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.
One good reason is when the noise of the resistor would reduce the SNR significantly. This happens in applications where the desired signal is at lowish frequency and the instability you're trying to prevent is further out. The bead's loss produces Johnson noise just like a separate resistor, but since it appears in parallel with the inductive reactance, which is small at low frequency, it hardly causes any SNR degradation.
The nice thing about ferrite beads is that they have very low DC resistance - much lower than a resistor offering the same impedance at RF - and very low parallel capacitance, comparable with an L-trimmed surface mount resistor, and appreciably less than a helically trimmed axial-leaded resistor.
They are also lossy at high frequencies, and tend not to have the embarassing self-resonance that you can get with a wound inductor. Keep in mind that they can have enough inductance to resonate with a filter capacitor at some frequency low enough that the losses in the ferrite don't damp the resonance. I got caught that way once, but early enough in the development process that it wasn't too embarassing.
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.
Deep Friar: a very philosophical monk.
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.
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.
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.
Deep Friar: a very philosophical monk.
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
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).
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.
On a sunny day (Sun, 27 Dec 2009 00:33:06 -0600) it happened "Tim Williams" wrote in :
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.
Frankly, it's more like wearing garlic to keep away vampires. When you suspect that a circuit will have noise coupling problems on Vcc rails, or that an opamp might rectify RF from the outside world, or emit RF and fail compliance testing, you design in some beads and some bypass caps. When it works, you congratulate yourself, and never know if it was really necessary. To be fair, most of us have found real situations where beads helped, and have applied them liberally since.
Surface-mount beads are cheap insurance, a few cents each, and can be used to isolate and/or measure supply currents, an added bonus.
Lots of opamps have ghastly supply-noise rejection at high frequencies. We recently got bit by one that had *gain* from V- supply to output, datasheet to the contrary.
We generally use ferrites and big ceramic caps to isolate low-level stages like photodiode amps, and use beads or monolithic filters (essentially beads+caps in a block) between the outside world and low-level stages that might rectify RF.
This doesn't analyze very well. Part models usually do a very bad job of simulation IC supply behavior, both current noise kicked out and supply rejection. And wiring/PCB resonances are major EMI issues, which Spice doesn't model well even if you can furnish the correct circuit. About all Spice can do is verify the gross rolloff and check for low-frequency resonances. We usually don't bother.
A few of us have found situations where they really hurt, too. I don't use them in supplies unless there is a reason to suspect problems.
We use 0-ohm resistors in front of all regulators. We recently replaced a few of them with beads when one of the other regulators (a "brick") was spewing 200MHz everywhere. The beads at least got it out of the other supplies.
You betcha. We're having problems finding a decent headphone driver now. The one we're using, a RoHS replacement, seems to detect 2.4GHz quite well. While we can't see anything, I suspect it's the input bipolars rectifying the 2.4GHz. It could also be on the power supplies, or both. Though we've put "y" supressors on the power supplies with limited success (sometimes it's there, sometimes not).
Yes, it's basically "FM", particularly when the carrier can't be seen.
As a rule of thumb anything longer than about 1/10 wavelength should be treated as a transmission line section. At 2.4 GHz, the free space wavelength is 13 cm, so 1/10 wavelength would be 1.3 cm and considering the velocity factor on conventional PCB materials, structures larger than 1 cm should be treated as transmission line sections.
Even if the device is intended for DC/audio applications, the EMC filtering should be designed as an RF circuit.
With GSM the trouble is more in the negotiating sequence where you see multiple bursts of carrier. This gets rectified even in circuits where you thought they'd never "see" 1.8GHz stuff. But they see enough to cause the rectified rat-tat-tat in the signal. All it takes is one BE junction, somewhere in the path.
Beads are typically not terribly effective up there, I use RC or dedicated GHz-mufflers such as the Murata line for that.