Sine wave multi frequency

[Aziz]

Oh well,

I have realised, that I can mix (add) two sines of frequency f1 and f2, where f2=3*f1. Amplitudes of about 0.65 each sine (related to full dynamic range of 1).
Aziz

[pito]

3 frequencies f1 = 12kHz, f2 = 4 kHz
Modulation
8kHz, 12 kHz, 16 kHz

[Olly]

OK, I've gotten very ambitious and have managed to generate and combine 8 x sine frequencies in the FPGA.
These are 1.2kHz, 3.2kHz, 9.6kHz, 12.8kHz, 16kHz, 19.2kHz, 51.2kHz and 96kHz.

The resultant waveform shown has equal amplitudes at each frequency but I can adjust relative amplitude of each.
I can also change frequencies on-the-fly in 100Hz steps - just so long as I keep them all fitting nicely within my synchronous demodulator sampling window which is 2.5mSec (1920 samples at 768k sample/sec).



Even though it kind of looks like a mess, the synchronous demodulator successfully extracts the correct amplitude and phase of each.
It's unlikely that I'll use all 8 simultaneously, but I can if I need to

[pito]

square wave has a lots of harmonic,






to select them band pass filter needed

then 10w audio amplifier

[moodz]

Generating sine waves and amplifying them is well known. But getting the signal into deep targets and determining VDI in difficult ground is not. Multi frequency attempts to resolve this issue but the algorithm is poor (ie still can't punch as deep as a PI )

Interestingly the PI detector has the answer to the shortcomings of VLF detectors ....so it is possible to build a VLF detector that actually exceeds PI performance and provides VDI as well.

[Olly]

This is mostly experimental at the moment. I wanted to go through some of the concepts introduced in the multi-frequency section of ITMD 3. In particular I wanted to look at the response of various metals and coins to different frequencies and also to check ground and wet, salty sand.... All in the interests of furthering my knowledge.

[pito]

. But getting the signal into deep targets .

https://www.youtube.com/shorts/CcuXwZVsfNo

[Olly]

square wave has a lots of harmonic,

The problem with using the harmonics is that the amplitude decreases with increasing frequency, whereas realistically we might need the amplitude to increase with higher frequencies.

[moodz]

Use a sinx/x or sinc transfer function and the spectral response is flat. Very easy using an FPGA.

[moodz]

https://www.youtube.com/shorts/CcuXwZVsfNo

Very good ...and the ferrite response..

[pito]

ferrite response..

I do that soon

[pito]

to increase with higher frequencies.

from 5V square waves you will have to big amplitude so you need potentiometer to adjust it to tens of millivolts = sensitivity of audio amplifier.

[EL NINO]

Generating sine waves and amplifying them is well known. But getting the signal into deep targets and determining VDI in difficult ground is not. Multi frequency attempts to resolve this issue but the algorithm is poor (ie still can't punch as deep as a PI )

Interestingly the PI detector has the answer to the shortcomings of VLF detectors ....so it is possible to build a VLF detector that actually exceeds PI performance and provides VDI as well.

Yes and no....but our tests on deeply buried targets..in more demanding terrain show that when tested VLF detector which has such possibilities -/Deep settings.../ can achieve even better results than PI..

AKA INTRONIK - VLF 2X Sinus 2khz /14 khz 13" coil : https://youtu.be/RGpGnc-tHHQ

LORENZ DEPMAX Z2 13" COIL: https://youtu.be/R8WvSev32d4

*Detection of deep targets and deposits is not the domain of modern multi-frequency metal detectors..because they are not specialized for this type of search..

EQ 800: https://youtu.be/gYnJts1TJDQ

PI can be more effective in really strongly mineralized terrain -where VLF really loses strongly the detection range..or detection stability.....Only there can PI have an advantage..

[Olly]

OK, I'm very happy with my sine wave generation and demodulation now! I'm driving this out of the FPGA via a 16-bit R-2R Ladder DAC.
I'd like to just hook this up to an off-the-shelf audio amplifier for lab testing, but these don't normally approach anywhere near the 96kHz I want to go up to so I'll probably use the LM1875 as a starter (Thanks for the recommendation Pito).
It doesn't go quite as high as 96K, but it does have a 70kHz bandwidth so it'll be good for initial experiments.

[Olly]

OK, I'm very happy with my sine wave generation and demodulation now! I'm driving this out of the FPGA via a 16-bit R-2R Ladder DAC.
I'd like to just hook this up to an off-the-shelf audio amplifier for lab testing, but these don't normally approach anywhere near the 96kHz I want to go up to so I'll probably use the LM1875 as a starter (Thanks for the recommendation Pito).
It doesn't go quite as high as 96K, but it does have a 70kHz bandwidth so it'll be good for initial experiments.

Found an LM1875 kit on Amazon, all parts plus PCB for under £6. Arriving tomorrow...