Hello.
There is an instument with HighZ load and this is connected via 50 Ohm BNC cable to Oscilloscope(OSC) with several impedance selections (HighZ, 75 and 50 Ohm).
People say I can choose 50 Ohm as OSC impedance then no reflection but huge attenuation of signal from the instrument is expected. Attenuation is understood as this is similar to a voltage divider but why is there no reflection without perfect impedance matching?
They also say that adding load terminal ub type with BNC tee or just in-line type) to the end of the cable at source side can control impedance matching. How? The impedance of the source, cable and OSC are already determined. How can adding load between source and cable help to match the impedance?
Lastly it looks like stub and in-line type of the load have different circuitry. Thus which is the best choice?
Please see the link where you can see the load terminal and what I meant.
https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=7312
Find the impedance seen looking back from the load to the signal generator. Then choose a load that is the complex conjugate of that. This will yield maximum power transfer.
This subject is not simply explained in a few words. My best advice to you is to use a 10X scope probe which will better match your high impedance instrument, and accept its attenuation which will at least be accurately known, and more importantly frequency response matched. Do not use 50-ohm cable, as it's capacitance will load your high impedance instrument and act as a low pass filter.
For a signal propagating in the 50 Ohm cable from the source to the oscilloscope there will be no reflection from the oscilloscope back in the source, if the input of the oscilloscope is set to 50 Ohm.
The other question is that connecting of the 50 Ohm cable to your source with "high Z" load would change an effective impedance of the load and, thus, change the signal at the load. In the most simple case this will just reduce the amplitude at the load.
Thanks for giving several things. but I may need some direct answers. My aim to this question is to obtain some understanding of practical way of impedance matching, rather than solving my system problem right now. Thus I need to know
1. HighZ instrument + 50 Ohm cable + 50 Ohm OSC is good for preventing signal reflection, although there is one impedance mismatch between instrument and cable.
2. How additional impedance with BNC tee (see the link in my question) will help for impedance matching? How can it changes effective impedance for one end of the cable if it is made to there?
Please be more specific on the signal source:
1) Is it a voltage source with an impedance larger than 50 Ohms, but in the kiloohms range, or is it a voltage source with really high impedance like a pH probe?
2) Is it a current source with nearly infinite impedance, e.g. a photodiode or a photomultiplier?
After having these data, you have to decide up to what frequency there should be meaningful readings. This would then allow us to make better "guesses" on the optimum wiring and termination.
Okay. we have high-voltage probe which can measure several tens of kV and its impedance is mentioned as HighZ which maybe equal or more than 1M Ohm, according to manufacturer product lists.
OK, thanks for the info. I guess, I do not have the product list, that the manufacturer means with High Z actually the input side of the probe. However, if it is a standard HV probe, the manufacturer also supplies the information on how to load the probe correctly on the measurements side.
As far as I understood, Your signal source is a current signal from the photodiode/photodetector and you would like to measure it with oscilloscope. Simplest solution is BNC T, high impedance Termination and High Z Voltage probe (passive voltage probes with 10MOhm Input impedance and 200-500MHz bandwidth are omnipresent and typically included with oscilloscopes). You connect BNC T to the Output BNC connector on your photodetector. From one BNC T terminal you connect high impedance Termination and on the other one you sense voltage with High Z oscilloscope voltage probe. The oscilloscope Input should be operated in high Z impedance mode. In such scheema you will have almost no reflections. Working principal of this scheema is following: current from photodetector will flow in Termination resistance (which should be much lower as impedance of the oscilloscope) and will be converted into voltage signal V=IR. Since BNC T is short, the reflections due to impedance mismatch will occur in time scales 100ps or shorter and will be invisible for oscilloscope input channel with bandwidth 1GHz or so. Oscilloscope will sense voltage across the Termination resistance. Typically voltage probes are matched to the 1MOhm oscilloscope Input impedance, therefore you should not have reflections in cable of the voltage probe. With best regards, S.
Sergey thus, in the case you mentioned, impedance is matched between Oscilloscope and high impedance terminal connected to BNC T so these two impedance are okay. What about other impedance matching such as high impedance terminal and cable 's characteristic impedance?
Termination impedance will not be matched to the BNC T and Output impedance of the photodetector, But... length of the BNC T is short 1-2cm, therefore reflections which will be due to impedance mismatch will be high frequency (>1GHZ). Roughly speaking You should care about matching only in cases when your cable/wiring system can create significant distortions for your signal. Typically its are following cases: long cables or very high frequency signals. In case described by me I supposed that frequency bandwidth of your signal is 200-500MHz maximally or lower. So if it is correct you can use mentioned above measurement scheema. If your photodetector provides more higher frequency signal: then you need special components: HF termination resistor, PCB with HF layout, HF voltage probes with bandwidth more than 500MHz, HF oscilloscope and so on... then measurements will be more complex for you.
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