---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Thu 12/03/15: 1 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 08:03 AM - Re: Re: Troubleshooting radio whine/noise on C essna 182P (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 08:03:39 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: Troubleshooting radio whine/noise on C essna 182P At 01:06 PM 12/2/2015, you wrote: > >I took some measurements of bus voltage using a Bluetooth >oscilloscope called a Mooshimeter. Here are 3 graphs. I'm also >including a fourth one which is picture of the voltage in my car >cigar lighter for comparison. > >Two things are noticeable: >1. The amplitude of the ripple in the aircraft is smaller by a >factor of 3 or 4. >2. There seem to be higher frequency components in the aircraft than >in the car. > >Does anyone have any thoughts about this? >I'm thinking that maybe the noise suppression capacitor is fried and >I'd like to test this hypothesis without shelling out $125 for the >Cessna part. Can anyone suggest a sensible capacitor I could try out? Keep in mind that the noise you're hearing is an 'audio rate' signal . . . else it would not be audible. You've already determined that the noise is not coming through radio-frequency stages of receivers so the noise is not a modulation riding on top of a radio-frequency signal. "Filters" as a class of product are seldom useful for attenuation of audio-rate noise . . . especially noise represented by alternator ripple. The DC power systems guys recognized a century ago that the DC bus in a vehicle is normally VERY noisy . . . Attentive designers recognized that there's no such thing as a noise-free power distribution system . . . unless perhaps your vehicle runs on flashlight batteries that operating nothing but light bulbs and heaters. What should one EXPECT or be prepared to TOLERATE in terms of bus noise? Here's a plot of what has become the Mil Standard for normal noises on a DC bus. From Mil-Std-704 we find . . . http://tinyurl.com/b3rhjwq This is for a 28v system, so we cut the numbers in half for a 14v system. Note that over the frequency range of `1000 to 5000 Hz, on should expect to tolerate 0dBV (1 volt RMS) noise in a 28v; 1/2 volt RMS in a 14v system. That is about 1.5 volts peak-to-peak in the 14v system. Note further that lesser but still substantial components of noise can exist above and below that 1-5KHz range with numbers that diminish as the frequency of interest moves further away from the center. Here's a plot of the bus noise on my old '95 GMC Safari observed on a 100mHz oscilloscope with a rather flow sweep. http://tinyurl.com/nkrwqhk Spreading it out for a closer look http://tinyurl.com/qzj7vbl Yes, that bus is pretty trashy but how bad? The only way you can quantify it is to do measurements with a spectrum analyzer that measures the energy in narrow slices along the frequency domain and hands you a series of numbers that can be first plotted and then compared with the Mil-Std-704 plot cited above. You can deduce very little about noise by observing bus perturbations on a 'scope, even less with a data acquisition system. Understand further that any effective filter of noise must present a high series impedance or low shunt impedance to the noise source AT THE FREQUENCY OF INTEREST. A filter that would effectively remove 100 Hz to 10KHz AUDIO RATE noise from the output of a 60 Amp alternator would be a huge device probably weighing more than the alternator itself. Hence the legacy resignation to designing to LIVE with alternator ripple. Having said that, there have been filters of various kinds added to alternators, motors and sundry devices over the years but these are almost always sized to remove RADIO FREQUENCY noises that serve as carriers of the AUDIO FREQUENCY noises that plagued radios, etc. If the observed noise is affected by the radio's volume control, then it is probably, not always but probably a radio frequency energy coming in through the the antenna. In this case, some filtering of alternators, motors, p-leads, etc. etc. may produce the desired remedy. Stock automotive alternators already have a high- quality 4-10uF capacitor tied right across the b-lead terminal to ground inside the alternator. If I recall the current thread correctly, the noise in question popped up after some changes to the system. That it's an audio rate noise that varies with rpm (alternator whine). It's a noise not affected by radio volume control setting. The task is to identify the propagation mode but it's a 99% sure bet that adding a filter is not going to do the job. Bob . . . ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.