Bits & Chips Benelux RF Conference 29. May 2024 Nijmegen

Lecture Sajjad Ahmed | Focus Microwaves: Sub-THz load pull and characterization of noise parameters

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Frequency Extenders from Virginia Diodes Inc.

Vector Network Analyzers (VNAs) are used in different HF technology fields to determine complex scattering parameters for a variety of components and modules. Therefore, they play an important role during development and production. Their measurement precision can be ensured by the use of high-quality sources and receivers. The calibration procedure plays an important role in this context.

Modern communication standards, like for instance 5G, place high demands on components (mixers, amplifiers, antennas, etc.), which have to be examined in high frequency bands (millimeter waves); the requirements include: broad frequency coverage, sufficient output power to operate the components under compression, high dynamic range and stability as well as a reduced dynamic content within the spectrum.

The VNA millimeter wave converters (VNAX) by Virginia Diodes (VDI) support the expansion of test capabilities and network analytics for components and antennas within millimeter wave bands from 50GHz to 1.5THz, with an outstanding dynamic range (>100dB at 500GHz) and excellent output power (> -30dBm at 1THz!).

Applications: Transmitter and receiver test, on-wafer measurements, antenna measurements etc.

They are available in different form factors (Standard, Mini and Micro). Thanks to their small footprint and reduced weight (21cm – ¼ of a standard module – and 1.8kg for a Mini VNAX module), these can be easily integrated on a prober for on-wafer measurements.  For antenna measurements, the Rx-Micro module is ideally suited for mounting on a robot arm.

Thanks to its small footprint, the thermal characteristics of the VNAX-Mini module prove to be especially advantageous. Its reduced mass helps to eliminate stability and phase shifts within the system, thereby enhancing the insensitivity of the measuring station regarding environmental temperature fluctuations.

The following measurement constellations are supported by the VNAX modules:

Tab: Available VNAX modules, with performance characteristics

Generally, the VNAX modules are suitable for use with all VNA models of well-known manufacturers, like e. g. Keysight, R&S and Anritsu. However, for older VNAs, we recommend to contact us in advance so we can precisely verify the compatibility of the existing devices.

Illustration: Measurement configuration to determine the S parameters of a 2-port component in the millimeter wave range

A minimum requirement with respect to the VNA is the availability of 2 sources and 4 ports.

In addition, Keysight devices require option 080 [Frequency Offset].

This option supports an offset between the PNA generator/source and receiver frequencies. The first source into the PNA is used for the RF frequency component of the signal, whereas the second source is used for the LO signal.

This setup is best explained by means of a drawing of a VNA module, as shown below:

– The VNAX module consists of a sender which is fed by the RF signal from the PNA, followed by two "back-to-back" directional couplers.

– One receiver is connected to each directional coupler. One is called the reference channel receiver ("reference mixer" – used for sampling the forward wave), the other is the measurement channel receiver ("measurement mixer" for the return wave).

– Both signals from the PNA (RF and LO) are setup with a small relative frequency offset, in order to shift the measurement signal to an intermediate signal, according to the heterodyne principle. It is essential that the magnitude and phase information of the measurement signal remains intact.

– The RF input signal is amplified within the sender, and a multiplier is used to convert the input signal to a higher frequency range, where the measurement objects are examined.

– The sender signal is transmitted to the DUT and back via two directional couplers. The first directional coupler samples the incoming signal of the measurement object, whereas the second directional coupler shows the signal reflected by the measurement object.  Another option, as shown above, is to use two VNAX modules and to perform a complete 2-port measurement. In this case, the signal transmitted by the measurement object is presented by the second VNAX module.

– The receiver consists of an amplifier and a multiplier to create the LO signal for a subharmonic mixer; the reference mixer determines the reference for the signal value and phase, whereas the measurement channel mixer captures the information regarding the measurement object.

– The output (measurement port) consists of a 1-inch waveguide port; its dimensions correspond to the relative frequency band.

For improved stability and noise characteristics, it is advisable to use a higher-quality external source (synthesizer, e. g. the Keysight PSG series – with UNX Option) instead of an internal source for measurements above 1000GHz.  The modules support operation with either a low or a high frequency (RF) signal. For the high frequency option, the 2x multiplication stage is removed; this can provide certain benefits above 500GHz regarding the harmonic content.

"Power Levelling" or "Power Sweeping" is also supported by the VDI VNAX module if the PM5 power meter by VDI is used.

For calibrating the VNAX modules, an appropriate calibration kit for the waveguide band is used. There are two calibration procedures available, depending on the frequency bands of the modules: TRL ("Through-Reflect-Line") from WR15 to WR3.4 and SOLT (Short-Open-Load-Through) from WR2.8 to WR1.0.

A cable set is required for each module, consisting of 1 x RF cable, 1 x LO cable and 2 x IF cables – (standard length 1.2m, longer cables up to 5m upon request). For antenna measurements, it is even possible to use cables up to lengths of 8-10m.

For some measurements, it is useful that the VNAX modules may be equipped with an optional variable attenuator (~0-30dB).

 For active load pull setups, the VNAX modules may be equipped with additional hardware for setting amplitude and phase, so they can be used as "electronic" tuners. Modules with the "High-Power" option have some minor restrictions regarding the available frequency range.

The Rx modules may also be configured as highly sensitive Rx-HS (for "High-Sensitivity"). These are ideally suited for measurement objects with substantial losses. The measuring arrangement in the next illustration shows a setup with amplification at the output of the Rx-HS signal.

The Anritsu VectorStar VNA series also offers a measurement solution for up to 1.1THz, the Millimeter Waveguide VNA System.

In order to operate the VDI modules with a device from the MS464xB VectorStar series, the following options are required:

-          Option 7 [Receiver Offset]

-          Option 80, 81, 82 or 83

-          3739C Broadband/Millimeter-Wave Test Set plus interface and cable

For operation at 1.1THz, a VectorStar model from 40GHz upwards should be used.

Millimeter-Wave Configuration Block Diagram