02-12-2008 06:07 PM - edited 02-12-2008 06:17 PM
Bonsoir EBL 🙂
>> the model of the fixture impairment (including the DACs, ADCs, cables, capacitors, wires, fingers, noses, and all) is one s-parameter matrix.
I'm sorry, but, I miss something. I understand what you explained, but I miss this sentence. What do you put in the term "fixture impairment", is it all the power loss due to these components ? In this case, this means that all the cables are inside a two port device. I suppose that on one port you put the DAC and on the other port you connect the dut. But if you model like this, where are connected the two ADC ? If you model all the circuit using a s-matrix, you have to choose a four port terminal (4x4 matrix). In the other case, you must assume that the two adc inputs are perfect, I think you do not make this assumption.
In the second model, I speak today, we can choose to have one two port device wich is only a 1k resistor and whose characteristics are known. And then, we consider that the fixture impairment is modeled by some measurements errors around this 1k resistor.
To my mind, but I'm quite novice, the doc of hp uses the second model. To my mind, but I'm absolutely not sure, you seem to work with the first case and consider that an 2x2 s matrix is sufficient to correct the errors. It is unimportant I'm right or wrong. But, I would really want to understand how you work. I will go on with one year of work based on sharp measurements. So, I need to know what I do. I think, I have to develop the two port calibration after I understand your app. So the discussion is not about maths, RF or labview, but how you model the fixture using s parameters. If you could do a figure... My misunderstanding cames from a little thing I miss.
In any case, merci beaucoup et a bientot 🙂
ps: today, I will speed up to retrieve my pc and have a look at your code
02-14-2008 12:15 PM
02-14-2008 06:21 PM - edited 02-14-2008 06:28 PM
03-26-2008 12:38 AM
03-26-2008 12:32 PM
05-20-2009 10:39 AM
05-20-2009 10:54 AM
Hi EBL, here is Misterdaq !! This was a long time ago. I do not forsake, but time was missing and difficulty was high. I succeed in finding the Pozar Book, then with your explanations and the last document you gave me, I think I now understand the calibration for VNA. I'm not very familiar with Labview and the spaghetti style is difficult to read. So I prefered to work with my pen. I undertand what you do in your VI for VNA calibration.
Nevertheless, I have a question about the computation of S11m, maybe my last question this time. I put the vi just above (simplified figure). The two vi on the right bothers me because I do not know precisely what they do. So I see you compute these two values V0/2 and V1-V0/2, the first seems to be the incident wave, and the other "your" evaluation of the reflected wave. After that, you compute something for S11...
if
I = Incident Wave = I e i(wt+phiI)
R = Reflected Wave = R e i(wt+phiR)
Do your S11 correspond to : S11 = R/I*e i (phiR-phiI) ?
thx
05-20-2009 10:58 AM
Here is the summary of your equations. Left here for the next generation 🙂
05-20-2009 03:55 PM - edited 05-20-2009 03:56 PM
Bonjour MisterDAQ,
Your understanding of the code seems to be correct. Indeed, s11m is basically the ratio of the reflected wave over the incident wave. There is only one slight complication: in order to reduce noise, I take a vector average of this ratio. You'll notice that in Refl Acq Multi-Tone.vi, there is a cross-correlation, which is a vector power measurement. By averaging these vectors, the noise in the reflected measurement that is not present in the incident measurement is reduced, depending on how many averages are taken.
Hope this helps,
Ed
12-13-2015 12:21 AM
Hello Ed,
I am very appreciative of your code and impedance measurement system. I have previously implemented it with a USB-4431 card with no hiccups whatsoever. Now I am trying to use a NI USB-6356, however, the analog input configuration is limited to differential. I am having problems with the system. The calibraion curves all oscillate and when I take a measurement of the load 1kOhm resistor, it is not nearly a straight line at 1000 as I have seen in the past. I have concluded, rightly or wrongly, that it is a grounding issue. In your impedance fixture circuit, AO, AI0 and AI1 all share a common ground. I am not sure how to wire this up with differential analog inputs. The pinout of the USB-6356 has AI+,AI-, AI_GND. As the fixture circuit is wired up, AI0- and AI1- and AO_GND are all connected. I have consulted the manual for the USB-4431 (the card on which I implemented you impedance measurement system successfully in the past) to determine what the difference betwen the USB-4431 and the USB-6356. The 4431 are 4 input/1 output BNC connections, with no obvious separate ground connections.
I am wondering if you could enlighten me on how I might succesfully implement your impedance analyzer with differential analog inputs. Differential is the only analog input configurtaion available for the USB-6356. I would have tried NRE if that was available.
Any assistance and direction you can offer would be appreciated.