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Antenna Amplifier Noise Figure
In message , Jeff writes
To me, NF refers to "noise floor". Lets see him go below that. GPS received signals are among the lowest "power" signals we currently grab. They sit just above the noise floor. It might to you, but in this context it means either Noise Factor or Noise Figure. But you have to be careful, as "noise factor" is a numerical ratio, and "noise figure" is in dB. Of course you can go below the Noise Floor, and in some circumstances and modes the signal is receivable and decodable. In the analogue cable TV world, the noise figure (in dB) can be looked at as the amount of noise power that (say) a real-world amplifier notionally has at its input in excess of that which would be generated from a perfect resistor as its source impedance. As a rule-of-thumb, in a 4MHz vision bandwidth, a perfect 75 ohm resistor generates -59dBmV. [Subtract around 48dB if you want dBmW.] The output of a noiseless amplifier would be -59dBmV + G, where G is the gain in DB. The output of a real-world amplifier would be -59dBmV + NF + G, where N is the noise figure. One method of measuring the noise figure is first to feed the amplifier first from a resistive source, and measure the output noise level. Next, feed the amplifier from a source containing a known amount of noise, and note the increase of output noise. The noise figure can then be calculated. In practice, the noisy source is usually a calibrated noise meter*. The first reading is taken with the noise meter set at zero additional noise output, and then the noise output is increased until the amplifier output level rises by 3dB. This means that the noise meter is now contributing the same amount of noise as the amplifier, and the noise figure can be read directly from its output display. [This conveniently saves having to do any further calculations.] *Usually, a noise meter has a calibrated output meter or other display, and this indicates the level of its noise output in a stated bandwidth - both as an absolute level, and as the equivalent in dB with respect to the basic minimum absolute level. In the cable TV world, the minimum would be -59dBmV (probably shown in microvolts) in a 4MHz bandwidth, or 0dB. If, to increase the amplifier output level by 3dB, the noise meter output had to be turned up to -49dBmV / 10dB, its noise figure would, of course, be 10dB. -- Ian |
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