Project: PC Controlled 2-250 MHz Network Analyzer

 

Featured in QEX, March 2002

 

Design Services Performed:

Description:

This demonstration project built on the 2-250 MHz VHF source and added a reflection coupler and three RF Log Amp detectors to make a full scalar - Reflection / Through network analyzer that can be built by nearly anyone at a fraction of the cost of a commercial unit. This was a very popular project that showed just how much could be done by a determined homebrewer! Built with the same - wide range, high resolution hetrodyned RF source concept. Using two PLL's and a DDS synthesizer built in source has 1 Hz resolution over the full 2 to 250 MHz output range. Excellent phase noise and spurious response performance is also achieved. Also included in the design is a novel output leveling circuit which provides -15 to 15 dB output range with 0.5 dB accuracy over the entire frequency range. The analyzer is really a virtual instrument with all it's functions being controlled by a PIC Microcontroller via an ActiveX enabled PC control program. Communications to the PC is via a speedy 19.2 k Baud RS232 link operating in a 4 byte packet mode for speed reasons. A full set of open, short, load and through calibrations can be made to bring the basic accuracy of the instrument to better than 0.5 dB.

This instrument platform is also a virtual - synthetic instrument. With the add-on software personalities the instrument can function as a full featured source, calibrated RF power meter, impedance analyzer and Q-meter to name just a few.

  • True calibrated Reflection / Through measurements from 2-250 MHz.
  • 70 dB plus dynamic range, better than 0.02 dB resolution.
  • 2-250 MHz output range with 1 Hz resolution.
  • -15 to +15 dBm output range (leveled).
  • Full frequency and output amplitude e-calibration built in.
  • PC Control via an ActiveX enabled virtual front panel.
  • RS232 communication (or USB) link to the PC.
  • On board PIC Microcontroller.
  • Spurious responses below -60 dBc.
  • Harmonics below -40 dBc.
  • Phase noise = -96 dBc/Hz at 10 kHz offset.
  • Wide band signal path, no switching from 2-250 MHz (7 octaves).
  • Shielded, modular design.

 

A full PCB set was published with the design. Above is the top and bottom of the Mixer / Output stage. High performance was achieved using a very basic 2 layer design mixing surface mount and through hole components as required to minimize the cost.

 

This is a screen shot of the analyzer measuring a 21.4 MHz crystal filter. The excellent dynamic range and low noise of the traces can be seen. The application programmed was written in Agilent VEE that was enabled by a custom ActiveX control.

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