On 10/19/2010 12:21 PM, Jorge Miguel wrote: > Hi Marcus! > > How do you get the 85 dB value? Is the Intermodulation distorsion of > the page 3 ADC datasheet? In the "features" list for the LTC2284:
72.4dB SNR, 88dB SFDR SFDR is "spur-free dynamic range". > > I am not an expecienced engineer (I am recent graduated) but I was > thinking about the maximun and minimum input powers to be linearly > detected in the USRP receiver. > > I thought the ADC as a good point to start with and then see what is > going on in the XCVR2450 transceiver > > At the ADC point, it is said that is has 2volts p-p dynamic range. > This value can give us the maximun power input at the ADC provided the > input impedance.(Good question. What is the input impedance? I cannot > see it in the ADC datasheet) > > P=(V^2)/R > > Is it right? In other post I can read:/"ADC's datasheet, we need 2Vp-p > to fully utilise its dynamic range. The input impedance of the ADC is > around 220ohms so this is equal to an input signal level of about > 6dBm. If you go above this you will get saturation."/ > I do the calculations and it doesn't match!!! The LTC2284 can be configured for either 1VP-P or 2VP-P. I believe the USRP2 uses 1VP-P configuration. Generally, with ADCs, you ascribe roughly 6dB of dynamic range per bit, this is a 14-bit A/D. > > > Before the ADC is the XCVR2450, with all RF components. Each one of > then has its Noise figure and some of them gain such us the power > amplifier or the Maxim. > > You said that /"The XCVR2450 does have gain control, but it is solely > under control of the host software"/. Besides the ones I mentioned, > is there any other place with gain? > The MAX2829 has roughly 93dB of RX gain-control range--that's right on the data sheet, and is "exposed" to the API inside Gnu Radio. When you create a source, you can specify the gain (actually, you can change it dynamically as well). > Is it the correct way of calculating the dynamic range? > I really need help. > > Thanks, > Jorge. > > > The LTC2284 A/D is a 14-bit A/D. The maximum input voltage the way it's configured is 0.707Vrms. Divide that by the 2^14, and that's roughly your minimum input voltage. In general, though you take a little bit off the top and add a little bit onto the bottom of the range. Google is your friend. I'd suggest looking up "ADC noise floor dynamic range". Plenty of articles out there. -- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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