I'm not sure I understand the problem with the probe impedance. You want the probe impedance to be as high as possible so that it does not affect the voltage measurement by drawing current.
If you are measuring the voltage across a 50 ohm impedance then the circuit will see the 50 ohms in parallel with the 1Mohm which is 49.9975 ohms - probably not enough of a difference from 50ohms to make a measurable difference.
Also, reading the manual you reference, it shows that the probes have a 10:1 impedance switch which I would interpret as meaning that you can actually get a 10Mohm probe impedance.
I'll not comment of whether it is worth it or not. The spec sheet says 1Gs/s in the hardware and that should be plenty for reliable HF waveform representations, even up to 50MHz. I have a hardware scope that also has 1Gs/s but my interests are more in the VHF area and it starts to struggles about 100MHz (the square waves start to be a bit rounded).
Susan
Of course - if you are measuring an open circuit then all you have is the scope probe impedance and the voltage can be completely unrepresentative of the voltage under load. I must admit that I assumed that you were referring to a 50ohm dummy load.You are right about the impedance things, but you assume that the equipment to be measured is internally matched to 50R. To see how homebrew unmatched equipment behave at 50R, a shunt resistor has to be connected at the input of the scope, so that the measurement is done at 50R. In other words in RF you have to match your devices to be measured to 50R if they are not.
You are actually measuring at 1M, or whatever your scope probe is, but the devices are loaded with 50R. So the end result is measuring at 50R. At least that is what I am thinking of.
Of course - if you are measuring an open circuit then all you have is the scope probe impedance and the voltage can be completely unrepresentative of the voltage under load. I must admit that I assumed that you were referring to a 50ohm dummy load.
Nyquist says that you need to sample at twice the frequency you need to reconstruct. However that only applies to sine waves. If you want to reconstruct other shapes then you need to look at the highest frequency that is present in the reconstructed wave and double that for the required sampling frequency. You are working this situation the other way so given a 1Gs/s then you can get a 500MHz sine wave but probably only a 50MHz square-ish wave. That should be fine for HF work (especially if you are looking at SWR etc. waveforms).
Susan VK3ANZ
I think I've mentioned picoscope here before. (The make I use)
They have a range online you could use to compare specs and prices with.
They are continually adding software features and provice free ongoing updates.
I've found them very good over time. (I bought my first one about 7 years ago)
You are right about the impedance things, but you assume that the equipment to be measured is internally matched to 50R..
I do not understand what you mean by "but you assume that the equipment to be measured is internally matched to 50R"- what is this 50R coming from?
Are these points right?
I suggest you to read the article:
https://www.qsl.net/va3iul/Impedance_Matching/Impedance_Matching.pdf
this is a good one, from a technical standpoint.
There is no need to use any special resistor in addition to the standard probe to measure potentials on a RF circuit board (if the voltage and frequency are within the limits...)
I do not know the output impedance of the circuit under test and since I need the circuit to operate with defined characteristics on 50R
What I am saying that you do not need any extra 50E resistor to measure the voltage on a scope. If the 50E resistor is part of the board, let it be. But you do not need anything extra. For example, I do not understand the meaning of the sentence:
The potentials at a given point in a board can be tested with a scope probe without any extra resistor. If you need to use the circuit with a 50E load, let it be there...
For example, many RF circuits use an antenna for power (some will say signal) transmission and reception and the circuit must be matched with the antenna, the antenna must be matched with the empty space...
These matchings are traditionally done at 50E and you will find it rewarding to study how the antenna are matched with empty space (and 50E impedance)
Thanks, but too much theory for me. All I am asking is practically, how to measure with the 1M scope input, a circuit (for example an oscillator that has a capacitor coupling on it's output) that has an unknown impedance and I want to measure it on 50R.
All the examples I have seen on the net, connect a 50R shunt resistor to ground (or a feedthough 50R terminator), so that the measurements are done at 50R, despite the fact that the scope has an input of 1M. Is that accurate enough?
When I used to work with RF, I always used a 50R feedthru with a female BNC on one end, a male BNC on the other.
We had some expensive Tektronix scopes which had an internal (selectable) 50 ohm termination. But the danger of forgetting about it and leaving it on was too great, so everyone around the lab always used those terminations instead.
Similar to these:
**broken link removed**
Measuring 1vpp at 50R is different than measuring 1vpp at 1M. By shunting a 50R, you measure the power of an oscillator at a 50R load.
Am I missing something here?
I do not see how you can get the power of an oscillator from a single point measurement at a 50R load; you will not even know the voltage!
Oscilloscopes shine when asked to show fast changing voltages!
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?