karesz said:@Zangetsu57,
(in, for me more sympatic, LCC20 package)
karesz said:Is it possible, that your R4/C8 arnt on GND_I dont see a Via!?
Yes, you can apply minimum a serial 47/50 Ohm termination_so you have a properly driven 50 Ohm system=2:1 attenuator and not a possible overload through 50 Ohm scope input...What about "AC(end) termination" pls?Zangetsu57 said:... the amplitude was not very high, but I think that's because it is designed to drive load 250 ohms or greater, not 50 ohm loads. Is there a better way to test this?
Be care pls with it!Zangetsu57 said:.
I'll consider the ADCMP573 in the future in redesigns too if I continue this work after graduation. I didn't know faster parts existed, and the best I could find was the LT1721.
Exactly, because your bypass Cs are really not perfect_pls refer to my earlier comments, then a tip GND-lead of 6" is nothing for pulsetechnique_sorry, you are in frequency domain of some GHz here_ you need GND-lead of cm, not 10-12cm....Zangetsu57 said:....I'll also try adding a XX pF capacitor for additional decoupling. I do notice periodic noise within the ground plane - spikes that correspond to the high speed transitions of the comparators. My best guess is these are high frequency components that aren't filtered out.
You must build your quasi HF- probe, or use a FET-probe!Zangetsu57 said:@karesz
The ground leads are about 6" if I were to guess. Probably too long. The probe capacitance is also 11.8pF if I remember correctly, and 1MOhm.
No problem, these is a good school stuff for youZangetsu57 said:I'm more of an electromagnetics/microwave engineering student so I'm still learning about high speed electronics and proper layout.
keith1200rs said:... as karesz said!
Also, for looking at the output you could use a simple T attenuator - say 250 ohms in, 50 ohms out to feed the scope 50 ohm input through some coax to avoid loading your circuit too much.
"short jumper wires about 5" in length" is an oxymoron! 5" jumpers are looonnnngg at these frequencies. You could try some coax and use a T attenuator there, although you will lose some signal. There is an example in the AD8637 data sheet on page 19. You could use the same values for a 'scope connection from the LT1721. Remember to terminate the coax at the LT1721 input.
You cannot really probe with normal 'scope probes at 10pF to 15pF. A T attenuator matched to some coax feeding a 50 ohm scope input would work though.
By the way, I use Eagle if you ever want someone to look over a board before you make it.
Keith.
These can be_sorry, but the life present it others!_I think you can agree with me?Zangetsu57 said:...I don't see how a 5" jumper is going to make a huge difference.
Pls dont mix two things!Zangetsu57 said:...The input signal has a rise time of about 10ns, and the comparators rise time is about 3ns. The maximum frequency component of the signal is about 0.35/Tr = 116 MHz (assuming a Gaussian response).
No, no materials has the same dielectric constant_because their are not the same materialsZangetsu57 said:..In FR4, this means a 5" cable (assume same dielectric constant as FR4 to be safe, even though it is probably less), has a wavelength of 0.1*lambda. This isn't *that* bad, right? I recall reading that a tenth of a wavelength is about when you start to care about transmission line effects, so this is right on the edge. But hey, I don't have the experience you guys do.
You has practically the answer yet-over that.Zangetsu57 said:...at what frequency should I start to care about designing transmission lines with a specific characteristic impedance for parts like this? And then, what should the impedance be?
keith1200rs said:When you need to use transmission lines depends on distance as well as
frequency. I have used transmission lines on video boards (a mere 4MHz). It
does no harm and I don't want to find out I should have done by getting a
problem when I test the PCB. On some of my high speed boards there aren't
any transmission lines because I place parts so close there is no need.
Also, the pads of high speed parts are too narrow for 50 lines on a double
sided FR4 so you need to fan out from the pad which is not always easy.
Normally my boards are 4 or 6 layer anyway.
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