Dynamic range of USRP2-XCVR2450

GNU Radio
1

Hi,

I am trying to find out the dynamic range of the system USRP2-XCVR2450.

Looking at the schematics it seems there isn’t automatic gain control in
the
whole RF chain, Isn’t it?
Thus the dynamic range of the system would be the ADC dynamic range,
which
is 2 volts

Am I right?

Cheers,
Jorge.

2

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

3

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

4

Hi Marcus,

-10dBm corresponds to 100W. With an input impedance of 100Ohms, since
Pmax
rms= (Vmax rms)^2 / R, --> Vmax rms= 0.1 volt

So, if I am right the maximum theoretical voltage at the input of the
ADC,
if 0 dB gain is set in the transceiver (XCVR2450 in my case) and noise
figure of all components were 0 dB, is 0.1 volt

However the gain of the transceiver is set to increase this [0 - 0.1]
volt
range to [0 - 1] volt range at the input of the ADC.

Is it right ?

Regarding the value of the ADC SNR (72.4dB), why is it important if it
doesnt depend on the input range? (SNR= 6.02*N + 1.76)

There is something I miss…grr !

Many thanks.
Jorge.

5

On 10/24/2010 01:07 PM, Jorge M. wrote:

Am I right?

Many thanks,
Jorge

I believe that it’s a 100ohm balanced input.

But the daughtercards all take a 50-ohm input, and convert to a balanced
output just before the
A/D.

With the exception of the Basic_RX and LFRX, the maximum input power for
any of the other
daughtercards is below -10dBm.

The BASIC_RX and LFRX have a maximum input power of about +10dBm.

–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

6

On 10/24/2010 04:27 PM, Jorge M. wrote:

I will have a look at the white papers from that manufacturers.
From your words, I imagine that the noise generated by the ADC will
set a limit in the noise floor at the input of the ADC, isn´t it?
Yes. The total signal power arriving at the A/D (signal+noise) must
exceed the effective
noise floor of the A/D by enough margin to allow “demodulation” of
your signal of
interest. Where “demodulation” is very loosely defined. Each
modulation technique has
its own SNR requirements. I do radio astronomy, where there’s no
demodulation, per se.
In fact, most of the time my “signals” are below the total noise
floor of the receiving system.
But I have the very great advantage of being able to maximize
bandwidth and integration
time in order to “see” those below-the-noise-floor signals.

–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium