The Power Calibration submenu configures and starts a source or receiver power calibration.
Source Power Cal... opens the configuration dialog for a source power calibration.
Receiver Power Cal... opens the configuration dialog for a receiver power calibration.
Power correction data can be stored to a cal group and applied to several channels; see description of the Calibration Manager. In remote control it is possible to read, modify and reload power calibration data. Refer to the description of the commands SOURce<Ch>:POWer<Pt>:CORRection:DATA and [SENSe<Ch>:]CORRection:POWer<port_no>:DATA.
Remote control:
SOURce<Ch>:POWer:CORRection... [SENSe<Ch>:]CORRection:POWer...
Opens the configuration dialog for a source power calibration.
Purpose of a source power calibration
A source power calibration ensures accurate power levels of the generated wave at an arbitrary reference plane in the measurement path. The calibrated reference plane is typically the input of the DUT.
In a frequency sweep, the power at the reference plane is maintained at a constant Cal Power value. The source power calibration eliminates frequency response errors in the signal path between the source and the reference plane. It is possible to introduce an arbitrary attenuation or gain into the signal path so that the Cal Power is not restricted to the power range of the source. A typical application for a power calibration in a frequency sweep is the measurement of the gain of an amplifier across a frequency range but at a fixed input power.
In a power sweep, the power calibration ensures that the power at the reference plane is either constant or a linear function of the stimulus power. A typical application for a power calibration in a power sweep is the measurement of the gain of an amplifier across a power range but at a fixed frequency.
The correction data acquired in a frequency or power sweep is re-used if a Time or CW Mode sweep is activated.
Calibration procedure
The source power calibration requires an external power meter, to be connected via GPIB bus, USB or LAN interface. The power sensor can be connected directly at the reference plane or to any other point in the test setup where the signal power is known to be proportional to the power at the reference plane.
The source power calibration involves several steps:
The analyzer performs a first calibration sweep at the source power that is likely to produce the target power (Cal Power) at the reference plane. A known attenuation or gain at the source port and in the signal path between the source port and the reference plane is taken into account.
The power measured at the reference plane is displayed in the calibration sweep diagram and compared to the Cal Power. The comparison serves as an input for the correction of the source power.
Steps 1 and 2 are repeated for a specified Number of Readings or after the deviation between the measured power at the reference plane and the Cal Power is below a specified Tolerance.
After the last calibration sweep, it is possible to perform an additional Verification Sweep in order to check the accuracy.
After a power calibration one can expect the power at the reference plane to be within the range of uncertainty of the power meter. After a change of the sweep points or sweep range, the analyzer interpolates or extrapolates the calibration data.
The Source Power Cal dialog provides the following control elements:
Calibrate – Source contains all analyzer ports or external generators which can be used as a source for the calibrated wave quantity. The list contains all analyzer ports Port 1 to Port n. Generators (Gen 1, Gen 2, ...) must be configured explicitly in the System Configuration – External Generators dialog before they appear in the list.
Take Cal Sweep initiates a calibration sweep for the selected source. The progress of the sweep is monitored in the calibration sweep diagram. After the necessary calibrations have been completed, a message in the trace list indicates the status of the power calibration.
Abort Sweep stops a running sweep. The data acquired in the aborted sweep is discarded.
The Settings panel displays the essential parameters of the source power calibration, to be defined in the Modify Source Power Cal Settings dialog.
Source Power Correction Off can be selected/deselected as soon as power calibration data have been acquired for the selected source. If the box is checked, the calibration data is kept in memory but not applied to the source. The Power Correction Off softkey disables the power correction for all receivers and sources.
Modify Settings... opens the Modify Source Power Cal Settings dialog.
Power Meter selects an external power meter that is used to measure the exact source power at the calibration point. Power meters (Pmtr1, Pmtr2, ...) must be configured explicitly in the System Configuration – External Power Meters dialog before they appear in the list.
Power Meter Config... opens the System Configuration – External Power Meters dialog.
Perform Verification Sweep after Cal enables or disables a verification sweep that the analyzer performs after the source power calibration. The sweep is also displayed in the calibration sweep diagram. Its purpose is to test the accuracy of the source power calibration after the final correction data have been acquired. The final power calibration results are applied starting with the first sweep after the verification sweep.
SOURce<Ch>:POWer<Pt>:CORRection[:ACQuire] SOURce<Ch>:POWer<Pt>:CORRection:COLLect[:ACQuire] SOURce<Ch>:POWer<Pt>:CORRection[:STATe] SOURce<Ch>:POWer<Pt>:CORRection:GENerator[:STATe] SOURce<Ch>:POWer<Pt>:CORRection:PMETer:ID SOURce<Ch>:POWer<Pt>:CORRection:COLLect[:ACQuire]:VERification[:STATe]
The calibration sweep diagram displays the power measured by the external power meter while a source power calibration is performed.
A trace is displayed for each calibration sweep.
If Perform Verification Sweep after Cal is enabled in the Source Power Cal dialog, an additional trace is displayed after the end of the calibration.
The diagram is automatically scaled so that the center corresponds to the selected Cal Power. For frequency sweeps, a horizontal trace close to the Cal Power indicates an accurate power calibration. For power sweeps, the ideal trace is a power ramp.
Below the sweep diagram, the status messages Calibration in Progress/Finished and Verification in Progress/Finished show the progress of the calibration procedure.
The Modify Source Power Cal Settings dialog displays the target power for the source power calibration and defines stop criteria.
The dialog provides the following control elements:
Port n Cal Power (where n is the calibrated source port selected in the Source Power Cal dialog) shows the target power of the calibration (Cal Power) and the Cal Power Offset. Both parameters can be adjusted in the Port n Cal Power dialog, to be opened via Modify Cal Power...
The parameters in the Power Meter Readings panel provide stop criteria for the calibration procedure: Maximum Number of Readings is a limit for the number of calibration sweeps. Tolerance denotes the maximum deviation of the measured power from the Cal Power. The calibration procedure is stopped if the Maximum Number of Readings is reached or if the measured power is within the Tolerance.
If Switch off All Other Sources... is checked, the power at all sources except the calibrated source is switched off during the calibration. This is advisable especially if the power calibration is intended for intermodulation measurements where different signals are combined.
SOURce<Ch>:POWer<Pt>:CORRection:NREadings SOURce<Ch>:POWer<Pt>:CORRection:COLLect:AVERage[:COUNt] SOURce<Ch>:POWer<Pt>:CORRection:COLLect:AVERage:NTOLerance SOURce<Ch>:POWer<Pt>:CORRection:OSOurces[:STATe]
The Port n Cal Power dialog defines the target power for the source power calibration. The settings are port-specific.
The Port n Cal Power settings serve two different purposes:
Port Power Offset defines a port-specific offset to the channel power. The actual output power at the port is equal to the channel power Pb (Channel – Stimulus – Power) plus the Port Power Offset. It is equal to the Port Power Offset (in dBm) if 0 dBm is selected instead of Pb. For power sweep and the selection Pb, the actual port power varies across the sweep, for other configuration the port power is constant.
Cal Power Offset specifies a gain (positive values) or an attenuation (negative values) in the signal path between the source port and the calibrated reference plane. With a Cal Power Offset of n dB, the target power at the reference plane is equal to the actual output power at the port plus n dB. The Cal Power Offset has no impact on the source power.
Use of an amplifier in the signal path
Assume that a DUT requires a constant input power of +35 dBm, and that the measurement path contains an amplifier with a 30 dB gain.
After a reset of the analyzer the channel power Pb is zero dBm. With a Port Power Offset of +5 dB at the calibrated source port and a Cal Power Offset of +30 dB, the source power calibration ensures that the constant input power of +35 dBm is maintained across the entire sweep range. The actual output power of the analyzer is +5 dBm.
Note that a power calibration with an appropriate Cal Power Offset will automatically prevent excess input levels at the DUT.
SOURce<Ch>:POWer[:LEVel][:IMMediate]:OFFSet <numeric_value>, ONLY | CPAD SOURce<Ch>:POWer<Pt>:CORRection:LEVel:OFFSet
Opens the configuration dialog for a receiver power calibration. The receiver power calibration data is acquired in a calibration sweep after appropriate selection of the calibrated wave quantity and the source.
Purpose of a receiver power calibration
A receiver power calibration ensures that the power readings at a specified input port of the analyzer agree with the source power level calibrated at an arbitrary reference plane. Typically, the reference plane is at the input of the receiver so that the calibration eliminates frequency response errors in the calibrated receiver.
A receiver power calibration generally improves the accuracy of power (wave quantity) measurements.
Calibration procedure and calibration sweep
The receiver power calibration is based on a wave with known power.
No external connection is needed for a receiver power calibration on the reference receiver (wave quantity an with Port n used as a source). Besides, the reference receiver is automatically calibrated together with any source power calibration of an analyzer port.
Calibration of a received wave bn involves an external test setup. The received wave to calibrate can be generated by the analyzer or an external generator.
The receiver power calibration involves a single calibration sweep. The calibration sweep is performed with current channel settings but with a maximum IF bandwidth of 1 kHz. Smaller IF bandwidths are maintained during the calibration sweep; larger bandwidths are restored after the sweep.
The analyzer measures the power at each sweep point, compares the result with the nominal power of the source, and compiles a correction table.
An acoustic signal indicates the end of the calibration sweep. At the same time, a message next to the calibrated source indicates the status of the receiver power calibration. After a change of the sweep points or sweep range, the analyzer interpolates or extrapolates the calibration data.
The Receiver Power Cal dialog provides the following control elements:
Wave Quantity to Calibrate provides two pull-down lists to select one of the transmitted waves an or one of the received waves bn for the calibration. Each power calibration only applies to the selected wave quantity in the active measurement.
Receiver Power Correction Off can be selected/deselected as soon as power calibration data have been acquired for the active wave quantity. If the box is checked, the calibration data is kept in memory but not applied to the wave quantity. The Power Correction Off softkey disables the power correction for all receivers and sources.
Used Source contains all analyzer ports or external generators which can which can be used as a source for the calibrated wave quantity. The list contains all analyzer ports Port 1 to Port n. Generators (Gen 1, Gen 2, ...) must be configured explicitly in the System Configuration – External Generators dialog before they appear in the list. Behind the source, a message indicates the status of the power calibration.
Take Cal Sweep initiates a calibration sweep for the selected wave quantity and source. The progress of the sweep is monitored in the Sweep progress bar.
The accuracy of the receiver power calibration depends on the accuracy of the source. Depending on the test setup and reference plane, it is recommendable to perform a source power calibration before the receiver power calibration.
[SENSe<Ch>:]CORRection:POWer:ACQuire [SENSe<Ch>:]CORRection:POWer:AWAVe[:STATe] [SENSe<Ch>:]CORRection:POWer:DATA [SENSe<Ch>:]CORRection:POWer[:STATe]
The status of a power calibration is indicated in the trace list and in the Used Source panel of the Receiver Power Cal dialog. The following messages can be displayed:
PCal denotes an active power calibration with unchanged sweep points (no interpolation or extrapolation).
PCai denotes an active, interpolated power calibration (at least one calibration step is interpolated).
PCa? denotes an active power calibration with changed step attenuator setting and disabled ALC.
PCao denotes an active power calibration with a changed source power (in at least one calibration step).
PCax denotes an active, extrapolated power calibration (in at least one calibration step).
Uncal denotes an uncalibrated source (not available for traces).
The labels appear in the trace list, provided that the measured quantity is an S-Parameter, wave quantity, ratio, or a derived quantity (stability factor, PAE), and that the necessary calibrations have been performed.
Necessary Calibrations
A trace is marked as power calibrated as soon as the analyzer assumes that the effects of an inaccurate source or receiver power have been consistently removed. The conditions for consistency depend on the measured quantity (see also Data Flow):
S-parameters Sxy represent the ratio of a received wave bx to a driving wave ay. The system error correction for S-parameters generally includes a receiver power calibration, however, a source power calibration can still improve the accuracy. Therefore S-parameters are assumed to be power calibrated as soon as the driving wave has been power calibrated. The same holds for quantities derived from S-parameters, e.g. stability factors.
For wave quantities, ratios, and derived quantities (e.g. the PAE), the driving waves and all receivers must be power calibrated.
The analyzer can interpolate and extrapolate power correction data so that a source or receiver power calibration can be reused after a change of the frequency sweep range:
At new sweep points within the calibrated sweep range, interpolation is applied to calculate the correction data. A label PCai in the trace list indicates an interpolated power calibration.
At new sweep points outside the calibrated sweep range, the correction values are extrapolated: Sweep points below the lowest calibrated frequency are assigned the correction value of the lowest frequency. Sweep points above the highest calibrated frequency are assigned the correction value of the highest frequency. A label PCax in the trace list indicates an extrapolated power calibration.
Applies all available source and/or receiver power calibrations to the active channel. This function is disabled if no power correction data has been acquired for the active channel.
Uncheck the ...Power Correction Off boxes in the Source Power Cal and Receiver Power Cal dialogs to enable an individual source or receiver calibration. Press Power Correction Off to disable the power calibration of all sources and receivers.
SOURce<Ch>:POWer:CORRection:STATe ON [SENSe<Ch>:]CORRection:POWer:STATe ON
Disables all source and receiver power calibrations of the active channel so that all power correction data is ignored. This function is disabled if no power correction data has been acquired for the active channel.
Check the ...Power Correction Off boxes in the Source Power Cal and Receiver Power Cal dialogs to disable an individual source or receiver calibration. Press Power Correction On to re-enable the power calibration of all sources and receivers.
SOURce<Ch>:POWer:CORRection:STATe OFF [SENSe<Ch>:]CORRection:POWer:STATe OFF