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An FM radio receiver, using a 1 kHz tone at 3 kHz deviation has a 12 dB SINAD (Signal To Noise and Distortion) of -116 dBm.

I would like to increase the 12 dB SINAD sensitivity of the receiver to at least -121 dBm to increase the weak signal performance.

Let's say I have an RF preamp, which, at the frequency of my receiver has 10 dB of gain with a 1 dB noise figure.

What process should I follow to calculate the new 12 dB SINAD sensitivity of the receiver with the preamp installed and whether it will provide sufficient amplification to increase my 12 dB SINAD sensitivity to -121 dBm?

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It's difficult to quantify the relationship between SINAD and noise figure. SINAD measures the signal quality at the output of the radio, so effects of the demodulator and sometimes even speaker are included.

Noise figure quantifies the internal noise generated by a component. This will degrade SNR, but the amount by which SNR is degraded depends on the input noise.

It's an apples to oranges comparison, with some additional unknowns thrown in. We'd need to know:

  • How the demodulated signal quality (SINAD) relates to the RF input SNR.
  • What the external noise (man-made and natural RF interference) is.
  • The receiver's noise figure.
  • Other details of your station, such as feedline losses and antenna gain.

But if you want to know generally how noise performance can be quantified and what your 1 dB NF LNA might be able to do about it, check out, see:

How can I calculate the effects of an LNA, antenna gain, etc. on noise performance?

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Phil's answer is sound from a technical perspective since there is not sufficient data to fully answer the question.

From a more pragmatic perspective, if you wish to increase the 12 dB SINAD sensitivity of your receiving system, you should:

  1. Minimize feedline losses
  2. Increase the gain of your antenna

Your required 5 dB improvement in sensitivity is usually well within reach by optimizing these two factors. Any improvement in gain or reduction of losses in these these two areas directly contribute to an increase in sensitivity with no degradation of your 12 dB SINAD.

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    $\begingroup$ I did seriously edit my answer, so I'd be interested to know if it's still technically sound :) $\endgroup$ – Phil Frost - W8II Jan 30 '18 at 14:05
  • $\begingroup$ 3. Place the preamp at the antenna, so feedline losses don't matter as much. $\endgroup$ – Mike Waters Jan 30 '18 at 14:23
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    $\begingroup$ @MikeWaters But if the noise factor of the preamp is worse than that of the first stage of the receiver, the preamp may not be beneficial for the application - i.e. a preamp is not a sure bet for improvement. $\endgroup$ – Glenn W9IQ Jan 30 '18 at 14:35
  • $\begingroup$ I assumed that you would assume that, Glenn. :-) EME operators have mounted relay-switched preamps with very low NFs on their antenna booms for decades. $\endgroup$ – Mike Waters Jan 30 '18 at 16:51

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