SNR at low levels?

Lasse Moell

During the past year that I have had my 6500, I have noticed that in certain instances the received CW signal tends to be noise modulated. Much more so than with my old TenTec Omni6+. Today I ran both from the same antenna using a 50 ohm splitter and tried to nail this thing down. My take is that both radios do copy weak signals just as good, but the Flex tends to have more noise on signals that are 10-20 dB above noise floor. Shouldn't the SNR go pretty linear from noise floor and up? The received signal sounds cleaner on the TenTec. I have to dig deeper into this, and try see if I can verify this using proper instruments. Right now I do not have a SINAD-meter. I am curious if anyone else have noticed this apparent less SNR? AGC-T nor equalizer seems to help, WNB; NB and NR are off. Test today used SSDR on PC and audio straight from radio.

Jim Gilliam

Out of curiosity what is the approximate level of your noise floor? Mine pretty much averages around -120 dBm Which is somewhere around an S-4 or S-5. So the modulation would begin at around an S-8 give or take?

Jim, K6QE

Lasse Moell

It depends on what band I am... today I noticed this on 14 MHz where noise floor is around -120 dBm (spectrum is at -130-ish dBm) depending on BW, signals from say -115 up to -100 are nosiy. One has to be a bit careful talking about noise floor, as the spectrum bins are a lot narrower than the demodulated signal BW.

But this noise modulation seems to occur on all bands and it is really more a SNR thing than an absolute number, i.e. say from 0 up to 30-35 dB above noise. I'll try get a signal generator out and mix its signal with the noise from the antenna and see if I can see this using Spectrum Lab s/w.

If I recall correctly S9 equals -73 dBm thus S1 should be -121 dBm
/Lasse SM5GLC