Real life active&passive filter ,Opamp g measurements Bode plot/potter diagram - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: September 05, 2024, 10:49:04 pm
content: In Spice, it is very simple to examine filters in the frequency domain / gain . You divide the output voltage by the input voltage and examine it in ac sweep.10 log10( (VOUT/Iout)/(VIN/Iin) )= 10 log10( (Pout)/(Pin) )20 log10( (VOUT)/(VIN )=in ex. If you can divide the diff ended output into the single ended input.There is no difference between the output differential and the input single ending, the logic is the same.Let's try this in the real world and examine it in the frequency/gain domain and draw bode plots.I would like to know to what extent I can do this, especially with the devices I have.VNA:I know I can draw bode plots of passive filters. Port0 applies a signal at a different frequency to the input while port1 shows how much the signal comes in.It is possible to save in touschstone files format. Contains magnitude and phase informationI am not sure if active filters can be measured with VNA (especially if the gain is greater than 0db, is it a problem?) When examining passive rlc components and PCB traces (there is no power no voltage thing), we generally examine 0 db and below.The VNA I have: cheap Nano VNAYes(need but need a attentuator)Spectrum analyzer:I am not sure if a single-input RF input Spectrum analyzer can produce a bode plot. It needs a signal generator that sweeps the input frequency and also shifts the frequencysynchronouslyNo for single spectrum analzyerContains magnitude no phase dataSpectrum analyzers with gen out:(tracking generator output?) I have seen that some models can produce bode diagrams of active and passive filters. But cant measure phase.Spectrum analyzers with gen out hold trace mode:It can be observed by applying a frequency sweep signal externally and putting it in sa hold trace mode.The SA I have: cheap Tinysa ultra (I do not know if bode diagrams can be produced with this)(no with tinysa ) but can maximum hold modeOscilloscopes with bode plot/potter diagram feature:I have an oscilloscope with this feature and I can produce bode diagrams of active and passive filters. One of the channels has the signal output feature and the second channel makes the measurement.Oscilloscope and manual measurement: I have seen applying signals at certain intervals in the time domain, noting their amplitudes and producing a bode plot in excel (I also tried it myself)Analog discovery 3:I bought an analog discovery3 with fft and many features. I am not that knowledgeable.network analyzer modeI don't know if I can do these with the devices I have, VNA, SA, and Analog Discovery 3. If possible, can you provide sample documentation?

author: jwet
author_rank: Frequent Contributor
timestamp: September 10, 2024, 11:16:34 pm
content: You can theoretically do what you want with any of those instruments given the details of your application- ie frequency, input and output impedances and gain/loss magnitude. Of the three instruments you have, the analog discovery is most applicable to what your graph is displaying.A few tips- I don't know what you mean by sample documentation- all these instruments have extensive app notes and tips that you can reference.The Nano VNA and Nano SA are 50 ohm input impedance instruments that are generally best suited for RF applications where 50 ohm I/O is the standard. You can work around this in some cases by putting passive 450 ohm (-20 dbV) resistors in series with the inputs. The instruments may have enough dynamic range to make this workable depending on you application.The Discovery 3 has high impedance inputs- somewhat like an oscilloscope and the amazing Digilent software provides LF network analyzer functions and a competent bode anlyzer which will give magnitude and phase out to a few MHz. This would be your best bet. There are exercises on the Digilent website making measurments like this.The Nano instruments could be applied but the the Dicovery will be much better.

author: uer166
author_rank: Super Contributor
timestamp: September 10, 2024, 11:41:07 pm
content: Without phase information you can't generate a bode plot directly. Ergo an SA can do a magnitude plot with a tracking gen, but not a full bode plot that includes phase.

author: radiolistener
author_rank: Super Contributor
timestamp: September 10, 2024, 11:50:22 pm
content: This is how NWT analyzers work. It don't have phase plot, just scalar amplitude response. But this is enought to test philter or amplifier response.You can find different versions: for 70 MHz, 200 MHz, 6000 MHz, etc.Internally it has sweep generator (different NWT use different circuits) and logarithmic amplifier with ADC. It also has linear input channel so you can use what you need.Technically it has PIC/STM32 MCU inside which set frequency and read ADC result, set next frequnecy and read ADC result, etc in a loop.

author: Smokey
author_rank: Super Contributor
timestamp: September 11, 2024, 12:04:36 am
content: Way back before I had dedicated equipment to do this sort of stuff, I made a Bode (https://www.eevblog.com/forum/eda/bode-plots-come-from-hendrik-bode-who_s-name-is-pronounced/) plotter from a function gen sine wave output and two oscilloscope channels doing measurement. Controlled everything with python. Captured the two scope channels with ch1 measuring input and ch2 measuring output. You can get both phase and amplitude from that.Bode plots is one of the feature sections in the analog discovery Waveforms software. It's built in.