"How does it react on hand touching antenna?" - Buggins
[dewster] With all coils electrical touching stalls but quickly restarts at withdraw.
With smaller coils the amplitude goes way down as I touch the cardboard insulated plate.
With my 19.3mH it actually stalls there.
Phase noise looks good, drive phase is somewhat delayed (didn't measure it but it is obvious).
Overall I think it's probably an improvement over the 8 transistor.
The same as observed by me with all simple oscillators.
Except simple current sensing oscillator based on comparator - it starts high frequency oscillation (10-15MHz) and does not return to normal functioning after removing the hand.
It think the only good way to prevent the oscillator from stalling is a drive signal from PLL - it should just go to the lowest frequency bound and do not stop the oscillation.
[dewster] I played with the sim of your transconductance AFE a bit (thank you!) - it doesn't seem to tolerate square wave drive very well?
Since the time I tried to design AFE I've improved my analog skills. Can try to rework it.
[dewster] I was sorta hoping for an AFE design that would allow the AFE to be on the main board, with coax or similar going the remote coil and antenna, but the added inductance and resonance of it would probably be too confounding. CMOS inverters are really difficult to avoid too, they're so symmetric and naturally rail to rail I/O. Re-approaching anything in this project takes a lot of effort to even get to the point of maybe improving something.
Easy experiment is possible. Use some current sensing oscillator (which connects to LC tank with one wire).
Try to connect inductor to oscillator with one unshielded wire and check performance. Try different wire lengths.
Try to connect inductor to oscillator with coaxial cable (with shield grounded) and check performance. Try different wire lengths.
So far, yet another current sensing oscillator. LTSpice model to play with is available on github: current_sensing_osc_v01.asc
Has more than 10 BJTs but it's still possible to build it on a breadboard.
Powered from 4.5V. With 3.3V the antenna swing will be lower.
It uses current feedback instrumentation amplifier on discrete BJTs for the sensing.
I've tried to generate squarish drive waveform - to maximize antenna voltage swing for the same drive swing.
Drive voltage and current:
Antenna voltage and inductor current:
