[Discuss-gnuradio] doubt about the tune()

[intermilan]

hi all:
       I recently run the test_usrp_standard_tx/rx.cc to do some test.and after I run the test_usrp_standard_tx.cc, I got the follow information:
which:    0
interp:   64
rf_freq:  2.5e+09
amp:      30000.000000
nsamples: 3.2e+07
Subdevice name is Flex 2400 Tx
Subdevice freq range: (2.3e+09, 2.7e+09)
mux: 0x000098
baseband rate: 2e+06
target_freq:     2500000000.000000
ok:              true
r.baseband_freq: 2504000000.000000
r.dxc_freq:      -4000000.000000
r.residual_freq: 0.000000
r.inverted:      0
xfered 3.2e+07 bytes in 4 seconds.  8.002e+06 bytes/sec.  cpu time = 0

      as you can see, I want to set the frequency at 2.5G(RFX2400) ,but there is a parameter called r.basebande=2.504G.I know it is the return valus of the tune().but I want to know what is the function of this parameter and how did  2.504G come out. can anyone help me to figure it out?
      Besides,why the frequenct of the DUC is set at -4M? how did it come out?

                                                                                                                                                                                                              Thank you in advance