| Electronics > Projects, Designs, and Technical Stuff |
| Low frequencies Vector Network Analyzer, arduino based. |
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| MasterT:
--- Quote from: xaxaxa on September 21, 2018, 03:09:29 am ---What directional coupler are you using for low frequency? --- End quote --- Not relevant, there is no any directional coupler. I used non RF approach, similar to analog device AD5933 |
| RoGeorge:
Nice, well done! :-+ Do you have any plans to publish the software, too? |
| MasterT:
--- Quote from: RoGeorge on September 21, 2018, 11:50:06 am ---Nice, well done! :-+ Do you have any plans to publish the software, too? --- End quote --- Difficult to say, have to clean it up first. It's like a big pizza, assembled out of "customized" libraries. For example, TFT, GFX - Adafruit libraries, tweaked internally to make it compatible with mcufiends ILI9341 shield. For rotary encoder same things, was not able to find "plug-in and use" arduino DUE version, have to re-write interrupt driven UNO version. DDS AD9833 again, tweaked to synchronize with adc start dma conversion. Customized I mean on direct port manipulation level, so pieces of code would not run on any other uCPU except sam3x. |
| ogden:
--- Quote from: xaxaxa on September 21, 2018, 03:09:29 am ---I would use an audio ADC, e.g. pcm1802. Cheap and has excellent linearity and dynamic range that no MCU ADC would ever hope to match. For higher frequencies (>1MHz) a mixer is a must. --- End quote --- You can't "undersample" with most SD ADC's so w/o mixer you are stuck with around 1/10 of sampling frequency - to get +/- meaningful phase/amplitude measurements. In general it means with such a slow ADC's you shall implement mixers, this also means added complexity. This basically means N2PK VNA or it's simplified variant with two AD9833's. --- Quote from: MasterT on September 20, 2018, 02:11:55 pm ---Feedback is welcome on a project, I'm having fun with. --- End quote --- Nice project indeed, but I would not name it VNA. It's more like RF impedance meter "with a twist". Also you shall write "theory of operation" or provide pointer to existing one - because many who read this thread possibly do not understand operation of instrument you are building. I would use DCT DFT instead of FFT. Your DDS is generating just single frequency, so there's no point to calculate all the bins of FFT that does not relate to frequency of DDS, you basically need to calculate just one which is what DFT does. With DFT you don't need so much memory and CPU resources as well. |
| MasterT:
--- Quote from: ogden on September 21, 2018, 01:57:58 pm --- --- Quote from: MasterT on September 20, 2018, 02:11:55 pm ---Feedback is welcome on a project, I'm having fun with. --- End quote --- Nice project indeed, but I would not name it VNA. It's more like RF impedance meter "with a twist". Also you shall write "theory of operation" or provide pointer to existing one - because many who reads this thread possibly do not understand operation of instrument you are building. I would use DCT instead of FFT. Your DDS is generating just single frequency, so there's no point to calculate all the bins of FFT that does not relate to frequency of DDS, you basically need to calculate just one which is what DCT does. With DCT you don't need so much memory and CPU resources as well. --- End quote --- Thanks. I should put a reference link to AD5933 in the first message, and since AD used "1 MSPS, 12-Bit Impedance Converter, Network Analyzer" as a title for theirs chip, I would keep network analyzer as a name of my project. I intended to get comments from the audience that already posses a knowledge "theory of operation." Manny people don't understand the absolute supremacy of the FFT on any other method of data processing. The question of "why using FFT instead of DCT?" is the same as "why to sample data by ADC and than process in DSP core, if we can drive a couple of MUXes by quadrature shifted clock?" Because FFT provides processing gain, increasing SNR ratio by >30 dB(!). Using hardware analogy with two MUXes, someone has to put a hardware preselector - 244 Hz wide band pass filter in front of the MUXes. Practically impossible to build such BPF with 40-80 dB roll off, adjustable over 1 kHz - 6.8 MHz. Not now, not ever in the future. |
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