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Show us your square wave
Dave_g8:
Hi Bill,
Yes, I expect the noise is from the current spikes that will occur during the transitions.
BillyO:
Just so everyone is aware, the design choices for this pulser were 1) make it cheap and 2) make it relatively easy to build and 3) make it as fast as 1) & 2) would allow so that it can be used to check out the rise time on (slower) scopes. With a bit of math it does a decent job with scopes up to 200MHz or so.
I know I could have made it perform much better using 74VLC, smaller SMD components and a much smaller footprint, but that would make it much harder for the newbie to build. Almost anyone who can solder can solder these relatively big SMD resistors.
TurboTom:
For comparison, here's a similar setup of the square-wave output of my rubidium frequency source, yet utilizing four inverter gates in parallel (no schmitt-trigger) of 74LVC2G04 variety, displayed on a Rigol MSO4000 scope (4GSa/s). The slope rise/fall times have got to be <500ps, considering the contribution of the scope. So my design of an inexpensive a cheap, fairly fast square wave source would consist of a digital crystal oscillator, followed by a single gate driver (maybe a 74VLC1G14), followed by two of the aforementioned dual inverter gates in at least SOT23-6 enclosures with lots of bypassing. I would consider Leo's fast pulser "inexpensive" and considering its performance far superior over any "cheap" solution one could possibly think of. If higher output levels are desired / required, the parallel LVC gate method may be a possible way to go, though.
FYI, this same source connected to an HDO1000 scope (2GSa/s) shows pretty well defined "Gibbs ears" on both sides of the slope, so the fast edges exceed the capabilities of that scope's sampling/display engine. Sorry, no screenshot of this, but neither spectacular either ;) .
Performa01:
Here’s the opposite of a cheap solution, but much better suited for lab use – a Siglent SDG7102A Arbitrary Waveform Generator programmed to output a 10 ns wide pulse at 10 MHz repetition rate with 1 ns transition time for both edges.
A quick check with a high bandwidth SDS7404A DSO reveals that the pulse shaper in the SDG isn’t quite accurate anymore at faster edges like this, hence the actual rise time is more like 900 ps:
SDS7404A H12_PR_W10ns_RT1ns_4GHz_Dots
A 500 MHz (actually 570 MHz) SDS2504X HD DSO by contrast, shows a bit of overshoot – this is the price we pay for analog AA-filtering, even when it’s still not very effective:
SDS2504X HD_PR_W10ns_RT1ns_570MHz_Dots
These measurements were taken in Dots display mode, in order to rule out any reconstruction artefacts. But of course, a properly implemented Sinc reconstruction does an equally flawless job:
SDS2504X HD_PR_W10ns_RT1ns_570MHz_Sinc
If you want to see Gibbs Ears, this won’t be achieved with a regular analog filter (as long as it is a minimal phase system). But it’s easily produced with a digital FIR filter, as in the variable bandwidth limiter provided by the SDS7404A. Here’s an example with 600 MHz acquisition bandwidth:
SDS7404A H12_PR_W10ns_RT1ns_600MHz_Dots
egonotto:
Hello,
Siglent SDG7102A and Siglent SDS7404A is very impressive. Here is a 1 kHz square wave with a 5 MHz but 16 bit PicoScope 4262.
Best regards
egonotto
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