Coils capacitors

[Daren]

I will need to build a IB DD coils. There are tunning capacitors: parallel to TX coil and RX in series with the coil. The question is: is it better to put capacitors into the coil housing so they will be right with the coils and not separated by connecting 4-5 feet cable or they can be with the MD PCB thus separated from coils by connecting cable. I would like to keep the caps with the PCB so I can tune coils to different freq if needed.
I'm just wondering if keeping them close to coils will give better results.

[Davor]

Keeping capacitor(s) at coil has advantage of minimising current in cable. Oscillators provide pulses for sustaining oscillation in order of ~10mA, while current in a tank is much larger ~100mA. So if you have a cable in between a coil and capacitor there are larger currents than in case coil and capacitor are together. It also facilitates using multiple coils if every coil is properly tuned on its own.
You'll find no benefit in tuning coil frequency in any span less than an octave, as the targets change their responses very little over such span of frequencies. You may, however screw coil tuning instead, so I'd vote for fixed frequency.
If you are buying your coils, just forget about what I wrote.

[Skippy]

There may be thermal benefits to having cap and L together in the same box. The trouble is, you need to know the thermal behaviour of the L in the case in order to choose your cap, which may be viable for mass-manufacturers, but hard for an amateur.

[kt315]

is it better to put capacitors into the coil housing

there are two types of the coils -
- tesoro idea, using a culpit generator and you can not re-move the cap from generator in coil housing, the TX cap is at board always
- whites idea, both case - at board and in housing, but whites is MASS manufacturing, easely to tune all coils on special stand, (for example - on 6.59kHz)
to tune the TX inductivity is some kinda nonsense leaving the cap at a board.
this is why the TX cap is INSIDE the housing.

so... WHAT you mean? WHAT type of generator you want to do the coil to with??

[Daren]

is it better to put capacitors into the coil housing

there are two types of the coils -
- tesoro idea, using a culpit generator and you can not re-move the cap from generator in coil housing, the TX cap is at board always
- whites idea, both case - at board and in housing, but whites is MASS manufacturing, easely to tune all coils on special stand, (for example - on 6.59kHz)
to tune the TX inductivity is some kinda nonsense leaving the cap at a board.
this is why the TX cap is INSIDE the housing.

so... WHAT you mean? WHAT type of generator you want to do the coil to with??

The coil is for Fortune-M; There is no resonant TX generator; coil is driven by DAC signal; frequency is selectable from a menu; you adjust frequency until coil resonant freq is found (max current usage at this point) and that is the frequency that is used. target TX is about 8KHz; RX is tuned to about (TX freq - 1.2KHz)

[Daren]

Keeping capacitor(s) at coil has advantage of minimising current in cable. Oscillators provide pulses for sustaining oscillation in order of ~10mA, while current in a tank is much larger ~100mA. So if you have a cable in between a coil and capacitor there are larger currents than in case coil and capacitor are together. It also facilitates using multiple coils if every coil is properly tuned on its own.

That what I was thinking... Why separate coil and tuning capacitor when they are designed to work in resonance.

[Davor]

Most people here use commercial coils, and many of those do not have a capacitor and you have to adopt their constraints.

[kt315]

The coil is for Fortune-M; There is no resonant TX generator; coil is driven by DAC signal; frequency is selectable from a menu; you adjust frequency until coil resonant freq is found (max current usage at this point) and that is the frequency that is used. target TX is about 8KHz; RX is tuned to about (TX freq - 1.2KHz)

boom boom boom... really the case forcing my mind thinking all pro and contra... the case is not trivial. if you stay the cap at board you can not
stay any firm coil from whites, fisher, bounty hunter, teknetics, garrett (or can but the TX frequency will be decreasing. you need in experiment), however you can easely use all tesoro ones.
so your choice is what you prefer.

[Daren]

I tested with and without capacitors in the coil:
Caps in the coil: RX signal much noisier; could null coil down to about 20mV
Caps on the far end of coil cable from the coil: much less noise on RX, can null coil down to 2mV
I'm keeping caps out of the coil.

[WM6]

Solution is EMI shielded cap in coil.

[Gwil]

I tested with and without capacitors in the coil:
Caps in the coil: RX signal much noisier; could null coil down to about 20mV
Caps on the far end of coil cable from the coil: much less noise on RX, can null coil down to 2mV
I'm keeping caps out of the coil.

Maybe the problem was due to capacitive coupling between the Tx and Rx cores in the cable. That can contribute a signal that can't be removed by adjusting the coils because it's a different phase. A good reason to use screened Tx and Rx cores.

[Daren]

I shielded and moved caps back to coil. I was able to null the coil back to 2mV P-P; Scotch 24 was used to shield caps and the coil. Interesting that even when caps' shielding was not connected to ground it seem to work, TX cap shielding may not have been necessary, but both TX and RX caps are now shielded with shielding grounded.
USB 2 cable was used for connection; It's surprisingly nice cable; There is significant difference between wire pairs; TX is used with USB power pair (black/red); RX with signal pair (white/green).
The cable with each pair shielded individually would have been better, but this may be just good enough