I there a minimum distance needed between the beads and the RF transceiver?

Like below ... the connections do not really matter, just if there are requirements regarding the distance between the bead and a nRF24L01 transceiver?

Through the bead MIDI data is send/received (31250 bps). I draw only 1 bead, there will be 18.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ @kva: I updated it with the circuit (official circuit, looks like mine). However, the beads are in between the MIDI connector and the microcontroller (so inside the box). \$\endgroup\$ – Michel Keijzers Jun 29 '17 at 11:07
  • \$\begingroup\$ If it isn't exactly your circuit then it tells nothing. Post your circuit and not some impressionist version. \$\endgroup\$ – Andy aka Jun 29 '17 at 11:24
  • \$\begingroup\$ @Andy aka ... I think it's not even related to the circuit ... it's more related if the distance between the transceiver and the beads are important, or do the beads only affect the cables through it? Not the receiver (antenna) which might be an inch (or more or less). \$\endgroup\$ – Michel Keijzers Jun 29 '17 at 11:48
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    \$\begingroup\$ Coupling is proportional to TWO key factors: rate-of-change and area. Your MIDI data is slow, with "slow" edges. And your area (of the victim or receiver of energy) should be very small, what with GND planes under the various radio PCB traces. Assume 100nS edges on MIDI data, charging 100pF over 5v. The current is 5mA. We'll assume the rise time is 50nS. Assume your loop area (in the radio) is 1mm by 1mm, at distance of 10mm. The induced voltage is 2e-7 * Area/Distance * dI/dT = 2e-7 * 1e-4 (A/D) * 10^5Amp/sec = 2 microvolts, at the MIDI data rate. \$\endgroup\$ – analogsystemsrf Jun 29 '17 at 15:25
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    \$\begingroup\$ The 2uV was the magnetically-induced voltage, at the MIDI bitrate, into your radio. \$\endgroup\$ – analogsystemsrf Jun 29 '17 at 16:59

The purpose of the beads is to keep common mode RF current off of the control lines.

The first line of defense is to place the beads at the transmitter end. However, depending on line lengths, shielding, grounding, termination impedances, and transmitter antenna placement, common mode currents can still be introduced into the interconnections. A best practice is to provide suppression at both ends of the cable.

It should also be noted that the ferrite material selection is important. It must have adequate suppression properties at the frequency of operation. But care must be taken that it does not provide any significant suppression at the frequencies used in the control cable.

  • \$\begingroup\$ Thanks, in my case I have it not in the cable (never seen beads in a MIDI cable), but in the device I'm going to make. I don't know if it is in the device connected to it (which can vary too). I checked the frequencies of operation and that should be ok. So let's hope it will work out well. \$\endgroup\$ – Michel Keijzers Jun 29 '17 at 13:33
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    \$\begingroup\$ The beads go just before the cable connector on the transmitter. Good luck with the project. If you have problems, open this thread again. There are more advanced methods that can be applied if necessary. \$\endgroup\$ – Glenn W9IQ Jun 29 '17 at 13:34
  • \$\begingroup\$ Thanks (again) ... it can take some time ... I'm new into electronics, and my (semi hobby) project 'boundaries' are getting bigger and bigger. But as a learning experience I want to add as much as possible 'good way of working' without making the project completely out of control. But some EMI/EMC protection is good (since I want to use it in life performances. \$\endgroup\$ – Michel Keijzers Jun 29 '17 at 13:37

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