[amsat-bb] XW-2D Returns to Life!

[Zach Metzinger]

On 2020-03-05 04:06, Hasan al-Basri via AMSAT-BB wrote:
> 1. Connect a 50 ohm resistor to the antenna or preamp input.
> 2. Set the rx for maximum sensitivity (RF Gain)
> 3. Adjust IF Gain so that the 50 Ohm resistor produces a signal level of
> -95 dBM
> 4. This creates a common noise floor level calibration point so that we can
> compare local noise (environmental) as well as satellite signal levels.

Hello Hasan,

Ah, noticed one more thing. This is only true if the measured thermal 
noise power bandwidth is the same on both setups.

Thermal noise power is k*T*B, where k is Boltzmann's constant (1.38E-23 
J/K), K is "temperature" of your source in Kelvins (290 is about room 
temp), and B is the bandwidth of the noise in Hertz (1/seconds).

We assume that there is no current flowing through the resistor for this 
example.

(1.38E-23 J/K)(290 K)(1 Hz) = 4.002E-21 Watts

[Kelvins cancel out, leaving Joules/seconds, which is Watts -- math is 
cool!]

However, this is an annoying figure to remember, so we convert it to dBm:

(4.002E-21 W)(1000 mW/W) = 4.002E-18 mW

10 * log10(4.002E-18 mW / 1 mW) ~= -174 dBm

(~= is "approximately equal", rounded)

Now that's an easy number to remember, and all radio amateurs should!

Notice that we initially computed this noise floor with Bandwidth of 1 
Hz. Not even the slowest CW would fit in that, so let's make it 
something more reasonable like 3kHz for voice.

Instead of doing all that math again, we can use logarithms to fix this 
up. Multiplication becomes addition when we're in a log scale (mmm, 
slide rules..), so:

-174 dBm + log(3000 Hz / 1 Hz) ~= -170 dBm

This is the thermal noise floor in a 3 kHz bandwidth from a noise source 
at 290 Kelvin (62 deg F).

Anyway, all of that was to show that bandwidth is very important when 
comparing noise power of two receivers.

If anyone is interested in learning more about noise floor, noise 
figure, or required SNR, here's a great article:

https://tinyurl.com/t7o4n34

--- Zach
N0ZGO