November 25, 2014
As many engineers know, in traditional harmonic sampling vector network analyzers (VNAs), samplers are gated by pulses generated with a Step-Recovery Diode (SRD) circuit, with the Local Oscillator (LO) and RF source phase locked to a common frequency reference. This conventional VNA design has its limitations, particularly at higher frequencies. An alternative VNA architecture based on Nonlinear Transmission Line (NLTL) samplers and distributed harmonic generators has been developed for a simplified VNA architecture and VNAs that are much more cost-effective than those employing fundamental mixing.
Samplers, harmonic mixers, or combinations of both are used in traditional VNAs to down-convert measurement signals to intermediate frequencies (IF) before digitizing them. Mixers tend to be the down converters of choice at RF frequencies, due mainly to their simpler LO drive system and enhanced spur-management advantages. At microwave and millimeter wave (mm-wave) frequencies – where receiver compression and cost are major concerns – harmonic sampling is often used.
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. Suppression of leaky signals requires the use of broadband isolation devices in the output arms of the power divider. Furthermore, these leaky signals are frequency dependent and cannot be removed via calibration, so they impose limitations on the dynamic range of a VNA. The result is that these VNAs can’t fully characterize highly reflective devices such as high-pass filters, and devices that have weak coupling among constituents must be measured as a function of frequency.
The short- and long-term stability and quality of broadband SRD-based VNA measurements may also be challenged by three factors:
1. Physically large and inhomogeneous measurement structures utilizing discrete components such as reflectometers, receivers, signal conditioning devices, interconnect cables, and waveguides
2. High-frequency multiplexing schemes
3. Complex receiver structures such as harmonic frequency converters and complex LO distribution networks
VNA NLTL Technology
NLTL technology has been leveraged in a new VNA architecture to provide enhanced performance over broad frequency ranges, and reduced measurement complexity when compared with existing solutions. NLTL-based samplers and distributed harmonic generators in VNAs, such as VectorStar™ and ShockLine™, overcome the aforementioned limitations of SRD-based sampling VNAs, and meet the needs for high-performance frequency-scalable VNA architecture.
How? Well, in general terms, NLTLs are distributed devices that support the propagation of nonlinear electrical waves such as shocks and solitons. 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. 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.
Measurement Benefits of NLTL-based VNAs
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. An essential ingredient in the pulse formation process is a differentiator circuit that transforms the step-like output of an NLTL into a pulse. Broadband distributed harmonic generation is achieved by leveraging the “harmonic growth” characteristics of NLTLs.
Figure 1
VNAs with NLTL-based samplers have 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 1). NLTL-based samplers offer a number of benefits to modern VNA architectures, as shown in Table 1, and provide engineers with an unparalleled value per GHz for their investment.
|
Parameter |
NLTL-based VNA Advantage |
Customer Benefit |
|
Simplified VNA architecture |
Monolithic reflectometer design reduces number of discrete parts and connectors |
Lower maintenance and operating costs; reduced down time |
|
Stability |
Integrated chip design greatly reduces temperature variation across reflectometer constituents |
Longer intervals between calibrations; better measurement accuracy and repeatability |
|
Bandwidth |
Extremely wide RF sampler bandwidth allows one sampler to cover broad frequency range |
Lower cost for making high-performance measurements over broader frequency ranges |
|
Dynamic Range |
Over 100 dB across all frequency ranges |
Better characterization of highly reflective devices and weak crosstalk |
|
Size |
High performance in very small form factor |
Direct connection to wafer probes; smaller footprint |
|
Cost |
Improved capability-to-cost ratio enables new applications |
Dramatic cost reduction for high-frequency testing |
Table 1
Our next post will delve deeper into how the NLTL-based VNAs can deliver such high performance, particularly in microwave and mm-wave designs. You can also learn more by downloading this VNA architecture white paper.
