Products > Test Equipment
Show us your square wave
Martin72:
Yep, just did it a few minutes ago...
jasonRF:
Now for something on the lower-performing side. This measurement is from a Picoscope 2204a (100 MS/s, nominal 10 MHz BW and 35 ns rise time) using the probes that were included with it; they are x1/x10 switchable with respective specified bandwidths of 15 MHz/60MHz. A NanoVNA was used for the source - I have no idea how clean the square waves are out of this cheap device but it goes up to 1.5 GHz so it must have much shorter rise/fall times than the Picoscope (EDIT: just to be clear, the nanoVNA only produces square waves up to 300 MHz; it uses the 5th harmonics to perform measurements the 900-1500 MHz range).
Hookup: nanovna 50 Ohm SMA output -> SMA to BNC adapter -> BNC to alligator cable (~1 meter) clipped across 50-Ohm resistor -> scope probe across resistor. This is admittedly a little flaky. Without the scope probe across the resistor, the nanoVNA measured a return loss of -30 dB at 1 MHz, -20 dB at 7.7 MHz, and -11.5 dB at 25 MHz (the highest I bothered to measure). Not so great at the higher end.
Anyway, the first image is with probe switched to x10, giving 12 ns rise and 13 ns fall. The second image is with probe at x1, yielding about 14 ns rise and fall, and some additional overshoot and ringing. Note the sampling time is 10 ns, so this is all the sinc interpolation at work. I'm not sure how much uncertainty this adds. Even if the rise time is 20 ns the Picoscope is better than I thought it was.
THDplusN_bad:
Good Day,
Ok, I realize that this is an old thread, but still an interesting topic :)
I was hooked up by the simplicity of the 74AC14-based fast rise pulse generator design. So, same as many other members I have built my own earlier this week. Nice!
I have used a simple double-sided copper board from the scrap pile and the results of this simple thing built in "dead-bug style" are pleasing.
I have just added a "proper" power connector, a diode to protect from reverse supply voltages and some stand-offs. Used a 100 nF cap. and a 10k resistor SMDs for the oscillator, as these 0805 size components fit nicely.
The BNC output connected is mounted on an old Tektronix assembly mount, which is a left over from an older oscilloscope repair.
Voilá - the simple thing worked right from the start and creates a nice 960 Hz square wave with an amplitude of about 3.4Vpp into 50 Ohms.
I have measured rise times between 1.8 ns and 2.1 ns (10% to 90%) per the attached screenshots. These were taken with an entry-level LeCroy Type Wavejet 334 DSO, which is spec'd at 1 ns typ rise time and 350 MHz BW.
The output signal overshoots by around 29%, but that's fine and the distinct peak makes it even useful when one uses it as a poor man's TDR.
And I had much fun when I was following Alan's (W2AEW) excellent video "#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter"
I was amazed to find out that this method allowed me some correct cable length measurements of two cables (an Aircell-5 and a plain-vanilla RG-58 made by HP) down to centimeter accuracy. Nice work, as always, Alan... @w2aew ! :-+
Cheers,
THDplusN_bad
joeqsmith:
You may find the following of interest as well where we look at several types of gates ran over various temperatures and voltages.
https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/
PartialDischarge:
Somewhere in a thread user ttggzz suggested the use of LVC gates in parallel, I made a board to test the idea and I'm getting less than 500ps rise and fall times with very nice shape, tested with a 1GHz scope:
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