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JT65, JT9, FT8 SNR Explained

Feb 4th 2017, 21:32

KC5RUO

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
JT65, JT9, FT8 SNR Explained
Have you ever wondered why JT65 reports a Signal-to-Noise Ratio (SNR) typically in the range -30 dB to -1 dB? SNR measurements in this range imply the received signal power level is below the received noise floor. But yet, you can clearly hear the musical Frequency Shift Key (FSK) tones and watch the FSK tones rise above the noise floor on your panoramic spectral display. Well, the real received SNR associated with the JT65 software’s ability to demodulate and decode those information bearing FSK tones is actually 29.7 dB greater than the JT65 reported SNR. You see, the JT65 reported SNR is referenced over a much wider noise bandwidth (2500 Hz) than the noise bandwidth that actually impacts the JT65 software’s ability to decode the digital data represented by the transmitted FSK tone. In fact, the JT65 noise bandwidth we really care about is only 2.6881 Hz wide, or a factor of 930 (2500 Hz/2.6881 Hz) times smaller. Hence, the actual “FSK tone” SNR that determines whether or not a received message will be successfully decoded is:
SNRFSKJT65 = SNRreportedJT65 + (10 x LOG (930))
SNRFSKJT65 = SNRreportedJT65 + 29.7 dB
For example, when JT65 provides a SNRreportedJT65 = -16 dB, the actual SNR is, SNRFSKJT65 = -16 dB + 29.7 dB = 13.7 dB, a SNR that reflects FSK tone power level well above the noise floor that we can see on our panoramic spectral display and hear on our Single Sideband Band (SSB) transceiver.
The same applies to JT9 communications. In the case of JT9, the noise bandwidth that impacts the JT9 software’s ability to decode data is even smaller, 1.736 Hz. Hence, the JT9 SNRFSKJT9 is:
SNRFSKJT9 = SNRreportedJT9 + (10 x LOG (2500 Hz/1.736 Hz))
SNRFSKJT9 = SNRreportedJT9 + (10 x LOG (1440))
SNRFSKJT9 = SNRreportedJT9 + 31.6 dB
The same applies to FT8 communications. In the case of FT8, the noise bandwidth that impacts the FT8 software’s ability to decode data, 6.250 Hz. Hence, the FT8 SNRFSKFT8 is:
SNRFSKFT8 = SNRreportedFT8 + (10 x LOG (2500 Hz/6.250 Hz))
SNRFSKFT8= SNRreportedFT8 + (10 x LOG (400))
SNRFSKFT8 = SNRreportedFT8 + 26 dB
It begs the question, why is 2500 Hz noise bandwidth used in JT65/JT9/FT8 SNR measurements? One of the best explanations I read is from Pieter-Tjerk de Boer, PA3FWM, in https://www.pa3fwm.nl/technotes/tn09b.html, The Signal to Noise Ratio (SNR) quoted for amateur radio modes is traditionally based on a receiver bandwidth of 2500 Hz, because these modes are usually received with a normal SSB receiver, whose IF filter is about 2500 Hz wide. The actual signal usually is much narrower, e.g. about 6 Hz in case of WSPR. So this is rather weird: we compare the power of a 6 Hz wide signal to the noise power received in the total 2500 Hz wide filter. It would make more sense to measure the SNR in the bandwidth that's really used by the receiver; but it may be hard to determine or define that "true" receive bandwidth.
So in short, your eyes and ears are not deceiving you. Those JT65/JT9/FT8 signals are very much so well above the noise.

Sep 9th, 11:51

AD4E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thank you for this explanation.

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