USRP2 - RFX900 DB - unwanted spurs

I am using GNU radio on SUSe. GNU radio and GRC installed properly with
some efforts. Could get the USRP2 to connect as well. I am using RFX900
DB. Using the GRC example for constant signal output
(USRP2_const_waveexample.grc) I am able to transmit a signal at desired
frequency. I want to test the RFX900 for the entire range frm 750MHz to
1050MHz. For this I am using variable slider from GRC to change
frequency and amplitude. Keeping amplitude such that the output power is
-40dBm as shown on the spectrum analyzer I am changing the output
frequency over the entire range in 10MHz steps.

what I observe is unwanted spurs described as follows (sample attached -
In the figures markers show the main transmit frequency)

  1. 900MHz DB results one unwanted Spur(residual) signal which has -60dBm
    power

  2. this residual changes in frequency with transmit frequency but remain
    constant in power for different signal frequencies as well as increase
    or decrease in signal power

  3. the frequency difference between the main transmit power and this
    residual peak is approximately 11.2 - 13.6 MHz.

  4. this residual peak is present even if the transmit power is 0
    provided the USRP is powered and GRC is running.

  5. there is a unwanted power emmission and some residual peaks for the
    890MHz-940MHz band which remain constant in frequency for different
    transmit frequencies but its amplitude and residual peaks change as per
    transmit frequency

  6. however the residual peak at 900MHz (-50dBm) is always present with
    almost constant power.

  7. this band power transmission is present even if the transmit power is
    0 provided the USRP is powered and GRC is running. 900Mhz peak residual
    is constant

  8. If the main transmit frequency is set to 870MHz to 930Mhz band, more
    residual peaks of high power arise in this particular band

  9. the cause of the residuals is suspected from the internal components
    of the USRP and DB but the exact cause is not known at this stage

  10. external sources are not a cause of these residuals as they cannot
    be seen if USRP is off and GRC not running

My questions
a. Is getting such spurs normal or is it this specific RFX900 DB?
b. if it is normal, is there a way to get rid of them or minimise them?
c. or if not - will i be doing something wrongly ?

kindly shed some light on this…Thank you

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