USRP2 and external RF tuner

GNU Radio
1

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
[10log10(|fftshift(fft(signalwindow))|^2)].

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

Thanks in advance

Luca

2

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.

Matt

3

Hi Matt,

I have connected the RF tuner that provides (nominal) a gain of 30 dB.
So
the chain is (SIGGEN)—>(TUNER)—>(USRP2).
The tuner itself introduces spurious signal (below -70dBm).
The output spectrum is:

a) without tuner (description is above the picture)
http://imagebin.ca/view/ZTARvXd.html

b) with tuner
http://imagebin.ca/view/KOYDSsHI.html

Do I need more amplification on the IF signal ?
Or does exist another way to obtain a clear spectrum with the injected
tone
only ?

Is it possible that some spurious are due to the downconvertion process
?

Regards,
Luca

2010/9/2 Matt E. [email protected]

4

Well, you haven’t really provided enough information here. Assuming
that your “RF Tuner” consists of some sort of frequency conversion and
an LO, you should know that you will always have leakage between the LO
input into the mixer and the IF output. Perhaps that is what you are
seeing? Does your RF Tuner have any sort of filtering? If the spurious
signal is on the IF, amplifying it will also amplify the spurious
products.

~Jeff

On 9/3/2010 11:45 AM, Luca P. wrote:

Hi Matt,

I have connected the RF tuner that provides (nominal) a gain of 30 dB.
So the chain is (SIGGEN)—>(TUNER)—>(USRP2).
The tuner itself introduces spurious signal (below -70dBm).
The output spectrum is:

a) without tuner (description is above the picture)
http://imagebin.ca/view/ZTARvXd.html

b) with tuner
http://imagebin.ca/view/KOYDSsHI.html

Do I need more amplification on the IF signal ?
Or does exist another way to obtain a clear spectrum with the injected
tone only ?

Is it possible that some spurious are due to the downconvertion process ?

Regards,
Luca

–
~Jeffrey L., K1VZX