nuckolls.bob(at)aeroelect
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 Posted: Thu Dec 03, 2015 8:03 am Post subject: Troubleshooting radio whine/noise on C essna 182P |
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At 01:06 PM 12/2/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "sacha" <uuccio(at)gmail.com>
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 . . .
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