Front End for use with Coaxial Coil

[Anonymous]

Here is the PPD1 front end schematic to use an auto-balanced stacked coaxial coil for fe/non-fe discrimination. This is the front end for the discriminator channel only. The TX coil has its own standard front end, sampling, dc amp. and audio generator. The output of the front end shown, goes on to its own two sampling gates, integrator, dc amp. and centre zero meter. There is an additional timing circuit to give the first discriminating sample pulse at 4uS after TX switch on. The second discrim. sample pulse is the same as the second sample pulse in the all metal channel.
The coils are balanced as far as possible electrically with a signal generator before being connected to the detector. The 2N5566 is a dual jfet similar to two j113

 

[Anonymous]

Hi Eric,
Thanks again for this kind of information. I guess if one thinks of the 2N5566s as working as voltage controlled resistors in parallel with the 39 ohms to tweak the balance in the two coils, like the 20 ohm pot does to start with, it makes the circuit a bit easier to understand.
Looks good, not sure what modern improvements could be made.
JC