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tggzzz:

--- Quote from: Folnia on September 27, 2024, 03:05:06 pm ---
--- 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!

--- End quote ---

It reduces the current that each device has to supply. That makes the transition faster, and reduces the consequence of lead inductance.

It reduces the consequence of each output's impedance being a function of the instantaneous output voltage. That makes the overall output impedance closer to 50ohms (i.e. (~7 + 143)/3 is closer to 50 than (~7 + 43).

Differential propagation delay is unlikely to be a noticeable issue with devices from the same batch. A bigger problem is decoupling (di/dt is 2e8A/s) and different wire lengths.
magic:
Another matter is that FET's aren't exactly linear resistors so their loss increases disproportionately with output current, and absolute maximum ratings are something to consider if you want it to work for long.
Folnia:

--- Quote from: tggzzz on September 27, 2024, 03:59:01 pm ---
--- Quote from: Folnia on September 27, 2024, 03:05:06 pm ---
--- 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!

--- End quote ---

It reduces the current that each device has to supply. That makes the transition faster, and reduces the consequence of lead inductance.

It reduces the consequence of each output's impedance being a function of the instantaneous output voltage. That makes the overall output impedance closer to 50ohms (i.e. (~7 + 143)/3 is closer to 50 than (~7 + 43).

Differential propagation delay is unlikely to be a noticeable issue with devices from the same batch. A bigger problem is decoupling (di/dt is 2e8A/s) and different wire lengths.

--- End quote ---

OK, it make sense, I'd like to build one and test. Thanks :-+
Gerhard_dk4xp:

--- 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 ---

Probably the same circuit with about the same results:

<    http://www.hoffmann-hochfrequenz.de/downloads/DoubDist.pdf    >

circuit on page 7, scope plots on page 12+.
Sorry for the blue color of the traces.
scope = Agilent 54846B  2.4GHz.

SA = Agilent 89441A
Folnia:
Hi Tggzzz,
I tested with my board, the rising edge measured is 900ps.  And the bandwidth of the scope is 525MHz.
So the calculation of the the pulse rising edge might be sqrt( 900² - (0.35/525M)² ) , around 604ps, much less than 256ps mentioned in your post. Did I miss something? (I don't find the spec of flip time in the datasheet of SN74LVC1G14DBVR so I guess it's just because of the different component...)
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