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Rev 2.02<br />
Noise Figure vs Sensitivity<br />
Noise figure is what the front end amplifier adds to the theoretical noise floor<br />
Theoretical noise floor is -174dBm in a 1HZ bandwidth. For a 500 HZ receiver bandwidth this would be:<br />
Noise Floor<br />
Noise Floor = -174 + 10*Log (500/1) dB or -174dBm +27dB or -147 dBm<br />
If in the Previous example our receiver has a noise figure of 8dB then the smallest signal you could possibly receive would<br />
need to be 8 dB larger. :<br />
Noise floor would be -147 dBm + 8 dB or -139 dBm or about .03µV<br />
Blocking dynamic range<br />
The blocking dynamic range of a receiver that has an 8 db noise figure and an IF bandwidth of 500 Hz and a -1 dB<br />
compression point of -20dBm is 119 db<br />
Theoretical noise floor for 500 Hz bandwidth is -147dBm<br />
Noise Floor = -174 dBm + (10 log (1/500) ))dB<br />
Noise floor = - 174 dBm +(10 log (.002))dB<br />
Noise floor = - 174 dBm +27dB<br />
Noise floor = - 147 dBm<br />
Add the +8 db Noise figure to the noise floor and get the actual noise floor of -139 dBm.<br />
Receiver Noise floor = theoretical noise floor plus noise figure<br />
Receiver Noise floor = -147dBm + 8dB = -139 dBm<br />
Subtracting the – 20 dBm compression point gives us a difference of 119dB which is the Blocking dynamic range.<br />
Blocking Dynamic range = Receiver Noise floor – 1 dBCompression point<br />
Blocking Dynamic range = -139 –( – 20 dB) = -119 dB<br />
Blocking Dynamic range is expressed as a positive number so<br />
the answer is 119 dB<br />
For a 2.8 KHz wide receiver the theoretical noise floor with a 5 dB noise figure would be<br />
-174dBm + 10 * LOG (2800/1) + 5 Or -174 + 34.5 + 5 or – 134.5 dB<br />
Jack Tiley <strong>AD7FO</strong> Page 106 3/15/2009