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Low-cost box converts PC into network analyzer

Posted: 10 Nov 2005     Print Version  Bookmark and Share

Keywords:model 6000  analog devices  ad9854  dds chip  direct digital synthesizer chip 

Here's a new RF VNA (vector network analyzer) that is low in cost but provides quite a few of the capabilities of pricey bench-top analyzers that cost thousands. Keep in mind that top-drawer VNAs provide very wide dynamic range (90dB to 120dB is typical), low noise, speed, great accuracy, lots of test channels, and many firmware/software options.

However, for routine lab tasks you can often forego extreme dynamic range and accuracy, and still get meaningful results.

Using your PC, you can use an instrument like this to put a lot of the burden on software. The PC can then manipulate and display your data. In this case, you're offloading many VNA functions to the PC, saving a bundle in the process.

You can use the $655 Model 6000 to test amplifiers, characteristics of coaxial cables, and measure and adjust filters. You can also use it to check antenna impedance vs. frequency, antenna SWR (standing wave ratio), and measure the length of transmission lines using a TDR (time domain reflectometer) feature.

In use, the VNA measures forward/reverse gain and phase response of circuits under test. It also measures input and output reflection (complex impedance) and provides you with S parameters. You can use it to make measurements of one-port networks (input, ground), and two-port networks (input with ground/output with ground). Finally, the VNA can also function as general purpose signal generator.

Coverage to VHF

The Model 6000 VNA covers 1MHz to 100MHz, with the capability to function with somewhat degraded performance up to 120MHz and down to 200kHz. Key to that range is the use of an on-board Analog Devices AD9854 DDS (direct digital synthesizer) chip and maxim integrated products 200MHz programmable video amplifier ICs.

These devices are under control of a Cypress EZUSB microcontroller, which is an 8051 derivative. If you need greater precision, the 6000's frequency accuracy, in either its analyzer mode or its signal generator mode, can be enhanced by feeding in an external reference.

Even without an external reference, the 6000 is no slouch. Resolution for amplitude measurements, for example, is within 0.1dB, with a 1° resolution when making for phase measurements.

Likewise, Model 6000's dynamic range in forward mode is 80dB at lower frequencies, dropping to only 75dB in its mid-range and higher frequencies. In reverse mode, dynamic range is -35dB at lower frequencies, and -25dB at mid-range and higher frequencies.

Output power is about +3dBm (about 2mW), and it will accept input levels of +10dBm (2-V P-P) into its TRANSMIT or RECEIVE front-panel coaxial connectors.

Windows plug-and-play

Your host PC talks to the VNA across a USB 1.1-compatible port (Universal Serial Bus). Thanks to USB, the set-up is plug-and-play.

The Model 6000's software runs on Windows-based PCs running Windows 98SE, Windows 2000, or Windows XP. Your system also requires the Windows installer program and .NET 1.1 Framework run-time package (which Ten-Tec provides on a CD-ROM).

Included with the Model 6000 VNA are a set of testing and calibration accessories, 1-meter-long and 3m-long cables, 10dB and 30dB attenuators, a 50-ohm non-reactive dummy load, a shorted connector, and a barrel connector. You also get a 100Vac to 240Vac 50/60Hz power supply, with a removable cord.

- Alex Mendelsohn


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