RF Attenuation and Signal Levels

This section describes how attenuation is handled by the ni5660 VIs. The user modifies attenuation through the reference level and attenuation parameters of the ni5660 Configure for Spectrum VI and the ni5660 Configure for IQ VI.

The auto attenuation option automatically selects an attenuation setting based on the user-specified reference level. Refer to the ni5660 Configure for Spectrum VI and the ni5660 Configure for IQ VI for details.

The Hardware

The RF downconverter module has five programmable attenuators: three RF attenuators at the beginning of its signal chain and two IF attenuators near the end of the signal chain. They are set up in the following sequence:

Attenuators are either set/asserted or not set/asserted. This allows a dynamic range of RF attenuation from 0–50 dB and a dynamic range of IF attenuation from 0–30 dB. Overall attenuation within the signal chain is the sum of all the attenuators set, for a range of 0–80 dB.

Reference Level and Mixer Level

Understanding the parts played by the reference level and the mixer level is key to setting the individual attenuators. NI–RFSA sets the RF attenuators first.

Let D be the difference between the reference level and the mixer level, such that

D = referenceLevel - mixerLevel

This formula correlates directly with the range of possible RF attenuator settings. Recall that the RF attenuators have a range of 0 - 50 dB. The coercions described in the previous section ensure that

mixerLevel < referenceLevel

and that

referenceLevel - mixerLevel ≤ 50 dBm.

D is then directly proportional to the total RF attenuation, a value between 0-50 dB.

The attenuators are set as follows (refer to the Attenuator Sequence table above):