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Question for the Oscilloscope Experts

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rsfoto, Oct 10, 2015.

  1. rsfoto

    rsfoto Member

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    Yes you are absolutely correct but there is the difference between a Profesional Astronomer and an Amateur Astronomer and the difference is

    The thickness of the Wallet

    Professional Astronomers build Observatories using our Taxes :wideyed:

    Amateur Astronomers build Observatories from what is left after Taxes :arghh: and where they can. We can not afford to cut off the top of a mountain and build something like below

    VLT from the ESO at the top of the Paranal mountain in Chile :wideyed:

    Observing_Platform_VLT.jpg

    My columns are sitting on a dense clay formation (no rocks available where I bought the house :arghh: ) This clay I remember being a young boy was really hard to dig. We had to use steel bars and inch by inch we were able to dig the trenches for the foundations of whatever was going to be build.

    So the intention of this, as I already wrote, not perfect and not elegant electronic circuit, is to make comparisons and show the people who want to build an observatory where perhaps good locations could be (of course within their building limits).

    But I appreciate any help to make it better as already happenes and if it would be possible with component values and some calculation so I can understand it better.

    So far everything has been Trial & Error :banghead:
     
    Last edited: Oct 16, 2015
  2. rsfoto

    rsfoto Member

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    Hi Tony,

    Thank You and absolutely no problem if you have no time.

    I am not asking that you design it for me. With all the tips here I have been trying to make it better and better and I know it is not that easy and even less when one is not a studied EE engineer, except perhaps you did not read message #69.

    I am not asking anything sophisticated for free ...

    As far as it is now I can live with it and will keep doing it with my Trial & Error method exchanging resistors and capacitors, adding and taking away those things and if it smokes, No Problem. next new IC and keep doing what I am doing

    Having a lot of Fun :happy:
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  4. dave

    Dave New Member

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  5. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    If you have time to make a geophone, suitable for seismic waves, you can bet far better results.
    Geophone is a spring suspended magnet inside a fixed coils the opposite of speakers.
    [​IMG]
    The output stage here controls the gain with feedback of 100K/100 =1k (1,000) gives max gain when pot=0
    .Geophones have a response < 1Hz. Subwoofer with no mass added ~ 20Hz , with mass added ?
    A Subwoofer with a mass attached to it and determines the low frequency response as long as the coils don't hit the magnet. Which is why it is better to have several magnet donuts inside a teflon lined aluminum sleeve with lots of coil windings around it. (crude description). Much more immune to noise than a piezo pickup.

    An audio bass frequency amp with 1k (60dB) gain will pickup a lot of ground vibrations.

    Like using a subwoofer connected to a phonograph input with a low pass filter.
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Notch A very small bandwidth filter (58-63 hz). e.g 60 Hz that attenuates power line frequencies.

    Tony mentioned "Stray hum". That would come from the power line (60 Hz). 120 Hz is also common because the rectification to get a DC output doubles the ripple frequecy.

    Tony mentioned "twisted pair shield". It's a type of cable. Shielding reduces RFI (Interference from radio stations) and twisting reduces (EMI) Electromagnetic interference.
    That happens when say a power cable is run parallel with a sensor cable that isn't twisted. Th e"twists" tend to cancel what's common to both wires.

    Tony mentioned " ferrite core around sensor" Think powdered iron donut. Commonly called a "Ferrite bead"

    Tony mentions a "CM Choke" CM is an abbreviation for "Common Mode" which also means the signal common to both wires. Chole is another word for aninductor.
    So, a CM choke is an inductor that makes the signals that are common to the system smaller

    CM = Common Mode (discussed)
    CMRR = Common Mode Rejection Ratio (usually expressed in db or decibels)
    db requires an entire topic, For now, a higher number is better,
    You can't cancel all of it that common.

    A high impedance antenna doesn't work well.
     
    Last edited: Oct 16, 2015
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  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Like most Cmos opamps, the LM6482 is fairly noisy with 5 times as much noise as a low noise OPAx134. It is not suitable for your very high gain preamp.
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    AG: I pointed to the right datasheet, but wrong number LMC6482.
     
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  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes I looked at the correct datasheet of the LMC6482 where "C" is Cmos. It is a noisy son of a bxxxx.
     

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  10. Mikebits

    Mikebits Well-Known Member

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    For his application does that really matter? What sort of noise numbers should he be looking for? I am just curious because nV/Hz seems pretty low to me, so I hope to learn something here.
     
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  11. rsfoto

    rsfoto Member

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    Hi KISS,

    Well if you look at the signals I compared from the amplified Piezo = Yellow line as well as from the unamplified direct signal = Green Line from the other Piezo, I see that the yellow amplified curve looks similar to a noise curve from a power supply from a video I just saw.

    So analyzing the strange positive / negative saw tooth curve of the amplified that comes from my DC Power supply. I have not yet measured the DC power supply noise. Will look in the internet how to do that

    comparaci├│n.JPG

    I read somewhere how to shield the inputs of the Oscilloscope Hantek 6022BE. Should I start there ?

    6022BE_Top.jpg

    Just saw a video of Texas Instruments how to properly measure with the probes

    Will keep doing my homework
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    His gain is 300 which is much more than a phono preamp. If you used a noisy opamp as a phono preamp then the noise that is heard and that would be displayed will be high.

    The scope trace produces 50us per division and the interference has 2.6 blips per division. Then each blip occurs every 19.23us so the frequency is 1/19.23us= 52kHz. Does something nearby have a switching power supply?
     
  13. rsfoto

    rsfoto Member

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    Hi AG,

    I am using this type of power supply for my experiments

    http://www.haitaik.com/products_Detail_classid_143_490.html

    which brings me to the idea of using a LiPo battery 3S = 11.1 volt and see the curves then ...
     
  14. rsfoto

    rsfoto Member

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    Another point is that I am using 3" cables with pins for my bread board connections ...

    1707.jpg
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Mikebits: The OPA is about 7x less noisy at 100 Hz.

    rsfoto: Your wierd sawtooth noise on a linear power supply battery explained: http://www.zen22142.zen.co.uk/Design/dcpsu.htm My way, the cap gets charged to a peak voltage and then acts like a discharging capacitor y when the signal goes away.

    I doubt you scope does FFT's (Fast Fourier Transforms)? That lets you look at signals by frequency component.

    How often in time to the higher spikes occur? 1/t = frequency.

    Look here: https://en.wikipedia.org/wiki/Shot_noise under the Title Electronic Devices and read the first paragraph only.
     
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  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes, it looks like your very high power Chang Chung power supply switches at 52kHz and also produces the other high frequency seen on your 'scope. You will not see the interference if that power supply is turned off and you use a battery instead.
    Each and every wire on your solderless breadboard is an antenna that picks up interference and causes coupling in the high gain circuit which might produce oscillation.
    You CANNOT use a solderless breadboard for such a high gain circuit.
     
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  17. rsfoto

    rsfoto Member

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    Hi KISS,

    Yes it has in the software a built in FFT function

    FFT1.JPG FFT2.JPG

    and here is the output of the oscilloscope when I connect the ground of the probe to the clip

    Osci-signal.JPG

    What is your veredict ? :nailbiting: :nailbiting: :nailbiting: :nailbiting:
     
  18. rsfoto

    rsfoto Member

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    Understand but how should then I test or experiment with different circuits ?

    So far I have not yet understood a simulator like LTSpice or similar software ┬┐?
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    What AG said in post #115.

    At this state you probably want a low noise linear power supply or a battery.

    Change to a batter or better supply. Then see if your breadboard in a grounded metal box would be the next place to go. EMI is from radiation and the power supply can cause that. So, all your wires act as antennas.
    The leads to that power supply act as an antenna. So, the power supply far away with say 10 feet of twisted pair shielded wire, with the shield at one end might also work.

    A linear supply MIGHT have issues too. The transformer vibrates at 60 Hz.

    The breadboards issues are primarily thermal offsets, contact movement, antenna pickups and capacitance. Right now, antenna pickup is the BAD thing.

    Even that power supply vibrates. The power cord provides a source of EMI and so do the wires. Fixible?, maybe. Shielded power cord. Shielded wires. Put ti in a shielded box.
    If you finally have electrical noise at 52 kHz, that can be filtered out. 60 Hz is everywhere. Your trying to DETECT subtle vibrations. Don't make any extra ones.

    Shielded boxes and short leads (high frequency antennas) go a long way,

    Oh, BTW, wiggling wires generate currents. Very tiny, but annoying sometimes. It's a wire moving in the Earth's magnetic field, so why shouldn't it?

    EDIT: Fixed typo
     
    Last edited: Oct 17, 2015
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  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I designed hundreds or thousands of special one of a kind circuits during my electronics career, some VERY complicated with MANY parts. I designed a compact stripboard layout for them and soldered each and every part and a few jumper wires that made half of a pcb. The copper strips were cut with a drill bit to be as short as possible and they formed the other half of the pcb. All circuits worked perfectly and the prototype was sold as the finished product.

    Computer simulations were not even dreamed about in those days.
     
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  21. rsfoto

    rsfoto Member

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    Thanks for the explanation. Do you have a photography of it so I can have a better idea ?

    I just searched under stripboard and OK that is how I make the circuita when they are tested, but what about when having to exchange components on it ?

    Desolder them and solder the new again into it ?

    :(
     

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