RFX1800 receiver problem

Hi all,
Using RFX1800 daughterboard and two computer, I sent a signal from one
to the other. The signal’s frequency is 100KHz and usrp sink at 1.7GHz.
Also usrp Source is at 1.7Ghz. Sample Rate is chosen 1M so decimation 64
and interpolation 128. Usrp sink gain is useles I guess, because I
it but nothing change at the receiver part. ( when signal’s amplitude is
1). Using a multiply constant(values : 10,20,30…etc) obtain increase
gain at the receiver part. Anyway this is not a big problem, My problem
when I transmit a signal at 100kHz, (no use Multiply constant) ,observe
two signals which is transmitted signal at 100Khz and the other is at 0
And this signal (at 0Hz) has 45-50dB gain and transmited signal has 10
gain. Without a multiply constant I couldn’t observe transmitted signal.
Do you know why I observe an extra signal at 0Hz.

Thanks Merve