VLF Newbie

[FreezonePK]

Carl,

This is great encouragement, tomorrow I will get the preamp implemented and let you know how it turns out. Also I will try adding some more turns on the bucking coil and move it around to get a better null.

Is a synchronous demodulator absolutely necessary? I planned to directly compare the receive signal to the transmitted signal using logic gates in which a phase difference would produce to a pulse on its output. The pulse width would correspond to a phase difference. I am up to changing this though, just need to make this decision sooner rather than later. *Edit: I am wondering if this is even necessary because I was just reading it is possible to do the synchronours demodulation with the ATMega32 that I plan to use.*

Cheers,

Justin

[Carl-NC]

If you perfectly balance the coil so the residual RX signal is zero, then a simple digital phase comparator might work. But a tiny target signal riding on top of a large residual signal won't give you much of any phase shift.

A typical VLF design uses demods to produce quasi-DC signals and high-pass filters to extract the dphi/dt. This filters out the large residual and all you see is the target signal.

[FreezonePK]

Hello geocommunity,

Sorry I haven't given an update on the progress of my metal detection adventures till now. School and research are bogging me down (last year of engineering).

The coils work awesome, I put 2 more turns on the bucking coil and ended up with a 2-3mV null (maybe too small). Now my big issue is my SNR, after I amplify the signal I end up amplifying a ton of noise which wrecks havic on my logic comparotor network. It is probably because this is being done on a breadboarded as a test. Is this noise filterable with say a 4 or 5 stage(less if I can) chehby low pass filter? If not, I will make use of a synchronous demodulator scheme, but I need to create the simplest detection scheme so that the MCU can do something with the signal. Any suggestions would be awesome and I will post a schematic once I figure it out.

Cheers,

Justin

[der_fisherman]

Looking at the first trace, you appear to have a lot of HF noise on the rx coil. What frequency is it? Expand the trace and measure it. This is not good!!

If I am right, you need to get rid of this, or thin down the trace on the scope!! It's far too thick.....