Measure the coupling at the distance you want to operate the coils to know where we are.
You will have to get rid of those parasitics because you do not know their values. Place output and input buffer (to reduce those 50 ohms from the source).
This works by impedance ratios. Can you make this work?
Note the source is 10 Ohms not 50. so the ratio of Load to source is 5:1 while coupling factor is 1:5 with a wide tolerance since it is low Q.
C1 on left controls 1st breakpoint and small C2 on right controls 2nd breakpoint with equal inductor coils.
Use low capacitance 10:1 probe to scope.
adverse variations
I am referring to the magnetic coupling factor "k" (not voltage ratio). https://www.daycounter.com/LabBook/Mutual-Inductance.phtml(1) how can I measure the coupling at the specific distance? Are you talking about measuring the peak to peak Vrx and Vtx?
You can use simple emitter follower. it depends on how handy does they come for you ..(2) For the buffers I can use, since they are dealing with pure AC signal, does that mean I need to use dual rail (for example, +/-5V vcc) opamp? It is OK for me to place one in the Tx coil, but it is hard to get one in the Rx side. How can I solve this in the output side?
There are different ways to describe the parameters of coupled inductors, and many ways to measure it. I don't think that you necessarily need a buffer for the measurement.
The problem is to decide which parameters you want to measure and to understand how can they be derived from your primary measurement values, e.g. voltage and current values or resonance frequencies.
you cannot use sig gen output, it must be buffered such as emitter follower with series R if you want to perform my design to match your specs, otherwise change your specs
The distance is fixed to 1 cm, or you can vary it as you want? You can vary the distance to where is best, or 1 cm is fixed?
What is the coupling factor at the distance you want to operate?
The distance is fixed to 1 cm, or you can vary it as you want? You can vary the distance to where is best, or 1 cm is fixed?
What is the coupling factor at the distance you want to operate?
In some case scenarios when the power coil is brought near the data coil, even when coils are coplanar, the coupling will influence the data coil. I guess some shielding would protect data coil and power coil but I know very little about that.It is good, but I am not confident if in some scenarios the noise coupling from the 6.78MHz power coil can impact the operation when data coil is between 1MHz to 5Mhz.
(1) do you think it could be a problem when my frequency separation between the data coil and power coil is only 6.78MHz-5Mhz=1.78MHz?
Additional filters after the receiver coil would not impede the receiver coil to pick up the 6 MHz noise. If you want to reject it after it was picked up in order to prevent it from reaching the other circuit stages, then I would say a passive band-reject filter is appropriate in this case, but make sure it is appropriately buffered in order to not disturb the receiver and the post stages (stages after the receiver).(2) Is there any quick way to modify this flat band circuits, by adding one or two poles around 6MHz to make a steep attenuation when the frequency is beyond 5MHz? So we will have much more attenuation in 6.78MHz. I was thinking of using the higher order active filter. However, it is difficult in processing the pure AC signal as its input when the opamp is supplied by 0V to 5V.
That would be an option if 13.56 MHz still performs well for your application.(3) If the frequency separation is not enough and no quick fix can be done. I am thinking to redesign my power coil and let it operate in 13.56MHz. From the existing bode plot, it has around 20dB attenuation between 5MHz and 13.56MHz, which looks like reasonable to serve as an LPF to reject the coupling noise from the power coil, is this correct?
In some case scenarios when the power coil is brought near the data coil, even when coils are coplanar, the coupling will influence the data coil. I guess some shielding would protect data coil and power coil but I know very little about that.
You may want to take a look at the Chapter 10, section 10.3 Shielding effectiveness : Near field sources, from the book Introduction to Electromagnetic compatibility by Clayton Paul.
Additional filters after the receiver coil would not impede the receiver coil to pick up the 6 MHz noise. If you want to reject it after it was picked up in order to prevent it from reaching the other circuit stages, then I would say a passive band-reject filter is appropriate in this case, but make sure it is appropriately buffered in order to not disturb the receiver and the post stages (stages after the receiver).
That would be an option if 13.56 MHz still performs well for your application.
Thank you CataM. I will read the book from Paul about the shielding.
Regarding your answer in question 3: theoratically, should we expect a 13.56MHz WPT introduce less noise coupling than a 6.78MHz for my case? If so, is it because we see much more attenuation in the bode plots?
Thank you!
It will introduce more attenuation in the frequencies in between 5 MHz and 13.56 MHz, but I think more or less the same at 13.56 MHz as it would be for the power coil operating at 6.78 MHz. It is because of the bode plot but not the Bode plot you have shown, instead, the Bode plot between power coil and data coil.Regarding your answer in question 3: theoratically, should we expect a 13.56MHz WPT introduce less noise coupling than a 6.78MHz for my case? If so, is it because we see much more attenuation in the bode plots?
I can not talk about shielding because I know very little about it. Maybe you should ask this on the RF section or Electromagnetic design section.Hi CataM, regarding the shield, if I am using 13.56MHz power coil, can I use the RF shield for RFID application for my board? I found a lot of RF shield for RFID application, which indicates 13.56MHz (such as “WE-FAS RFID Flexible Absorber Sheet” in https://katalog.we-online.de/en/pbs/WE-FAS-RFID?m=t&sq=rf+shield&sp=https%3A%2F%2Fwww.we-online.com%2Fweb%2Fen%2Fwuerth_elektronik%2Fsearchpage.php%3Fsearch%3DRF%2Bshield&_ga=1.249364743.10007329.1457366634). Do you think can I use this sheet around the power coil to minimize its interference with the other parts of the circuits?
It will introduce more attenuation in the frequencies in between 5 MHz and 13.56 MHz, but I think more or less the same at 13.56 MHz as it would be for the power coil operating at 6.78 MHz. It is because of the bode plot but not the Bode plot you have shown, instead, the Bode plot between power coil and data coil.
As you have seen by adjusting this coupled data coils, it will depend on both the power coil impedance, and data coil impedance how much of attenuation you will have.
I can not talk about shielding because I know very little about it. Maybe you should ask this on the RF section or Electromagnetic design section.
Thank you CataM. I am going to try bode test in a coplanar structure to how the AC injection in the power coil can cause the change of data coil. If there is no big difference in the Bode plot, can we conclude from the noise coupling perspective, either frequency in the WPT is OK? Thank you!
Unfortunately the hidden parts of your schematic contain the cause for the actually observed magnitude diagram (e.g. capacitive and possibly resistive load). As in previous diagrams, the generator impedance isn't shown.
I won't be able to derive anything from this measurement.
Not quite sure why you want high frequency power link although the coils are optimized for Qi 100 kHz range? 13.56 MHz is an internationally regulated ISM frequency may be preferred over 6.78 MHz, don't know what are the applicable standards in your region. Larger distance to 5 MHz eases selective filtering.
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