Electronics > Beginners
Impeadence matching vs high impeadence buffering?
rwgast_lowlevellogicdesin:
Ok im not sure if i am using all the right technical terms, so forgive me if my symantics are off. So I understan AC therory and reactence/reluctance, impeadence real and imaginary. But i having hard time grasping something more practical i need to understand for a 3mhz current probe im working on.
So in RF and audio/video we use impeadence matching at 50/600/75 ohm, respectively. This is to insure maximum power transfer. Test gear usuall has much higher impeadences in the mega ohm region, and this is done to ensure your gear doesnt load the DUT and effect the measurement.
Now why is it if you connect a 50ohm function gen to a 10megaohm scope input using cables well shorter than 1/4 wavelength, without using a 50ohm passthrough on the scope you get power loss and reflection? Wouldnt the scopes input have no effect on the signal, as if the function gen were not even connected? When deigning a device designed to plug in to a scopes BNC shoud the device end have a 10mega ohm termination to match the scopes impeadence? Is just using a current buffer/voltage folllower enoough or is none of this neccisary at 3mhz, and if not what about at 100mhz?
Kirill V.:
I also want to learn a little more about the topic of radio frequency devices.
Termination on the source and receiver side as well as the cable is all a single system called the transmission line. Everything is important in it. And the mismatch of cable and termination impedances will not allow the transmission line to function correctly. This is my personal knowledge in this area at the moment. They must be right:)
TimFox:
With a 50 ohm source impedance, an arbitrary length of 50 ohm cable, and an open-circuit (high impedance) termination, the following happens for ideal resistors and transmission lines:
Imagine the source putting out a narrow pulse (much less than the time delay of the cable). If the source makes 2 V before the internal source impedance, it would deliver 1 V into a 50 ohm termination.
At the source output connection, a 1 V pulse starts traveling through the cable. When it reaches the open-circuit termination, there is a positive (non-inverted) reflection which travels back to the source, where it is absorbed in the matched 50 ohm source. At the open-circuit termination, it adds to the original 1 V pulse to produce a 2 V pulse at the oscilloscope input. If the impedance at the oscilloscope were 50 ohms, there would be no reflection, and a 1 V pulse at the input.
Replacing the 2 V pulse (before source resistance) with 2 V DC would give 2 V into an open circuit and 1 V into 50 ohms.
The sine wave case is left as an exercise for the student.
Zero999:
--- Quote from: rwgast_lowlevellogicdesin on February 08, 2020, 09:41:52 pm ---Now why is it if you connect a 50ohm function gen to a 10megaohm scope input using cables well shorter than 1/4 wavelength, without using a 50ohm passthrough on the scope you get power loss and reflection?
--- End quote ---
You don't. Try connecting your oscilloscope to the output of a signal generator set to a 1kHz sine wave output and you'll see a perfect waveform.
--- Quote ---Wouldnt the scopes input have no effect on the signal, as if the function gen were not even connected?
--- End quote ---
It doesn't. If you have a true RMS multimeter capable of working up to 1kHz, and connect it to the output of the signal generator, the voltage will hardly change between the oscilloscope being connected and open circuit.
Kirill V.:
Wideband oscilloscopes can have both 50 Ohms and 1 MOhm input impedance
Conductors for pulse signals may not be transmission lines if their length is small enough
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