On 23 December 2012 22:09, pianovt <pianovt@...> wrote:
On the 8752/8753/8720 etc., and pretty much all newer VNAs, even what you call "uncalibrated" is actually calibrated at the factory or at the service center when you send the analyzer for calibration.
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.
The problem is that you don't know what cables were used for the factory calibration. Hence, two analyzers that were calibrated with different cables will show different results giving the impression that the hardware is vastly different.
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.
I think, the 8510 was the last VNA which showed true uncorrected hardware performance when the instrument wasn't calibrated. And it was impressive!
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.
This factory calibration is in a way similar to the user calibration, but the correction constants are saved in non volatile memory and protected with a jumper on the processor board. This then is the default state of the VNA when it powers up without a user calibration.
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. I'm sitting in bed at this minute with the
laptop - it has just gone midlight local time. Hence I can't be
bothered to go downstairs and switch the VNA on to see what it shows.
Vladan
Dave
