EEVblog > EEVblog Specific

EEVblog 1415 - Reverse Engineering the DP10007 Differential Probe

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--- Quote from: Just_another_Dave on October 06, 2021, 07:52:52 pm ---Yesterday I had a bit of free time, so I build a prototype of the tool for switching between both photos mentioned in the video on reverse engineering the probe. It doesn’t support marking the components yet and the web version has a bug that impedes loading photos (Godot doesn’t provide access to the native file system and its filedialogue component does not work properly in html5 exports).

Although I’m planing releasing the source code when the application is completed, I’ve uploaded an early prototype to itchio just in case someone is interested in trying it:

Edit: corrected a typo

--- End quote ---

I’ve just seen that it has been mentioned in the eevblog Twitter account, so I wanted to thank you for giving it visibility. I’m currently working on solving the known bugs, as well as implementing a system to allow adding marks. Thankfully, one of the bugs has been solved in a great open source tool called pixelorama, so the web version will probably work in the near future

David Hess:

--- Quote from: free_electron on August 31, 2021, 12:04:32 am ---H-bridge driver for a latched relay. One reason for having a latched relay is signal integrity. An energized coil leaks enough magnetic field that his has an impact on the signal.
An ac signal traveling through an magnetic field sees a complex impedance. This throws a spanner in the gain flatness if that relay is used to switch gain settings. The bandwidth will change with energized/non energized state. This behavior is frequency dependent too.
i found this out the hard way when making test equipment for ADSL. even though the signals there are only up to the 1.3 MHz range, traveling through an energized relay was enough to have a few kilobit impact on the line. So i switched to dual coil relays (set-rest type) and the problem went away.
when i left one of the coils energized you could clearly see the drop.
--- End quote ---

In the 1970s Tektronix was making their own relays, and selling them to others, for use up to at least 100 MHz in low and high impedance circuits which did not have that problem, but later they used latching relays if only for power conservation.  The distinguishing thing about their relays at the time was that the contacts were located at the bottom of the frame limiting wire length and capacitance, which apparently became a design feature in later "telecom" style relays.

--- Quote from: KT88 on August 31, 2021, 11:03:47 am ---Why are they using a relay in the first place? The gain switching happens at a low-impedance node behind the differential stage. An analog CMOS switch would be perfectly fine for the job at less power, size and less cost...
--- End quote ---

It is difficult to get sufficient frequency and phase flatness out of CMOS switches, which raises the question about how solid state channel switches were designed in the past without relays.  They used current switching instead of voltage switching which works find with bipolar transistors or diodes.


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