External Attenuation Table

The spectrum measurement allows you to view a signal in the frequency domain within the frequency span that you have configured.

Accuracy of the spectral measurements performed on a radio frequency (RF) signal is limited by several random and systematic sources of error [1]. Examples of random sources of error are, Additive White Gaussian Noise (AWGN) [2], quantization noise [3] etc. Examples of systematic sources of error are; incorrect or outdated signal or spectrum analyzer calibration, impedance mismatches of RF ports, and uncompensated frequency response of external RF components like cables, power splitters, switches, attenuators etc.

Impact of random sources of error can generally be reduced by measurement averaging, but systematic errors must be explicitly compensated.

The following illustration shows the frequency response of a 10 dB RF attenuator. Note that the attenuation is not exactly 10 dB for all frequencies, introducing systematic errors in measurements.

External Attenuation Table in RFmx

In RFmx, to compensate for systematic errors due to frequency response of RF components in spectral measurements, External Attenuation (dB) and the corresponding Frequency (Hz) parameters should be specified as a table.

If the RF signal is attenuated by the RF components in the signal path, you need to enter a positive value of the Attenuation (dB) in the table. Similarly, if the RF signal is amplified by the RF components in the signal path, you need to enter a negative value.

Specifying Calibration Planes and External Attenuation Tables

Every external attenuation table should be associated with a unique string, the Table Name.

The signal path can consist of one or more calibration planes, where each calibration plane represents one or a group of RF components. In a calibration plane that is indicated by the selector string, you can configure multiple external attenuation tables, but you can select only one Active Table per calibration plane.

If you do not mention a Table Name and a Selector String to be used for the calibration plane, a default is used.

The configuration of the external attenuation table and the selection of the Active Table in the calibration plane can be done as follows for the preceding example.

If the Active Table does not contain a attenuation value for a particular frequency, it is calculated by linearly interpolating the available entries in the table. For frequency values lesser than the minimum frequency specified in the table, attenuation value corresponding to the minimum frequency in the table is used. Similarly, for frequency values greater than the maximum frequency specified in the table, attenuation value corresponding to the maximum frequency in the table is used.

A single table configured on the calibration plane makes the table active by default.

Amplitude Correction Type

There are two ways of compensating the acquired spectrum in all spectral measurement; RF Center Frequency, and the Spectrum Frequency Bin.

RF Center Frequency

Compensates the entire acquired spectrum with a constant attenuation value that is obtained by interpolating all the active tables for the RF acquisition frequency. This is the default amplitude correction type.

In case of measurements which are required to acquire at multiple frequencies like IM, Spur, Harmonics and ACP with the Measurement Method property set to Dynamic Range, the external attenuation corresponding to that acquisition frequency is used in every segment.

Spectrum Frequency Bin

Compensates all frequency points in the acquired spectrum with the respective attenuation values obtained by interpolating all active tables.

References

[1] Taylor, John. Introduction to error analysis, the study of uncertainties in physical measurements. 1997

[2] Johnson, John Bertrand. "Thermal agitation of electricity in conductors." Physical review 32.1 (1928): 97

[3] Gray, Robert M., and David L. Neuhoff. "Quantization." IEEE transactions on information theory 44.6 (1998): 2325-2383