Hello Test Tiger,
That noise level of 0.200V does not sound very reasonable to me. The first thing that I would check is to make sure that you are using the latest version of the driver,
NI-RFSA 2.0.1. I tried this out with a PXI-5600 and PXI-5621, and was able to trigger off signals as low as 5mV without noise interfering with the trigger signal. One thing I have noticed is that the noise level will depend on the reference level you set with the ni5660 Configure.vi. This reference level determines how the attenuators in the PXI-5600 downconverter are set, which ultimately determines the magnitude of the signal being passed from the downconverter into the PXI-562x digitizer. What reference level are you using?
One good test to try would be to view the raw power spectrum acquired by the PXI-562x digitizer with the niScope SMT Power in Band.vi example program, found under C:\Program Files\National Instruments\LabVIEW 8.x\examples\Spectral Measurements Toolset\niScope. On the front panel of this example program, under the Configuration tab, enter the Device Name for your PXI-5620 (i.e. DAQ::1), enter a center frequency of 16M and span of 32M. Under the Averaging tab, select RMS averaging for the type, and continuous for the linear weighting mode. Under the Units tab, place a checkmark in the box for PSD. With these setting, run the example program with and without the output of the PXI-5600 connected to the PXI-5620. This will give you a good idea of the noise floor of the PXI-5620. The peaks in the lower frequency ranges (0-5MHz) are due to dithering being enabled on the PXI-5620. You can disable this dithering by configuring the niScope Property Node in the SMT Config niScope for Spectrum.vi. With dithering disabled, I was able to see a noise floor of around -125dBm with the downconverter attached, and around -125dBm without the downconverter attached. If you see a drastically different noise floor, it might be helpful to attach a screenshot of your power spectrum graph in a reply to this post.
I hope this helps,
Travis G.
Applications Engineering
National Instruments
www.ni.com/support