What do you mean it would destroy the valuable part of the signal?
If the signal has a carrier (like in AM or FM) or some other regular pattern (e.g. digital signal) then the typical the way to recover it from under the noise floor is to use a narrow-band PLL.
I’m experimenting with it right now and just found that a quartz/ceramic oscillator pulled by a varicap controlled by a PLL gives pretty good results - low phase noise, good noise rejection and ability to recover the carrier from a very noisy signal. But for that to work, the signal has to be shifted to a constant frequency through heterodyning first.
If the signal has a carrier (like in AM or FM) or some other regular pattern (e.g. digital signal) then the typical the way to recover it from under the noise floor is to use a narrow-band PLL.
I’m experimenting with it right now and just found that a quartz/ceramic oscillator pulled by a varicap controlled by a PLL gives pretty good results - low phase noise, good noise rejection and ability to recover the carrier from a very noisy signal. But for that to work, the signal has to be shifted to a constant frequency through heterodyning first.