The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 6: Technology

Why an SSB Receiver?

Dear Dr. SETI:
When following the SETI station block diagram on the website, I notice that the microwave downconverter drives an SSB VHF receiver, typically at 144 MHz. Must the 144 MHz receiver be SSB? Why not AM? Or why can't I use my existing VHF FM scanner? After all, it tunes 144 MHz.

Eric (via email)

The Doctor Responds:
I'm assuming you're using a kit or commercial downconverter, Eric, to shift the microwave signal received by your radio telescope down into the VHF spectrum. Since the signal coming out of your downconverter is intermediate in frequency between the microwave signal entering your antenna and the audio signal driving your computer, we call it an Intermediate Frequency, or IF. The receiver itself (which typically operates at VHF, often in the 2-meter or 144 MHz ham radio band) would then be called an IF receiver. This is the most common scheme employed in homebrew SETI stations. The IF receiver which you choose to use following the converter should contain a single-sideband (SSB) detector. Either upper sideband (USB) or lower sideband (LSB) may be used. I recommend against using AM or FM mode in your IF receiver. Here's why:

We do not contemplate that extraterrestrial civilizations will necessarily employ SSB modulation. Rather, an SSB receiver will convert whatever signal it happens to receive, with whatever modulation -- or even no modulation at all -- into audio tones. These tones can be digitized by a computer's sound card, and analyzed in software for the hallmarks of artificiality, those characteristics which confirm their extraterrestrial intelligent origin. The output from an FM or AM detector, on the other hand, may not contain audio components (especially in the case of an unmodulated carrier, which is a likely scheme for interstellar beacons), and thus would produce no useful output from a computer's sound card.

VHF receivers containing SSB detectors (sometimes called Multi-Mode or All-Mode receivers) are readily available through ham radio retailers, and commonly used by serious VHF/UHF amateurs. Check with your local ham radio club for specific recommendations.

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