experiment pi: dd rx is strange

[Punkt]

Hey guys working right now on a DD PI at arround 750hz. i did my tx signal work just fine. now on the rx i struggle abit. Image A show red(tx) yellow (rx) without any circuit small wave idle more wave with a hammer 15cm away.

now moving to the schematic just ignore the second opamp. when i dont connect the bottom site ob the coil to ground i get a signal like (ba) after the opamp but when i connect it i just get a 12v high.

do i did a misstake on my circuit?

[Carl-NC]

You have a 6V "virtual ground" applied to the top of the coil, if you then apply ground to the bottom of the coil then you are shorting out the 6V (through the coil) and, yes, the opamp will likely rail out at 12V due to the DC resistance of the coil. The schematic exactly as you show should work, although you normally want to buffer the virtual ground.

[Punkt]

how to buffer because the signal is pretty weak... after the opamp have tryed till 1m ohm

[Carl-NC]

The signal out of the preamp is usually not very strong. You then demodulate it and apply A LOT of gain in the back end circuitry.

[Punkt]

idk because i have it on the beadboard.
i have limit the puls to arround 8% of the whole periode time and my power supply can delivery more thats 2A at 12v
so i can not rais it alot

[Carl-NC]

I guess I don't understand what the problem is. If all you have is a preamp then the signal will be weak. You need to add the rest of the circuitry.

[Punkt]

all good so far this is the probaly test circuit.

i dont have build it up yet.

on the bottom the transistor mosfet gate will be switched later for a IGBT driver

its all on a pretty early strage right now. and still work in progress

[Punkt]

a quick update cuz i got right now my laboratory psu the system take right now only 1.1w so 0.092a and i get allrdy a 1€ coin stable detected at 15cm so i think when i rais the possible a to .5-1.5a on the tx side the detection shold go way more up or do i think wrong?

[Carl-NC]

Maybe 20cm.

[Punkt]

Maybe 20cm.

i hit indoor now 25cm air. at 8khz frequency have to get it lower when i get the same decoulping quality thats i got on 8khz with some mix settings them i have the right way
right now i amplify the signal with tl072

[Punkt]

was not able to edit the post befor to insert the image

first one is idle signal the both spikes are prety high but dont matter.
second image = wow signal

[Punkt]

befor i starting a new thread.

follow question:
what cable thickness do you use? example with mono as well as with DD.
I find 0.4mm copper lacquer a bit too thin as only a small amount of winding is possible.
Before the inductance increases too much. At 0.63mm it looks a little better. or am I on the wrong path?

I also see that some of the work is done with quite thin PVC cable. 0.2-0.3mm maybe someone can explain it to me a bit pros and cons