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Scalar Network Analyzer

Scalar Network Analyzer (SNA) are wonderful tools. They are relatively cheap, easy to build and use but are very valuable measurement tools when repairing or building rigs. A scalar network analyzer consists of a VFO and a logarithmic amplifier. The former generates an about +0dBm signal at the output of the SNA while the latter measures the incoming RF power. That is all.

Obviously, such a device can act as a signal generator and power meter but can also measure the frequency profile of filters accurately. Additionally, together with an SWR bridge, such a device is an antenna analyzer. The applications are manifold.

With the advent of chap DDS chips (e.g., AD9850), micro-controllers (e.g., ATMega328) and integrated logarithmic amplifiers (e.g., AD8307) an SNA can be build easily. The only additional circuitry is a simple amplifier to boost the power of the DDS signal to about +0dBm, using two cheap 2N3904.


Circuit of the scalar network analyzer.
The circuit of the Scalar network analzer (SNA) consists of a ATMega328 MCU, DDS generator and buffer, a logarithmic amplifier and an RS232 interface.

As mentioned above, the SNA circuit consists of a DDS module (AD9850) controlled by an ATMega328. The latter also measures the voltage form the logarithmic amplifier (AD8307) which is proportional to the logarithm of the input power delivered into 50Ohm. The micro-controller is interfaced in old-school style via RS232 using a MAX232 level-converter. You could also interface and power the complete circuit via USB using the V-USB library and emulating a serial interface over USB.

Board-layout of the scalar network analyzer.
The circuit can be squeezed on a 5cm by 7cm (about 2in by 3in) perf-board.

Stacking the DDS module above the micro-controller allows to build the complete device onto a small 5cm by 7cm (about 2in by 3in) board. This compact construction has the disadvantage that the logarithmic amplifier will pick-up some noise from the DDS generator and micro-controller, thus loosing some sensitivity.

Photo of the SNA inside its tiny case.

None the less, a compact constructs allows for a small device.


Screen-shot of the client application in action.

The Hardware CAD files, Firmware and Software for the SNA can be found at my GitHub page.

Best & 73,
Hannes, DM3MAT