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| > 1 GHz DIY differential probes |
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| dietert1:
If we are still considering DIY, a 3 mm "antenna" of thin wire soldered vertical to one end of a 0603 resistor helps. Then you can adjust compensation by bending the loose end in one direction or the other. Also "solder blobs" were mentioned above. |
| nctnico:
The question is how low do you want to get the input capacitance. On one of my prototypes I measured less than 0.5pf between the tips (and that is with some uncertainties). The actual capacitance between the tips may even be lower. This also implies that aiming for a very high differential input impedance isn't going to do much good because at several GHz the input capacitance will dominate the input impedance anyway. |
| 0xdeadbeef:
--- Quote from: nctnico on October 26, 2018, 11:55:11 am ---This also implies that aiming for a very high differential input impedance isn't going to do much good because at several GHz the input capacitance will dominate the input impedance anyway. --- End quote --- While it's true of course that a capacitance becomes low ohmic at high frequencies (e.g. 1pF at 1GHz equals ~159Ohm), your signal typically consists of a wide range of frequencies where the (real part) resistance will matter the more, the more low frequency components you have. |
| dietert1:
This will be similar to any other scope frontend. Once you know how to DIY you can solve the DC impedance problem using AC coupling or using an adjustable DC offset source or by adapting the division ratio. Depends on the application. For myself i want a high impedance 10:1 probe, so i designed a small 4-layer board with 2x HEMT buffers plus DC servos plus a LMH3401 difference stage plus low noise +/- 2.5 V regulators. A HEMT easily oscillates above 10 GHz and the probe will have two of them next to each other, so i already know i will suffer. Probably i should first try a THS4303 single ended 10:1 probe, will be much easier. |
| Marco:
The quality to which Baluns can be made is quite impressive by the way. Marki has one which does 40 dB CMRR out to 6 GHz ... they of course go higher, but for those the CMRR is reduced across the board. What's inside these things? I suspect ferrite on coax. |
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