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

[Edward R Cole]

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