At 02:24 PM 8/6/98 -0400, Dave Emery wrote:
This is expected of hard limiters below threshold (such as in a reasonable FM IF system fed Johnson noise). The spikes occur on phase discontinuities in the limiter output - what you are seeing is the input Johnson noise intermittantly inducing phase jumps in the output of the ringing IF filters shock excited by the broadband input noise. The IF filters tend to ring somewhat coherently until hit by another burst of noise which sets them off on another phase. Each phase jump shows up as a spike on the discriminator output. The narrower the bandwidth of the filters, the longer the spike and interspike intervals. Thus one needs an IF bandwidth considerably wider than the sample rate.
Given that the cheapo radio was 10 cm from a 21" monitor, I don't think you need to invoke noise-driven resonances in the humble radio. I was/am interested in robust methods that don't depend on exquisite analog equalization, etc. Zero crossings are analogs (pun intended) of RFC1750's #2 method (via Shannon): map 01 -> 1 map 10 -> 0 00, 11 are no-ops. Note that for a digital signal you are detecting level-changes. (Vdd/2 changes?) Waiting for zero crossing will give you perfect 0:1 ratio, but you may have to wait arbitrarily long (but will almost certainly not, in the formal sense ---there's a decaying exponential in there).