> 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 <[email protected]> To: [email protected] 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: [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
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