discuss-gnuradio
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Discuss-gnuradio] Dynamic range of USRP2-XCVR2450


From: Jorge Miguel
Subject: Re: [Discuss-gnuradio] Dynamic range of USRP2-XCVR2450
Date: Sun, 24 Oct 2010 19:07:36 +0200

Hello,

I am trying to find out the minimum/maximun input power of the ADC LTC2284.
I know that the maximum input voltage is 1 Volt, 0.707Vrms, but what is the input impedance? so that I can calculate the maximum input power as:

minimum input power   (SNR ADC) = Maximun input power
( [(0.707)/(2^14))^2]/Rin) + 72.4 dB = Maximun input power

Am I right?

Many thanks,
Jorge

On 19 October 2010 19:18, Marcus D. Leech <address@hidden> wrote:
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


reply via email to

[Prev in Thread] Current Thread [Next in Thread]