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| Kirkby calibration kit alternatives? |
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| rfspezi:
--- Quote from: G0HZU on March 20, 2018, 12:53:47 am ---Don't forget that your cal kit SHORT and OPEN is at the far end of either a male-male SMA adaptor or a female SMA adaptor. The typical delay in the F-F adaptor is about 42ps and the M-M is probably about 58ps. So the moment you put your cal kit OPEN back on the VNA after a calibration the open end of your OPEN will be either 42ps or 58ps away from your reference plane. So the smith chart on the VNA will show a 0-6GHz line going maybe halfway around the smith chart. It's effectively measuring the SMA F-F barrel. My guess is that you have tried to do a 1 port calibration and told the VNA the wrong gender for the cal kit. So the 42ps and 58ps are muddled. Try again for the 42ps female OPEN (and don't forget the VNA refers to this as OPEN (M) ) and it might look like this: --- End quote --- Thank you for the explanation! So I definately used the wrong sex and need to think at the reference plane. If i understood right, i should be able to verify a correctly performed calibration by e.g. connecting the SHORT-reference and applying a port extension that exaclty matches the one specified in the calibration data for convergence? Concerning the loads the calibration file says: --- Code: ---** Male load. ** Minimum frequency: 0 MHz Maximum frequency: 7000 MHz Offset Zo: 50.0 ohms Offset loss: 1000 Mohm/s Offset delay: 0 ps ** Female load. ** Minimum frequency: 0 MHz Maximum frequency: 7000 MHz Offset Zo: 50.0 ohms Offset loss: 3000 Mohm/s Offset delay: 0 ps --- End code --- Shouldn't the resistor values match the truely measured ones? (although measured with DC) I guess the correct way would be to get better 50 Ohm references, right? :) |
| drkirkby:
Hi, I'm Dr. David Kirkby, the director of Kirkby Microwave. Since a few people asked, I can confirm that the attenuator supplied in the SMA kits http://www.kirkbymicrowave.co.uk/support/Kits/85033/ is measured with an Agilent 85052B 3.5 mm cal kit, not the SMA kit supplied. There are 5 measurements made on the SMA attenuator: * S11 * S21 * S12 (in theory the same as S21) * S22 * Reflection coefficient at the male port, with the female port unterminated S11 and S22 measurements can't be expected to be very accurate at low frequencies, as the return loss of the attenuator is very good. A high return loss means a high measurement uncertainty. As the frequency rises, so the return loss of the attenuator deteriorates, and the S11 and S22 measurements are more meaningful. But the S21 measurement is useful at all frequencies. The measurement of a reflection coefficient at the male port with the female port unterminated gives a return loss that will be met in many practical circumstances. On the N kits http://www.kirkbymicrowave.co.uk/sales/85054/ we also measure the reflection coefficient at the female port, with the male port unterminated. But we don't do that on the SMA kits as the nut on the male SMA wobbles around, changing the capacitance to the inner conductor. Since such a measurement is potentially unstable, we don't do it. There's no doubt the attenuator is not a perfect test of a VNA, but it has in the past highlighted problems when people have entered the coefficients incorrectly. There's a review of our kit on Amazon UK by Dr. Robert Watson at Bath University. He describes how he used airlines, and a Beatty Standard and compared the kit with various other kits. See https://www.amazon.co.uk/Kirkby-Microwave-85033-calibration-verification/dp/B01BAWR70M/ref=sr_1_1?ie=UTF8&qid=1467301985&sr=8-1&keywords=85033+sma+calibration+kit/ I have no connection with Robert Watson, other than he bought a couple of kits, and another UK university bought one when they were collaborating with him. I never studied at Bath University, and have never met him, but he has agreed if anyone has any questions, he may be contacted. You can find his details on the Bath University website. As for the comments by someone about the DC resistance of the loads, we are less concerned about their DC resistance than their RF performance. The loads are tested to have a return loss of at least 32 dB from 50 MHz to 7 GHz in a standard kit. (They are usually much better). We have an HP 3457A 6.5 digit multimeter (currently at Keysight for calibration), so could easily measure the DC resistance using 4-wire Kelvin connections. However, I feel that's irrelevant, as it does not indicate the RF performance at all, because the real part of the resistance is not independent of frequency. You will never see a specification for the DC resistance (other than 50 or 75 ohms) in any cal kit from Agilent. Of course, if the DC resistance is well away from 50 ohms, the return loss at low frequencies would be poor, and we would reject the load, irrespective of what its performance is like at microwave frequencies. On some loads, the return loss at 50 MHz (the lowest frequency we measure) will be worst than at 7 GHz. Whilst this is the exception, rather than the rule, it does indicate that the DC resistance is not so important. We do now provide measured S-parameters on the opens, shorts and loads so if one has a VNA that allows the S-parameters to be imported, that data could be used. Someone asked why when they loaded the floppy disk into an 8753, it indicated a maximum frequency of 3 GHz. That is because we find it quicker to create the files on our 8753ES, which is a 3 GHz unit. The actual coefficients are valid to 7 GHz, and a 6 GHz 8753 will calibrate up to 6 GHz. I have never seen one, but there's some information in one of the data sheets from Agilent that suggest there were versions of the 8753 up to 8.5 GHz. I guess these were "specials". If you did have such a unit, a standard SMA kit would not calibrate it beyond 7 GHz. We can do SMA kits up to 12 GHz now, but they are more expensive, as very few loads have suitable performance at 12 GHz. I strongly suggest if anyone does have any queries about how to use our kits, it is better to ask us, than ask on a forum like this. I see people asking if the SMA_M_M file is the right one to use in a particular circumstance. Oh, and one more thing. The Agilent N9912A FieldFox is a very poor VNA. If you ask Agilent, they will tell you it is only designed for basic measurement. It can not even measure the phase of S21 - only the magnitude. When I first played with an N9912A I thought there was a fault on it, as the dynamic range is only about 50 dB. I'm told that is normal. The N9912A really is a brain-dead instrument. Dr. David Kirkby, Kirkby Microwave. |
| drkirkby:
--- Quote from: Safar on February 18, 2018, 02:31:08 pm ---Some sellers from China on ebay sell kits with PNA trademark. They wrote in description: "Accurate electrical models are needed of the opens and short circuits. A number of eBay sellers sell low-cost calibration kits, but these are effectively useless, as they don't have the mathematical models of the opens and shorts necessary to load the constants into a VNA, and we doubt they have optimal phase difference between open and short. This kit is different, in that it is designed, rather than just assembled. With this calibration kit, there is a support page with information on what coefficients to use for Agilent/HP,R/S,ADVANTEST,ANRITSU...". --- End quote --- Note those words are a direct copy from the Kirkby Microwave auctions! :) They did not copy the link to our website! The Chinese have just copied what we wrote. I did think of complaining to eBay, but decided not to bother. Dr. David Kirkby. |
| drkirkby:
--- Quote from: rfspezi on March 19, 2018, 08:06:25 pm ---I could need some advice on what i am doing wrong with my Kirkby cal-kit. (Please appologise stupid questions, since VNA measurements are totally new to me ;) ) My setup consists of: -) HP 8753E VNA (6 GHz) -) Kirkby 85033 SMA calkit -) 50 cm RG316D koax I followed the instructions on the Kirkby 8753 page here: http://www.kirkbymicrowave.co.uk/support/VNAs/HP-Agilent-Keysight/8753/ On the floppy disk there are 3 files (SMA, SMA_M_M, SMA_F_F). I selected "SMA" and loaded it with "RECALL STATE". Is that the correct file to choose? After that, the frequency range is automatically changed to 300 kHz ... 3 GHz. Shouldn't it be set to the full range of 30 kHz - 6 GHz of my VNA? --- End quote --- If you need advice, you should contact us! The reason the span is set from 300 kHz to 3 GHz on your 6 GHz 8753ES is that we actually create the files to go on the floppy disk using an 3 GHz 8753ES, which works from 300 kHz to 3 GHz. This is less tedious, and less error prone than trying to use any Agilent software, like the Cal Kit Manager 2.1 software. The coefficients are valid to 7 GHz though. If you load the SMA kit, and look at the frequency range of any of the standards, you will find the minimum frequency is 0 GHz and the maximum 7 GHz. Someone else has already pointed out some other errors you made, so I will not repeat them, but I thought I'd explain why you see a span of 300 kHz to 3 GHz. You will just need to set the start and stop frequencies to what you want. Dr. David Kirkby. |
| TheSteve:
It is great to have you here David. I have been playing with more VNA's. I have something that is full 2 port now to 6 GHz. With a full 2 port OSLT cal my Mini Circuits VAT10+ looks quite reasonable when compared to the datasheet. Much better then the results my N9912A was providing. |
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