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> 1 GHz DIY differential probes
nctnico:
--- Quote from: dietert1 on October 18, 2018, 07:14:15 am ---By the way, i received a LMH3401 evaluation board. That is an excellent difference stage, like +/- 1 dB up to 2.9 GHz. Image is boring.
--- End quote ---
That sounds promising. Did you test single-ended in and single-ended out? I did some testing with an LMH6703 (dead-bug style) and that looks promising as well albeit with less bandwidth. At least there is no difference between reversing the inputs.
dietert1:
Yes, the LMH3401 was tested with the HP 8560A again, so it is single ended in (TG) and single ended out. The curves from both inputs to the same output appear flat +/- 1 dB up to at least 2.9 GHz. I would not trust my setup to better than a dB.
In the meantime i experimented a little with the ADA4927 probe from Mr. Rosenkränzer. I modified the difference stage to the standard setup with 4 301R resistors (inputs = Gnd). The 2K5 0603 input resistors were replaced each by a 2x 620R 0402 resistor pair. I also removed two pads and brought the coax cable closer to the amp.
Now it works better, but the asymmetry of the ADA4927 still exists. With our WR64Xi and a Lattice FPGA measured risetimes are 345 psec on indirect input, 306 psec on direct input. The direct trace still shows more overshoot/ringing at 1.4 GHz .
nctnico:
OK. I think I'm going to re-design my board with an LMH3401 then instead of the LMH6703.
David Hess:
Bob Pease's probe with 0.29 picofarads of input capacitance is shown below. The printed circuit board stiffener adds another 0.08 picofarads but drilling holes in it reduces this to 0.06 picofarads of added capacitance.
Old designs like the Tektronix P6202 (which is what I would start with if I wanted to design a general purpose probe) use an input divider to reduce the capacitance of the input device, 2.0 picofarads in this case.
Cerebus:
Both designs are well and good in themselves, but don't forget that the title contains >1GHz and differential in its title.
Both are single ended, the JFETs in Bob's design run out of steam at ~200MHz and the P6202 is a 500MHz design.
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