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).

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    \$\begingroup\$ We don't have it locally but I wonder if it's en.wikipedia.org/wiki/HD_Radio. \$\endgroup\$
    – PeterJ
    Commented Jul 12, 2013 at 4:41
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Jul 12, 2013 at 5:03
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jul 12, 2013 at 5:10
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    \$\begingroup\$ Update the Wikipedia article then, your images are much clearer, though it could do with a larger font. \$\endgroup\$
    – jippie
    Commented Jul 12, 2013 at 6:24
  • \$\begingroup\$ @Connor Wolf If it's not a secret, which SDR dongle did you get? \$\endgroup\$
    – AndrejaKo
    Commented Jul 12, 2013 at 7:40

2 Answers 2


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

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    \$\begingroup\$ Tis sounds such a good answer, even if it turns out to be wrong it gets a +1 \$\endgroup\$
    – Andy aka
    Commented Jul 12, 2013 at 10:11
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Jul 12, 2013 at 11:10
  • \$\begingroup\$ It certainly matches up with the emissions limits on page 6 of this doc: nrscstandards.org/SG/NRSC-5-B/1026sE.pdf \$\endgroup\$
    – mng
    Commented Jul 16, 2013 at 22:24

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


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