VLF MD with digital signal processing : Bee-Buzz 1

[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,

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]

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,

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,

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

[moodz]

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

Get one from an old switchmode supply ... ppl dont throw out computers where you are ??

[Atul Asthana]

where can I find fixed point (preferably q16.16) goertzel library optimised for single known frequency detection, for use with stm32f103c8t6?

[Aziz]

Get one from an old switchmode supply ... ppl dont throw out computers where you are ??

Sure it's a good idea. But there is a legal issue. At the moment of disposing it belongs to the recycling company. I may not pick them up - besides I pay for it.
These good old days are gone!

Aziz

[Aziz]

Hi all,

if I can not find a good ring core, L1 could be part of the TX coil. Two TX-coils (less coupled to each other of course)! Induction balancing gets difficult as the RX coil must be nulled against TX1 and TX2 (L1). Processing the signals are becoming more complex - but can be done anyway!
Or making the choke L1 air core above the plastic coil stick (through the plastic coil stick for better holding) so it's position does not change relate to TX-coil.
Anyone with unique idea?


Aziz

[Atul Asthana]

Sure it's a good idea. But there is a legal issue. At the moment of disposing it belongs to the recycling company. I may not pick them up - besides I pay for it.
These good old days are gone!

Aziz

I've used old/defective cfl (compact fluorescent lamps) pcbs to extract ring cores, some of them work well upto 500 khz.

[Atul Asthana]

where can I find fixed point (preferably q16.16) goertzel library optimised for single known frequency detection, for use with stm32f103c8t6?

it seems, in the case of Bee-Buzz 1, instead of goertzel algorithm, a digital lockin amplifier will have better out put, be simpler snd computationally lighter.

this stems from the fact that I am any way doing most of the steps of a lockin amplifier, before or after the implementation of goertzel algorithm.

please opine.

[Aziz]

it seems, in the case of Bee-Buzz 1, instead of goertzel algorithm, a digital lockin amplifier will have better out put, be simpler snd computationally lighter.

this stems from the fact that I am any way doing most of the steps of a lockin amplifier, before or after the implementation of goertzel algorithm.

please opine.

Hi Atul,

Goertzel is much simpler and more efficient:
Goerztel: 1 multiplication and two add or subs per sample
Lockin: 2 multiplications and two adds per sample

The internet is full of Goertzel examples.
Aziz

[Atul Asthana]

Hi Atul,Goertzel is much simpler and more efficient:
Goerztel: 1 multiplication and two add or subs per sample
Lockin: 2 multiplications and two adds per sample

The internet is full of Goertzel examples.
Aziz

thanks Aziz,
yes, I agree with you.

however, this little extra computational load of lockin amplifier seems to give me about 1.5-2 enob more than goertzel for narrowband and if integrated over many cycles, 10-15 dB snr improvement.

and, this is important for the design.

so, I will try both.

[Olly]

Ultimately we're most interested in the I and Q demodulated signals of each of the respective TX frequencies. Perhaps a digital narrowband demodulation system as described on pages 467 and 468 of Carl's latest book (ITMD 3rd edition) would be the way to go? Very effective and not overly complex...

[JoyJo]

Let me be curious. Is it possible to see photos of these pages (467, 46?