RF Attenuation and Signal Levels

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 architecture 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 above 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).

Attenuation in NI-RFSA

You modify attenuation using the Attenuation property or the NIRFSA_ATTR_ATTENUATION attribute.