On 10/19/2010 12:21 PM, Jorge M. wrote:
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)
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.
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.