>> So the reduction in SNR (assuming equal noise power) is >> 3.01 dB, not 6 dB.
>That's the best case if the noise power is equal. If the >"other" receiver has higher noise power (wider bandwidth, >more interfering signals, etc.) the S/N reduction is greater. >Even 3 dB reduction in S/N is a big hit if the DX Station >you're trying to hear is at or just below the noise level. >It would be a shame to turn a top performing radio into a >mid-pack device by mixing the audio - because of some old >wife's tail. Let those who want mixing do it externally >so it doesn't impose a S/N penalty otherwise. There seems to be a fundamental mis-understanding on how uncorrelated and correlated noise works. If two receivers are listening to the exact same signal and use the same antenna, then the short answer is that summing the output from these two receivers together will produce the same signal at a level 6 dB higher with no signal to noise change. This assumes the receivers themselves are identical and do not contribute noise (more on this later), so the two receiver outputs will be identical. Band noise from one receiver at any instant in time will look exactly like band noise from the second receiver. The desired stations signal will look the same from both receivers. The point is that magic is not involved, and that neither of these two receivers can tell the difference between band noise and the desired signal and thus will process both the desired station and the band noise the same. Thus, there would be no degradation or improvement in signal to noise ratio as long as we are talking about ideal receivers. However, if the situation is a weak signal situation where the receiver noise floor is at least partially masking the desired signal, we have a different situation. Band noise and stations on the band will both be correlated coming out of both receivers and thus get a 6 dB improvement. On the other hand, the internal noise produced in each receiver is independent and thus uncorrelated. While correlated signals add voltage wise (V + V = 2V or 6 dB gain), uncorrelated noise adds as the square root of the sum of the squares sqrt(V*V + V*V) = 1.4V or 3 dB gain. This means what when adding the output of two identical receivers, the band noise and the desired station signals will increase by 6 dB while the receiver noise contributions will increase by only 3 db. You have in essence created a single composite receiver with a lower noise floor. If you added just enough RF pre-amplification to overcome the signal splitting loss to N receivers, adding more and more receivers in parallel will produce a composite receiver that has a better and better noise floor. This is in essence what the space telescope folks do. They gang many dishes and many receivers together across a very large area to get an enhancement on the signal and space noise and a suppression of the effective receiver noise contribution. Thus, if the noise floor of the receivers are the limit, listening to the same signal with two identical receivers will have the effected of reducing the receiver noise contribution by 3 db. Conversely, that also means that if you are listening on 40m where the band noise is way higher than the receiver sensitivity, reducing the receiver noise contribution will be of no benefit. - Dan Tayloe, N7VE _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
