Thanks for the interesting info Chen. Your post did not make it to the list for some reason so I'm leaving it attached below for any (including Elecraft) that may be interested.
73, Bill W4ZV On Fri, Aug 22, 2008 at 12:31 PM, Kok Chen <[EMAIL PROTECTED]> wrote: > All sound cards that are connected to a QSD-based SDR have an absolute noise > floor. If a signal that is being > converted is weaker than this A/D noise floor, it will not move the least > significant bit of the A/D converter; > i.e., the software demodulator receives no signal. Indeed, the signal needs > to be a couple of bits stronger > than the noise floor, or you are going to get unwanted spectral byproducts > that degrades the demodulation > capabilities of the SDR. > > Thus, you want the noise floor of the K3's I.F. output to be at least as > strong as the noise floor of the SDR. If the > K3's noise floor is stronger, you can always use a pad in between. It is > only when the two noise floors are matched > that you will get the maximum dynamic range out of an SDR based system. > > The only time you can use an A/D converter down to, and below, the least > significant bit of the A/D is if the input > noise to the A/D converter is Gaussian (i.e., there are no other strong > signals anywhere in the passband). You > will then be able to use "noise linearization" to obtain a perfectly > undistorted spectrum. This is known as the > Van Vleck theorem, which also appears in Probability books as the "Arcsine > law" of joint Gaussian processes. > However, you will need some software to apply a correction, plus you always > lose 2.1 dB in recovered SNR. The > noise linearization technique had been put to good use by Van Vleck in WWII > radar and Sandy Weinreb's > "autocorrelation receiver" in radio astronomy, but I am not sure it will > work well in H.F. radios, and certainly > not when listening to a pileup. > > 73, > Chen, W7AY > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
