Recent Posts

1

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

« Last post by gf on Today at 10:06:12 pm »

If you look carefully at the 450MHz screenshot Performa01 posted, you'll see that the displayed signal can't be right. Just look how the histogram for the frequency measurement shows a wide spread instead of a single frequency! And also look at the amplitude compared to the 408MHz signal. IOW, the screenshot doesn't show what is actually happening. Just stick to testing fs / 2.5 + 2% or so. DSOs can show fantasy signals in such circumstances!

I also have the feeling that the interpolation already suffers a bit in the 450 MHz screenshot.

2

Test Equipment / Re: Siglent SDS3000X HD and upgraded SDS1000X HD

« Last post by tv84 on Today at 10:02:08 pm »

The new to come 6&8 GHz models ?

First time I see BW references inside Siglent's FW going up to 16GHz !!!

3

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

« Last post by gf on Today at 10:00:08 pm »

No, it does not have to be periodic. "Perfect reconstruction" only requires that the original signal sampled by the ADC sampled was bandwidth-limited.

Nope.

The signal in this image is bandwidth limited (5Hz signal, 11Hz sample rate) but sin(x)/x won't reconstruct it unless the filter is infinitely wide and the signal is periodic (which can't happen in practice).

That signal has components above 5.5 Hz.

How can a 5Hz sine wave have components about 5.5Hz?

I wrote "above", not "about".  The wave as shown is not a continuous sine wave.  It is a continuous 5Hz sine wave gated on by a pulse 1 second long.  That is Amplitude Modulation which produces additional frequencies above and below 5Hz.

If it is meant to be a one-time burst of 5 cycles (and zero outside the burst), the spectrum has even an infinite extent. The attached plot shows the spectrum up to 100 Hz.

4

Beginners / Re: uCurrent Gold Schematic

« Last post by uf29857 on Today at 09:53:23 pm »

With only 2 switches (FETs) and 3 shunts one would have the drop from the FET also with the lowest shunt. The voltage sense would not be through the 10 and 10 K resistors, but the direct way on the top. So the 3 rd Fet is required. (This was a comment from one guy here and dave agreed to this).
Which tvs diode i should use? Any recommendation?

5

Test Equipment / Re: Was your first meter a "trashy" meter?

« Last post by floobydust on Today at 09:51:44 pm »

My allowance was $7/month so I did not buy much. Visiting great-grandparents and they would give me $20.

I remember the range-doubler was slowwww for the meter pointer to settle. That, and having to be awake to know to half the reading oh yeah and there is no fuse... when measuring current like all manly multimeters of the day, do not screw up!

6

Projects, Designs, and Technical Stuff / Re: more modern version of tl494?

« Last post by Hiemal on Today at 09:49:51 pm »

You can use any PWM IC if you just short the FB pin to ground, and then drive the COMP pin with an external error amplifier.

This is exactly what most isolated switchers do actually. The good ol' TL431 acts as the error amplifier on the secondary side, which drives an optocoupler that exerts itself on the COMP pin, which changes the duty cycle output. The comp pin is usually high impedance, and thusly will follow pretty much whatever voltage you command into it with an op amp. Most (good) datasheets will have a parameter or a note somewhere telling you what voltage on the comp pin corresponds to what duty cycle.

So, you can achieve the same exact thing just using an op amp to drive the comp pin directly, using whatever PWM chip you want.

7

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

« Last post by wasedadoc on Today at 09:46:03 pm »

No, it does not have to be periodic. "Perfect reconstruction" only requires that the original signal sampled by the ADC sampled was bandwidth-limited.

Nope.

The signal in this image is bandwidth limited (5Hz signal, 11Hz sample rate) but sin(x)/x won't reconstruct it unless the filter is infinitely wide and the signal is periodic (which can't happen in practice).

That signal has components above 5.5 Hz.

How can a 5Hz sine wave have components about 5.5Hz?

I wrote "above", not "about".  The wave as shown is not a continuous sine wave.  It is a continuous 5Hz sine wave gated on by a pulse 1 second long.  That is Amplitude Modulation which produces additional frequencies above and below 5Hz.

8

Test Equipment / Re: Siglent SDS3000X HD and upgraded SDS1000X HD

« Last post by tautech on Today at 09:44:02 pm »

The app contains references to a new Siglent device's internal name:

HULUWA_2CH
HULUWA_4CH

The new to come 6&8 GHz models ?
https://www.siglent.com/products-overview/sds7000a/

9

General Technical Chat / Re: A serious (as in not troll) requestfor help w/ultrasonic transducer

« Last post by watchmaker on Today at 09:42:27 pm »

I GOT BUBBLES!!!

It was the capacitor array.  I say it cost $35; my wife sez it cost at least $5K to repair. 

10

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

« Last post by shapirus on Today at 09:41:24 pm »

The same applies to 490 MHz. It was meant to be 49% of the sample rate, so it would correspond to 612.50 MHz @1.25GSa/s.
If you want to retry it, use 10ns/div do see at least one period of the envelope, and better capture a single shot since I don't expect a stable trigger.

600+ MHz isn't really an option, because the amplitude gets too low at this point. We can try 612.5 * 0.49 = 300.1 MHz at 612.5 MSa/s, though. Here you go. And a bonus: 312.5 MHz at 100 us/div.

edit: mistake! that was supposed to be 625, not 612.5. I'll keep that screenshot and attach a new one: 625*0.49 = 306.25 MHz at 625 MSa/s. That's the same thing in principle, though.