Coil Front End

[Daves]

Hello,
What I want to ask about the RX coil;
1) Why are R1 36ohm and C1 650nf added? What is the intended use?
2) Why are RX and TX combined with C2 22nf?
3)How is the connection made from TX to ground? Do you scrape the enamel and add a wire from there to ground?
Note: 30cm DD Coil for Garrett Ace

[kt315]

this is NOKTA? what is it?

[Daves]

this is NOKTA? what is it?

The person who shared it makes DD coils for Garrett Ace.

[Carl-NC]

1) Looks like the cap is for series-resonating the coil. The resistor might be a PTC-type to compensate for TX phase shift over temperature.
2) Don't know
3) I think it indicates that TX(-) connects to the shield. It connects to ground via the cable to the circuit.

[Daves]

1) Looks like the cap is for series-resonating the coil. The resistor might be a PTC-type to compensate for TX phase shift over temperature.
2) Don't know
3) I think it indicates that TX(-) connects to the shield. It connects to ground via the cable to the circuit.

Thank you Carl, which filter should be used in the first opamp that RX enters? BPF or LPF and how should the range of these filters be chosen? For example, should it be 1-3khz above the operating frequency for LPF? Or should it be at a different value?

[Carl-NC]

Your choice, really. Look at the VLF schematics that have been posted, some use LPF, some use BPF, break points are all over the place. I would start with LPF and a generous BW, then later look at alternatives when you have a good way to measure performance.

[Daves]

Your choice, really. Look at the VLF schematics that have been posted, some use LPF, some use BPF, break points are all over the place. I would start with LPF and a generous BW, then later look at alternatives when you have a good way to measure performance.

What solution do you suggest in analog terms for the noise that will occur due to coil resistance?

[Carl-NC]

The noise from coil resistance should not be an issue.

[Daves]

The noise from coil resistance should not be an issue.

Carl, can you give some information about the resistance and capacity usage here? filter, etc.

[Carl-NC]

First, let's draw a decent schematic:


The coil is series-resonated by C2, the resonant frequency depends on the inductance of the coil. If, for example, the RX inductance is 2mH then resonance is 24kHz. R2 provides a load for the resonant circuit and sets the Q. C3 and R2 create a high frequency pole at 177kHz, probably to help suppress noise. R1 & C1 form a pole at 142kHz but I don't know the exact purpose, might be some kind of phase compensation.

The opamp gain appears to be 15k/47 = 319 but it's not. The feedback pole (R4*C5) is at 5.3kHz and the feedforward pole (R3*C4) is at 22.6kHz. Since the feedback rolls off before the feedforward pole breaks, the peak of the response occurs at about 10.9kHz (the geometric mean) and is roughly 12dB lower than 319, or about 80. We normally don't set the peak of the preamp response equal to the operating frequency so I'm guessing this preamp design is used on a detector that runs somewhere less than 10kHz.

If you are trying to understand circuits like this, I urge you to start with something simpler like this:


and work your way up.

[Daves]

First, let's draw a decent schematic:


The coil is series-resonated by C2, the resonant frequency depends on the inductance of the coil. If, for example, the RX inductance is 2mH then resonance is 24kHz. R2 provides a load for the resonant circuit and sets the Q. C3 and R2 create a high frequency pole at 177kHz, probably to help suppress noise. R1 & C1 form a pole at 142kHz but I don't know the exact purpose, might be some kind of phase compensation.

The opamp gain appears to be 15k/47 = 319 but it's not. The feedback pole (R4*C5) is at 5.3kHz and the feedforward pole (R3*C4) is at 22.6kHz. Since the feedback rolls off before the feedforward pole breaks, the peak of the response occurs at about 10.9kHz (the geometric mean) and is roughly 12dB lower than 319, or about 80. We normally don't set the peak of the preamp response equal to the operating frequency so I'm guessing this preamp design is used on a detector that runs somewhere less than 10kHz.

If you are trying to understand circuits like this, I urge you to start with something simpler like this:


and work your way up.

Thanks for information Carl,

what I want to ask;
*This detector works at 14khz.
*Series resonance capacity does not seem to be the case. The sine at the RX input is broken, but there is a 22nf cap parallel to the input. When you add it, the sinus improves.
*I set up the same input circuit and use the same coil, but when I examine the Rx raw input, I cannot see the same result. Parallel 22nf cap. After adding noise present on the sine what could this be causing?
*How are phase shifting and gain on feedback determined? What should be the lp filter value on the feedback?
*What is the high frequency polar filter used for?
*Used for temperature-induced changes
*Is R1 and C1 a hpf?
*How to calculate opamp peak point?
*How about Opamp Gain Bandwidth calculation?
*What should be included in the Rx front end design?
*In addition, when I increase the gain in the opamp after the analog demodulator, I see worse performance (loss of depth) even though it should perform better (expecting a deeper response).