Birdies on the Wing: Questions about Clock Dithering and the FAA


Is power supply/class-D clock dithering sufficient reason for the FAA re relax regulations about passenger equipment use during critical flight segments?



Several years ago, when I wrote about a new product – a Class-D amplifier for headsets that relied on the inductance of the output cable to filter out the (dithered) harmonics of the device’s clock signal, I got an email from a reader, a senior RF guy, who took issue with the whole business of clock dithering. 

As closely as I can recall the email, his beef was that we were, in general, “trading a few stationary birdies for a general rise, globally, across the whole noise floor.”

I keep thinking back to that email, especially when I hear about the FAA relaxing its rules about in-flight use of portable devices.

Here’s where I need help:  I’m looking for readers’ comments on how seriously to take this issue.  By myself, I know just enough to research and write reasonably interesting articles. I have an EE, I used to have a First Phone (before the FCC made that license obsolete), and I have an Amateur Extra -- all of which give me just enough of an RF vocabulary to ask semi-intelligent questions. 

So here’s one: What if 300 people on an airplane are all using multiple gadgets, each of which raises the noise floor by just a little bit?



Here’s some background.  We can talk about clocks in digital circuits, switch-mode power supplies, and certain audio amplifiers.  In switching power supplies, the clock controls the rate at which the switching devices turn the current on and off. Class D amplifiers resemble power supplies in which the (supersonic) duty-cycle of the clock is varied as a function of the audio input. (Then the clock is filtered out, leaving the amplified audio.) 

In both cases, the catch is that clock waveforms are essentially square waves; they create and radiate odd harmonics of their fundamental frequency.



Among the myriad of Federal Communications Commission (FCC) regulations, there is one, Title 47, Part 15, Subpart B (Title 47 of the Code of Federal Regulations, Unlicensed Transmissions, Unintentional Radiators).

In essence, it says that if you want to build something that could generate RF interference, you have to test it, and, even though it’s not a radio per se, the RF it emits it still has to be below certain limits.

Paragraphs 15.109 of the document contain the radiated interference limits. Subparagraph (a) says, “(a) Except for Class A digital devices, the field strength of radiated emissions from unintentional radiators at a distance of 3 meters shall not exceed the following values:


Frequency of Emissions (MHz)

Field Strength








Above 960




There is also a standard for the actual process of measuring Part 15, Class B devices: ANSI C63.4-2003, “American National Standard for Methods of American National Standard for Methods of Measurement of Radio- Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz,” It’s essentially about test equipment and procedures.



Apparently, what’s been inferred from these documents is that it’s okay to time-average field strength measurements.

Thus, if the harmonics being observed come from a clock whose fundamental frequency is constantly being swept across a certain range, the time-averaged value is less than any peak value.  (At least, that’s how it has been explained to me many times.)

From which, I infer that, in some instances, peak values might have busted the spec if they stayed at one frequency. I also infer that because there are some dithering profiles that actually have IP patent protection.



This brings me back to that email from the unhappy RF engineer. He was concerned about what happens when you rob Peter to pay Paul. (See the figure)

From a static standpoint, dithering in effect reduces the amplitude of clock harmonics while raising the overall noise floor.

“Birdies” is an old term.  I encountered it in my earliest ham days, when they were just whistles in my superhet receiver caused by the local oscillator or some other piece of equipment.  You tuned them out with a variable audio filter, just like any other interference, and you got sore if they were covering up that guy on St. Helena you’d been trying to get in your log.

The noise floor was something different.  If that guy in St. Helena was buried down under the noise, that was it.  That feeling about the noise floor is also backed up by some of my early experience writing qualification-test reports at aerospace companies.



Should I be worried about this stuff?  I accept that GPS is already working below the noise floor, thanks to the magic of autocorrelation, but is that good enough to deal with a Jumbo full of people, each with three or four pieces of personal electronics yodeling away during a tricky approach?  Please tell me somebody really did some worst-case susceptibility testing with a bench full of laptops and cellular phones.  If they did, what kind of safety margins did they find? 

Thanks for your help.

Discuss this Blog Entry 9

on Oct 9, 2013

Don, my background is similar to yours but I never got the ham ticket. One of the reasons everyone's so concerned about GPS is even on the large jetliners due to drag issues there's usually only one GPS-specific antenna and LNA for the whole aircraft, so even when there's redundancy by having multiple GPS receivers they'll probably still get the same degree of signal contamination. The other systems that would be likely to suffer would be TCAS, UAT and ADS-B but I don't "have the numbers" to demonstrate how susceptible they could be, nonetheless it's clear that missing or misinterpreting the position of any large traffic either in the air or on the ground could cause significant safety issues and potentially an accident involving substantial loss of life, and it could take that to make folks think how important finishing that online "Words with Friends" game really is.

on Oct 9, 2013

The thing is, we don't know anything about the quality of shielding on the devices. They might have been adequately shielded when they were certified/qualified, but does each production device have adequate shielding? What happens if someone drops their device and it ruins the shielding? If there is a little interference, does it really matter? We are dealing with statistical probabilities, not certainties, so human judgement has to be applied. The safest thing to do is to prohibit the use of electronic devices on planes.

Cell phones are a different story. If used in the air, they mess with the networks on the ground. Imagine 300 cell phones passing over Enid, Oklahoma at 550 knots. Wouldn't that swamp Enid's cell towers? Then those 300 phones would get jerked to a different tower as the plane proceeds. Planes could have satellite-based cell repeaters installed to prevent this. HOWEVER, I sure don't want to listen to idiotic cell phone conversations for the duration of flights, and people sure need to drop their phones and pay attention to flight attendants at times!

on Oct 14, 2013

Real world verse testing: Imagine you have ten pieces of equipment, each susceptible at a unique frequency. If your netbook emits on one frequency, you may interfere with one of those ten devices: if it dithers its clock(s) you can 'hit' each and every one of those ten devices albeit intermittently. If they are designed to recover and the recovery is fast enough, MAYBE it doesn't matter.?. Dithering is designed to pass a test: a slow moving narrow bandwidth-averaging filter, period.
Too often, I have seen things that should not interfere, interfere. Distrust 'tests' and listen to real life experiences. Turn the stuff off during critical maneuvers.

on Oct 14, 2013

The auto-correlation of pseudo random codes used by GPS is a very powerful technique for signal recovery in the presence of noise, however it is possible for the small cross-correlation of a powerful noise signal to overwhelm the auto-correlation of a weak GPS signal. Hopefully the politicians at the FAA have consulted the GPS community about where those thresholds are.
On a different note, only the newest airliners use GPS, the rest use legacy inertial guidance systems. I know I'll consider this before booking a flight if the FAA approves the use of electronic devices.

on Oct 14, 2013

Yeah but the outfits that are still using Delco Carousels and the like all have the premise of converting to more modern systems on their near-term agendas. Delco System Operations closed many years ago, have you ever tried to get one of those older units fixed or aligned? - and each aircraft had to have three of them! This wouldn't be a very practical long-term solution I'm afraid.

on Oct 14, 2013

Switching supplies and class-D amps use PWM. The waveforms are not square waves and contain even-order harmonics.

Isn't the real issue how much RFI actually makes it to the antennas and other avionics systems? Without that information, how can you know how much interference is likely to occur?

on Oct 16, 2013

In general, if you have 300 white noise sources of equal strength, they add as the square root of the number of sources, so you raise the noise floor by a factor of 17 or almost 25dB. GPS signals are low power density signals and the FCC ensures that they are in a relatively 'quiet' part of the spectrum. However, the processing gain of a 1K sequence is only 30dB and raising the noise floor only makes the received GPS signal to noise ratio that much lower.
You are in a hollow tube so you might think that Gauss's law works in reverse: what is inside stays inside. Except for all of the conductors laced through the airframe 'leaking' signals to the outside. I was able to receive AM and FM radio in the interior of a workspace on the Enterprise due to the pickup on all of the AC lines. (Surrounded on all six sides by 1/2" steel!!!)
As for cell phone use on an airplane, I probably would get arrested. The person next to me has no expectation of privacy and I would just carry on a conversation as if they were talking to me......They can't allow cellphone use on an airplane - it's already miserable enough....


on Oct 16, 2013

I subscribe to another less esoteric reason for turning off electronics on take off and landing.
Have you seen how engrossed some of these users are?
In an emergency I want everyone to think of only one thing, how to listen to directions and how to get the h*** out of the seat and the plane.

on Oct 18, 2013

The most important lesson any engineer can ever learn, and one that too many have forgotten, is the law of unintended consequences. i think that all electronics should be off during any portion of critical flight due to this reason alone. In my design experience, any individual electronics device, even a cell phone, does not have the power density to interfere with an aircraft, but put lots of them together, with beat frequencies and power peaks in the totally unknown range, and I become concerned. I have seen too many times where multiple devices "gang up" to create problems. Besides, UpstateNY is probably right, the people who use these devices are like on crack...they become affixed to them and ignore everything around them.... including reality; and that makes them dangerous.

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Don Tuite covers Analog and Power issues for Electronic Design’s magazine and website. He has a BSEE and an M.S in Technical Communication, and has worked for companies in aerospace,...
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