There is some very interesting amateur radio work (and computer programs) with super weak signals for VLF on the internet. A key to it's success is accurate frequency resolution. From what I've read, even 1hz off can make a difference sometimes. Regards, Mike ----- Original Message ----- From: "Peter L. Peres" To: Sent: Saturday, August 04, 2001 8:10 AM Subject: Re: [EE]: Amplify then filter ? > > I once used a pll to respond to sonar signals in noise, it worked with a > > signal 6db lower than the noise. on a scope you could not even see the > > signal in the noise, and this was with no extra filtering. > > The signal was 6 dB lower than the noise floor, but not in the capture > band of the PLL, otherwise it would not lock ? Afair the magic 'getting > signals from below noise level' done by PLLs is due to their capture > bandwidth being a sort of filter. This correspondingly improves the S/N > ratio for the wanted signal. A scope will not show the singal in the noise > but I am almost certain that a scope with a prefilter with the same > bandwidth as the PLL capture loop will show it. > > Peter > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu