NI 5661 Theory of Operation

The NI 5600 downconverter module performs two primary functions: frequency shifting, or downconversion, and input signal conditioning. Frequency shifting is performed using a tunable oscillator in the superheterodyne signal chain.

Input signal conditioning is accomplished using three stages of mixer conversion and two sets of gain attenuators whose levels are programmable. The first set of attenuators can be set to minimize distortion and other spurious signals when input levels are high and to minimize noise when input levels are low. Attenuator levels are set using the Attenuation property or the NIRFSA_ATTR_ATTENUATION attribute.

The second set of attenuators is in the second intermediate frequency (IF) path before the third mixer and ensures an appropriate output signal level even if the first mixer is intentionally driven into compression. These attenuators are also set when performing linearity measurements. For more information on proper attenuation levels, refer to Guidelines for Making Accurate Measurements.