Question for the Oscilloscope Experts

[rsfoto]

I have seen a lot science going into eliminating resonances in many past fields of aerospace and HDD design/manufacturing

The best foundation is deep connected to bedrock, next is 3m deep where soil is compressed at least 4x density. I saw a new house garage design with every room in the basement with 8" thick concrete walls including the garage which had 2'x3'x8' concrete monolith over the concrete garage basement pad then filled with crushed rock then a ground level pad over this.

Buildings in Japan are built on low Q <<5) rubberized dampening material to float during an earthquake.

A large architectural struts will of course have high Q resonance perhaps at a few hundred Hz but then floating on plastic clay dampens the coupling from surface waves but not so much from vertical seismic waves.

If you wanted to suppress vibration, it starts by accurate instrumentation then defining the amount of attenuation, the define the structure, then consider solutions.

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

Amateur Astronomers build Observatories from what is left after Taxes 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



My columns are sitting on a dense clay formation (no rocks available where I bought the house ) 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

 

[rsfoto]

lots of EMI on source from supply perhaps, lots of DC offset from mismatch R and high gain and lots of resonance around 50Hz and higher from window resonance.
-generally poor signals, but good enough for your purpose.

I would use a small piezo accelerometer mounted with bees wax on telescope, semi-rigid coax to pre-=amp , dc coupled with a precision balanced ultra-low input current Charge amplifier.

But no time to design it for you.

Good luck

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

 

[KeepItSimpleStupid]

AG: I read it as an LMC6482: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAAahUKEwiHrrqE-cfIAhUJVT4KHbgVDx4&url=https://www.ti.com/lit/ds/symlink/lmc6482.pdf&usg=AFQjCNELALcB5BW6KF5-Q10lE8Tf7ZPPlg&sig2=9OTnXmNwaLMvzF7SMa_jnA

EDIT: OOps: LMC6482 not LM6482 - fixed. Link was correct.

 

[Tony Stewart]

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.
**broken link removed**
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.

 

[KeepItSimpleStupid]

The only thing you have to measure is the low frequency noise. If you get stray hum add a notch filter, which is common as the last link showed a 100Meg load resistor try twisted pair shield with ferrite core around sensor cable or a CM choke from a telephone modem card. Raising the CM impedance improves the CMRR significantly on high impedance sources with longish cables acting as antenna at uV to mV signal levels.

The only thing you have to measure is the low frequency noise. If you get stray hum add a notch filter

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

Raising the CM impedance improves the CMRR significantly on high impedance sources with longish cables acting as antenna at uV to mV signal levels

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.

 

[audioguru]

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.

 

[KeepItSimpleStupid]

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

 

[audioguru]

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

 

[Mikebits]

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.

 

[rsfoto]

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

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



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



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

Will keep doing my homework

 

[audioguru]

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.

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?

 

[rsfoto]

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?

Hi AG,

I am using this type of power supply for my experiments

**broken link removed**

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

 

[rsfoto]

Hi AG,

I am using this type of power supply for my experiments

**broken link removed**

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

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

 

[KeepItSimpleStupid]

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

rsfoto: Your wierd sawtooth noise on a linear power supply battery explained: **broken link removed** 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.

 

[audioguru]

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.

 

[rsfoto]

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

rsfoto: Your wierd sawtooth noise on a linear power supply battery explained: **broken link removed** 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.

Hi KISS,

Yes it has in the software a built in FFT function



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



What is your veredict ?

 

[rsfoto]

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.

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 ¿?

 

[KeepItSimpleStupid]

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

 

[audioguru]

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

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.

 

[rsfoto]

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.

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 ?