Recent Posts

1

General Technical Chat / Re: What did you buy today? Post your latest purchase!

« Last post by paulca on Today at 08:57:34 am »

here is some interesting math about the tritium keychain
https://www.gammaspectacular.com/phpBB3/viewtopic.php?t=889

the skin exposure divided by area is interesting to me. thats a big number

But if you lose a hand due to this, you always have your teeth to make up for it.

The point in reading the above that I went, "Ah... hang on a minute there."

Was here:

If 2.7mS/y is for whole body exposure to background radiation and the odd higher source like a medical x-ray maybe we can assume that the entire area of the skin is exposed to this amount on average, so we can find the average exposure of 1cm of skin as follows:

I am fairly sure it doesn't work that way.

Also the experiment is invalid without measuring the xrays where are NOT absorbed but pass straight through.

If you have a choice to eat a radioactive cookie, ALWAYS pick the gamma cookie, if that has gone, pick the xray cookie.

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Microcontrollers / Re: Attiny1606 burning out when connected to a drv8251

« Last post by lavaman on Today at 08:57:22 am »

I am skeptical too. Power usage on the 3v3 rail, when drv is under load, is around 20mA. Not sure if there was some kind of problem with the voltage regulator.

3

Microcontrollers / Re: SD Card reliability in SPI mode

« Last post by iMo on Today at 08:51:42 am »

For proper handling of an sdcard while streaming data on it you may need a FIFO implemented. The sdcards have got so called "write latency", which may last for up to 200ms. During this time the sdcards do the housekeeping, like wear leveling etc. During the WL the sdcard is not accessible.
That 200ms is max as per the sdcard standard, afaik, and the duration of WLs is usually xx ms, but rather random occurrences.
It works without fifo as well but then you may need a specially prepared fs.
The fifo should be as deep as is your data rate and expected WL duration like.

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Beginners / Re: PCB review and RF layout

« Last post by T3sl4co1l on Today at 08:51:29 am »

No you don't match to your trace, because you make the trace 50 Ohm. If your trace is not 50 Ohm, you absolutely have to match to a trace which is any considerable length.

So you categorically deny the applicability of the feedline length trimming example?  Mind I don't mean as a practical basis, practically it's stupid, but on a basis of impedance. Impedance is easy to work with, and should be easy to prove or disprove.  Can you demonstrate with equations, Smith chart, etc. how it does not work?

It's like trying to use a 75 Ohm coax for a 50 ohm system, you get reflections. You might even need to open up the ground plane under a component, if the size of the component pad is larger than your 50 Ohm trace. Do you think they make these recommendations based on nothing? Nobody ever say that "Oh, don't worry about that component pad, it's shorter than quarter wavelength".

Pads..?  These... aren't 20GHz signals here. I'm not sure what you're getting at.

The old rules of thumb don't work in practice when talking about GHz signals, especially when GPS is involved with it's -120dBm signal strength.
Here is a measurement of a RF system, that I messed up by having just 600 femtofarad extra capacitance in the connector. That's just the effect of an extra 0.4mm * 4mm pad. Much shorter than the wavelength, that's ~40mm@4GHz.

I guess you forgot an attachment, oh well.

What difference is signal strength? Impedance is linear, it works even if it's Johnson noise.  I'm talking system dynamics, raw impedance.  Your error is apparently either a matter of impedance mismatch (600fF at 4GHz is 66Ω -- hardly negligible) or bandwidth (not also adjust nearby inductor to compensate).  Which includes the wideband case (tuning of limited use or impossible).   
I also guess that you had a multilayer board, as at K = 4.6 for example, those dimensions give a 0.1mm substrate height.  Which is also around 30 ohms microstrip, which isn't crazy, but that's a dB or two off if you don't have other compensation for it (like above).

What is the corresponding mismatch length and impedance ratio for this frequency?

Tim

5

Beginners / Re: The old "Replace blue LEDs with red LEDs" 7 segment question (74HC595)

« Last post by Kleinstein on Today at 08:49:11 am »

When driven from an HC595 chips the LEDs should get way less than 20 mA. With a 5 V supply one would have the 220 ohm resistor + some 50 ohm resistance for the HC595 and about 3 V for the LED. This would be some 5-7 mA per LED. At that low current the LEDs should not wear out.  Some 7 mA also also the maximum one should use with the 595 chips to keep the current for the supply pins reasonable.

For the reduced intensity it could still be LED (a bad series), the resistor (cheap carbon type), the 595 chip and also the supply that aged.
With RED leds one may want to increase the resistors to something like 390 ohm. If the displays are bright to start with one could also use less current / larger resistors. So it could be worth testing how much current is wanted.

Dimming is likely via PWM.

There are a few common sizes for the 7 seg displays. 10 mm hight is already on the small end, many are larger.

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

« Last post by ebastler on Today at 08:49:10 am »

And 2GSps is really something, its should be possible to look at near 1GHz RF, such as 433 or 800MHz china RF module digital ASK modulation, something 1GSps scope is incapable, even if frontend BW is hackable. But then which type of guy are you? 

I would not stress potential hacked, extreme-bandwidth applications here. Very few users will try to go there. But even without considering hardware hacks, it is worth mentioning that 2 GSa/s (as in the SDS800X HD) are sufficient to support 200 MHz bandwidth in 4-channel mode, while 1.2 GSa/s (DHO800/900) are not.

Also, if one has any plans to use the logic analyser option in the future, the Siglent scopes will still have 2 GSa/s available for the analog channels, while the DHO900 will cut that sampling rate in half. (But the Siglent has other limitations, due to the fact that digital data are acquired by an external FPGA subsystem.)

7

Beginners / Re: The old "Replace blue LEDs with red LEDs" 7 segment question (74HC595)

« Last post by pcprogrammer on Today at 08:48:36 am »

If possible change the 220 Ohm current limiting resistors to 470 Ohm when replacing the 7 segment displays. This will limit the current to about 6mA instead of 13mA the 220 Ohm allows. The 13mA won't kill the red ones but they might be to bright with it and it can kill the 74HC595A because with all segments lit the current will be above the max Icc of 75mA given in the datasheet.

Simple rule is to take the LED forward voltage and subtract it from the supply voltage and divide that by the current limiting resistor to get the operational current. So with a Vf of 3.8V and a supply of 5V and a 220 Ohm resistor it is 5 - 3.8 = 1.2 / 220 = 0,005454 Ampere. Will be a bit less due to the output voltage of the 74HC595A not being equal to the supply voltage.

The 74HC595A has an output enable. Controlling this with a PWM signal can dim the display. The outputs will only supply current when enabled and on average lower the current through the LED thus dimming it.

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Projects, Designs, and Technical Stuff / Re: Software/Spreadsheet for transformer calculation

« Last post by arjen on Today at 08:47:26 am »

Have you considered creating a database to store and manage transformer data?

9

Repair / Re: Tek FG502 fails at high frequency end of dial - help needed!

« Last post by gcwills on Today at 08:47:03 am »

Thanks very much for taking the time to respond and for your suggestions.
I did try reseating the opamps and also replacing them all to no effect.
I did check the resistors associated with the multiplier and they were all spot on apart from R150 and R186 which read a little over 4M. Replaced them with 3.9M 1% resistors with no effect.
And yes, the signal dropout can be seen at the gate of Q200 so the problem occurs in the circuit before Q200.
I did replace U140 and U175 and tested Q140 and Q175 - all good.
I did find that the associated carbon comp resistors R140 and R175 were reading around 575 ohm - higher than their value of 510 ohm and were replaced with 510 1% metal film - no difference!
I did check all of the timing caps and thought I had a leaky C154 but it turned out to be fine. The caps were tested for value and for leakage with a very sensitive leakage tester - all good.
So the hunt for the elusive issue continues!

10

Test Equipment / Re: Siglent SDM3045X Boot Hang

« Last post by somen on Today at 08:46:43 am »

I had the same issue with my SDM3045X, however I somehow got out of the bootloop by doing the following:
1. Switch off the power supply (or unplug the power cable)
2. Press the on/off button and keep it pressed while switching on the power supply

After this it would boot up and I could install the latest firmware.

I did the manipulations from this post. Unfortunately it didn't help me