Agung,
If the problem is that the USB-SA44B has some LO feedthru which is causing interference, there are several ways to mitigate this.
1) Use a reference level of -30 dBm. This will turn on the RF preamplifier, which also blocks LO leakage. Your leakage should be well below -50 dBm in this state.
2) Add an external 20 dB pad to the input of the SA44B and increase the sensitivity by lowering the reference level. With a 20 dB pad, and a reference level of -40 dBm, your LO leakage should be around -70 dBm.
3) If this is not enough, switching to the BB60C is recommended. Its LO feedthrough is below -80 dBm in all modes, and the LO frequency is >2.4 GHz higher than the signal, so it is very unlikely to interfere.
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