DD coil damping resistance

[GT Blocker]

Hey all,
A quick DD coil(s) question:
Given two identical coils (hypothetically speaking), would the damping resistance be the same for both, TX and REC?
As a follow up question:
If so, would the damping resistance change if one of the same coils was used as a mono coil?

Probably a dumb question, but I was wondering, thanks.

[Carl-NC]

Hypothetically, and usually practically, yes. There is a small difference in parasitic capacitance between the TX switch and the RX preamp, but coil + cable should still dominate. Don't understand the follow-up... you mean a mono coil identical to one of the DD coils? Same R.

- Carl

[GT Blocker]

Thanks Carl...

Yep...that's what I meant...I kind of thought it would work that way, but things aren't always what I expect with this project!
Thanks!

[homefire]

GT; If both coils are Identical, I would assume the Damp R would be the same. Or close enough to work with. Jim

PS> Sorta hard to hand wound two coils that close I would think.

[GT Blocker]

Guess I asked the wrong question

Hey all,

I specifically stated 'hypothetically' as far as two coils being the same, so I guess all I accomplished was to confuse the issue.

So here's what I guess I meant:

I noticed that it was fairly easy to use a pot across the damping resistor to find the final value.
I'm in the mono coil setting on my HH atm, so....
I wondered if that same value would translate across if I moved that coil to the receive coil in a DD configuration. Which it appears it would. Cool.

Hope that makes more sense...
Thanks, GTB

[bbsailor]

A quick DD coil(s) question:
Given two identical coils (hypothetically speaking), would the damping resistance be the same for both, TX and REC?

The simple answer is no for the following reasons. While two identical coils would appear to require the same value of ramping resistor (Rd) for both the TX and RX, in reality they are loaded by different loads and see different capacitance values that require damping.

The TX coil of a DD coil set sees the coil driver MOSFET capacitance while the RX coil does not. The RX coil sees the value of the first amplifier input resistor (Rin) be effectively placed in parallel when the clamping diodes are conducting while the flyback voltage is oscillating down to about 0.7V.

As a follow up question:
If so, would the damping resistance change if one of the same coils was used as a mono coil?

Yes, but not much more than 100 to 200 ohms lower compared to the adjusted DD coil. The mono coil would need a lower resistance to compensate for the additional TX capacitance of the MOSFET. Remember, the more capacitance that the coil sees either within the coil itself, the coil shield, the coax capacitance or the virtual load of Rin all contribute to the final value of Rd.

Probably a dumb question, but I was wondering, thanks.

This is not dumb question but one that I have pondered myself. You can apply the detail information of making a fast mono coil in my atricle on this site and apply that to making a DD coil. First, the DD TX coil needs to be damped and then the RX coil. Typically, DD coils will operate a few microseconds faster than mono coils as the value of Rd will be a little higher thus allowing for a slightly faster coil and faster sampling.

Carl is right in stating that the coax is a large contributor to coil capacitance. In fact, the largest capacitance is contributed by the MOSFET COSS (output capacitance) and coax capacitance of about 25 pf per foot.

Once the value of Rd is adjusted for a particular coil here is an interesting little trick that will help you mentally visualize the relationship of the Rd value to the number of turns added or subtracted from any home made coil.

Consider that a coil is effectivly damped by an Rd value of X ohms with Y number of turns. Now multiply the value of Rd times the number of turns and visualize Y number of resistors, one per turn, all connected in parallel at one X * Y resistor value per turn which has the effective value as the initial Rd. Now add an additional turn and the add one more Y *Y resistor in parallel with the others. The Rd value is now effectivly lowered by the addition of one more resistor.

The values of Rd do not stay linear but are close to the same value when adding or subtracting one or two turns. Removing one coil turn effectivly removes one parallel resistor (in that mental model) for that turn which increased the effective value of Rd. In reality adding or removing on turn has only the effect of changing the value of Rd by 15 to 20 ohms.

DD RX coils typically have more turns (about 1.5 times the TX coil number of turns) in the RX coil to obtain a little more sensitivity.

I hope this helps?

bbsailor

[bbsailor]

Correction

"add one more Y *Y resistor in parallel with the others."

it should read " add one more X * Y resistor value in parallel with the others.

bbsailor