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Setting RF Attenuation on the NI 5660/5661

Hardware: Modular Instruments>>RF Measurement Devices>>PXI-5600, Modular Instruments>>RF Measurement Devices>>PXI-5660, Modular Instruments>>RF Measurement Devices>>PXI-5661, Modular Instruments>>RF Measurement Devices>>PXIe-5601

Problem:
I am trying to override the NI-RFSA default attenuation setting. How can I explicitly set the level of attenuation on my NI 5660/5661?

Solution:
The RF attenuation level is automatically set by the NI-RFSA driver based on the reference level and mixer level using the following equation:

RF Attenuation [dB] = Reference Level [dBm] - Mixer Level [dBm]

NI-RFSA automatically selects an optimal mixer level value given a reference level to offer the best compromise between distortion and noise; therefore, in most cases, specifying the input reference level is enough to set the appropriate attenuation level. For reference levels above -20 dBm, the optimal mixer level is set to -20 dBm.

Both the NI 5660 and NI 5661 use the NI 5600 downconverter, which has five programmable attenuators: three RF attenuators and two IF attenuators. The RF attenuator settings are the only ones that you can modify. The following table shows the different attenuators and their values.

 

Using a combination of these attenuators, RF attenuation levels can range from 0 dB to 50 dB in increments of 10 dB, as shown in the table below.

where D represents the total RF attenuation.

It is important to notice that for every 10 dB of RF attenuation, approximately 10 dB of noise is added to the system. Therefore, in some instances you may want to explicitly control the RF attenuators and mixer level to achieve better noise performance at the expense of distortion, or vice versa. Changing any two parameters in the RF attenuation equation sets the third parameter. For example, setting the reference level to 0 dBm and setting the attenuation to 30 dB sets the mixer level to -30 dBm. In general, higher attenuation levels increase the noise level while decreasing distortion, whereas lower attenuation levels decrease the noise level while increasing distortion.

Before modifying the attenuation values, first query the NI-RFSA driver for the default attenuation value for a given reference level. Starting from this value, you can override the default attenuation value in increments of 10 dB by using the Attenuation property node or the NIRFSA_ATTR_ATTENUATION attribute. Changing this setting also changes the mixer level. For optimum performance, the mixer level should be set at -20 dBm. Notice that RF attenuation can be manually configured only with the NI 5600/5601 (downconverter only mode) and the NI 5661.

Note: NI recommends you use the default settings for the attenuator control most of the time by setting only the reference level. These settings provide optimum performance for most measurements.

Note: Incorrectly setting the reference level can cause severe damage to the hardware. Ensure that the reference level is never lower than the maximum expected power of the input RF signal. NI is not liable from any damage resulting from improper signal connections.

Related Links:
Product Manual: NI PXI-5660 Specifications (January 2008)
Product Manual: NI PXI-5661 Specifications (November 2007)

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Report Date: 02/20/2009
Last Updated: 03/10/2009
Document ID: 4UJIESJQ

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