Products > Test Equipment
Scope Current Probe Deskew Fixture PCB Project
Jay_Diddy_B:
Hi,
Given the results from _Wim_ it looks like a rise and fall time in the 10 -20ns are is required to deskew the probes.
I would suggest selecting the MOSFET carefully. Do not oversize the MOSFET this is will slow it down.
You are looking for low gate charge.
The RDS(on) could be 1 \$\Omega\$.
Regards,
Jay_Diddy_B
Ribster:
So i've made a design based on wim's prototype.
I holds USB C input of 5 volts, also a connector for wired 5V0.
The mosfet is a GaN mosfet with high switching parameters (EPC2014C)
I also put in a selector switch for the internal oscillator with the NE555, or selection for an external clock.
The clock input is fed via an inverter to a mosfet driver to switch the mosfet as hard as we can.
Will order today and post with the results later
steve1515:
Thanks for all the great suggestions!
I wanted to update everyone on my project. For this revision, I'm going to leave it with just a USB micro input. I waffled on the power input for a long time. In my case, I usually have a USB micro cable around vs other cables and I wanted to use this board as a learning experience for trying to place some parts I haven't used before.
I did modify the voltage reading to be after the coil as Jay_Diddy_B and _Wim_ mentioned. Good catch! :) I'm a little embarased that I missed that. |O
The large resistors are WSC4527's from Vishay. I ended up with those by reviewing the Agilent U1880A fixture. The were pretty easy to source for me.
If I do another revision, I'll likely change it to the 1206 resistors in parallel and add an alternate power input as that does make sense. I also like the idea of a pulse input!
_Wim_, what FET did you use to get such fast rise times?
_Wim_ and Ribster, would you be able to post schematics of your projects?
Jay_Diddy_B:
Hi,
I would suggest looking at the BSS316N MOSFET. This is in a more friendly SOT-23 package.
If I compare gate charge:
BSS316N
EPC2014C
The BSS316N should be faster with the same gate drive.
Hot loop
Also watch the layout. The 'hot loop', where the high speed currents are is from the 5V supply to the ground connection at the bottom resistor. The decoupling should be optimized for this loop.
Regards,
Jay_Diddy_B
_Wim_:
--- Quote from: steve1515 on August 23, 2020, 10:10:24 pm ---_Wim_, what FET did you use to get such fast rise times?
_Wim_ and Ribster, would you be able to post schematics of your projects?
(Attachment Link)
--- End quote ---
Hi, I used just some parts I had in my parts bin. The FET is an FDS4559 (N-channel used, complementary P-channel unused) driven by an abused 74LCT2524 clock driver (is a 3.3V part, but I am running it at 5V...)
Attached is a schematic (made in Microcap 12), but clock driver is not shown as that part was not available in Micro-cap and I was too lazy to add it. A very crude drawing of the prototype layout is also shown, as is the Micro-cap simulation file.
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