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pocketVNA

pocketVNA. What is a Vector Network Analyzer? In oversimplified terms, a Vector Network Analyzer (VNA) measures the complex impedances of electrical networks at RF frequencies.

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pocketVNA

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  1. pocketVNA What is a Vector Network Analyzer? • In oversimplified terms, a Vector Network Analyzer (VNA) measures the complex impedances of electrical networks at RF frequencies. • For a given frequency span, step through the range, apply a signal of known amplitude and phase to the network, and measure the amplitude and phase of the response of the network at each frequency. Gather all of this data and present it to the user (e.g. as a Smith chart).

  2. pocketVNA How are Vector Network Analyzers different from Antenna Analyzers? • Most antenna analyzers can only measure the impedance of an antenna, which is a one-port device. • Most VNAs can measure the impedances of a two-port network (i.e. a filter, attenuator, amplifier, etc.) • But in practice, many Antenna Analyzers are really one-port VNAs

  3. pocketVNA Why do we need Vector Network Analyzers? • To test RF things other than (single) antennas. To test things that have more than one port. Almost all “Antenna Analyzers” are one port VNAs. This discussion is about two port VNAs. • To measure things like S-parameters that more fully characterize the device under test (DUT).

  4. pocketVNA What do Vector Network Analyzers do? Most VNAs run some meta-code like this: For each frequency from lower to upper by step size Apply a signal of known amplitude and phase to the DUT Read the incident signal amplitude and phase Read the reflected signal amplitude and phase Read the transmitted signal amplitude and phase Calculate the complex impedance versus frequency and display it

  5. pocketVNA How do Vector Network Analyzers work? All VNAs need the following components: • A signal source • A receiver • A directional coupler or bridge • A user interface Most low-cost VNAs use DDS chips to implement the signal source and receiver. They are usually implemented using a Transmission/Reflection test set, and so they need to be manually “turned around” to make full two-port measurements.

  6. pocketVNA (Thanks to National Instruments for the graphic.)

  7. pocketVNA Signal Sources • The VNA needs a signal source to provide a stimulus to the DUT. • Most modern VNAs use a synthesized signal source • Ideally, the signal source should have low noise and be stable • The output power of the signal source limits the dynamic range of the VNA

  8. pocketVNA Signal Separation • VNAs need to measure both the generated signal that is incident on the DUT, and the reflected signal coming from the DUT. • This requires a component that is sensitive to the direction of the signal passing through it: Two common options are: • Directional Coupler • Directional Bridge

  9. pocketVNA Signal Separation • The performance, or “directivity” of the signal separation component (called the test set) largely determines the dynamic range and performance of the VNA. • Directivity is a measure of the ability of a signal separation component’s ability to separate signals flowing through it in opposite directions.

  10. pocketVNA (Thanks to Agilent for the graphic.)

  11. pocketVNA Signal Separation • Low cost VNAs usually use Directional Bridges, as Directional Couplers have several issues: • They are expensive, especially when they work over a broad frequency range • They are more difficult to keep in calibration • However, Directional Bridges are lossy and reduce the dynamic range of the VNA.

  12. pocketVNA Why a Transmission/Reflection (T/R) test set? • S-parameter test sets require extensive RF switching. This is both expensive and difficult to keep in calibration. Most “professional” VNAs use S-parameter test sets. • T/R test sets have to be manually “turned around” to measure full S-parameters, and hence don’t need switches. They can only measure S21 (forward gain) and S11 (VSWR) parameters.

  13. pocketVNA What is it? • The pocketVNA is a (relatively) low-cost Vector Network Analyzer (VNA)

  14. pocketVNA Why the pocketVNA? • I could go out and get an Agilent/Keysight or Rohde&Schwarz VNA, but they typically cost as much as a new car. The focus here is on a “cheap” VNA (< $500). The pocketVNA costs about $430, which is roughly -20 dB less cost than professional equipment. • I wanted a VNA that could do both precision measurements at HF, but would be adequate for antenna work at VHF/UHF frequencies. • I wanted something with software available for systems other than Windows.

  15. pocketVNA What else is available? • miniVNA, $500, http://miniradiosolutions.com/minivna-pro/ • N2PK VNA (kit), $400, http://www.makarov.ca/vna.htm • VNWA, $670, https://sdr-kits.net/index.php?route=web/pages&page_id=11_11

  16. pocketVNA Specifications • Two port T/R test set, measures S21 and S11. • Frequency range: 500 kHz to 4 GHz, 1001 measurement points per span • Dynamic range: up to 70 dB below 350 MHz, up to 40 dB below 4 GHz • Output power: 14 dBm • Impedance measurement range: 3 to 1000 Ohms • USB powered • Windows, Linux, Mac OS X software

  17. pocketVNA This is what the innards look like.

  18. pocketVNA Why the pocketVNA and not something else? I liked the pocketVNA for several reasons: • Software for platforms other than Windows (Mac OS X and Linux) • Modest cost • Good frequency range • Acceptable dynamic range (at HF/VHF) • USB powered • Provides an API interface for third-party software to use

  19. pocketVNA Cons • Very limited dynamic range and stability at high (GHz) frequencies • Closed source software

  20. pocketVNA How good is it? • I didn’t have time to compare the pocketVNA to professional VNAs, but I was able to show some functional abilities: • Measure the performance of a commercial low-pass filter • Identify the characteristics of an unknown RF component

  21. pocketVNA Case one: Performance of a known RF component • Device is a Mini-Circuits 70 MHz, 50 ohm low-pass filter • I calibrated the VNA over the range of 30 to 100 MHz • I then swept the DUT over the same range

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  25. pocketVNA Case two: Identification of an unknown RF component • Device is an unknown component salvaged from some old HP test gear • I knew the test gear was for cellular phone use, so I had a hunch about the frequency range • I calibrated the VNA over the range of 1.4 GHz to 2.1 GHz • I then swept the DUT from 1.5 GHz to 2.0 GHz

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  28. pocketVNA Example: WiFi Antenna Characterization • This is a sample measurement “project” from the vendor • It is a one-port sweep of a WiFi antenna from 2320 to 2460 MHz

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