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Uncorrected performance of 8720D VNA - is this a fault?
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David Kirkby
If I do the following on my HP 8720D VNA (50 MHz to 20 GHz)
1) Press PRESET 2) Connect the two test cables together 3) Measure S12 or S21 without calibrating I see this (sorry about my poor Excel skills!) One can expect ripples in S21 plot, as I've not calibrated the instrument with a calibration kit, but is the fact the uncorrected S21 falls off so much at low frequencies to be expected? I've been trying to make some measurement of fringing capacitance of an open connector after doing a calibration kit. I see to have issues with this at low frequencies. I don't know if this is because the reactance of such a small capacitance is too large to measure, or if there's some fault in the VNA. Dave |
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Dave,
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a smaller span would show much more detail at the low end. It seems to start down at some -20 dB at 50 MHz and go up to normal level at around 0.5 to 1 GHz. I do not know how much a 8720 is specified to roll off at it's low end, but to me it appears to be too much. This might be caused by a bad contact or hair crack somewhere in the sgnal path. I would first measure S11 and S22 with a cal load at each test cable. If one side is bad, S11 or S22 will ne near 0 dB at the low end. If that is so, remove the cable from that test port and connect the load directly. Then you know if the cable is bad or the directional bridge / coupler of the integrated S-paramater test set. Adrian --- In hp_agilent_equipment@..., David Kirkby wrote:
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Dave,
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I would check your test port cables first. One of them might have an open. Vladan --- In hp_agilent_equipment@..., David Kirkby wrote:
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David Kirkby
On 9 January 2013 23:17, adrian_microwave <adrian_microwave@...> wrote:
Thank you Adrian. I got a message on one of the Agilent VNA forums from an Agilent member of staff to say this is normal. The couplers fall off below 840 MHz. Checking the data sheet it says: =========================================== System dynamic range 0.05 to 2 GHz 100 dB (note #1) 2 to 8 GHz 100 dB 8 to 20 GHz 100 dB #1. Rolls off below 840 MHz to 77 dB at 50 MHz ============================================ So a 100-77=23 dB rolloff at 50 MHz is to be expected. I guess that is one of the limitations of using a microwave VNA at lower frequencies. Perhaps the better Agilent VNAs switch in different couplers as the frequency range is changed. I've no idea if that is the case. I thought the whole purpose of the default calibration of the instrument was to correct for this sort of thing in software, so I'm surprised that the rolloff can be seen at all. Dave |
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Hi Dave,
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I stand corrected, the 8720D did not have internal correction constants for the coupler. It has some other corrections, so backing them up is still useful, but the roll off is there on IP. Sorry if my old post from the other thread caused you anxiety. I guess, since the D has a floppy drive, you can do a user calibration and save it there. As long as you don't change cables, you can always recall it and be in business without the tedious calibration process. Vladan --- In hp_agilent_equipment@..., David Kirkby wrote:
I got a message on one of the Agilent VNA forums from an Agilent |
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David Kirkby
On 10 January 2013 17:14, pianovt <pianovt@...> wrote:
Hi Dave,Hi Vladan, I stand corrected, the 8720D did not have internal correction constants for the coupler. It has some other corrections, so backing them up is still useful, but the roll off is there on IP. Sorry if my old post from the other thread caused you anxiety.It was partly your post, but I thought the rolloff odd for some time. Now I know it is ok. I guess, since the D has a floppy drive, you can do a user calibration and save it there. As long as you don't change cables, you can always recall it and be in business without the tedious calibration process.Or one can save it to the internal memory, which is simpler. I never change cables as such, as I have the proper Agilent test cables, which cost a fortune. Sometimes I add connectors from 3.5 mm to N, but I have cal kits for both N and 3,5 mm, so obviously save them as different names. But if a measurement is critical, I will always recalibrate since I don't have a temerature controlled lab, so a little drift is inevatable. But it is pretty damm small. The 8720D seems a pretty decent VNA overall. I've never used a PNA-X, but can't afford one anyway, so the fact they will be better is pretty irrelevant to me. VladanDave |
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Hi Dave,
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The 8753 and 872X series are excellent. If you are just making S-parameter measurements they are probably all you need. I certainly don't have a PNA-X either, but from conversations with people who have them, I have learned that the big step forward are the "apps". These are applications and options you can buy, such as a noise figure meter option or automated swept third order intercept measurements, including measurements on mixers, etc. The PNA-X have become multi-purpose boxes that can do almost anything. They certainly are state of the art stuff. They are also so loaded with features that you need a training course to learn how to use them to their full capacity. Vladan --- In hp_agilent_equipment@..., David Kirkby wrote:
The 8720D seems a pretty decent VNA overall. I've never used a PNA-X, |
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