EEVblog #652 – Oscilloscope & Function Generator Measurement Trap

Dave explains a big trap in high frequency measurement with your oscilloscope.
Based on a viewer request, Dave demonstrates how to incorrectly and then correctly measure the signal output level over frequency of your function generator using your oscilloscope.
Some whiteboard transmission line theory is thrown in as well.
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  1. I would have though you would have mentioned the internal series resistor between the rigol driver and it’s output connector. A sure fire clue is the generator measures twice the amplitude.

    Keep up the great work. Oh yea from here the sine waves look upside down from Australia.

  2. I has waiting for this great video.Thanks Dave.This explains some things in my mind but now I have a doubt about it.You said when you add the extra cable unterminated it reflects the signal since it does not have the 50 ohms terminator, ok I understand that but since the other cable was right terminated with 50 ohms, doesn’t it will affect the entire impedance of the cables if I add the 50 ohms to the unterminated cable?
    Because when I add it in theory it will be two 50 ohms resistors in parallel right?

    • If you watch the other video I think Dave shows that electrically coax cable is equivalent to a lot of series inductors with capacitances between the inductors & earth (like a ladder). If the resistance is at the wrong end it will have a completely different load & therefore affect on the transmission line. The simple answer is that above approx 10MHz nothing is quite what it looks like on the schematic – all those wires and PCB tracks (& bits of coax!) become complex networks of inductance, capacitance & resistance 🙂 RF design is an art all of its own.

  3. I remember, from the days of analogue goodness, using an unterminated delay line stub as an attenuator. A source of colour subcarrier at 4.4336 MHz at 2V pk-pk was overloading some broadcast equipment requiring CSC at 1V pk-pk, as a suitable 75ohm attenuator was not at hand, a T-piece was used to create a spur consisting of an unterminated switched delay line. Later when the delay was required elsewhere it was replaced with a simple length of 75ohm unterminated coax, cut to match the measured delay.

  4. Sniff'n the crotch

    This guy does explain it much more serious.
    Search for phase anomalies for different length of non terminated RG58 stubs on his blog. He explaining RF-reflections without the blah blah.
    w2aew has better informations on rf stuff.

  5. The very next time I read a blog, Hopefully it doesn’t fail me just as much as this particular one. After all, Yes, it was my choice to read, nonetheless I actually thought you’d have something interesting to say. All I hear is a bunch of moaning about something you can fix if you weren’t too busy seeking attention.

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