I've finally started work on my metal detector project and after a fair start, I'm a bit stuck.
The circuit is based almost entirely on the buccaneer VLF circuit to be found here. I've made some alterations, I've upped the voltage to 9V and made the necessary adjustments to account for that. I've also increased the coil diameter to 20cm (the original was 11cm), but I've kept the inductances more or less the same (my coils measure 2.9mH, whereas the original was somewhere in the region of 2.3mH). With the Tx and Rx circuit built (on the breadboard - yes, I know it's not the best, but I didn't want to commit to a full build just yet) and the Tx oscillator up and running, I tried nulling the Rx coil. I must have spent hours trying to get near where I thought I should be at (millivolt level) without much success.
In the screenshots below, I set the p-p Tx voltage at about 40V by trimming R6. This is considerably less than the expected output of the original circuit (80V on my simulations) and yet the Rx coild is showing just over 2V p-p (1.8Vrms), which is certainly a far cry from the millivolt level I was expecting.
The give away here, is that in the original circuit the pre-amp gain is set to be roughly 10. When I tried that kind of gain, my signal railed badly, so I reduced the gain to 5 but it's still clipping.
So, my first question is this: just exactly what should I expect when I "null" my coils and do my results look normal?
Secondly, I wasn't expecting a perfect induced signal but the Rx coil has these annoying witches hats that appear at twice the operating frequency. If you look really closely at the Tx signal you can just make out the glitch that may be coupled into the Rx coil, but I'm not sure. What can be causing this? One thing to note is that the "hats" move around the signal as I squeeze the coils to try to null them. It's really screwing up my Rx signal.
I know there's a lot to take in here and if you need more info I'll post that up.
UPDATE: Incidentally, for those who'd like to know more about the Tx Oscillator, see this post