My SMW 555 TX-470nf - 7.7Khz- 8.6 Vac/ RX- 1nf - 8.2 Khz
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This version of oscillator is way better than the typical oscillators in popular VLF builds, but there is also a catch... you can't use it with any coil. The coil for this Tx must float, so it must not have any of the ends connected to a shield.
In case you build your own coil it is not a problem at all. The op amp takes care that the conduction is precisely 50:50 so the 2nd harmonic will be greatly suppressed, and purity of signal will depend mostly on how the supply voltage is pure.
The same job would be performed even better with a real balanced oscillator, as you have to have a Tx coil floating anyway. The advantage of a balanced mode operation is that a net E-field of such coil in operation is zero, so capacitive feedback from Tx to Rx coils is off the table and better balance is achieved. With single ended operation a net E-field of a coil is half the voltage across the Tx coil, and capacitive feedback limits the depth of achievable balance.
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Where is the problem
in my SMW I have problem There is an inverse relationship between sensitivity and volume key . When the sound is low Detecting coin in the air 40cm . When the sound is high Detecting coin in the air 29cm. I use BC517 Instead of MPSA13. help.Attached Files
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