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Re: [Discuss-gnuradio] ADC Specs

From: Scott A. McDermott
Subject: Re: [Discuss-gnuradio] ADC Specs
Date: Mon, 15 Dec 2008 17:17:44 -0500
User-agent: Thunderbird (Windows/20080421)

> "The full range on the ADCs is 2V peak to peak, and the input is 50 ohms
> differential. This is equates to 40mW, or 16dBm."
> Please may someone enlighten me, how the 40mW and 16dBm values were
> calculated?

Hello Aadil ---

The impedance (effective resistance to ground) seen from the input pin
on the USRP coax, is 50 ohms.  The calculation you quote assumes a
sinusoidal input at Vpp = 2 Volts peak-to-peak, which means the root
mean square amplitude is Vrms = (Vpp/sqrt(2)) = (2/sqrt(2)) = sqrt(2).
This RMS voltage is going through a 50 ohm resistor, so the power
dissipated is Vrms^2/R = (sqrt(2)^2)/50 = 2/50 = 0.04 Watts, or 40
milliWatts.  To convert linear mW to logarithmic (decibel) mW, or dBm,
we calculate 10*log(40) = 16dBm.

That's the short answer, please feel free to ask for more detail.

--Scott McDermott

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