If you ever run a transmitter off of a car battery and its signal is hum free but go to use an indoor power supply and the problem pops up you may seriously want to consider opening up the supply and adding snubbing capacitors, but ONLY after you ruled out other potential hum causing issues...
Something that I stumbled upon that I never knew about before is this little idea called "snubbing" capacitors which in HIFI audio amplifiers and radio transmitters are usually high value (0.01 to 0.1uF) capacitors across each power supply diode. In a standard 4 diode bridge circuit used in most power supplies this means there would be a capacitor wired in parallel with each diode for a total of 4 diodes and 4 capacitors. In split supplies there would be 2 diodes and 2 capacitors wired.. once again.. in parallel.
Now here is the reason for this:
A diode is a basic switching device in that when rectifying AC it will be on only for part of the AC cycle. When the diode is at the part of the AC cycle where it's just about to be turned off there is some distortion which can leak into the large filter capacitors which can get into the audio sections of a HIFI amplifier. Or at least that's my understanding of this. The theory goes a bit deeper than that but it should be understood that when used in HIFI amplifiers the purpose is usually to help filter even more AC hum out of the circuits and protect power supply current characteristics.
With radio transmitters it's actually the other way around! When you are generating RF some of that RF path can get rectified for part of the 60Hz AC cycle which switches part of your antennas grounding that is shared with the power supply ground on and off at the same rate as the wall AC which gets into your broadcast audio. This problem becomes much more noticeable if you are in the same building as your transmitter and walking around with a radio. You will notice in some setups that when you move the radio around there will be spots where there is no hum in your audio and other spots (usually by appliance cords, or house wiring) that you get very loud hum.
When placing high value capacitors across the rectifying diodes it allows the RF to flow past the diode without rectifying it but since the value of the capacitor is high enough it will have no detrimental effects to the AC to DC rectification. In other words the capacitor makes the diode ineffective at RF, which is what we want!
Learning this information made a lightbulb go off in my head as I have dealt with this problem for a while now and my power supplies were hand built so I decided to add one 0.022uF capacitor across each diode in the power supply bridge and it reduced the hum down to almost nothing while walking around in the house with the radio.
This should be a warning to anyone here using cheap power supplies, or homemade ones. Many of those old radio shack power supplies were never built with them either. You can have all the ripple filtering in the world on the end of a power supply but it still may not be as effective as this because hum on your modulation may have never been the issue in the first place but rather your signal mingling with the AC wiring of your house.
Here is a quick example schematic I found where snubbing capacitors are used to help paint a better picture of where to place the parts...
In this case the capacitors C2, C3, C4 and C5 are all of the same value (0.01 or 0.1uF) and rated at twice or higher the maximum transformer output voltage to be safe.
In the RF world, the signal will propagate into circuits unless RF bypassing is used...ie the snubbers. In the circuit example above, the caps lets the RF shunt to the rectifier's ground point.
In high end audio as well as RF, it is not uncommon to find the unit being powered by a toroid core power transformer with a large metal shield on the top and bottom. Toroid cores keeps the magnetic field on the inside of the core, and has very low loss.
Unshielded traditional transformers tends to emit the inner magnetic fields outward, and yes embeds onto nearby circuits. Even with well designed filtering, the hum can make it's way through by means of induction.
The old catch phrase applies here..."shortest path to ground". If there is no short path, the RF floating around with the power supply 60 cycle signal mixes and travels down the power cord into the electrical wiring, hence you get hum in various places when moving the radio around.
Moral to the story..a well designed unit..audio or RF, will have adequate bypassing in key areas. And it all begins with the power supply!
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
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