DD coil question, The coil speeds

[GT Blocker]

Hey all,
I have a couple of questions about DD coils specifically.
When it comes to short sample times (i.e. 10uSec and lower), do the coils need to be matched for resonance (or should they always be close)? Does it matter?
If they don't need to be closely matched, is it more important that the TXmit be the 'fastest', or do they both need to be as fast as possible?
I guess the real question here is- is there any relationship between the two coil speeds in a DD coil design?
Probably a stupid set of questions since I couldn't find any references to them, but what the hey, I wanna know....anyone?
Thanks, folks, GTB

[KingJL]

Hey all,
I have a couple of questions about DD coils specifically.

I am not an authority on coils DD or otherwise, but I'll communicate my experiences with the DD coil that I built.

When it comes to short sample times (i.e. 10uSec and lower), do the coils need to be matched for resonance (or should they always be close)? Does it matter?

I don't think it matters greatly. I think Rex has stated that he thought that his DD with equal spec coils was less susceptable to noise than those he built that had higher inductance in the RX. My coil was 300uH TX and about 650 RX. It works great and I sample down to 6usec.

If they don't need to be closely matched, is it more important that the TXmit be the 'fastest', or do they both need to be as fast as possible?

The TX needs to be as fast as possible. Care must be taken to keep capacitance as low as possible. This means you have to pay attention to the type wire (type and thickness of insulation) used for the coil, the type and length of coax for connection to MD (I used miniature dual coax), the type of shielding used (I used scotch copper mesh) and how it is applied (type and thickness of spacer).

I guess the real question here is- is there any relationship between the two coil speeds in a DD coil design?

There may be some, but I have yet to discover them. I think any disadvantage there is in having a higher RX inductance is overcome by the increased signal (up to a point) of a coil with more windings.

Probably a stupid set of questions since I couldn't find any references to them, but what the hey, I wanna know....anyone?

Give bbsailor a yank on his chain. He is one of the most credible authorities regarding coils used in PIs.

Regards,
J. L. King