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- Thread starter daniel1211
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- #2

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In most applications, you wish to understand what your signal to noise ratio is which is simple if your modulation is simple. Mulitplexed and pulsed modulation schemes become very difficult as the RMS voltage is dependent upon your modulation.

You may wish to look at the RMS background noise and multiply it by 6.6 to convert that to a peak-peak background noise.

Then, measure your peak-to-peak from modulation.

SNR (dB) = 20 * log (peak-peak_mod / peak-peak_noise)

For a part to do monitor your output strength, you may wish to look into the LTC5507 low frequency peak power detector.

Fun project. I used to mux signals in high school, and acquired a fiber optic link in college to send stereo using PWM :)

- #3

tech99

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I think you mean the signal power of one analogue output. If the modulation is sinusoidal, you can find the RMS voltage using an oscilloscope. You must have the correct load resistor in place. Then the power in the load (Pr) is Vrms^2 / R. If wanted, you can then measure the analogue signal power into the TX (Pt) in a similar way (you need to know the input resistance). If you want overall loss in decibels it is 10 log (Pt/Pr).

- #4

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However, if your sensor's output reflects cable power, then the 10 log (Vout / Vnoise) reflects the signal to noise in terms of cable power.

If your addressing the noise power in terms of output voltage, then the 20 log(Vout / Vnoise).

Tech99's suggestion holds valid for computing the effects of fiber changes (i.e. more cable), so I suspect it is most valid.

Mike

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