Nonlinear Transmission Line (NLTL) technology has historically been used for pulse shaping applications and in digitizing oscilloscopes. Over the years it has proven itself to be a highly credible, robust technology. It has been refined by Anritsu for high-frequency use, and complemented with novel monolithic broadband directional bridges, multiplexers, and other key components, resulting in NLTL-based samplers and distributed harmonic generators that:
- Overcome the limitations of SRD-based sampling VNAs
- Meet the needs for a high-performance frequency-scalable VNA architecture
In general terms, NLTLs are distributed devices that support the propagation of nonlinear electrical waves such as shocks and solitons. Shock wave propagation along an NLTL closely mimics the motion of water waves just before breaking on the seashore. In their most basic form, NLTLs consist of high-impedance transmission lines loaded with varactor diodes that form a propagation medium whose phase velocity, and thus time delay are a function of the instantaneous voltage across the diodes (Figure 1). The lower the voltage, the lower the phase velocity and the longer the time delay of a waveform propagating along the nonlinear transmission line. Conversely, the higher the voltage, the greater the phase velocity and the shorter the time delay. When acting on a section of a trapezoidal voltage waveform applied to its input, an NLTL compresses the waveform’s front, resulting in a step-like voltage that is highly rich in harmonics.
Figure 1. The falling edge of an electrical wave undergoes compression as the wave propagates along the
nonlinear transmission line. This effect is analogous to that of a water wave before breaking on the seashore.
By leveraging the fall-time compression characteristics of an NLTL, a train of very narrow gating pulses can be generated at microwave and mm-wave frequencies for sampling receivers starting from a CW signal (Figure 2). An essential ingredient in the pulse formation process is a differentiator circuit (not shown) that transforms the step-like output of an NLTL into a pulse. On the other hand, broadband distributed harmonic generation is achieved by leveraging the “harmonic growth” characteristics of NLTLs. Since two primary functions of any VNA are generating signals and sampling them, NLTL technology is especially well suited for use in such instruments.
Figure 2. Non-uniform NLTLs enhance fall time compression, and result in a train of step-like waveforms when
driven by a CW signal. Step differentiation results in a train of pulses that are used for sampler gating.
Due to their attractive features, NLTL-based samplers have been developed for use in multiple families of Anritsu VNAs. These features include RF and LO frequency scalability, and high channel-to-channel isolation. High isolation is key to achieving high dynamic range. It is carried out by means of amplifiers, filters, and other isolation elements (Figure 3). The isolation between test channels 1 and 2, for example, may be enhanced further by adding additional isolation elements.
Figure 3. A sampling VNA based on nonlinear-transmission-line (NLTL) samplers. Leakage between
channels is suppressed by means of devices such as amplifiers, filters, and isolators
Limitations of Prior VNA Architectures
VNAs make use of samplers, harmonic mixers, or combinations thereof to down-convert measurement signals to intermediate frequencies (IF) before digitizing them. Such down-conversion components play a critical role in VNAs because they set bounds on important parameters like conversion efficiency, receiver compression, isolation between measurement channels, and spurious generation at the ports of a device under test (DUT).
Mixers tend to be the down converters of choice at RF frequencies, due mainly to their simpler local oscillator (LO) drive system and enhanced spur-management advantages.
At microwave and millimeter wave frequencies (where receiver compression and cost are of major concern), harmonic sampling is often used. VNAs have traditionally relied on Schottky diodes as switches and on SRDs for pulse generation. Implementations have been used extensively in a range of instrumentation, including microwave VNAs, sampling oscilloscopes, and frequency counters.
In an SRD-based sampling VNA, the dynamic range of transmission measurements is often limited by the bandwidth of devices used for isolating test channels. Channel isolation can be best understood by considering the SRD-gated sampling reflectometer shown in Figure 4. Note that suppression of leaky signals requires the use of broadband isolation devices in the output arms of the power divider.
Figure 4. A VNA based on a step-recovery diode (SRD) often has leakage between test channels that limits the VNA’s dynamic range.
Benefits of NLTL Technology
The use of NLTL-based samplers offers a number of benefits to modern VNA architectures (Table). These benefits provide customers with an unparalleled value per GHz for their investments.
||NLTL-Based VNA Advantage
|Simplified VNA architecture
||Monolithic reflectometer design reduces number of discrete parts and connectors
||Lower maintenance cost, reduced down time and operating costs.
||Integrated chip design greatly reduces the temperature variation across reflectometer constituents
||Longer intervals between calibrations, better measurement accuracy and repeatability
||Extremely wide RF sampler bandwidth allows one sampler to cover broad frequency range
||Lower cost for making high-performance measurements over broader frequency ranges
||Over 100 dB across all frequency ranges
||Better characterization of highly reflective devices and weak crosstalk.
||High performance in a very small form factor
||Direct connection to wafer probes, smaller footprint in manufacturing, light weight field solutions
||Improved capability-to-cost ratio enables new applications
||Dramatic cost reduction for high frequency testing in engineering, manufacturing and field
A major advantage of NLTL based VNAs is the high level of monolithic integration of the various constituents. These include the sampling receivers, distributed harmonic generators, directional bridges, and other key components. The resulting reflectometer modules share the same thermally stable mass, and are miniature in size, thus greatly reducing temperature variations. These in turn, result in highly optimized short- and long-term stability, and less frequent VNA calibration. In addition, the elimination of microwave connectors between the various reflectometer components enhances performance (e.g. lower loss, less reflections) while improving system reliability and stability.
The compact nature of the NLTL-based reflectometers enables several key applications for VNAs such as:
- High-frequency on-wafer testing with the prime advantage of locating the VNA closest to the DUT. By directly connecting the reflectometer to the wafer probe, directivity, port power, and system stability are enhanced.
- Dense multi-port on-wafer measurements.
- Very low cost solutions for testing components in production environments.
- High-frequency handheld VNAs for field applications.
In addition to their miniature size, these reflectometers provide highly attractive features such as short and long term thermal stability due to the vanishing thermal gradient across the modules, high amplitude and phase stability, and raw directivity to mention a few. Most importantly, placing the sampling directional bridge closest to the AUT/DUT provides long term amplitude and phase stability. It is these features in particular that lend themselves well to antenna measurements whether these are performed in a near field, far field, or compact range scenario. In order to reduce the cable complexity and address opportunities in high-frequency 5G communications where multi-port measurements are required, we introduced an E-band version of the reflectometers based on a modular AXIe format.
Over the years NLTL has proven itself to be a highly credible, robust technology. It has been refined by Anritsu for high-frequency use, and complemented with novel monolithic broadband directional bridges, multiplexers, and other key components, resulting in NLTL-based samplers and distributed harmonic generators that overcome the aforementioned limitations of SRD-based sampling VNAs and meets the needs for a high-performance frequency-scalable VNA architecture.
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