When are ferrite beads the most approproate component to use?
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From: Paul Keinanen on 27 Dec 2009 02:43 On Sun, 27 Dec 2009 00:33:06 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"mook johnson" <mook(a)mook.net> wrote in message >news:4AAZm.358$yy2.59(a)newsfe01.iad... >> For lowish frequency stuff (say sub-10MHz) in what applications would a >> ferrite bead be the bees knees? > >AFAIK, none. They're for the >10MHz range, and ~100MHz decade especially. >That doesn't mean you don't want them; if your circuit is sub-10MHz and you >want to *keep it that way*, you might need some LCs for filtering. At least since the introduction of GSM mobile telephones (which is TDMA and hence large amplitude variations), it has been quite clear that in order to survive in the real world, the design of electronic equipment must address the EMC issues. For instance a non-inverting op-amp amplifying the signal from an external DC/LF sensor can suffer from RF-signals picked up by the external wiring. The open loop voltage gain for the whole op-amp drops below 0 dB at quite low frequencies (a few MHz), so the feedback does not help. However, the input transistor in the input differential pair can have a quite significant fT, so a strong RF signal entering the non-inverting input gets amplified and driving the differential input stage into saturation on one or both half cycles, causing DC bias (which is harmful for DC amplification) or if the signal is amplitude modulated, will cause audio rectification. For this reason, it is very important even for DC amplification stages to keep any RF out of the circuit. To keep the 800-2500 MHz signals out of the circuit, ferrite beads are nice in order to increase the series impedance of the source at those frequencies and make it possible for the bypass capacitors to do their job properly. Of course, good quality surface mount capacitors with proper PCB RF layout should be used for the signal entry, even in DC/LF equipment. However, these capacitors are useless, if the interference source impedance is extremely low, so it is important to increase the source impedance artificially with a series impedance e.g. with ferrite beads. >Story: the college radio station antenna is right on top of my dorm. You >know, that station that no one wants to listen to. Well, despite being FM, >it's forcing its way into my powered speakers. Either there's enough AM on >it as-is, or it's so strong that it's being discriminated despite the low >efficiency. Whatever the cause, I added ferrite beads and ceramic caps on >all inputs, which shut it up quite excellently. If this was a vertically polarized antenna, the radiation pattern has a null directly downwards. To reach your room, the signal would have reflected from the surrounding buildings, creating a large number of multipath nulls (selective fading). With one such null at or close to the transmitter frequency, the received signal amplitude will change depending the instantaneous frequency, which varies with the audio waveform. Trying to find the reason for the interference to a repeater receiver located in a tower with lot of antennas, hooking the spectrum analyzer directly to the repeater receiver antenna showed that all FM broadcasts stations (vertical polarized antennas about 20 m higher) had a nice spectrum, but one showed quite irregular spectrum across the FM broadcast signal. I intended to call the station to check their transmitters, but fortunately hooked a short wire to the spectrum analyser to pick up the signal in the equipment room and the spectrum was clean also for this station. Selective fading (multipath) can convert even some constant amplitude signals into amplitude variations, which could cause audio rectification in audio equipment. Some old Revox FM-receivers even had an oscilloscope CRT driven by the audio voltage (instantaneous frequency deviation) on the X-plates and the corresponding RF/IF signal strength on the Y-plates. With no multipath, there would be a horizontal line, but with multipath, the IF-amplitude varied within the FM-channel.
From: Tim Williams on 27 Dec 2009 04:27 "Paul Keinanen" <keinanen(a)sci.fi> wrote in message news:ma1ej51jo38s7s2utdfa28f1tpci4uevnj(a)4ax.com... >>Story: the college radio station antenna is right on top of my dorm... > > If this was a vertically polarized antenna, the radiation pattern has > a null directly downwards. To reach your room, the signal would have > reflected from the surrounding buildings, creating a large number of > multipath nulls (selective fading). With one such null at or close to > the transmitter frequency, the received signal amplitude will change > depending the instantaneous frequency, which varies with the audio > waveform. <snip> > Some old Revox FM-receivers even had an oscilloscope CRT driven by the > audio voltage (instantaneous frequency deviation) on the X-plates and > the corresponding RF/IF signal strength on the Y-plates. With no > multipath, there would be a horizontal line, but with multipath, the > IF-amplitude varied within the FM-channel. Oh, so multipath causes AM? That's neat... I'm about 70 feet (~23m?) below the antenna, which I think is a vertically oriented dipole. I assumed near field was either blasting its way down the building, or conducting through various dubiously grounded structures (I have reason to believe the steam radiator along the outer wall carries more RF than the nearby outlet). Oh well, in the city there's certainly no shortage of reflection paths. There's an apartment building the same height just a block away. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Paul Keinanen on 27 Dec 2009 05:46 On Sun, 27 Dec 2009 03:27:03 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: > >Oh, so multipath causes AM? That's neat... In a very simplified example e.g. with a multipath caused by a single ground reflection, the received signal strength resembles the frequency response of a comb filter http://en.wikipedia.org/wiki/Comb_filter Unfortunately, the diagrams in this article are drawn on a linear scale, but when drawn on a decibel scale, the nulls can be quite narrow and very deep, so the amplitude response over the 200 kHz wide FM channel can vary tens of decibels, hence the constant amplitude FM signal will contain both amplitude as well as frequency modulation at the receiver. This is very similar to using an AM receiver to receive narrow band FM signal by slightly detuning the receiver (slope detection). >I'm about 70 feet (~23m?) below the antenna, which I think is a vertically >oriented dipole. I assumed near field was either blasting its way down the >building, or conducting through various dubiously grounded structures (I >have reason to believe the steam radiator along the outer wall carries more >RF than the nearby outlet). Oh well, in the city there's certainly no >shortage of reflection paths. There's an apartment building the same height >just a block away. The most likely path is through reflections from the surrounding buildings, especially if the received signal strength varies greatly by moving the receiver a few centimeters.
From: Jan Panteltje on 27 Dec 2009 07:41 On a sunny day (Sun, 27 Dec 2009 00:33:06 -0600) it happened "Tim Williams" <tmoranwms(a)charter.net> wrote in <NaDZm.13709$wC3.8147(a)newsfe07.iad>: >"mook johnson" <mook(a)mook.net> wrote in message >news:4AAZm.358$yy2.59(a)newsfe01.iad... >> For lowish frequency stuff (say sub-10MHz) in what applications would a >> ferrite bead be the bees knees? > >AFAIK, none. They're for the >10MHz range, and ~100MHz decade especially. >That doesn't mean you don't want them; if your circuit is sub-10MHz and you >want to *keep it that way*, you might need some LCs for filtering. > >Story: the college radio station antenna is right on top of my dorm. You >know, that station that no one wants to listen to. Well, despite being FM, >it's forcing its way into my powered speakers. Either there's enough AM on >it as-is, or it's so strong that it's being discriminated despite the low >efficiency. Whatever the cause, I added ferrite beads and ceramic caps on >all inputs, which shut it up quite excellently. > >Tim Flank detection, if your system has a non-even frequency response in the tranmitter range (likely) then a FM signal will cause a changing bias because of RF detection due to non-linearity in the input stages.
From: mook johnson on 27 Dec 2009 08:15
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:sbndj5hvn62d2je8jo8tkvoi0scv9tvkb0(a)4ax.com... > On Sat, 26 Dec 2009 21:33:16 -0600, "mook johnson" <mook(a)mook.net> > wrote: > >>I've never found an application where a I felt a ferrite bead as head ahd >>shoulders over a resistor or some other means filtering the signal. Most >>of >>the develpments Im involved with are power conversion and data acquisition >>(ADCs and DSPS)from various sensors. >> >>For lowish frequency stuff (say sub-10MHz) in what applications would a >>ferrite bead be the bees knees? >> >>Just want to make sure I'm not overlooking a useful electrical component. >> >>thanks guys. >> >> > > They're great for keeping high frequency stuff out of opamps and such. > They have low DC resistance, so don't mess up low frequency circuits, > and they have miserable Qs, so kill resonances. > > ftp://jjlarkin.lmi.net/Ferrite.JPG > > This bead reduced the RFI sensitivity of a thermocouple front-end by > 20 dB or so, specifically by killing some wiring resonances in the > 100-300 MHz range. > > Surface-mount beads are also good, cheap power supply decouplers. Most > of them act like lossy inductors in roughly the 5 uH range. > > John Thanks John, What processes do you use to predict the action of applying a ferrite bead. Is is all cut and try? Can you spice it in a fairly straight forward way? Any other analysis techniques? |