28 and 35cm Coil Calibration and Ferrite Zerro

[Daves]

Hi,

For a single frequency (14khz) metal detector that works with direct sampling,

We produce 28cm and 35cm coils.

We try to keep the tx inductance of both coils as close as possible, but they cannot be 100% the same. But I have a problem;

The raw phase angle response given to the ferrite by the 28cm elliptical coil and the 35cm coil is different, and therefore I cannot make a standard ferrite adjustment.

For example:
When I make phase adjustment using a 28cm elliptical coil, I get 0 for the ferrite, but when I pass the ferrite coil using a 35cm dd coil, I get a raw phase response of 177 degrees, that is, there is a 3 degree difference between the two coils.
How should I solve this?

28cm elliptical dd coil values;
(LCR Meter 10khz)

TX:
Lp:613uH
Q:17.3
Rp: 662 ohm
X:38.3 ohm

RX:
Lp:6.45mH
Q:9.076
Rp: 3.68 kohm
X: 392 ohm

35cm circle DD coil values;

TX:
Lp:618uH
Q:15.15
Rp: 587 ohm
X:38.6 ohm

RX:
Lp:6.60mH

[dbanner]

6.6mH and 6.45mH will give slightly different ferrite reject points.

Tune them both to exactly identical frequency.( I am assuming that your Rx coils are tuned parallel LC) You will need to tweak the C for one of the Rx coils with variable C trimmer which will be with the coil permanently. Make the resonance the same as other Rx coil.

[Carl-NC]

What ferrite are you using and how is it oriented with the coil?
What is your master phase reference, the TX voltage or TX current?

[gurdal]

Hello everyone,
According to the information in the picture, can you support me with the capacitor values required for the tx and rx operating frequency that you can recommend for the admiral classic circuit?
Also, my master dbanner gave some information about Tx and Rx resonance. Could you explain it step by step?
Thank you very much
Best regards

[Daves]

What ferrite are you using and how is it oriented with the coil?
What is your master phase reference, the TX voltage or TX current?

TX Voltage using for reference.
I am using this ferrite. I check the phase response of the ferrite by pumping a little bit above the coil.



28cm Coil Values;
TX:
Lp: 610uH Q: 17 Rp: 660ohm X: 38ohm
----------------------------
RX:
Lp: 6.33mH Q: 8.6 Rp: 3.42kOhm X: 390ohm

Big Coil Values;
TX:
Lp: 583uH Q: 14.7 Rp: 542ohm X: 36.5ohm
----------------------------
RX:
Lp: 6.56mH Q: 7.7 Rp: 3.20kOhm X: 405ohm

Note: Hantek LCR Meter 10kHz

The phase difference between the two coils is around 2 degrees for ferrite. This causes something like this: I cannot detect the most precious metal. Because there is already a phase difference of about 2 degrees between the most precious metal and the ferrite. I can detect it on whichever coil I adjust it to, but not on the other.

[Carl-NC]

Normally there is a 90° phase shift between the TX coil current & voltage but coil resistance alters this a little. From you coil numbers I can see a potential difference of 0.5° but not 3°.

Also, if one coil produces 0° and another 177° it sounds like one of the RX coils is connected backwards. That is, if you have a 3° error then the ferrite phase should be 3° or 357°. not 177°.

[Skippy]

I suggest lowering the TX Q-factor of the higher Q coil ( 28cm ) so that it matches that of the larger coil. Do this by adding a resistor in parallel with the TX winding.