Friday, November 6, 2009

Signal to Noise Ratio - Rainbows in the Dark

In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB). If the incoming signal strength in microvolts is Vs, and the noise level, also in microvolts, is Vn, then the signal-to-noise ratio, S/N, in decibels is given by the formula;

S/N = 20 log10(Vs/Vn)

If Vs = Vn, then S/N = 0. In this situation, the signal normally borders on unreadable, because the noise level severely competes with it. In digital communications, this will probably cause a reduction in data speed because of frequent errors that require the source (transmitting) computer or terminal to resend some packets of data. If Vs is less than Vn, then S/N is negative. In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination (receiving) computer or terminal.

And this was the world we lived in for most of time, but recently (last 40 years) even HAMs can communicate under the noise. As you may know if you have been following my blogs here, I have been smitten by WSPR and chalk up -30db contacts.

Here are my findings to date (with the equipment at hand)

Mode Min SNR

CW@20WPM +3db (machine decoded by MFJ-461)
CW@20WPM +1db (machine decoded by fldigi or CWget)
HFpacket (300baud) +1db
RTTY45 -5db
CW@20WPM -7db (other claim like less, like -13db*)
PSK63 -7db
FELDHell -7db
PSK31 -10db
Olivia 64/2000 -13db
Olivia 16/500 -14db
WSPR -30db (maybe less)

some talk of minimum detectable signal (MDS), but I mean where one can actually decode it, not just hear it.

I prepared some files with simulated noise so I could judge for myself, now you can click on the links and judge for yourselves.

(I left them as .wav files because they sounded better, and are SMALLER than the .mp3 files)

Signal to Noise Ratio Files 091115 // CW string of 'ki4SGU em73ox' // CLICK TO LISTEN
  1. Postive 9
  2. Postive 7
  3. Postive 5
  4. Postive 3
  5. SNR of 0 (Zero)
  6. Negative 3
  7. Negative 5
  8. Negative 7
  9. Negative 9
So here is the deal, if you are using WSPR (and you should be by now), and you make contact with me, drop an email and lets sched up a real QSO using some digital mode CW, PSK31 or Olivia and see if my estimates are right, or just full of hot air. 73 ki4SGU


  1. I would like to try different modes with you as you suggest.

    We have QSO'd on WSPR recently.

    Drop me a line!

  2. Eric; We sure have - the range of SNRs is bewildering from a +11db to a squeaky -27db!

    We are both EST TZ, so that should make easier, How does SUNDAY evening sound to you? - I'm curious if WSPR reports one SNR, we then try it on say PSK31 (or some other digital mode of choice) if would in fact connect, we will have to find a adjacent frequency as to not bother the normal WSPR traffic, but not too far as to throw the results.

    73 ki4SGU

  3. Eric /AB3LI and I (/ki4SGU) were able to conduct our initial WSPR vs PSK31 comparison this last Sunday night. At around 1930EST I started up my station on WSPR/40m to establish a baseline, with both stations running 20w, after a half hour we noticed my signal was very long, lots of stations in EU, but none is the USA - Eric is near DC, and I'm near Atlanta, so forty was the wrong band, Eric suggested dropping to 80m and we switched. This was better but not by much, we made only one WSPR contact at 2100EST @7+db after a few minutes of re-confirmations we switched over to PSK31/@3585kc, but there was a strong Olivia station I could not pickup Eric - More re-adjustments I bump power to 40w, Eric switched radios @10w and we finally made PSK contact, but @12+db (calc, +4 off of the WSPR provided number and copy was not 100%) - So we will have to try again because we wanted to prove that "a PSK31 QSO with a SNR >0db would be 100% and have WSPR measure the path first."

    We will try again middle of this week.

    PS. We also tried some CW test - again the results we inconclusive.