NI 5660 Theory of Operation

NI PXI-5660

NI 5660 Theory of Operation

The NI 5600 downconverter module (block diagram) 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 ni5660 Configure for Spectrum VI and the ni5660 Configure for IQ VI.

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.