There have been prior posts regarding hum in a part 15 FM signal and this one is to add to the body of knowledge.
There have been prior posts regarding hum in a part 15 FM signal and this one is to add to the body of knowledge.
My Ramsey FM 25 is enclosed in a metal box and has a clamp-on ferrite choke on the power lead. To date I have been able to minimize hum by dressing the power cables from the tx. and other nearby devices. This hit and miss technique is not a great solution but it works. Last week I decided to clean up the wiring mess behind my electronics shelf by replacing the daisy chained outlet strips with a 22 outlet home made bolt on the wall strip with the recepticals mounted vertically to accommodate the wall warts so they don't block other outlets. This was a great improvement but it removed my ability to dress the power leads to reduce hum.
Some hum was back after this change so I took a different approach. The tx. antenna was the supplied whip extending from the top of the box and I had installed a BNC jack for testing or future antenna use. I have a home made mag mount 2 meter whip antenna with a telescoping element that I made years ago for emergency and portable operation. I removed the Ramsey whip and connected the mag mount to the BNC and positioned the mag mount on a metal cabinet about ten feet away from the tx. and other electronics on my shelf. The hum is now inaudible. It has been reduced to below the white noise as assessed by listening on a Walkman and headphones. It is good practice to keep the radiating element away from audio and power cables either by moving the antenna as I did or my relocating the transmitter.
This fix has been mentioned by others but I though a quick refresher on the topic with a real world example appropriate.
Neil
Hi Neil Radio 8z:
After thinking about your post for awhile a question has arisen.
It seems to me the obvious reason you put the Ramko FM25 in a metal box, since of course it comes originally in a plastic box, is to shield it from leaking or receiving RF.
But it sounds like the hum problem is a matter of the antenna being too close to the transmitter, which would suggest the RF is penetrating the metal box.
Although separating the two improves the situation, why can't the metal box be the solution in the first place?
Good question, Carl, and I don't know if I have a good answer regarding the FM situation since the problem could have been with something other than the RF getting back into the transmitter. It has been reported by some that wall warts and chargers can pick up the carrier and modulate and re-radiate it. This is consistent with moving the wires around to reduce the hum.
The transmitter was mounted in the metal box for shielding but, in general, a metal box is not as good a shield as it would appear. I learned this the hard way when I built a 2 meter repeater. I tested the shielding by keying my handheld next to the receiver box with the antenna shorted and it heard the signal loud and clear despite being in a closed metal box. Effective shielding required feed thru capacitors and silver plated braided "gaskets" at all joints with screws every inch along the covers. I finally achieved good shielding but it wasn't easy.
Fortunately this effort is not needed for part 15 FM because of the low power involved but the first thing I would do (and did) with a hum problem is put the transmitter in a metal box.
Neil
I am only compounding the problem I know, but why do so many devices work adequately with only a plastic box?
"why do so many devices work adequately with only a plastic box?"
Because the circuitry has isolation filters and/or is of a type that isn't affected much by RFI, or has thin aluminum (almost foil) backing sheets inside the enclosure? Just a rough guess.
Hum is usually generated by loss of AC ground isolation in some way; IOW it leaks. if you have any gear or power supplies that are having a problem that is affecting transmission but not audio lines, moving not only the antenna, but also the transmitter itself, away from power supplies other than its own would make sense.
Virtually all my serious audio and radio gear have metal cabinets except for the front, which are usually plastic, and often have that thin aluminum backing I spoke of.
I've found the only time I hum is when
I'm running off back-up power on the UPS.
I assume that has to do with the loss of
AC ground that Ken was talking about.
I'm not sure how this relates to the previous discussion in this thread, but a while back I introduced a monumental hum problem to my EDM FM transmitter. Here is how it went. I had a beautiful, hum-free signal going when feeding the TX from a Sansa MP3 player operating on its rechargeable battery. I then bought an el-cheapo USP power supply wall-wart from now defunct Circuit City, thinking I could keep the player running 24/7 independent of a computer USB port. The 24/7 part worked, but the resulting signal had hum/buzz that obliterated the audio. When I had the player connected to the computer USB port, there was no hum. Why didn't I just leave it plugged in to the computer? The computer is on the other side of the room from the stereo system where I wanted the player located so I could select other audio sources to the EDM transmitter.
Lesson learned: Don't power the audio source from a crappy, cheap USB power supply.
Lesson to be learned: how to find a good USB power supply that actually complies to FCC specifications!
Yep, most of these little things are, pardon the expression, RF hell. Actually, so is the insides of most computers, which is why you should use an external A/D converter, preferably one with its own DSP (so it doesn't tax the computer's CPU).
The only solution I've found is to go for the best most well-filtered external PS you can get your hands on. I think I may have mentioned elsewhere that I replaced my TH PS with an HP Printer PS. The thing is a 2 lb. brick, but it doesn't cause hum.
Something else I noticed ... the goofy little 400w inverters I use in the car and on the boat don't seem to generate hum. Also, if you don't have a USB or FW A/D converter, some laptops don't hum much either.
Hi Phil,
Please tell us more. Do you like your EDM unit? What's your experience with it? I've been toying with the idea of installing one in my car, driving up a nearby hill and see how far I can get, or having fun with it while waiting in ferry lines.
Ken speaks the truth about the RF hash inside a computer case. I will defer to his experience re A/D conversion for audio but I can relate mine for scientific A/D. I use a PCI card A/D 12 bit system for temperature measurement/control and the best performance, limited by digital hash, is noise equivalent to .025 degrees C in a system with about this resolution. This is good but there are frequent noise spikes ten times this amount, especially with disc activity, which cause problems in a closed loop control system.
Back to part 15 stuff, I have several p15 devices perched on a shelf above my work area, including a rain gauge and an outdoor thermometer. On occasion the displays blank due to signal loss (temporary interference) so I tolerate this. I added a PIR driveway alert system and the problem became worse with very frequent "black outs". Missing the obvious, I replace the batteries in the units but there was no improvement. I eventually saw through the trees that the new driveway alert receiver was, by inference, radiating a signal which interfered with the other receivers. This is probably due to a super-regen receiver and relocating this receiver about 10 feet away solved the problem. The lesson is that even receivers can, and usually do, radiate RF and are subject to p15 regulations.
This driveway alert system bears no FCC identification and, though it works, it appears that these are being dumped on the market at a very low price ($13) perhaps to clear inventory before someone at the FCC notices.
Neil
I purchased one of the 10mw EDM transmitters around 4 years ago, so my comments may be out of date and they may have improved it. But here are my impressions.
First, it is not certified, so if you use it, you take your chances.
And I found, at least with mine, that there are some good reasons other than just field strength that it isn't certified. With my Decade MS-100, the Panaxis ACC-100, the Ramsey FM100B, and even a PCI Max that I fooled around with, there was no splatter at all into adjacent FM channels listening with my car radio. There was significant splatter with the EDM (i.e., I transmit on 88.7, and could hear interference quite a ways away from the transmitter on 88.5 and 88.9). Close to the transmitter it affected even 88.3 and 89.1.
It can only transmit in stereo, unless you purchase a replacement circuit board chip for mono.
And of all the transmitters I've used, it was the most finicky in requiring a clean power suppy; otherwise, you'd get hum and other noise. I never could get all the hum out, even when using a well regulated power suppy that other transmitters operated cleanly with.
Some appear to have achieved good results, at least from their perspective, with the EDM, but I wouldn't even consider using it. There are just so many better (and if you care, sometimes legal) transmitters out there.
Ken,
I have had the EDM for a couple years. It performs very well. I use it inside the house and keep it on the low power setting. The signal is very strong on all the radios. The sound is superb, no hum or distortion. Of course, I'm not talking about the hum caused by the previously mentioned USB power adapter. It is hum free from the computer and the MP3 player on battery.
It's built like a brick. The antenna is a short wire about 2 ft long and doesn't seem sensitive to placement. I bought it because of its additional power after reading many reports about other transmitters not being capable of covering a house. I often use it when we are out on the deck in the summer. It's only on occasionally.