I want to measure the SWR of a device. There is a Narda switch between the NA port and the device. If I perform the cal with the standards placed after switch, will that remove the SWR of the switch from the measurement?
the NA port and the device. If I perform the cal with the standards placed after switch, will that remove the SWR of the switch from the measurement?
I do not think it will help as stated. That approach is too simplistic.
?-)
port and the device. If I perform the cal with the standards placed after switch, will that remove the SWR of the switch from the measurement?
How could it not?
the NA port and the device. If I perform the cal with the standards placed after switch, will that remove the SWR of the switch from the measurement?
It comes down to whether the Narda switch and the cables/adapters used to connect it are of a higher expected quality than whatever you are trying to measure. Are they? At HF, it would not usually make sense to worry about de-embedding microwave-grade interconnects. At higher frequencies closer to the accessories' rated limits, it can be a bigger deal.
By definition, the measurement's reference plane is defined by where you placed the standards when you did the calibration. In general, the farther these components depart from an ideal 50-ohm transmission line at the frequencies of interest, the less likely that you can move the measurement's reference plane to the downstream side simply by calibrating after the additional components. At some point, you will need to de-embed the accessory hardware by characterizing it separately, in order to remove its influence from the final measurement data. It's hard to say exactly when/where this is necessary.
For instance, when you take a wideband sweep at a remote reference plane, you are relying on your analyzer to interpolate its error- correction terms between discrete calibrated points. Often, cables and accessories will exhibit a lot of VSWR ripple that the calibration process will 'undersample.' Calibration alone can't take the accessories out in this case, but a good deembedding algorithm can. If you are making a narrowband measurement, or a measurement at a single frequency, you will probably be fine with calibration alone.
Joel Dunsmore, who has worked on VNA stuff at Agilent since the 8753As in the mid-1980s, just came out with a nifty book on the subject:
But there are still no "Yes, you can do that," or "No, don't do that," answers to be had, in the general case.
-- john
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