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Scalar Network Analyser

Scalar Network Analyser using DDS, log amplifiers and ADC.

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This project is a simple scalar network analyser using a DDS, log amplifiers, multi channel ADC and a generic I/O interface. The pinout matches a Raspberry Pi, but any micro-controller may be used to program the DDS and read the ADC.

The analog bandwidth has been deliberately set at 30MHz. Note how the measured response matches the theoretical simulation very closely. There is approximately 0.02dB variance between 1kHz and 30MHz. The noise floor averages around -69dB with shielding fitted.

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.

An example of a single stage R-C low pass filter response, theoretical cutoff f = 1/(2*pi*RC) = 31.8kHz :

An example of a single stage L-C high pass filter response, theoretical cutoff f = 1/(2*Pi*sqrt(LC)) = 1.18MHz :

Measured response of a 10.7MHz ceramic resonator :

Measured response of a 4MHz crystal:

Added a new software mode that does a very fast pre-scan but with a reduced frequency resolution :

Also added an ASCII output mode for very fast pre-scan testing without a graphical display, axis ranges are automatically scaled:

scalar_source_code.c

C source code for Raspberry Pi

x-csrc - 6.24 kB - 03/30/2019 at 00:10

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Adobe Portable Document Format - 26.22 kB - 03/29/2019 at 23:41

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  • 1 × AD9850 Direct Digital Synthesizer
  • 1 × ADS1115 16 bit ADC
  • 2 × AD8307 Logarithmic Amplifier

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Jarrett wrote 03/30/2019 at 04:25 point

I like this a lot.

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Dan Maloney wrote 03/29/2019 at 15:35 point

So, dumb question: aside from the obvious, what's the difference between a scalar network analyzer and a vector network analyzer? This seems much simpler than any VNA build I've seen. 

Nice build, BTW. Very clean PCB layout - love the way the RF section is closed off from the ADC section.

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Steven Merrifield wrote 03/31/2019 at 06:56 point

A VNA measures amplitude and phase, a SNA only measures amplitude.

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