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Re: [Discuss-gnuradio] USRP2 and external RF tuner

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] USRP2 and external RF tuner
Date: Thu, 02 Sep 2010 14:13:02 -0700
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The BasicRX has no amplification. Its full scale amplitude is +10dBm. The ADC in the USRP2 has a rated SFDR of 88dBFS. That means you may see spurs at levels high as -78dBm equivalent when using a BasicRX. In the plots you sent me, your signal is -50 dBm and the spur is more than 40 dB lower, putting it at better than -90dBm, or more than 12dB better than the spec. That is what you need to expect from a real-world ADC.

Normally, if you want to look at a -60dBm signal, you will amplify it a lot before you put it into the ADC. That will make it much stronger than the spurs. This is why boards like the RFX, DBSRX, and WBX all have amplifiers. I suggest amplifying your signal by at least 30dB.


On 09/01/2010 06:10 AM, Luca Pascale wrote:
Hi all,

I am working with a usrp2+BasicRx and an external tuner with a 21.4 MHz
IF output and a bandwidth of about 20 MHz.
The same setup but with usrp1 (instead of usrp2) it's working well.

Now I am injecting a tone at 21.45 MHz (-60 dBm) at the input of the
BasicRX and set the usrp2 freq at 21.4MHz (with "set_rx_freq()" UHD API).
The problem is that:

a) with sample rate > of about 3.125M I have 2 strong spuriouses. The
first in the middle of the FFT (FFTDIM/2) and the second one at about
the middle of the second half of the FFT

b) with a sample rate <=3.125 the second spurious disappear but the
first at FFTDIM/2 is still present.

In both case I can also see the tone at 21.45MHz.
To compute the spectrum I use matlab

Am I doing mistakes or someone have the same problem ?
Suggestions ?

Thanks in advance


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