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I recently bought an inexpensive USB software-defined radio dongle, and have been poking around my local spectrum:
enter image description here

The above image is one of the local FM radio stations (88.7 Mhz, to be specific). I am wondering what the large, rectangular side-bands are. They don't seem to be involved in the actual radio content, as this is the only station that has them, and all the other stations seem to be received fine.

It's also not the stereo component of the signal, as again, stereo demodulation for other stations that lack these sidebands works without issue.

Each sideband is 125 Khz out from the center of the signal (i.e. the inner edges of the rectangular sidebands are 250 Khz apart).

Connor Wolf
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    We don't have it locally but I wonder if it's http://en.wikipedia.org/wiki/HD_Radio. – PeterJ Jul 12 '13 at 04:41
  • @PeterJ - It certainly looks like that's what it could be, but the spectrograms on the wikipedia page for HD radio are really, really terrible quality. – Connor Wolf Jul 12 '13 at 05:03
  • Oh, now that I knew the specific term to look for, KUSC (the radio station in question) specifically mentions they broadcast in HD on their website. Add your comment as an answer, and I'll accept it. – Connor Wolf Jul 12 '13 at 05:10
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    Update the Wikipedia article then, your images are much clearer, though it could do with a larger font. – jippie Jul 12 '13 at 06:24
  • @Connor Wolf If it's not a secret, which SDR dongle did you get? – AndrejaKo Jul 12 '13 at 07:40
  • @AndrejaKo - One of the cheapie RTL2832-based USB dongles. Mine has a R820T tuner IC for the frontend. ~$18 including S&H, from china. For the money, it's ridiculously awesome. It's tunable from 24 Mhz to ~1.7 Ghz. – Connor Wolf Jul 12 '13 at 11:09
  • The antenna it comes with is crap, unsuprisingly. Fortunately, it just uses a MCX connector for the RF in, so now I get another interesting project to build. – Connor Wolf Jul 12 '13 at 11:11
  • @Connor Wolf Thanks for the info! I found a no-name one with such combination on dealextreme, so I'm definitely going to get it. – AndrejaKo Jul 12 '13 at 12:39
  • @AndrejaKo - the magic words for these things is "rtl-sdr". Resources: http://sdr.osmocom.org/trac/wiki/rtl-sdr http://www.rtl-sdr.com/ The software I used for the image in my post: http://sdrsharp.com/ – Connor Wolf Jul 12 '13 at 22:53
  • @Connor Wolf Yeah, I managed to find the site buried somewhere in the thousands of my bookmarks (which I can't use, since Firefox keeps choking on them). In any case, thanks for the links! – AndrejaKo Jul 12 '13 at 23:07

2 Answers2

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The spectrum seems to match up fairly well with HD Radio even though the image from that article is not all that great. It might be worth checking if that particular station is broadcasting HD Radio.

enter image description here

PeterJ
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    Tis sounds such a good answer, even if it turns out to be wrong it gets a +1 – Andy aka Jul 12 '13 at 10:11
  • @Andyaka - The radio station in question specifically mention they also broadcast in HD, so I'm fairly confident that the sidebands are indeed the signature of a HD radio broadcast modulated onto the normal FM transmission. – Connor Wolf Jul 12 '13 at 11:10
  • It certainly matches up with the emissions limits on page 6 of this doc: http://www.nrscstandards.org/SG/NRSC-5-B/1026sE.pdf – mng Jul 16 '13 at 22:24
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I think this is correct. The attached graphic is a GNURadio Frequency Sink for 103.5 WEZL, Charleston SC. This station is an HD station. The bandwidth of an FM station is specified as 200 kHz. Note the main signal clearly shows 200 kHz from 103.4 - 103.6. There are also sidebands that are about 100 kHz each at 103 - 104 and 106 - 107. This violates the 200 kHZ spec but conforms nicely to the HD spec. See https://www.sigidwiki.com/wiki/HD_Radio_(FM)

enter image description here

pcomitz
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