[Elecraft] [K3] Perfect for Field Day S&P ... Limited Production 700 Hz 8-Pole Filters

[Wes (N7WS)]

Joe is right on.

Back when EME was "real", i.e copied by ear, it wasn't unusual to copy signals 
with a negative SNR.

A little story:  I went to work for Hughes Aircraft here in Tucson when I was 25 
years old.  At the time I was given a hearing test as part of my physical exam. 
(So they told me later).

About 15 years later, at the time I was working two-meter EME,  I had to attend 
a meeting in another building.  After the meeting, as I exited that building and 
headed to the security gate a Plant Protection Office ran to his truck, jumped 
in and proceeded to turn on the siren just as I walked by.  I was nearly 
deafened by the sound.

The next day my ears were still ringing (to this day I have tinnitus) so I went 
to the Medical Department and reported an on-the-job injury. As part of their 
evaluation they gave me another hearing test.  The test was almost like 
listening for EME signals; detecting weak tones buried in noise and pressing a 
button when I heard them.  Their conclusion was that not only where my ears 
fine, my hearing had actually improved since I took the first test!  Clearly 
impossible, but training made it seem so.

My receiver (in fact with the exception of the coax, the whole station) was HB 
and designed for high linearity and low distortion.  The final hardware BW was 
200 Hz, provided by a passive, LC audio BPF.  The final BW was set by the 
tracking filter between the ears.

Wes  N7WS


On 3/3/2014 12:59 PM, Joe Subich, W4TV wrote:
>
> > If B = 2.7 KHZ, LogB = 44.3 dB
> > If T = 290K (room temp), LogT = 24.6
> > then Pn = -198.6+24.6+44.3 = -129.7 dBm
>
> All of which means absolutely nothing since the proper number to use
> for "B" (bandwidth) is the final detector bandwidth which can be as
> little as 50 Hz *for a trained human ear*.
>
> A conventional product detector following an IF where the selective
> element is a crystal filter after the mixer has a detector bandwidth
> of several KHz (the noise bandwidth of the IF) yet the real bandwidth
> that determines MDS is the bandwidth of the user's ear/brain filter not
> necessarily that of the IF filter or the product detector.  It is quite
> easy for the trained ear to detect coherent (e.g. CW) signals that have
> a negative SNR of one were simply measuring signal power vs. integrated
> noise power in the product detector output.
>
> 73,
>
>    ... Joe, W4TV
>