In a VNA a swept sine-wave signal source is used to sequentially stimulate the ports of the interconnect or device under test. The amplitude and phase of the resultant transmitted and reflected signals appearing at both VNA ports are then received and measured. To wholly characterize a 2-port device under test (DUT), six pairs of measurements need to be made: the amplitude and phase of the signal that was emitted from both ports, and the amplitude and phase of the signal that was received at both ports for each source. In practice this can be achieved with a reasonable degree of accuracy with a single source, a transfer switch and two receivers; the latter inputs being switched through a further pair of transfer switches. Alternatively three receivers can be used with an additional input transfer switch or, as in the PicoVNA, four receivers can be used.
Using four receivers eliminates the receiver input transfer switch errors (chiefly leakage and crosstalk) that cannot be corrected. These residual errors are always present in two- and three-receiver architectures and lead to lower accuracy than that of the Quad RX design.
Support for 8 and 12-term calibration and the unknown thru
Almost all vector network analyzers are calibrated for twelve error sources (six for each signal direction). This is the so-called 12-term calibration, which experienced VNA users are used to performing fairly regularly. In a four-receiver design some error sources are so reduced that 8-term calibration becomes possible, along with an important and efficient calibration technique known as the unknown thru. This gives the ability to use any thru interconnect (including the DUT) during the calibration process, vastly simplifying the procedure and reducing the number of costly calibration standards that need to be maintained.
Advanced vector network analyzer users will be pleased to know that internal a-wave and b-wave data can be exported for diagnostic use.
Bias-Ts are often not provided, or available as costly extras, on other VNAs. Use the PicoVNAs' built-in bias-Ts to provide a DC bias or test stimulus to active devices without the complexity and cost of external DC-blocks. The bias is supplied from external power supplies or test sources routed to the SMB connectors adjacent to each VNA port.
Test cables and calibration standards
A range of high-integrity RF and Microwave accessories are available from Pico Technology. Test cables and calibration standards strongly influence the overall performance of a VNA, so we recommend that you select your accessories and perform your calibrations carefully.
Cables and standards are often the weakest links in a VNA measurement, generally contributing significantly to measurement uncertainty despite their high cost. At the lowest levels of uncertainty, costs can be significant and measurements can be compromised by seemingly quite minor damage or wear. For these reasons, many customers hold both premium-grade items for calibration, reference or measurement standards, and standard-grade items as working or transfer standards and cables. Pico Technology can now offer cost-effective solutions in both grades. In general we recommend PC3.5 interfaces for premium or reference use and SMA interfaces for working use.
The PicoVNAs may also be used with third-party calibration kits, such as the Keysight/Agilent 85032F, that are characterized by polynomial coefficient models either in written form or as a text file.
Phase- and amplitude-stable test leads
Two test cable types and grades are recommended and provided by Pico Technology. Both of high quality, with robust and flexible construction and stainless steel connectors, the main difference between them is the stability of their propagation velocity and loss characteristic when flexed; that is, the degree to which a measurement could change when the cables are moved or formed to a new position. Cables are specified in terms of flatness and phase variation at up to 8.5 GHz when a straight cable is formed as one 360° turn around a 10 cm mandrel.
The PicoVNA 108 delivers an exceptional dynamic range of 124 dB at 10 Hz (118 dB for the PicoVNA 106) and less than 0.006 dB RMS trace noise at its maximum operating bandwidth of 140 kHz. The instruments can also gather all four S-parameters at each frequency point in just 182 μs (PicoVNA 106) or 189 μs (PicoVNA 108) or or S11 + S21 in less than 100 μs. In other words, a 201 point 2-port .s2p Touchstone file in less than 38 ms or up to two .s1p files in less than 20 ms.
Their low price makes them cost-effective as deep dynamic range scalar network analyzers or single-port vector reflectometers as well as full-function dual-port, dual-path vector network analyzers. They are affordable in the classroom, in small businesses and even in amateur workshops, yet capable of meeting the needs of all users up to the laboratory or production test technician or the metrology
The low-cost PicoVNA 106 opens up the potential for every student or trainee to learn through the use of a full-function professional-grade vector network analyzer. The Pico PQ186 Network Metrology Training Kit builds on this opportunity and supports a wide variety of learning and experiment. Central to the kit is the separately available PQ189 Network Test PCA. This printed circuit accessory hosts a variety of example lumped element, active and passive and transmission line DUTs and end-of-line SOLT (short, open, load and through) calibration standards.
Used with the PicoVNA 106, the kit supports teaching objectives around reflection and transmission measurements, S-parameters and other standard measurement quantities. These can be presented and interpreted as log, linear, phase, real, imaginary, polar and Smith chart formats and derived quantities group delay and time domain transmission and reflection. Additionally, by including an active broadband amplifier element (+5 V DC power required, 2.1 mm jack), nonlinear compression measurements such as P1dB and AM to PM (phase due to amplitude modulation) can be explored using the PicoVNA 106’s built-in measurement utilities.
Measurements and calibrations are made via industry-standard SMA connectors. These support measurements out to 6 GHz and the teaching of best interconnection practice and the importance of secure, repeatable and robust connections. Using the on-board calibration standards the student can practice calibration for reflection, transmission and 8- and 12-term corrections using short-open-load and the known and unknown through methods.
PCA includes example attenuator, broadband amplifier, 25 Ω mismatched
line, resistive power divider, low pass and bandpass filters and a user chip component site, plus short, open, load and thru calibration elements.
Also included in the kit is a set of budget SMA(f) SOLT calibration standards (PQ190). With these the student can calibrate at cable ends rather than on board. This supports teaching and experiment around reference plane shift, normalization and the de‑embedding of feedlines and connections on the PCA. Assumed ‘ideal’ and typical calibration data for these standards can be downloaded from picotech.com, along with the comprehensive user’s and trainer’s guide and referenced instrument settings files. Students with access to the AWR Design Environment can also download the Microwave Office design project for the kit PCA. Pico’s Cadence AWR DE interface wizard can then import real-world measured data directly to the project to allow measurement enhanced simulation or comparison with the design simulations. Software development kit examples are also available from our GitHub pages to import and work with measured data in other CAD, test and programming environments such as MATLAB, LabVIEW, C, C#, C++ and Python.
The Pico PQ186 Network Metrology Training Kit includes N(m) to SMA(f) inter-series adaptors, SMA(m-m) test leads and fixed SMA wrenches – all that the student will need to pair with the PicoVNA 106 (or any other VNA) to begin their practical learning. Printed circuit layout of the network test PCA is generic to support modification to alternative passive network and components.
PQ187 calibration kit
Two Network Metrology Demonstrator kits are also offered. These include Pico’s now widely respected Standard SMA or Precision PC3.5 professional grade test leads, a female SOLT calibration kit and an SMA non-insertable female to female Check Standard; both with reference data, traceable back to national standards. Either of these kits can realise and verify the full measurement capability and accuracy of the PicoVNA 106. High-quality, low-uncertainty measurements can then be made, suiting this training investment to much wider application within research and doctorate projects. Accurate measurement can also establish accurate reference and error terms in the measurements that students are making when using the PQ186 Training kit.