Doubt about the tune()

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

On Wed, Aug 04, 2010 at 05:08:44PM +0800, intermilan wrote:

mux: 0x000098
Besides,why the frequenct of the DUC is set at -4M? how did it come out?
It moves the DC offset 4MHz away from the center of your desired
spectrum. r.baseband_freq + r.dxc_freq = target_freq.

Eric

From: [email protected]
To: [email protected]
Subject: RE: [Discuss-gnuradio] doubt about the tune()
Date: Thu, 5 Aug 2010 09:10:50 +0800

   Thank you for the answer.then I have another quertion.why the 

value is 4M? why not other values like 3M or 5M?and can you tell me
where is the code to set the DC offect?

                                                                                                                                                                 Thank 

you

Thanks again.

I have found the code.But what I want to know is why the value of
the LO offset is automatic set at 4M. Why not other values?Is there any
reasons for it?

                                                                                               Thank 

you.

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