I’ve recently been building a link budget for my ground station. I notice that no matter what given the noise bandwidth of the SDR I’m using, the signal to noise ratio is negative. That would imply there is more signal than noise and as such nothing useful is received. How is it that people are still capable of making observations from low power transissions from birds such as Havelsat using an omnidirectional antenna which gives a negative SNR? Does the SDR do something to the signal to retrieve information from the noise? How does that work? Any ideas?
SNR is a ratio no idea how that can be negative.
SNR depends on the bandwidth, so I guess you are using incorrect bandwidth in your calcuations.
Ah i got confused, I was calculating the SNR using decibels and forgot to convert back out of decibels. My bad! Thanks for reminding me!
I was under the impression that the bandwidth used for the noise calculation when using the SNR method was the bandwidth of the receiver itself. Is that not so?
Yes, but the receiver includes a software part, which has a narrow digital filter tuned to the bandwidth of the signal you are interested in.
Interesting. So this reduces the noise bandwidth of the receiver? And thus the overall noise production of the SDR? Thank you for taking the time to educate me, by the way. It is really very much appreciated.
Yes, a filter has the same function from a signal theory point of view regardless of whether it is analog or digital. We like digital filters running on a computer because they can have better performance, but we need the analog filters for high frequency front end filtering – at least until we can afford gigahertz sampling and processing