Reference Impedance (Input Analog Tab)

The Reference Impedance parameter specifies the reference impedance "Z(ref)" value that the VSA uses to calculate the power in dBm deciBels referenced to a milliWatt: dB relative to 1 milliwatt dissipated in the nominal input impedance of the analyzer for the selected Logical Channel. The VSA uses the following equation to compute dBm. dBm is the power in decibels referenced to a milliWatt.

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The Reference Impedance can be specified to match the Input Impedance by selecting Same as Input or set separately by clearing the Same as Input checkbox and entering an impedance in the text box.

This parameter can be selected for All channels or individually for each available channel. The available channels are displayed in Logical Channel located at the top of the dialog box. If the selections for individual channels are different, the arrow button () after the Reference Impedance text box will be orange. Click the arrow button to view the Reference Impedance selection for each available channel.

Measurement Concepts

When the DUT Device under Test: An acronym used to describe some type of electrical apparatus connected to test instrumentation. The apparatus can range from a single component to a complex subsystem such as a mobile phone, base station or MSC. and measurement hardware input impedance are the same, the Same as Input checkbox is selected. No changes are required to the Input Impedance or Reference Impedance parameters.

When there is an impedance mismatch between the DUT and the input of the measurement hardware, the Same as Input checkbox is cleared. The Reference Impedance parameter is used in conjunction with the Input Impedance parameter to calculate dBm. The VSA algorithm uses the measured voltage and the Reference Impedance value to calculate the power in dBm. Also, any impedance matching device insertion loss can be compensated for using the User Correction Gain parameter (Input > User Correction > Amplifier/Attenuator).

The following steps show how to set up a measurement when there is a DUT and measurement hardware input impedance mismatch.

Set the Input Impedance parameter to match the impedance of the impedance-matching device that is connected to the input of the measurement hardware.
Clear the Same as Input checkbox.
Specify the Reference Impedance to match the DUT output characteristic impedance.
If an impedance matching device, attenuator, or measurement probe that has insertion loss is used, compenaste with the parameter:
1. Click Input > User Correction.
2. Click the Amplifier/Attenuator check box.
3. Specify the insertion loss compensation value in the Gain parameter.

Reference Impedance Measurement Examples

If the device under test has an output impedance that matches the measurement hardware input impedance, set Input Impedance to match the DUT impedance and select Same as Input. Because the two impedances match, no external amplitude calibration is required.

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If the output impedance of the device under test and the input impedance of the measurement do not match, use an impedance-matching device. The impedance-matching device must have an input impedance equal to the output impedance of the device under test and an output impedance equal to the measurement hardware's input impedance.

In this case, set the following parameters:

Set the Input Impedance to the output value of the impedance-matching device (50W in this example).
Set the Reference Impedance to the value of the device under test output impedance (75W in this example).
The impedance-matching device introduces some amount of insertion loss, so the User Correction Gain must be specified to compensate.

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The VSA corrects the amplitude based on the correct Reference Impedance and Gain settings but the Input Range is set relative to the VSA input and not the impedance converter input.