Products > Test Equipment
Sniffing the Rigol's internal I2C bus
clifford:
--- Quote from: mightyzen on December 27, 2013, 04:29:50 pm ---Very interesting. What program did you use for this?
--- End quote ---
This is Qucs (http://qucs.sourceforge.net).
clifford:
--- Quote from: m-joy on December 27, 2013, 06:09:41 pm ---Well this is pretty much theory. This is the case if the coax cable impedanz is perfectly 50ohm which is not the case and if the Transmission line inside is perfectly and if the Terminator is perfectly 50ohm ... I would worry more about tolerances than this line theory
--- End quote ---
The argument stays the same: It all happens within a few centimeters of the receiver.
But you are right: Screw the theory and lets make a simple experiment. :-/O
This is the fastest rise time I could generate within a few minutes for a simple test (that's the trigger output of my DG1022). So maybe someone with access to a faster rising edge could create a better test case.
The white trace (R1) is with the coax going directly into the scope input. So technically this is unterminated. But there is 50 Ohms termination on the other end, so it should more or less look the same as if we had 50 Ohms termination in the scope. The yellow trace (1) is with a BNC T connector with a 50 Ohms terminator on the second port.
We can see that it wants to overshoot a bit, as we would have expected from the simulation. But because the signal itself rises not fast enough we only end up with a trace slightly above the trace from the first case.
This is on my DS2072 (HW 2.0, SW 00.02.01, "upgraded" to a DS2303) with averaging of 1024 triggers.
PS: Yes, the trigger level is a bit awkward. I only realized that after I have created the screenshot. (I think that is simply the level I had already set and I did not bother to touch it because the trace looked fine.) The behavior does not change if I move the trigger down a bit. I've tried that, I just did not bother to create new screenshot.
EV:
Try scopes trigger out signal. It is very fast
--- Quote from: clifford on December 27, 2013, 06:57:08 pm ---
This is the fastest rise time I could generate within a few minutes for a simple test (that's the trigger output of my DG1022). So maybe someone with access to a faster rising edge could create a better test case.
--- End quote ---
Wim13:
--- Quote from: clifford on December 27, 2013, 06:27:29 pm ---
--- Quote from: mightyzen on December 27, 2013, 04:29:50 pm ---Very interesting. What program did you use for this?
--- End quote ---
This is Qucs (http://qucs.sourceforge.net).
--- End quote ---
One thing you have to add to your simulation.
The scoop entry is also a low pass filter, in this case a 300 Mhz low pass filter,
thats why signals going to the 300 Mhz always look always like a sine wave, because of the low pass filter.
All the harmonics are gone after 150 Mhz, all is left is a sine wave..
So a scoop of 300 Mhz is usefool to 20 Mhz.
clifford:
--- Quote from: Wim13 on December 28, 2013, 09:04:49 am ---One thing you have to add to your simulation.
The scoop entry is also a low pass filter, in this case a 300 Mhz low pass filter,
--- End quote ---
This is tricky because the 16pF / 1 MOhm scope input is already part of the 300 MHz spec. But I've created bode plots of the original circuit and with an added first order 300 MHz low pass:
http://imgur.com/a/sMYNH
This set of images also contains a transient simulation of the circuit with the added additional filter. As you can see, the rise time is slightly lower now but it does not really make any difference regarding the qualitative effect of the short transmission line between the termination resistor and the scope input.
--- Quote from: Wim13 on December 28, 2013, 09:04:49 am ---thats why signals going to the 300 Mhz always look always like a sine wave, because of the low pass filter.
All the harmonics are gone after 150 Mhz, all is left is a sine wave..
So a scoop of 300 Mhz is usefool to 20 Mhz.
--- End quote ---
I think you are confusing bandwidth and sampling frequency here. All DS2000 scopes have a sampling frequency of 2 GHz.
Also: The harmonics above the nyquist frequency (1 GHz in this case) are not gone or magically filtered by the sampling. They show up as aliasing frequencies. You have to actively filter those components out using an anti-aliasing filter. If you sample fast enough you already have significant low pass characteristics on your input path and don't need to build a filter, its just implicitly there. That's the 300 MHz in this case.
But those filters never do have an ideal sinc impulse response. So you will never see a signal just morphing into a pure sine wave when approaching the filter edge frequency. (You can build such filters in a DSP of course: Just perform an FFT, mask out the frequencies you do not want, and run an IFFT. But you will never see the equivalent of that in an analog filter.)
There is this rule of thumb that you should have at least a factor 10 between sampling frequency and bandwidth. It is a good rule of thumb, but it is not the ultimate answer. The minimum factor between sampling frequency an signal bandwidth depends on the kind of signal you are interested in, the kind of aliasing filter you are using and the interpolation method you are using. In most RF applications you can get pretty close to the nyquist frequency, because you have extremely band-limited signals, use high order filters and you effectively use a sin(x)/x interpolation (you will never actually look at the signal in the time domain, but the algorithms work with an equivalent representation).
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version