Sometimes engineers need to measure the network parameters of electrical networks, which is where a network analyzer comes in handy. In other cases, they may need to gain critical insight into devices by measuring the amplitude properties of the device under test (DUT), which is where a scalar network analyzer shines. Like any diagnostic tool, they can be purchased on the market, but why buy one if you can build it cheaper and still retain accuracy?
Electronic engineer Steven Merrifield went with the second option and built his own scalar network analyzer using an AD9850 DDS (Direct Digital Synthesizer), an ADS1115 ADC, a pair of AD8307 log amplifiers, and a generic I/O pinout that matches the Raspberry Pi, although Merrifield states any microcontroller can be used to program the ADC.
Beyond the electronics mentioned earlier, Merrifield also outfitted the SNA with shielding cut from a 0.5mm copper plate, which will help limit noise for more accurate readings. Merrifield states, “This circuit is a little different from most in that the filtered DDS output is measured in addition to the input voltage from the EUT. This allows a more accurate measurement as it doesn't assume the output is constant, any variations due to loading effects are automatically compensated for.”
The analog bandwidth is deliberately set at 1KHz to 30KHz for increased accuracy, while the noise floor averages -69db with the shielding added. Merrifield has provided an overview of his DIY scalar network analyzer on his website, along with links to the PCB schematic and source code for those who would like to copy his build.