We have previously discussed the potential interference problems created by switching power supplies but I have just experienced another facet of this which I will call "Phantom Trash". Phantom trash is unwanted signals in a system which are not readily apparent but which can affect the system operation.
While developing and testing an electronic system unrelated to radio I noticed a subtle problem. The system measures pulse amplitudes of low amplitude at a high rate (similar to audio digitizing) and processes the signals for display on a screen. One of the key requirements is "resolution" of the signal peaks which should be as high as possible.
All seemed to work according to the theoretical maximum resolution predictions but when I attached a small nanocomputer system to help in system calibration the resolution decreased by several percent. In fact the decrease was seen when only the ground connections were made with no signal connection. By using software to tap into the raw input signal I was able to observe a 10 dB decrease in the signal to noise ratio when the nanocomputer ground was connected to the system ground. The nano was powered by a 9 volt switching "wall wart" supply.
Replacing the switcher with a linear supply solved the problem and restored the original S/N ratio with the nano connected leading to the conclusion that the switcher was inducing noise into the system.
The striking thing was that the effect of the noise was not apparent other than through a subtle decrease in resolution. Extrapolating this to an audio or radio system it can be concluded that negative effects which are not apparent but degrade performance can happen in part 15 radio and audio applications.
Perhaps the audio signal to noise ratio is affected or aliasing or mixing distortion is produced. Another effect which would not be readily noticed is that if the switcher noise is getting into the transmitter it can produce spurious emissions. In the case of the system I am building the noise was at a frequency of around 35 kHz which would not be audible but which could cause weird artifacts in signals.
Perhaps there are "quiet" switchers available but for now I am using only linear supplies for my noise sensitive projects.
Neil
I "Had" a 12/18.5 voltage converter that I used for my laptop computer. It put out so much crap that I couldn't use an FM radio in the house! It even wiped out my car radio as I approached the house! Needless to say, that converter ended up in the trash bin..
Had issues with switching supplies and VHF commercial radios messing up the rcvr. sensitivity. A good 'ol Astron linear supply fixed that..
I am the defrense team for switching supplies, asking the question...
Is it possible to design and build a switching-supply that works properly in every way and does not produce unwanted noise?
Or should we ban all switching power supplies.
Yes, it is possible to design a "quiet" switcher. I have had no problems with the switchers inside my computers and have used lab equipment with internal switchers which caused no interference. I have an older 13.8 volt at 2.5 amp switcher that I used with my Ramsey AM-25 and other equipment which worked without causing interference.
The offender in my original post was a 9 volt wall wart rated at 900 mA and obtained for $6. To make this retail price point something(s) had to be compromised in the design and filtering and shielding were likely the first to be cut.
The problem is not the technology but rather is cost driven design where minimal effort is applied.
Neil
I too have found that the block type power supplies like the ones used with desktop computer monitors and laptops are best. With FM transmitters they are the only ones that work without noise and hum.
Mark