What Marcus and Tom are telling you is that when a signal reaches a USRP Sink in GNURadio it is transformed into an integer representation that is weighted so that the integers represent values -1 <= x < 1.
Input values with magnitudes larger than these will be saturated to these limits, which of course alters (destroys) the signals spectrum.
Ideally your flow graph should try to deliver signals to a USRP sink that have been scaled so that they are slightly within these constraints to maximize the dynamic range offered by this (16Bit) integer representation, hence Marcus's suggestion that you target 0.8 as a maximum amplitude of your signal.
The various USRP daughter card products have different available programable analog gain ranges for the TX amplifiers, and these are controlled by the Gain parameter of the USRP sink. Generally the highest gain values will completely saturate the analog output stages of the Daughter card, given a full range digital input signal. You will need to empirically determine a gain value that obtains the maximum rated power from the daughter card, given your input signal, without driving the output stage into saturation.
-Ian
On Dec 1, 2013, at 8:24 PM, atools_cook <liwenpuxjtu@gmail.com> wrote:
> Marcus，
>
> Thanks for your advice and we do use the "very old" code exactly.
>
> Would you please elaborate the "backing things off" things because I can't
> your point of it.
>
> Best
> Lee
>
>
>
> --
> View this message in context: http://gnuradio.4.n7.nabble.com/trouble-with-gr-multiply-const-ff-tp45018p45039.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
>
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