qt gui frequency sink problem with PSD level

[Ali G. Dezfuli]

Hi all,

I wonder if this is a bug or what:
I just make a QPSK constellation on the unit circle with a quite random
data and
look at the spectrum by "QT GUI Frequency Sink" block in GRC.
With these parameters:
    FFT size = 1000
    Window type = None OR Rectangular
I should see a flat line as the spectrum with -30 dB (= -30 dBW) but
surprisingly it
shows -33 dB !!!
I use GNU Radio Companion 3.7.13.4 and ubuntu 16.04.
Would be grateful if you could help me in this matter.

regards,
Ali