By measuring the amplitude and angle of the conducted RF, thru the bead, you can determine the L and R of the bead. An oscilloscope can be used for this but a VNA is a better method. At any rate, the reduction in radiated energy is due to the reduction in antenna current flowing on the cables and such and not so much that the bead "absorbs" the RF. An example is when the number of beads is doubled. We can use the example of a zero-impedance RF source. The RF current is halved and the radiated energy is reduced by a factor of four. The losses in the beads is also reduced by a factor of four. So, we have a reduction in radiated energy yet the beads are "absorbing" less RF energy themselves. Dave Cuthbert Micron Technology -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Ken Javor Sent: Thursday, December 18, 2003 7:40 PM To: Price, Ed; 'Grasso, Charles'; '[email protected]' Subject: Re: Measuring a ferrite performance
As usual, Ed's advice is practical and economical to implement. I would add the following to make it more comprehensive. Ed's test measures insertion loss. You can calculate impedance from that. At frequencies where the impedance is high Ed's test works well. At frequencies where the impedance is low, it does not provide sufficient accuracy. For low impedances, connect a wire from center conductor to ground with the bead in place around this ground wire. When the bead adds no impedance, the wire acts like a short to ground and you see little signal. As the bead impedance begins to increase, the short becomes a higher impedance short and you get more signal at the other end. Again, from this you can calculate the bead impedance. Either of these techniques becomes questionable at frequencies where the bead acts like an absorber rather than a reflective mismatch. You will see loss >from the bead, but I'm not sure it can be related to a specific impedance when the bead is absorbing the rf. I think here you might have to carefully measure current into the wire with and without bead. The difference would be vswr-related and again could be used to determine impedance. From: "Price, Ed" <[email protected]> Reply-To: "Price, Ed" <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Thu, 18 Dec 2003 11:42:53 -0800 To: "'Grasso, Charles'" <[email protected]>, "'[email protected]'" <[email protected]> Subject: RE: Measuring a ferrite performance -----Original Message----- From: Grasso, Charles [mailto:[email protected]] Sent: Thursday, December 18, 2003 9:40 AM To: '[email protected]' Subject: Measuring a ferrite performance Greetings all: I have a requirement to test the performance of ferrite bead (one that that can fit over a power cord) to 500MHz for qualification of alternate vendors. I have an impedance analyzer - but it only goes to 40MHz. Does anyone have a good way to verify ferrite impedance.?? Thanks Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: [email protected]; Email Alternate: [email protected] Chuck: Assuming that you have a signal generator and a spectrum analyzer, you could try the following: 1. Get a small RF project box. Mount two N or BNC or SMA coax connectors on opposite walls of the box. Solder a short wire directly from one coax connector center pin to the other. 2. Decide on system impedance. If 50 ohms is OK, then just connect a 3 dB attenuator to each of the coax connectors. If you want another impedance, then put appropriate resistors (carbon comp preferred) into the project box. 3. Connect the signal generator to one attenuator, and the spectrum analyzer to the other. 4. Now set the generator to 0 dBm and sweep or step the frequency range, noting the spectrum analyzer response. 5. Now open the box, break the wire, thread the bead onto the wire, and re-solder the wire. 6. Repeat step 4. The amplitude differences will be the insertion loss due to the bead performance. 7. Repeat steps 5 & 6 with any other bead samples. This isn't an exhaustive test, but it's a good A to B comparison of RF loss. Ed Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty
