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Show us your square wave

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norbert.kiszka:
74LVC14AD from Texas Instruments and modified Rigol DHO924S (removed 250 MHz low pass filter on channels 3 and 4).

Sadly this scope has a sinc interpolation that can't be disabled (until I hack this someday). It goes up to 450 MHz and ~600 ps rise time (800 ps per sample), according to this scope.

jundar:

--- Quote from: norbert.kiszka on August 09, 2024, 02:45:17 pm ---74LVC14AD from Texas Instruments...

--- End quote ---

Does this exact "device" in the first photo generate <nS rise time signal? Could you please share the schematic of it? Thank you.

norbert.kiszka:

--- Quote from: jundar on August 18, 2024, 03:47:49 pm ---
--- Quote from: norbert.kiszka on August 09, 2024, 02:45:17 pm ---74LVC14AD from Texas Instruments...

--- End quote ---

Does this exact "device" in the first photo generate <nS rise time signal? Could you please share the schematic of it? Thank you.

--- End quote ---

If You zoom in first photo, You will probably see that I just shorted 3 (three) pins together - 11, 10 and 9. So one of gates became a ~450 MHz generator connected to input of other gate, output of this second gate (pin 8) is soldered directly into very small BNC female socket - I used "sex adapter" to connect it into scope. Pin 7 is GND and distance between 7 and 8 is good enough to solder both of them into mentioned BNC. Also THT 1uF ceramic capacitor (100 nF is not enough to make that fast rise time). That of course will make something more like sinus than square wave.

After that, I removed this wire (3 pins) and connected external generator (DIY on a separate PCB) ~17 MHz via short as possible wires (both power and signal) into pin 9. Used scope has 1.25 GS/s so there is 800 ps between samples - very likely real rise time was much lower than 800 ps (I guess 600 ps or maybe even faster).



PartialDischarge:

--- Quote from: jundar on August 18, 2024, 03:47:49 pm ---
--- Quote from: norbert.kiszka on August 09, 2024, 02:45:17 pm ---74LVC14AD from Texas Instruments...

--- End quote ---

Does this exact "device" in the first photo generate <nS rise time signal? Could you please share the schematic of it? Thank you.

--- End quote ---


Read this https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg4510763/#msg4510763

Folnia:

--- Quote from: tggzzz on October 18, 2018, 01:55:26 pm ---OK, here's the edges from a very simple pulse generator:

* the source is 5V with no load, but obviously it is used to drive 2.5V into the scopes' 50ohm input
* 4GHz LeCroy HDO4904, connected directly to the scope input with a BNCfemale-BNCfemale adaptor
* 1GHz Agilent MSOS104A, connected via a 1m piece of coax of untested quality
It looks like the 10%-90% risetime is 256ps with a 6.3% overshoot, and the falltime is 453ps with a 3.8% overshoot.

Considering the simplicity of the circuit, that is remarkably fast. It is a simple demonstration that modern jellybean logic (74LVC1G*) generates significant power into the microwave waveband - and hence RF practices are appropriate.

In this circuit a major contributor to the performance is the decoupling capactors, especially the 0V/5V planes and short wide wires, and not forgetting that MLCCs have a very significantly reduced capacitance when there's a DC bias voltage.

My apologies for the quality of the photos; they had to be taken relatively quickly and in non-ideal conditions.

--- End quote ---

Hi Tggzzz,
I'm not sure about the purpose of having three 74LVC1G in parallel here (it should not be only to get 50ohm resistance).  If one is faster than other two, it seems there might be voltage dividing?
Thanks!

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