Recent Posts

1

Test Equipment / Re: Vevor SDS1104 for first oscilloscope?

« Last post by rhb on Today at 05:28:14 pm »

I have the Hanmatek label 2 & 4 channel versions.  The best way to describe them is Tek 465 with some extra features such as stored waveform.  The FFT is great, but the labels are wrong.

The PCB shows it's an Owon SDS1104.  I plan to power mine with a LiPo pack.

They are no BS 140 MHz BW scopes which is an amazing.  The limited memory makes them a bit dubious for digital work, but for analog applications they're quite good.

I'm old enough to remember when all the computer techs carried 465s.  There were always 2 or 3 in the hall at the window where I dropped off punch card decks.

2

Projects, Designs, and Technical Stuff / Designing a binary output current sensor

« Last post by tmd63 on Today at 05:28:06 pm »

I had a design from years ago that had reed switches with a wire wrapped around them. If current was flowing (depending on the wire gauge and number of turns) then the magnetic field would cause the reed to switch on and cold be used to indicate current flowing.
This was great but in a high vibration environment, The reeds could still bounce and doesn't help with todays low current LED devices.
Is there a simulator that can be used to work out which diodes could provide a 1V Vf for a very wide range of currents If? (Typically 20mA to 2.5A or more)2.5A on a 12V system gives a power of 30W, but it is the very low Vf at If=20mA that is an issue for a single diode.
I need to reliably trigger a transistor with a Vf> 0.6V but I don't want a high Vf at higher currents (1V is good enough but 2V is too much)

3

Beginners / Strange noise signal on oscilloscope

« Last post by DanielDaniel on Today at 05:27:11 pm »

Hello,

I'd like to measure the current noise of a laser diode current source. The working point of the laser diode is about 100 mA at about 1 Volt.

I've learned that to measure the current noise one drives a resistive load (instead of a laser diode) and measures the resulting voltage across the load resistor with a scope. I can also set the oscilloscope to generate a FFT of the signal to see the frequency components of the noise. At least that's the theory.

So I connected a 10 Ohm resistor to the outputs of the current source as a shunt. Also I attached a coaxial cable (RG174) to these outputs. I soldered the braided shield to the output that is connected to GND and the center wire to the other output.

This "probe" looks like this:


Now when I connect this "probe" to the current source without connecting the current source to anything else (no power supply, no serial connection) the scope shows a strange signal in the FFT window:



The top part of the scope shot shows the signal in time domain (orange), the bottom part shows the signal in the frequency domain. There, the pink line is the FFT of the signal averaged over 16 measurements, the white line shows the noise floor when the probe is not connected to anything (except the scope).

I wonder what signal I'm receiving with my probe: The orange line shows that the noise is not always present in the signal. Noise drops drastically for about 1.2 ms. A separate measurement shows that the noise drop periodically (every 100 ms).
Regarding the lower part, there's a prominent noise that covers the frequency range from about 1.04 MHz to 2.60 Mhz.

What could that be? I switched of my PC and nearby devices but the noise is still there. However, the amplitude of the noise changes when when I move the PCB (current source).

Any ideas? Thanks.

4

Test Equipment / Re: Choosing between entry-level 12-bit DSOs

« Last post by awakephd on Today at 05:27:02 pm »

Re-hacking after a firmware update is normal on many oscilloscope hacks. The popular ones get scripts to update them - just run the script after a firmware update. It's not as if there's firmware updates every day...

Things like the FFT hack are separate apps and won't be affected.

Also very helpful. Thanks!

5

Test Equipment / Re: SDS800X HD Actual Use Cases

« Last post by mawyatt on Today at 05:24:42 pm »

Here's another interesting use case with the SDS800X HD. If one wishes to see a low frequency plot of capacitance vs frequency then an expensive LCR meter is usually required. There were various threads about using the built-in Bode Function to plot Impedance and Admittance using a DSO, see these for details.

https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745

https://www.eevblog.com/forum/testgear/admittance-measurements-with-dso-awg-with-bode-function/msg4491952/#msg4491952

Following the analysis in the above threads, the Bode Plot is defined as:

Bode Function Display = Vo/Vi where Vi is the signal created by a coupled signal generator, and Vo is the voltage across the DUT.

If we add a series impedance Zr with the DUT (Z), then (neglecting DSO sensing currents) the DUT current is:

I = (Vi-Vo)/Zr, where Vo is the output or voltage across DUT, and:

I= Vo/Z, and equating I:

Vo/Z = (Vi-Vo)/Zr

Z = Zr(Vo)/(Vi-Vo)

If Vo is << Vi (meaning Zr >> Z), then:

Z ~ Zr(Vo/Vi)

Normally we make Zr a Reference Resistor, but what if we make Zr a Reference Capacitor, what then?

Z ~ Zcr(Vo/Vi), where Zcr is reference capacitance impedance and what if we are interested in Z being just capacitance, thus:

Zc ~ Zcr(Vo/Vi), where Zc = 1/jwC and Zcr = 1/jwCr

1/jwC ~ (1/jwCr)Vo/Vi, and:

C ~ Cr*/(Vo/Vi)

So if we can "fool" the Bode Plot and swap the Ch1 (Vi) with Ch2 (Vo), then:

C ~ Cr*Bode Plot!!

Of course we can just set the Bode Plot Input and Output to Ch2 and Ch1 respective and not need to reverse the probes and "fool" the Bode Plot!!

Ok, now that we've got thru all that, does it work!!

Yep, sure does

Here's an example of a Precision Polystyrene 1uF Reference Capacitor (1.01594uF) and a 100uF Electrolytic Capacitor.

The DUT Capacitance is directly displayed in dB to the Reference Capacitor, so the resulting DUT Capacitance is the product of the display and Reference Capacitor. Note how the effective DUT capacitance begins to drop off with increasing frequency. Results agree well with Lab Bench LCR Meter (TH2830).

DUT Capacitance result is Cr * 10^(Reading/20)

Not a replacement for a plot displaying LCR meter, but in a pinch might help.

Anyway, hope some folks find this useful.

Best,

6

Beginners / Re: XOR gate problem with GND

« Last post by Ian.M on Today at 05:23:34 pm »

That's clever, but the simplification makes it  non-cascadable and, as you noted,  not suitable for simple switch inputs.  Add two input buffers and an inverter on the output, to get XOR rather than XNOR, and and you are back up to five transistors.

7

Test Equipment / Re: LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source)

« Last post by _Wim_ on Today at 05:23:20 pm »

can you also state which picoscope model you are using?

8

Test Equipment / Re: LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source)

« Last post by _Wim_ on Today at 05:22:02 pm »

ok so i have skipped the probe för the generator side, THANKS , then from the generator, I have tryed 2 options (like in the manual and in your revommendation):

option 1:
Gennerator to B probe "red" and then also from Gennerator thru the ref resistor (120 ohm).
on the other side of the Rref I have the A probe"blue" and also from the other side of the Rref i have connected the DUKin.
then thru the DUK to DUKout, to GND clip on proBe A and B.

same result :-/

then i have altso tryed the  ;
option 2:
Gennerator to A probe "blue" and then also from Gennerator thru the ref resistor (120 ohm).
on the other side of the Rref I have the B probe"red" and also from the other side of the Rref i have connected the DUTin.
then thru the DUT to DUTout, to GND clip on probe A and B.

odd result :-/

the DUT has red and black cables in the picture, same as before,

If the software is still configured as in the attached screenshot, then you should use option 2. I am however not sure what is exactly wrong, but a few suggestions to figure is out:

- 1) while everything is connected like in option 2, just use your picoscope software and generate a 1kHz sine signal of 1V. Take a screenshot of the signals you see on channel A and channel B. Repeat while generating a 20kHz sine signal. This should give us an idea what the scope actually sees and hopefully make the issue more clear.
- 2) instead of connecting an RLC as the DUT, please only use a known resistor or known capacitor and measure the impedance. This will also make it easier to understand the results.

9

Programming / Re: Linux Dependency Black Hole

« Last post by coppice on Today at 05:21:29 pm »

The current version of the ITU G.1050 spec is at https://www.itu.int/rec/T-REC-G.1050-201607-I/en . Its from 2016. Not exactly new, but not exactly ancient. Try using the software provided there. It requires numerous things, mostly open source, but it requires specific revisions of many of those things. It looks like within 2 years of the publication of that spec you couldn't find some of the necessary versions on the internet, and now you can find almost none. So much for the notion that with open source you just publish and somewhere on the internet it will be effectively archived.

Demanding too much backwards compatibility is a bad thing. You learn a lot about how to design something well by getting a first weak pass out the door. If you can't be incompatible you are locking in a lousy design forever. You do need to keep the old stuff relevant for a very long time, though. ataradov referenced libusb totally breaking compatibility. gstreamer and other software has been through the same thing. Some of those things struggled to move people on to the better solution, but that did discipline them to keep the old version usable and relevant. Many people are too keen to let the old stuff rot when they are able to carry a good number of people forward to the new version.

10

Metrology / Re: WinGPIB - Windows GPIB App (multi-purpose)

« Last post by IanJ on Today at 05:19:04 pm »

New version in Post #1 available for download.
WinGPIB_V3_173.zip
[Virus scanned using BitDefender AntiVirus]
Need help installing, upgrading or moving to a new PC, then see instructions in Post #1

- Fixed slight bug in the Playback Chart, the Manual Refresh and Zoom All buttons now refresh the CSV without upsetting the x-scale labels and Time in mins.
- Add simple instructions/info on the GPIB Command Line tab.

Ian.