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

[Joe Subich, W4TV]

 > 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


On 3/3/2014 1:22 PM, Edward R Cole wrote:
> I wonder if any of you have seen this:
> Pn = KTB
> Pn noise power
> K = Botzmann's Contant = 138 E-20
> T = temperature in Kelvin (0C = 273K)
> B = bandwidth in Hz
>
> convert to dB:
> Pn = Log (KTB) = -198.6 + LogT + LogB
>
> 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
>
> which a pretty good MDS for a HF receiver.
> MDS defined as minimum detectable signal or when SNR = 1
> Ps = Pn
>
> So what if you use a 500-Hz filter?
>
> that merely becomes the Log (0.5/2.7)*Pn = -7.3 -129.7 = -137 dBm
> Does anyone recognize this as close to the published MDS for the K3?
>
> So Pn reduced 7.3 dB when the bandwidth was reduced from 2.7 to 0.5 KHz!
>
> Hello!  The noise reduced.  Why? Because the noise bandwidth was reduced.
> It is to be expected!
>
> Now your homework is to calculate Pn for 2.1 KHz to 250 Hz reduction. ;-)
>
> This is accurate if there is no AGC operating which will change receiver
> gain, including both Pn and Ps.  For that math I suggest you contact
> Elecraft.  No, I am not serious about the last comment, unless you are a
> communications engineer and plan to design a receiver.
>
> 73, Ed - KL7UW
> ---------------------
> From: Bill Frantz <frantz at pwpconsult.com>
> To: Elecraft at mailman.qth.net
> Subject: Re: [Elecraft] [K3] Perfect for Field Day S&P ... Limited
>          Production 700 Hz 8-Pole Filters
> I am using:
>
> 13 KHz: 0 dB
> 2.7 KHz: 0 dB
> 2.1 KHz: 0 dB
> 250 Hz: 6 dB
>
> When the 2.1 and 250 filters switch in, the signal level in the
> earphones doesn't seem to change, but the noise level does
> indicating an improvement in S/N. Perhaps they are sharpening
> the cutoff of the DSP filter removing some of the off-frequency noise?
>
> Cheers - Bill, AE6JV
>
>
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
>      "Kits made by KL7UW"
> Dubus Mag business:
>      dubususa at gmail.com
>
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