant frequencies.
Aziz

[Aziz]

Aziz! Let's open a can of worms!
https://www.aliexpress.com/item/1005...Cquery_from%3A

That's a nice radio!!!

We don't need a radio.
We need a good next gen VLF/LF metal detector. With lots of precise and very sensitive digital decoders. We don't even need DSP functionality. All math can be reduced to simple +, -, *, / operations.
Aziz

[ivconic]

Aziz! Let's open a can of worms!
https://www.aliexpress.com/item/1005...Cquery_from%3A

[Aziz]

Hi all,

lets reduce the transmitter power a bit and make things much easier (and low cost). We are leaving the inductive mixer (the mixer choke Lm) and duing the mixing pure capacitive. Reducing the coupling capacitors so the external usb sound card gets not too hot. We really don't need much power to the TX. It is still enough power.
See what happens in the following schematics.

The TX-coil voltage is still swinging between +50 and -50 V.
And we don't need the mixer choke.
The capacitive voltage divider must be adapted (increase or lower C4) to get the TX-reference voltage into the reasonable range. In this case, we have to decrease it to 10 nF to be in range of +2.5/-2.5 V.
Aziz

[Atul Asthana]

I am still looking for the range of filter values for separating target and ambient/ground response that are generally used by vlf mds.

I have assumed target response to be around 100 hz and ambient/ground around 1 s to 5 seconds.

also, what should the target detection threshold value be, and derived from which parameter?

[Aziz]

The TX coil current was missing.. You can see, that there is enough bang to the TX coil.

Of course, you may drive the TX on single frequency too (either lower or higher). Just provide same level, frequency and phase on left and right output channel of the headphone output. You can make it chooseable.
- Frequency 1 (single low),
- Frequency 2 (single high),
- Frequency 1 + Frequency 2 (dual)
No changes required to the transmitter to implement the three modes.

Aziz

PS: Choke L1 is a high voltage choke. It must handle voltages up to 200 V and more!

[Aziz]

Hi all,

this is a preliminary version of the dual frequency VLF/LF transmitter for Tablet PC + external USB sound card solution. I may reduce the output power for the final version. This is, what is easily possible with the G6 headphone output. You can reduce the TX coil current by reducing the coupling capacitors C1 and C2. The resonant frequencies are defined by Cs, L1 and Cp, LTX. C3/C4 is a capacitive voltage divider to get the TX reference voltage down to reasonable voltage levels for line input of the sound card. C3/C4 has low frequency determining effect to the resonant frequencies. Use zener diodes (D1 .. D4) to clamp the high voltage level to protect the line input of the sound card. Below is the spice simulation file for your convenience. And some pics.

The dual frequency VLF/LF transmitter schematics:




That's all for now.
Aziz