Converter Control (Option R&S ZVA-K8)  

Measurements at frequencies beyond the analyzer's operating range (mm-wave measurements) are achieved by combining a frequency-converting measurement with an external test set (frequency converter). Rohde & Schwarz offers frequency converter types for various frequency ranges. E.g. the frequency converters R&S ZVA-Z110 and R&S ZV-Z110E extend the frequency range of R&S ZVA or R&S ZVT network analyzers to a range between 75 GHz and 110 GHz.

The test ports of the frequency converters and the connecting elements are rectangular waveguides. For the R&S ZVA-Z110(E) converter, the dimensions of the waveguides are according to EIA WR-10, equivalent to RCSR WG-27. Refer to the Quick Start Guide and the "Specifications" supplied with your converter for detailed information about the frequency range and the waveguide type of your converter model.  

Two different converter types are available:

• Converters without electronic attenuators. The analyzer cannot control the output power of these converters.

• Converters with electronic attenuators (and a capital "E" in the type designation, e.g. R&S ZV-Z110). The output power of these converters is controlled by the analyzer; a power calibration and power sweep is possible.    

Required equipment and options

Test Setup and Principle of the Measurement

The frequency converters use frequency multipliers to transform the RF source signal from one of the network analyzer ports into a high-frequency stimulus signal. A dual directional coupler separates the reference and measurement channels from the waveguide test port. A second signal (Local Oscillator, LO) is used for down-conversion of the reference and measurement channels. The LO signal can be provided either by a second analyzer port or by an external generator. The down-converted signals are fed to the REF IN and MEAS IN input connectors of the analyzer port providing the RF source signal.

The schematic test setup for a two-port transmission measurement using four analyzer ports (no external generator) is shown below.

A measurement using converters without electronic attenuator involves the following stages:

1. Selection of the converter type and of the test setup in the Frequency Converter dialog

2. Connection of the frequency converters

3. System Error Correction using a suitable waveguide calibration kit

4. Connection of the DUT and measurement

For converters with electronic attenuators, the following additional stages are required:

• Entry of power coefficients (when the converter is used for the first time)

• Power and frequency settings

• Power calibration

Frequency Converter

The Frequency Converter tab belongs to the System Configuration menu (System – System Config). It selects the frequency converter type and the external test setup as well as the power transfer model (especially for converters with electronic attenuators).  

The dialog provides the following control elements:  

• Type selects a frequency converter type.

• Use of External Sources provides the possible test setups for each converter type. The test setups differ in the way how the LO signal is provided (from the analyzer port or from an external generator).

• The Power Transfer settings define the model by which the analyzer controls the output power at all converter ports. The exact meaning of the power transfer modes depends on the converter model; see Power Transfer Modes.   

• Apply activates the selected test setup and adjusts the analyzer settings; see information below.

• For frequency converters with electronic attenuators, pressing Apply enables the Power Coefficients button. This button opens an additional dialog to enter the power coefficients for the electronic attenuators.

The frequency-converter settings belong to the global resources; they are not changed upon a Preset or *RST.

Analyzer settings with active frequency converter

Power Transfer Models

The Power Transfer (All Converter Ports) settings in the Frequency Converter tab define how the analyzer controls the output power at all converter ports. The general settings can be changed for individual converter ports (Channel – Mode – Port Configuration – Source Power – Converter Port <n> Power – Power Transfer).

Changing the power transfer settings in the Frequency Converter tab will overwrite the individual port settings.

The basic power transfer model (Use Advanced Model off) is simple to configure and suitable for most standard applications.

• Converters with electronic attenuators: The converter output power is controlled by the analyzer, using the power control cable and the electronic waveguide attenuators in the converters. The power coefficients of each converter must be correctly entered in the Power Coefficients dialog. A power calibration is possible. In addition, port-specific power settings allow you to account for devices with known attenuation/amplification in the test setup.  

• Converters without electronic attenuators: The converter output power can be varied by means of the power adjustment screw which is available for most converter models. No power calibration is possible, however, port-specific power settings allow you to account for devices with known attenuation/amplification in the test setup.  

The advanced model may be based on a Data Set or on a Linear Approximation of the relation between the converter input power and the converter output power.

• Data Set is suitable for converters with electronic attenuators. The analyzer uses a set of power calibration data to ensure accurate powers at the reference plane. The calibration data set is acquired by the R&S ZVA Frequency Converter Leveling Tool, a software utility which is available for download on the R&S ZVA product pages (http://www.rohde-schwarz.com/product/zva). For detailed information about the calibration procedure refer to the leveling tool's help system.
  The leveling tool can automatically acquire calibration data over a wide range of powers and frequencies. This power transfer model is recommended for best accuracy, in particular if large sets of calibration data are required. If Data Set is active, the Read Data Set button allows you to select the calibration data folder (Calibration Subdirectory) for the current channel, in accordance with the definitions in the leveling tool.

• Linear Approximation is suitable for all converter types. Converters with electronic attenuators are controlled by a combination of the RF input power (i.e. the source power at the analyzer test port providing the signal fed to RF IN) and the electronic waveguide attenuators in the converters. At sufficiently high converter output powers, the electronic waveguide attenuator settings are fixed. In the low converter output power range, a selectable portion of the attenuation is achieved by the waveguide attenuators. This advanced power transfer model provides a maximum dynamic range while maintaining the spectral purity of the converter output signal in the low-power range.  
  Converters without electronic attenuators are controlled by the power adjustment screw. It is possible to adjust the screw to a desired attenuation value and use this value as a correction factor for the port power.  
  The settings for the linear approximation appear in the Power Transfer dialog.  

The following table shows an overview of the different modes.

Power Coefficients (for Converters with Electronic Attenuators)

The Power Coefficients dialog is used to enter the power coefficients of converters with external attenuators. It can be opened from the System Configuration – Frequency Converter tab after the converter is activated.

For accurate control of the converter output power, the R&S ZVA analyzer must know the (non-linear) current-power characteristic of the converters. The characteristic is sufficiently described by a third-order polynomial. A label with the four polynomial coefficients c0, c1, c2, and c3 is affixed to each converter.

• The number of rows in the table is equal to the number of analyzer ports. Each row contains the power coefficients for the converter connected to the NWA port in the fist, non-editable column. Overwrite the coefficients in a row whenever you connect a new converter to the corresponding NWA port.  

• Use default coefficients overwrites the entire table with a set of default power coefficients, providing acceptable output power control for arbitrary converters. For accurate measurements, use the actual power coefficients of each converter.  

Connecting the Frequency Converters

Each frequency converter must be connected to the DUT, the network analyzer, and the power supply. The DUT is screwed on the waveguide flange at the front of the converter. The remaining connectors are located on the rear panel of the converter:

Connection to the network analyzer comprises the following RF input and output signals:

1. RF source signal (input): Connect port 1 or port 2 of the analyzer to RF IN of the converters.

2. LO signal (input): Connect port 3 or port 4 of the analyzer to LO IN of the converters. Alternative: Connect an external generator signal to LO IN; see Frequency Converter dialog.

3. Reference signal (output): Connect REF OUT of the converters to the REF IN connector of the analyzer port providing the RF source signal. The reference signal corresponds to the incident wave (a-wave) applied to the input of the DUT.

4. Measurement signal (output): Connect MEAS OUT of the converters to the MEAS IN connector of the analyzer port providing the RF source signal. The measured signal corresponds to the b-wave caused due to reflection or transmission at the DUT.  

For converters with electronic attenuators, an additional USB control connection between the output connector of option R& ZVA-B8 (EXTATT CTRL) on the front panel of the analyzer and the 3-pin control connector at the rear panel of the converter is required. Observe the port assignment of the NWA output connector.

The input signal at the frequency converters must be relatively stable over the entire sweep range. Use high-quality cables for the connection of the RF and LO input signals. Appropriate cables for the REF OUT and MEAS OUT signals are supplied with the frequency converters.

In view of the port arrangement at the front panel of the analyzer, it is advantageous to use test ports 1 and 2 as RF sources, ports 3 and 4 as LO sources. The frequencies of the RF and LO signal must be independent, therefore do not combine a pair of ports as RF and LO sources that is supplied by a common generator (ports 1 and 2, 3 and 4; see Coupled test ports).  

To supply the frequency converter, connect the external DC power supply provided with the converter to the 9 V / 0.5 A DC input. The power supply supports input AC voltages between 100 V and 240 V and frequencies between 47 Hz and 63 Hz. A complete test setup for a 2-port transmission measurement using converters without electronic attenuators is shown below.

Frequency converters and true differential mode
To generate a true differential signal with two frequency converters, a modified test setup with three independent sources is required. See background information inBalanced Ports and Port Groups – True Diff Mode.

Power and Frequency Settings

While the frequency converter mode is active, the Channel – Stimulus settings of the network analyzer control the frequency and power range of the converters. In addition the Channel – Mode – Port Configuration dialog shows an additional row for each converter. The Frequency and Power settings in the Source section of the dialog serve different purposes:

• The output power for each converter can be selected in the Power column. For frequency converters with electronic attenuators, a source power calibration of the converter port ensures that the analyzer will generate the selected source power.

• The source frequency at the converter ports is essentially determined by the port frequency of the analyzer. The converter frequency settings in the Port Configuration dialog define the frequency axes for the source power calibrations, however, they do not actually affect  the  converter output frequencies. For best accuracy, ensure that the correct converter frequencies are set, especially if the test setup contains additional frequency-converting components.

Example: In the example below, the frequency at the NWA Port 1 has been increased by a 50 MHz offset. The converter source frequency six times the NWA source frequency, therefore a 300 MHz offset has been entered for Converter Port 1. This ensures a correct frequency axis during the power calibration.

Calibration (for Converters without Electronic Attenuators)

The output power of the frequency converter can be set manually (at the converter) only, therefore the standard source power calibration eliminating frequency response errors in the signal path between the source and the reference plane (external power meter) is not possible. A power calibration of the reference receiver (a-wave) using an external power meter, however, is possible and recommended for measurements concerning the wave quantities a and b. Proceed as follows:

1. Ensure that the output power of the frequency converter is not attenuated (adjust the knurled knob at the top of the converter to 2 mm).

2. Connect an appropriate external waveguide power meter to the waveguide flange and open the Channel – Calibration – Start Power Cal – Source Power Cal dialog (see Source Power Cal).  

3. Click Modify Settings and disable Flatness Cal, leaving Reference Receiver Cal checked.

4. Start the calibration sweep.

This power calibration procedure ensures a reasonable accuracy of the reference power readings over a wide range of converter output powers (i.e. even if the adjusting knob is used to reduce the powers).

A Receiver Power Calibration of the b-waves (without external power meter) is possible after completed power calibration of the a-wave. Moreover, you can use the advanced power transfer model with linear approximation to adjust the screw to a desired attenuation value and use this value as a correction factor for the port power.

After the power calibration procedure a system error correction is recommended. Due to the physical properties of the mm-waves and the waveguides, measurements with frequency converters require a special calibration kit for system error correction. Rohde & Schwarz offers special waveguide calibration kits for this purpose, e.g. the calibration kits R&S ZV-WR03, R&S ZV-WR10, and R&S WR-15. The standards in the calibration kits allow all one-port and two-port calibration types supported by the network analyzer except TNA. Refer to the documentation of the calibration kit for details.  

Calibration (for Converters with Electronic Attenuators)

A source power calibration for a frequency converter requires an appropriate external power meter, to be connected to the converter's waveguide flange. Waveguide power meters are configured in the ordinary way using the System Configuration – External Power Meters tab.  

To perform the source power calibration, proceed as follows:

1. Connect the waveguide power meter and open the Channel – Calibration – Start Power Cal – Source Power Cal dialog.   

2. Select your converter and source port from the Source pull-down list (e.g. Conv 1 for a frequency converter connected to NWA port 1).

3. Click Modify Settings and ensure that both Flatness Cal and Reference Receiver Cal are checked.

4. If your test setup causes strong nonlinear effects, you can choose a Convergence Factor different from one.

5. Start the calibration sweep.

To ensure an accurate source power calibration and quick convergence, use the correct power coefficients. A receiver power calibration of the b-waves (without external power meter, using the “Receiver Power Calibration” dialog) is possible after completed source power calibration.

After the power calibration procedure a system error correction is recommended. Due to the physical properties of the mm-waves and the waveguides, measurements with frequency converters require a special calibration kit for system error correction. Rohde & Schwarz offers special waveguide calibration kits for this purpose, e.g. the calibration kits R&S ZV-WR03, R&S ZV-WR10, R&S ZV-WR12, and R&S WR15. The standards in the calibration kits allow all one-port and two-port calibration types supported by the network analyzer except TNA. Refer to the documentation of the calibration kit for details.

Measurement

After power calibration and system error correction, the mm-wave measurement can be performed like any other network analyzer measurement. The Port Configuration settings (together with the Stimulus settings) determine the sweep range of the converted signals (i.e the input and output frequencies at the DUT ports). All measured quantities (S-parameters, wave quantities, ratios etc.) and other trace settings are available.

The following restrictions hold for measurements with external frequency converters without external attenuators:

• The measurement is performed at fixed RF source and LO power. No power sweep is possible.

• To reduce the actual output power of the converters (e.g. for measuring wave quantities or testing compression effects), turn the adjusting knob on top of the converters clockwise. You will see an effect on the output power within the last 2 mm of the adjustable range.

• Note the special analyzer settings listed in section Frequency Converter.

The following example shows the transmission and reflection coefficients of a bandpass filter in the frequency range between 75 GHz and 110 GHz, i.e. the measurement range of the converter R&S ZVA-Z110.