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> 1 GHz DIY differential probes
dietert1:
Yes, definitely 10x gain is better than 5x. Also very interesting: The THS4303 datasheet contains a schematic diagram of the amplifier.
When we compare the large signal gain curves (Figure 3), both amplifiers show the -3 dB point at about 1.4 or 1.5 GHz. The THS3402 has a minor bandwidth advantage only with small signals.
My own 1.8 GHz bandwidth estimate above was "almost large signal": -10 dBm into 50 Ohm = 200 mVpp, 5x => 1 Vpp.
David Hess:
--- Quote from: Cerebus on October 20, 2018, 08:07:01 am ---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.
--- End quote ---
I linked them as examples of what is involved. Both serve as benchmarks for what is possible without resorting to hybrid construction.
I think a >1GHz differential probe is unrealistic without hybrid construction but performance better than the famous and very long lived Tektronix 100MHz P6046 differential probe should be feasible. I would take a very close look at maximum performance design using the AD8130 difference amplifier as the differential to single ended conversion stage to get better than P6046 performance.
If there is a better part than the AD8130 for differential probes, I have not found it yet.
dietert1:
The amplifier testkits i wrote about are no hybrids but standard PC boards. The LMH3401 board has four layers with the amplifier side made from RF material. Its common mode is near perfect up to 3 GHz, its differential mode extends up to 7 GHz (datasheet spec). The HEMT fet buffer i tested should be good for 5 or 10 GHz as well. The wiggles in my measurements are reflections and mean nothing. Some days ago i found better patch cables and some SMA attenuators which make the measurements smooth to about +/- 0.2 dB.
If you prefer AD over TI, the differential amplifier ADL5565 mentioned above seems to be very similar to the LMH3401. Or the ADL5569 with 2x 20 dB gain on a single chip and good common mode suppression up to 3 GHz.
If you want to make hybrids, please search for "agilent ghz die" at ebay. Wellcome to 2018!
Marco:
AFAICS it's impossible to guess what the CMRR is for the ADs for single ended output. Only the LMH3401 has anything in the datasheet about it (Fig. 42, Common-mode input, common-mode output transfer function).
nctnico:
--- Quote from: David Hess on October 24, 2018, 09:08:57 pm ---
--- Quote from: Cerebus on October 20, 2018, 08:07:01 am ---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.
--- End quote ---
I linked them as examples of what is involved. Both serve as benchmarks for what is possible without resorting to hybrid construction.
I think a >1GHz differential probe is unrealistic without hybrid construction but performance better than the famous and very long lived Tektronix 100MHz P6046 differential probe should be feasible. I would take a very close look at maximum performance design using the AD8130 difference amplifier as the differential to single ended conversion stage to get better than P6046 performance.
If there is a better part than the AD8130 for differential probes, I have not found it yet.
--- End quote ---
One way to get in the ball park of 1GHz is to use 2 closed loop buffers (from TI for example) and drive a regular opamp or differential amplifier with these to get the differential signal. It should be possible to get over 20k Ohm of differential input impedance and more if you make it a 1:10 probe but I think the capacitive divider will be tricky to get right.
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