Re: [Discuss-gnuradio] sampling artefact because of two step mixing?

[Luis Simoes]

Hi Don,

the problem is that the second peaks are not at DC in baseband. For example: I 
generate a sine tone at 2.444 Ghz. Then I tune the usrp to centre frequency 
of 2.448 GHz. Now I can see the peak at -3 Mhz in base band. Ok, this is 
correct because what Iam seeing is the original sine. When I tune the USPR to 
centre frequency 2.450 Ghz I see an attenuated peak at 1 MHz in baseband what 
correspond to 2.451Ghz. This second peak is 6 Mhz above the original sine.

If I rise the gain in the usrp_fft.py tool there appear more undesired peaks 
but I am still feeding the USRP with olny one tone. If I decrease the gain to 
eliminate all secondary peaks my original signal gets too weak and it is less 
than 10 db over the noise floor.

I am trying several setting to find out what is the reason of all this. The 
last try was:
1 sine tone at 2.488 Ghz feed into USRP
The result was:
one peak at 2.488 Ghz and one alt 2.432 GHz (20 dB's lower)
Now the secondary peak is 56 MHz away from the original tone!!!

Please, can anyone help me? I am getting desperate...

Luis   

On Wednesday 18 October 2006 20:12, you wrote:
> I have observed a similar phenomenon with the LFRX daughterboard.  It seems
> there is often (or always?) a peak at DC in the baseband (USRP output)
> spectrum, regardless of the tuned frequency of the USRP.  I suspect it is
> due to rounding down in the CIC decimation filter.  Adding a value of
> e*complex(1.0,1.0) where 0.5 <= e <= 0.8 to the USRP output will cancel it.
>
> -- Don W.
>
> ----- Original Message -----
> From: "Luis Simoes" <address@hidden>
> [snipped]
>
> > As I feed the USRP with a single sine tone with a frequency of 2.444 GHz
> > and
> > an amplitude of -50dbm I saw on my plot a nice peak at 2.444 GHZ but also
> > a
> > second peak at 2.452 GHZ but attenuated by 15 db's when daughter board is
> > tuned to 2.452 Ghz.