The Intermod Distortion Meas submenu controls the intermodulation distortion measurement including power calibration. The intermodulation distortion measurement requires option R&S ZVA-K4. The results can be selected from the Trace – Measure – Intermod Distortion Quantities submenu.
Intermodulation measurement, test setups
An intermodulation measurement is performed with two RF signals of equal power but different frequencies termed the upper and lower tone. The purpose of the measurement is to test the properties of a DUT that is supplied with a signal that covers a frequency band, typically a modulated RF channel. To simulate this scenario, the frequency difference ("tone distance") between the upper and lower signal is chosen to be small compared to the frequencies of the two tones:
fL ≈ fU or Δf = fU– fL << fL
A nonlinear behavior of the DUT causes emissions at frequencies which correspond to sums and differences of the upper and lower tone frequencies and their integer multiples. These intermodulation products can be in the vicinity of the upper and lower tone frequencies, provided that their order is odd.
The analyzer measures the intermodulation products of kth order IMk (where k = 3, 5, 7, 9) at the lower tone frequency minus (k – 1)/2 times the tone distance and at the upper tone frequency plus (k – 1)/2 times the tone distance. For an R&S ZVA analyzer that is equipped with option R&S ZVA-B16, Direct Generator/Receiver Access, a test setup of the following type is recommended:
The lower tone signal is generated at port 1, the upper tone is provided by a second source (port 3). Both signals are combined externally and fed to the SOURCE IN connector at port 1. Thus the superimposed signals are available at test port 1 and can be fed to the DUT input. The intermodulation quantities can be measured at the DUT input (reflected wave a1) or at the DUT output (transmitted wave b2). It is possible to change the port numbers and to use external generators for the upper and lower tone signals.
On R&S ZVA-Z67 network analyzers, all ports have independent internal sources. You can perform intermodulation measurements with an arbitrary combination of two source ports.
The following alternative test setup does not require an external coupler but provides a reduced dynamic range.
The lower tone from port 1 and the upper tone from port 3 are combined using the directional coupler of port 3. Ports 1 and 3 are connected as follows: SOURCE OUT (1) to MEAS OUT (3) and Port 3 to SOURCE IN (1). The superimposed signals are available at test port 1 and can be fed to the DUT input. The intermodulation quantities can be measured at the DUT input (reflected wave a1) or at the DUT output (transmitted wave b2). It is possible to change the port numbers and to use external generators for the upper and lower tone signals. For more details about the test setup see Intermodulation Power Calibration and Level Handling.
If you use an external combiner but no options R&S ZVA-B16, you can still measure the intermodulation products at the output of the DUT.
With an R&S ZVT20 network analyzer that is equipped with option R&S ZVT-B11, you can combine the two source signals internally without using the SOURCE IN/OUT connectors and an external combiner. You can also replace the external combiner by an Extension Unit R&S ZVAX with installed option R&S ZVAX-B211.
Intermodulation measurement, results
The intermodulation measurement provides two different types of results.
1. In the swept measurement, the analyzer performs a frequency or power sweep of the two-tone stimulus signal and displays the selected intermodulation quantities as a function of the lower-tone frequency or power.
2. In the intermodulation distortion spectrum measurement, the frequency and power of the lower and upper tones is kept constant; the analyzer displays all intermodulation products in the vicinity of the signals up to a selectable order.
Intermodulation measurement with frequency conversion
The intermodulation distortion measurement can be extended to frequency-converting DUTs. E.g. it is possible to feed the two-tone source signal to the RF input of a mixer and measure the intermodulation distortion of the IF output signal, after conversion with an additional LO signal.
The Define Mixer Measurement dialog can be accessed from the intermodulation distortion measurement dialog. You can also use the Port Configuration dialog to configure an arbitrary frequency-converting measurement.
Define Intermod Dist Meas... opens a wizard that you can use to configure the intermodulation measurement. The remaining entries in the submenu are available as soon as a valid configuration has been defined.
CW Mode Intermod Spectrum... opens a dialog that you can use to configure the intermodulation distortion spectrum measurement.
Intermod Dist Meas Power Cal opens the power calibration wizard for the intermodulation measurement.
Reset Frequency Conv and Intermod disables the intermodulation measurement mode and switches back to normal (non frequency-converting) mode.
The Define Intermodulation Distortion Measurement wizard defines the source and receiver settings for the intermodulation measurement.
Frequency range for intermodulation measurements After a preset, the lower tone is swept across the full frequency range of the analyzer, which means that the upper tone frequency is out of range. Click Set Frequencies and Powers and reduce the sweep range to establish compatible settings. The intermodulation products of kth order (IMk) are measured at the lower tone frequency minus (k – 1)/2 times the tone distance and at the upper tone frequency plus (k – 1)/2 times the tone distance. These receiver frequencies must be within the frequency range of the analyzer, too. The analyzer displays a message if the source frequency (as defined in the Port Configuration dialog) at one of the used source ports is different from the channel base frequency (the frequency-converting / arbitrary mode is active). Disabling the arbitrary mode ensures a valid port configuration and facilitates the interpretation of the intermodulation distortion results.
Lower Tone selects an analyzer port or an external generator as a source of the lower tone signal. If Port 1 is selected as the Two Tone Output, then Port 1 must also provide the lower tone. This condition ensures a consistent receiver power calibration for the intermodulation products at the DUT output port (e.g. Port 2, for details refer to the description of the power calibration procedure in section Intermodulation Distortion Measurement Power Calibration).
Upper Tone selects an analyzer port or external generator as a source of the upper tone signal. The source must be different from the lower tone source. This means that you cannot combine coupled generator ports (port 1/2, port 3/4 etc.) for the lower and upper tones.
Two Tone Output defines the source of the two tone signal which is fed to the input of the DUT. For analyzers without option R&S ZVA-B16, Direct Generator/Receiver Access, and without an internal combiner , the setting Ext. Dev. (external device) is fixed: The analyzer assumes that the lower tone and the upper tone signals are combined by external means. If one of the options above is available, the lower tone source Port 1 may be selected alternatively. The two basic test setups are described above; see Intermodulation measurement, test setups. The Two Tone Output setting affects the fast power calibration method (see Modify Source Power Cal Settings dialog, Use Reference Receiver After First Cal Sweep: On): If Ext. Dev. is selected, the actual source power is derived from the reference waves (a waves) of the lower and upper tone ports. If Port 1 is selected, the source power is derived from the lower tone reference wave a1. Select the Two Tone Output in accordance with the test setup to ensure an accurate fast power calibration.
Config Ext Generators... opens the System Configuration – External Generators dialog for adding and configuring external generators.
Tone Distance defines the frequency difference between the upper and the lower tone. The tone distance is kept constant across the entire sweep.
Use Internal Combiner activates the internal combiner for network analyzers R&S ZVT20 that are equipped with option R&S ZVT-B11. The internal combiner requires the lower tone from port 1, the upper tone from port 3.
The Receiver – Measured Wave Quantities section adjusts the intermodulation measurement to the test setup; see Intermodulation measurement, test setups. At DUT Output shows the wave quantities which are fed to the DUT input, depending on the source settings (Lower Tone, Upper Tone, Two Tone Output). Wave quantities from external generators are not listed. The analyzer port which provides the combined two-tone signal is shown in brackets. At DUT Output selects the receive port for the signal from the output of the DUT.
The remaining Receiver settings define the bandwidth and selectivity of the IF filter at test port 2; see section Meas. Bandwidth.
Prepare Measurement of IM Order selects the order(s) of the measured intermodulation products. Each order requires an additional measurement at a different frequency, therefore the measurement time increases with the number of selected orders. If a higher-order IM is selected, its frequency may exceed the upper or lower frequency limit of the analyzer. Use the Fit Frequency Range button in the Set Frequencies and Powers dialog to restrict the sweep range as required.
Define Mixer Measurement... opens the configuration dialog for a scalar mixer measurement. Use this dialog if you want to determine the intermodulation distortion of a mixer which receives the two-tone signal at its RF input. , to be performed in parallel to the intermodulation measurement. The Lower Tone signal serves as the RF signal for this mixer measurement; the IF signal is provided by an additional source.
Set Frequencies and Powers... opens a dialog where you can define the sweep ranges.
Remote control:
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The Set Frequencies and Powers dialog defines the (frequency or power) sweep ranges for the lower tone. The frequency of the upper tone is equal to the lower tone frequency plus the Tone Distance defined in the Define Intermodulation Distortion Measurement dialog. Its power is equal to the lower tone power.
The sweep range settings depend on the active sweep type (frequency or power sweep). The frequency and power settings for the lower tone are identical with the Channel – Stimulus settings.
The Frequency Range table shows the frequency ranges for the lower and upper tone and for the highest-order measured IM product, depending on the current frequency settings. E.g. the lower IM3 product is measured at the lower tone frequency minus the tone difference. An orange background in the Range column indicates that the frequency range exceeds the upper or lower frequency limit of the analyzer.
If one of the ranges exceeds the analyzer limits, the Fit Frequency Range button restricts the lower tone sweep range so that the analyzer can measure all selected IM products.
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The Intermodulation Distortion Measurement Power Calibration wizard controls the power calibration for the intermodulation measurement. A power calibration ensures accurate levels at the input of the DUT (source power calibration for the lower and upper tone) and an accurate power measurement of the intermodulation distortion quantities (receiver calibration). The necessary calibration steps are automatically performed across the entire frequency or power range of the intermodulation measurement.
Power calibration procedure and example
Due to the different frequency ranges of the lower tone and the upper tone signals, the intermodulation distortion measurement power calibration must be performed in several steps. For a scenario where the directional coupler of port 3 is used to combine the lower tone from port 1 and the upper tone from port 3, and where the output of the DUT is connected to port 2, the following test setups are required.
1. Source power calibration for the lower tone signal. A power sensor is connected to port 1. Ports 1 and 3 are connected as shown below (SOURCE OUT (1) to MEAS OUT (3) and Port 3 to SOURCE IN (1)). The RF power (red) is calibrated over the selected sweep range. During the lower tone source power calibration, the upper tone is switched off. If a frequency-converting measurement is active, a second source power calibration is performed at the converted lower tone frequency (receiver frequency). This second source power calibration is the basis for the receiver calibration in step 3.
Power considerations The directional coupler introduces a coupling loss of approx. 10 dB. Adjust the port power settings to account for this attenuation and possible additional effects; see Level Handling during Power Calibration.
2. Source power calibration for the upper tone signal. The test setup from step 1 is maintained. During the upper tone source power calibration, the lower tone is switched off. For frequency-converting measurements, no new source power calibration at the receiver frequency is performed.
3. Receiver power calibration for the measured intermodulation product(s). No external device is needed. The receiver (port 2) is calibrated at the frequency of the intermodulation product(s) using the source signal from port 1 (lower tone signal, red) calibrated in the first step. If a frequency-converting measurement is active, a second receiver power calibration is performed at the converted lower tone frequency.
The calibration steps for the other test setups are analogous. Refer to the indications in the power calibration dialog for a correct connection of the power meter and the receive port.
The calibration procedure is controlled by means of the three buttons on the left side.
Take Cal Sweep in the three numbered panels start the calibration sweeps for the upper and lower tone source and the receiver calibration, respectively. The calibration sweeps are performed at the test settings shown on the right side (see also Source Power Cal and Receiver Power Cal dialogs). The progress of the calibration is shown in the calibration sweep diagram and the messages below. Abort Sweep aborts the calibration.
The control elements in the right part of the dialog give access to various settings:
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 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. For the remaining buttons in the Power Meter panel refer to Power Meter Settings.
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 has been acquired. The final power calibration results are applied starting with the first sweep after the verification sweep.
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The figure below shows the signal flow in the test scenario where the lower tone from port 1 and the upper tone from port 3 are combined using the directional coupler of port 3. In this scenario the directional coupler introduces an attenuation (coupling loss) of typically 10 dB for the lower tone signal.
To compensate for the coupling loss and obtain upper and lower tone signals with equal power, modify the source power settings for port 1:
Open the Intermodulation Distortion Measurement Power Calibration dialog and click Modify Settings... – Modify Cal Power... for port 1 to open the Port 1 Power dialog.
Select a 10 dB Port Power Offset to increase the source power, compensating for the 10 dB coupling loss.
Select a –10 dB Cal Power Offset to ensure that the calibrated power at the reference plane (Cal Power) remains at the channel base power pb (Cal Power = pb + Port Power Offset + Cal Power Offset).
Larger Port Power Offset values (and hence smaller compensating Cal Power Offset values) may be required e.g. to account for an additional cable loss in the test setup. The maximum source power (depending on the analyzer model and the frequency; see data sheet) must not be exceeded. If necessary, reduce the channel base power (and thus the Cal Power).
Coupling loss for microwave analyzers
The directional couplers in network analyzers which cover the microwave range (R&S ZVA/ZVT20 and above) have a larger coupling loss, especially at very low frequencies (e.g. 20 dB loss at 10 MHz). If the (possibly converted) lower tone (or an intermodulation product of interest) must be measured at low frequencies, a reduced channel base power may be required.
Example: Lower tone frequency: 1 GHz; the lower tone is fed to a mixer, together with an arbitrary upper tone and a 990 MHz LO signal. The converted lower tone in the mixer output (IF) signal is measured at 10 MHz. The lower tone frequency is >> 10 MHz. However, in the first power calibration step, the analyzer calibrates the port 1 signal at the converted frequency, so the maximum channel base power is equal to: pb, max = <maximum source power> – 20 dB = – 5 dBm (typical value)
Possible remedy: Select port 3 as a source port for the lower tone.
In a test setup with external coupler, the NWA settings should account for the coupling loss in an analogous way.
The Def CW Mode Intermodulation Spectrum dialog configures the frequency range for the intermodulation spectrum measurement.
The intermodulation spectrum measurement is performed at fixed frequency of the lower tone (CW Frequency) and the upper tone (CW Frequency + Tone Distance). The measurement comprises intermodulation products up to a selectable order (IM Order). The channel settings differ from the swept intermodulation distortion measurement, therefore it is possible to create a new channel when the spectrum measurement is activated.
The following example shows an intermodulation spectrum with the following settings:
CW Frequency: 1 GHz (lower tone, position of marker M3)
Tone Distance: 1 MHz (defines the upper tone, position of marker M1)
Max. IM Order: 9. The peaks correspond to the intermodulation products lowerIM9, lower IM7, lower IM5, lower IM3 (marker M4), lower tone, upper tone, upper IM3 (marker M2), upper IM5, upper IM7, upper IM9.
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This softkey disables the intermodulation measurement mode and switches back to normal (non frequency-converting) mode.
A Mix Frq RF label in the channel list indicates that a frequency-converting mode is active. A MixDly label in the trace list indicates an active mixer delay measurement.
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