1. Verify that the discrepancy between the NI 5663 and the source is within the error limits of the devices.
    1. Verify the absolute amplitude accuracy of the NI 5663 using the appropriate value.
    2. Verify the level accuracy of the input signal into the receiver.
  2. Check for cable loss, which can be substantial. For example, an RG58 coaxial cable loses about 2.1 dB of signal amplitude per foot at 2 GHz. Unless you are using high-quality cables, expect losses when working with high-frequency signals.
  3. Perform a self-calibration for the NI 5663.

If errors continue, contact NI technical support or visit ni.com/support.