Products > Test Equipment
Cheap and Small Dynamic Signal Analyzer?
TopQuark:
--- Quote from: Berni on August 20, 2020, 05:11:17 am ---
--- Quote from: gigavolt on August 19, 2020, 09:54:05 pm ---
--- Quote from: electrolust on August 19, 2020, 01:25:17 am ---NanoDSA!
--- End quote ---
;D my thoughts exactly. I'm surprised nobody has tried to do it yet, but I suppose that the combination of low frequency and low noise makes it difficult.
--- End quote ---
Guess more of a problem is that very few people actually want one. They are perfectly happy with there existing Rigol scope.
The low frequency and noise is not that much of a problem, you just pick a really nice opamp for the AFE. More of a difficulty is getting a spurious free response over the whole range. The noise floor is so low that even the tinyest of signals show up as a spike. This means a great deal of care must be taken in the shielding and PCB design to avoid any power supply noise or digital signals from getting in there and showing up. For example in my diy sound card design i had a LDO become just slightly unstable, not enough to cause obvious problems or a huge sine wave on the output when when poked with a scope, but it was singing enough to produce a small spike in the FFT, was quite a hunt to find where it was coming from.
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I do have a need for a DSA. The question is more about if I will do it as a one off or am I trying to build a "NanoDSA".
If I am just building one for myself, I'd just wait for the AD4630-24 eval board to ship, plug it into my fpga dev board, write just enough code to have the project do what I want to do, and be done with it.
On the other hand, if I am building a "NanoDSA", I'd go with a couple of AD7768-1, or even a stereo audio ADC, pair it with a STM32 (or clone, even), and build something down to a cost.
I think I'll build some form of a DSA, because I genuinely have a need for one, just not sure if I have the will and motivation to do a "NanoDSA" project.
maxwell3e10:
If you think of making it as a project, I would go for at least 1-2 MHz sample rate. This would put it clearly above the sampling rate of audio products, which otherwise are hard to compete with in terms of hardware performance. Software is another question. It would be useful to take input from an audio interface (ADC+DAC), which often have a USB connection and standard drivers, and implement all software functionality of an SR785.
David Hess:
--- Quote from: gigavolt on August 20, 2020, 03:26:56 pm ---
--- Quote from: Berni on August 20, 2020, 05:11:17 am ---Guess more of a problem is that very few people actually want one. They are perfectly happy with there existing Rigol scope.
--- End quote ---
That too, because ultimately the reason I would want it is for better noise performance/higher dynamic range, but in a much smaller bandwidth (<100 kHz) than my scope. I'm guessing most hobbyists can "get by" with 8 bits.
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Exactly, and it is too specialized of an instrument. For most network analysis, 8 bits is plenty and a DSO with signal generator only needs appropriate firmware. There are some relatively low cost products intended for the audio market that operate up to 192 or 384 kS/s like the QA403. At higher cost, Cleverscope operates at much higher frequencies, but with lower resolution and an oscilloscope input stage will compromise performance.
I have thought about designing a more general bidirectional 2-port low frequency VNA for low frequency network analysis, but the bidirectional design would compromise performance as a dynamic signal analyzer because of limited common mode rejection, and I am not sure that is a problem worth solving.
TopQuark:
--- Quote from: David Hess on October 27, 2022, 01:28:23 am ---
--- Quote from: gigavolt on August 20, 2020, 03:26:56 pm ---
--- Quote from: Berni on August 20, 2020, 05:11:17 am ---Guess more of a problem is that very few people actually want one. They are perfectly happy with there existing Rigol scope.
--- End quote ---
That too, because ultimately the reason I would want it is for better noise performance/higher dynamic range, but in a much smaller bandwidth (<100 kHz) than my scope. I'm guessing most hobbyists can "get by" with 8 bits.
--- End quote ---
Exactly, and it is too specialized of an instrument. For most network analysis, 8 bits is plenty and a DSO with signal generator only needs appropriate firmware. There are some relatively low cost products intended for the audio market that operate up to 192 or 384 kS/s like the QA403. At higher cost, Cleverscope operates at much higher frequencies, but with lower resolution and an oscilloscope input stage will compromise performance.
I have thought about designing a more general bidirectional 2-port low frequency VNA for low frequency network analysis, but the bidirectional design would compromise performance as a dynamic signal analyzer because of limited common mode rejection, and I am not sure that is a problem worth solving.
--- End quote ---
https://www.testunlimited.com/pdf/an/5988-6774EN.pdf
According to page 20, a 2 channel DSA (one ch for ref, one ch for DUT output) with a noise source should make for a blazingly fast if not "real time" network analysis / bode plotting tool. I think if I want to include this function, I will indeed want to have at least 2Msps for the ADC so that the bode plot can cover lets say 100Hz to 1MHz in a single sweep.
DC2390A eval board from ADI (https://www.analog.com/media/en/technical-documentation/user-guides/DC2390AF.PDF) would be the perfect solution paired with a FPGA board, with two LTC2500-32 and two fast DAC. Shame the connector on the board is a HSMC which doesn't plug into anything I own.
David Hess:
--- Quote from: TopQuark on October 27, 2022, 01:51:26 am ---https://www.testunlimited.com/pdf/an/5988-6774EN.pdf
According to page 20, a 2 channel DSA (one ch for ref, one ch for DUT output) with a noise source should make for a blazingly fast if not "real time" network analysis / bode plotting tool. I think if I want to include this function, I will indeed want to have at least 2Msps for the ADC so that the bode plot can cover lets say 100Hz to 1MHz in a single sweep.
--- End quote ---
An impulse source is even faster. Or an edge can be used as the source and then the signal differentiated before the FFT. Limited only by the FFT execution time and the time to capture the acquisition, the display of the network response can be real time. I am actually disappointed that the recent oscilloscope bode plot generators cannot use this method.
--- Quote ---DC2390A eval board from ADI (https://www.analog.com/media/en/technical-documentation/user-guides/DC2390AF.PDF) would be the perfect solution paired with a FPGA board, with two LTC2500-32 and two fast DAC. Shame the connector on the board is a HSMC which doesn't plug into anything I own.
--- End quote ---
There are a bunch of products which almost do it, but at least for me the software is not trivial either.
I have been thinking more in terms of a VNA because it would be so easy to use as a super LCR meter and I have never seen one that operates in the way that I am thinking. Of course maybe there is a good reason for that. HP published an application note on the subject which discusses the advantages and disadvantages of the various designs, and I should reread it.
I am not really interested in distortion measurement. There are too many products which already handle that well so making another distortion analyzer is less interesting.
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