Dave,
Is that the same data you are supposed to back up to a floppy? The
reason I ask, is that the manuals seem to imply this will make
restoring the data much quicker if you have it on a floppy in the
event of a failure. But you are suggesting it would be changed when
the unit went for cal, suggesting to me there's not much point having
it on a floppy. Or perhaps I'm wrong.
Yes, backing up on floppy will save those calibration constants (and other stuff as well). That way, if you have to replace the processor board, you don't have to recalibrate everything. You can replace the board in a few minutes and restore the constants. No need to send the analyzer in.
In the case of the 8720D, there is a set of cables available (Agilent
t 85131F) for this, with the NMD connectors, to ensure a rugged
connection to the VNA. I assume it would be calibrated with them. I
know the length of them is nominaly 24.5", but at 20 GHz, with a
wavelength of 15 mm, slightly differnt cables would give quite
different phases.
You can do the internal calibration with those cables, but also with other cables or omit one of the cables altogether. Whatever you pick as your default cable setup will then become the calibrated state when you power up the analyzer.
I assume several of the internal cables were selected to give it this
performance. Did the 8510 have a computer inside to do automatic error
correction, or was it a manual process? I've used a few VNAs, but
never and 8510. I own an 8720D and 8753A.
The 8510 had top notch hardware and relied on it for pretty good raw performance at the plane of the test set ports on the front panel. So, if you connect something to either port directly, you can get a decent measurement of return loss even without a cal. For S21 and S12, you need a test cable, and that cable is an unknown until you do a calibration which is similar to the user cal on your 8720. In the back of the test set, you can attach a pair of delay lines that are hopefully of similar length to your test port cables. That's as close as you can get on the 8510 as a default state.
The 8753 and 8720 are newer designs, and they made more use of digital correction and non-volatile memory, thereby allowing for less expensive hardware. The main provision is that you need stable hardware, and that is the case.
At least on my 8720D, I'm sure it is nowwhere near 55 dB as one of the
VNAs reported in this thread. I'm not sure it is even 35 dB. But I
always do a user cal.
If you were to send the analyzer for a "Internal Calibration" (this is the factory/service center job) and you asked them to use your supplied cables and calibration standards, in theory you would get the analyzer to have perfect calibration when you turn it on and test it with your cal standards. You would not need to do the manual calibration. In practice, connectors are not perfectly repeatable, and you might see something like 40-50 dB of return loss or even better, depending on repeatability. However, remember that your measurement is only comparing the unknown against your calibration standard. Hence, seeing 60dB of return loss only means that the unknown is almost identical to the calibration standard you used. In practice, you could have calibrated the analyzer with a poor load that is 45 Ohms instead of 50 and the analyzer would "think" that a 45 Ohm unknown is a perfect 50 Ohms.
The "internal calibration" procedure is in the service manual for the 8753, and I assume that it's also in the manual for the 8720. I don't have an 8720, so I am speculating. If you find it, you will see that a portion of the procedure is quite similar to the regular cal that you do all the time.
Vladan
