THz Spectrum Analysis With A Vector Network Analyzer
Device characterization can be challenging and complicated when venturing into the terahertz frequency range. To further complicate the situation, achieving an insightful understanding of component performance and behavior often requires two instruments: a vector network analyzer (VNA) and a spectrum analyzer (SA). During a typical measurement session, the need to frequently connect, disconnect and reconnect the device under test (DUT) is both inconvenient and time-consuming. It can also introduce measurement errors, extend measurement time, and damage the probes, the test cables and even the DUT.
One solution is to incorporate VNA and SA capabilities into a single instrument. Recently, faster digitizers, digital signal processors (DSP) and central processing units (CPU) have enabled Keysight to implement an SA capability that is fast enough to accelerate crucial — and often tedious — measurements, such as the search for spurious signals. In the analog portion of the block diagram, the next step is extending SA capabilities into the terahertz region while retaining the expected functionality, performance and accuracy of VNA and SA measurements.
VNA test solutions that measure below 67 GHz are usually implemented as a single, integrated instrument. Extending VNA capabilities to higher frequencies is typically achieved by using what is called a distributed architecture. This requires the use of frequency extenders that up-convert stimulus signals and down-convert response signals to support DUTs that operate into the terahertz range. A mmWave VNA can be implemented as a preconfigured solution or as a user-integrated system built around an existing VNA.
For example, Keysight offers an integrated system under a single model number, the N5251A mmWave network analyzer. This configuration covers 10 MHz to 110 GHz and currently offers extensions to 1.1 THz. The core instrument is a Keysight PNA microwave network analyzer. These solutions can be configured in two ways: one supports single sweep measurements through 1.0 mm coaxial connections; the other supports a variety of banded measurements via waveguide. The single sweep configuration is based on a 67 GHz PNA and includes a pair of companion mmWave controllers that support two- or four-port measurements (Keysight N5261A or N5262A, respectively). These connect to broadband frequency extenders, providing the interface between the mmWave test-head modules and the network analyzer. The extenders provide a 1.0 mm coaxial interface to the DUT up to 110 GHz, as shown in the product photo, and waveguide is used above 110 GHz.
The banded configuration supports a variety of frequency extenders from OML and Virginia Diodes (VDI). These use waveguide for frequencies above 110 GHz and in some frequency bands between 67 and 110 GHz (see Figure 1). The latest version of Keysight’s optional “SA on VNA” capability now supports all of these configurations, enabling integrated spectrum analysis into the terahertz range on the PNA and PNA-X network analyzers.
BUILT IN SA
The optional spectrum analyzer mode includes a user interface that presents the typical array of setup parameters: center frequency and span, start and stop frequencies, step size, resolution bandwidth (RBW), detector shape, averaging and receiver attenuation (see Figure 2). One important note about using SA in the distributed configuration: because the internal receiver attenuators are bypassed, external attenuators may be required when testing high power DUTs.
The integration of SA capabilities enables quick handoffs from the VNA mode without changing the physical test setup. For example, if an anomaly crops up in a VNA trace, the user can place a marker at that point and press “Marker to SA” to initiate a spectrum measurement. The measurement appears in a new window, enabling further observation and analysis of spectral content and behavior. The PNA and PNA-X also include a calibrated stimulus that can be directed to any and all DUT ports. Through tight control of frequency, amplitude and DC offset, this provides a very accurate test solution for the characterization of harmonics and intermodulation products. In addition, internal pulse generators and modulators enable characterization of DUTs with pulsed RF stimuli. The net result is the ability to evaluate DUT behavior under a wide operating range and in a variety of operating conditions.
The Keysight implementation of spectrum analysis is built on its existing VNA architecture. A typical spectrum analyzer includes a microwave pre-selector (i.e., a filter) that blocks high level signals while measuring low level signals as well as unwanted mixing products; this removes receiver harmonics and image responses. The SA-on-VNA design relies on custom hardware and software to provide spectrum analyzer performance. The PNA series uses custom RFICs designed in proprietary processes, coupled with software algorithms, to eliminate images. The hardware design also compensates for remote up- and down-converter topologies and nonlinearity.
The same techniques can be utilized in the distributed architecture VNA configuration used for mmWave measurements. The only additional consideration is a special calibration of the SA receivers to ensure accurate measurements. This calibration must include the frequency extender heads as well as all associated hardware, cabling and fixturing. Because the user may change one or more of these elements to suit a specific frequency range or setup, two types of calibrations must be performed any time the test configuration changes: power level and IF receiver. To simplify these situations, the new high frequency SA options include functionality that automates the calibration processes and guides the user.
The integration of SA capabilities into a VNA offers two key advantages over the multi-instrument approach: multiple simultaneous measurements and calibrated accuracy. Through its multiple test ports, a VNA enables multi-channel spectrum analysis that is synchronized with the internal swept signal generators. A PNA or PNA-X, through a single connection, provides simultaneous measurements on all DUT ports. The range of possible measurements includes input spectra, output spectra, channel power, gain compression, feedthrough, reflections, conversion gain, harmonics and intermodulation (see Figure 3). This simplifies characterization of devices such as mixers, frequency converters, amplifiers, high frequency modules and subsystems. VNA calibration and de-embedding techniques are essential to the accuracy of in-fixture and on-wafer measurements. The process corrects for the instrument’s systematic errors, and it removes cable and fixture effects. It can be used with frequency extenders, and it is also applicable to the SA-on-VNA capability. In addition, the power compensation features can be used to deliver a stimulus of known power to the DUT, compensating for known loss in the fixture or probes. The resulting improvement in measurement accuracy enables a deeper understanding of a DUT’s true performance.
Spurious are unwanted signals — harmonic or non-harmonic — that may cause interference from transmitters, false responses in radar systems or reduced dynamic range in communications receivers. Spurs must be identified and measured before a designer can reduce them to sufficiently low levels, as defined by a system or device specification. The search for spurs presents two challenges: time and complexity. The process of checking spurious performance is time-consuming, especially when searching for low level signals over a broad frequency range. Characterizing spurs over the operating range of typical mixers and frequency conversion devices tends to be tedious and complicated, and it often requires external control software. With the integrated high performance SA capability, a PNA or PNA-X can perform fast spurious searches across a broad frequency band, improving test time compared to a stand-alone signal analyzer. Speed does not degrade accuracy: measurements results are comparable to those obtained with today’s most sophisticated spectrum or signal analyzers (see Figure 4).
Working at mmWave and sub-mmWave frequencies can be challenging. As implemented in the Keysight PNA and PNA-X microwave network analyzers, the optional addition of integrated SA capabilities to a distributed VNA architecture makes it possible to characterize component performance and behavior into the terahertz range with a single test setup. The integrated stimulus, with the ability to perform spectrum analyzer measurements on multiple channels simultaneously, offers research and design engineers new insights in much less time and with excellent accuracy.
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