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I want to test fm preamplifer 88~108 mhz.
This is to receive fm transmission.
I wish to check noise & sensitivity along with gain.
For noise i will use noise figure meter.
For gain & return loss i will use network analyser.
Now i want to know the sensitivity of preamp so that i can decide which will work better at low fm signals.
I think you are doing everything right. The sensitivity of preamp depends on your spec and usually calculated as sum of kTB+NF+SNR where SNR is required signal-to-noise ratio. It looks like your application is for FM broadcasting stereo receiver, in this case you probably need to use SINAD instead of SNR, but it is practically the same calculations.
In kTB part of equation k is Boltzmann’s constant 1.381*E-23 (joule/K); T is the receiver temperature in degree Kelvin (K); and B is the effective noise bandwidth of the receiver (Hz). Because noise figure is defined only for 290K T must be equal to 290K or you cannot use NF in this calculation and must switch to the noise temperature. The product kT will provide you with about -174 dBm/Hz noise power density. Then you calculate effective noise bandwidth B as 10*log(B[Hz]) and add it to -174 dBm/Hz to arrive with the noise power kTB. Now you add noise figure NF to get the total receiver noise floor kTB+NF. This value often call as MDS – minimum discernible signal. This is the value of signal at receiver input which is equal to the receiver noise floor. In order to receive the data with some predetermined quality we need to have input signal to be higher than noise floor by the value called SNR – signal-to-noise-ratio. By adding required SNR to MDS we arrive with receiver sensitivity S=kTB+NF+SNR. For FM receivers usually SINAD used instead of SNR and SINAD is SNR + distortion. Very often it is equal to 12 dB.
Some books have this definition with +3dB, but most of the books have just kTB+NF. It has more sense when it referred to the level with equal power, then you may apply SNR and PG. But if you like more definition with +3dB it is okay, especially if your particular application is better suited for it.
Of course I did not emphasized like or not like. The MDS is a tool and we use it slightly differently depends on particular application. Single carrier signal like AM is definitely different from CDMA of UWB signal. Any way it is no more than artificially created tool that help us to evaluate the system noise performance. More often that not in today's radio communications MDS is loosing sense because it cannot represent the actual noise performance for many practical cases. Even classical kTB can be corrected today despite it seems to be untouchable. I think when RF engineer is doing system analyzes he or she should find the best approach and then verify it with thorough lab tests.
And of course you are right: signal never can be discernible 100% at any level, not only at the noise floor. Just because MDS have business with thermal noise whereas there are so many other sources. And all of them are noise by definition.