- July 13, 2007 at 1:13 pm #6963radio8zSenior Moderator
Total posts : 247
Recently my community had a tornado warning and I tuned into the local SkyWarn amateur radio net from my “bear cave” in the basement. Much to my dismay, I had a S9 noise level on my receiver and had to switch from my indoor antenna to my roof mounted outdoor antenna….not the best option with lightning flying around.
So, since this setup worked before, what went wrong? My “bench” used to be powered by a homebrew 13.8 V linear supply but to modernize I replaced this with a switcher a few months ago. The switcher is putting out so much “hash” that my indoor antenna is useless. This is an OEM device and bears no FCC part 15 certification label. Guess there is a reason for part 15 limits after all. Back to the good old linear here.
Since part 15 AM and FM operators are mostly concerned with transmitting rather than receiving my lesson may not be relevant. I have used a switcher for AM part 15 transmitting in the past with no problems and have never used one for FM so there may be no problem powering a transmitter with one. But, I thought it would be worth a mention about this in case you are getting noise on your part 15 FM signal. It might be coming from a switching power supply.
NeilJuly 13, 2007 at 1:15 pm #15804scwisGuest
Total posts : 45366
I had a similar experience with 6W $5 switching PSUs from CUI available from Digikey. Was going to use them on my new TX design, but had to can the idea due to intermodulation effects. These PSUs were rated 12V 500mA. When I tried one of their larger 12C 1.25A (more expensive switchers) all was OK. It’s these cheapy ones that are lacking. I opened up the 6W supplies and noticed that the component count was an absolute minimum. I’m not surprised they cause problems. Oh well back to good old linears.
GerryJuly 13, 2007 at 1:15 pm #15803scwisGuest
Total posts : 45366
Switching power supplies can be designed so they don’t radiate or induce noise. Modern TVs use switchers and there isn’t any fuss over them.
It is troubling that cheap, made in China, switchers get away without compliance. As long as there are no complaints to the FCC, the FCC probably isn’t even aware of the existence of many of these products. I think 99.9999% of the users are not even aware of the applicable FCC rules, so no complaints are filed.
I personally wish the FCC would address noise problems from switchers (and lamp dimmers and IBOC and …) instead of picking on little part 15 transmitters. The latest targets of a slew of FCC actions are directed to retailers who fail to inform customers that their TVs are not digital ready! Actually, this may be a good thing because it keeps the FCC busy elsewhere rather than picking on part 15. But. on the other hand, they certainly won’t be picking on devices that radiate interference!
You should file a complaint to the FCC on that noisy switcher. That won’t help your noise problem, but it might make you feel better 🙂
PhilBJuly 13, 2007 at 1:16 pm #15806scwisGuest
Total posts : 45366
My switcher is made by Lambda which is a long recognized mfg. of power supplies. The Lambda linears I have used over the years have been flawless.
This one was intended to be mounted in an industrial control cabinet as a component and as I mentioned does not have a FCC label probably because it is an OEM component, though I would expect that such devices would be subject to part 15 rules anyway.
The other switcher I sometimes use for AM is just fine. It is as 13.8V, 3A unit and is a “semi wall wart” in that it is in a pod in the cord. It does have the FCC part 15 sticker and reg. number so apparently it was tested and passed.
My 2 meter transceiver requires 13.8V at 7A which makes for a fairly large linear supply, but since I have the homebrew unit ready to go this is not going to be a problem here. For part 15 AM I use the SSTRAN supplied wall wart, and for FM I use the Ramsey supplied wall wart. Both work fine.
Regarding the FCC cites for retailers of “improperly labelled” TVs, this appears to be a real cash cow since the fines are substantial.
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