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| David Hess:
Source impedance is a major problem with wide bandwidth differential probes and a major limitation of performance. If you are probing a differential transmission line, then it may be better to build a test point into the circuit with 50 ohm outputs to connect directly to the oscilloscope which can then do the differential measurement itself. |
| joeqsmith:
--- Quote from: David Hess on August 17, 2016, 09:53:14 pm ---Source impedance is a major problem with wide bandwidth differential probes and a major limitation of performance. If you are probing a differential transmission line, then it may be better to build a test point into the circuit with 50 ohm outputs to connect directly to the oscilloscope which can then do the differential measurement itself. --- End quote --- :-// I guess that depends on what you consider a major problem. My old DC coupled diff probes are 300 ohms at 4GHz. Tip capacitance is a 1/10th of a pf. |
| David Hess:
--- Quote from: joeqsmith on August 17, 2016, 11:37:02 pm --- :-// I guess that depends on what you consider a major problem. My old DC coupled diff probes are 300 ohms at 4GHz. Tip capacitance is a 1/10th of a pf. --- End quote --- How much source impedance mismatch are you dealing with though? A 300 ohm differential probe is not intended for general purpose measurements; it requires closely matched source impedances to maintain common mode rejection ratio. This Tektronix article, Making Single-ended Measurements with a Differential Probe, refers to the problem on page 11: A 50 ohm source impedance mismatch for a single-ended signal measurement has only a minor effect on the DC CMRR of a probe with a 50 Kohm probe input impedance. As the signal frequency increases however, the probe input impedance begins to decrease and eventually this 50 ohm source impedance mismatch for a single-ended measurement can become quite significant. High frequency AC common mode voltage transients that may be largely rejected by the relatively high CMRR in a differential measurement may show a noticeable effect due to degraded CMRR in a single-ended measurement. |
| tronde:
--- Quote from: JohnG on August 13, 2016, 03:37:19 pm ---I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design. --- End quote --- I don't want to break Elektor's copyright, but this is the specs: Technical specification • Attenuation: 10:1 with a differential signal and 50 ? termination in the ‘scope • Differential input resistance: 5 k? • Single-ended input resistance: 2.5 k? • Output resistance: 50 ? • Bandwidth: 1.9 GHz (–3 dB) • Rise/fall time: 300 ps • Power supply: ±8 to 12 V DC. They also write: For readers who are interested the author offers ready-to-use and tested PCB modules, also a kit consisting of case, RF cable with BNC connector and power supply lead with plug. Further information from: alfred_rosenkraenzer@gmx.de. |
| joeqsmith:
--- Quote from: David Hess on August 18, 2016, 06:31:04 am ---How much source impedance mismatch are you dealing with though? A 300 ohm differential probe is not intended for general purpose measurements; it requires closely matched source impedances to maintain common mode rejection ratio. --- End quote --- Differential digital, LVDS etc. |
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