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My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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I uploaded the results of my testing the SG503 012-0482-00 cable. In
addition I compared it to two other similar cables: a Tektronix 012-0057-00 50惟 cable, and a Pomona Electronics 50惟 cable. It is located at: /g/TekScopes/files/Evaluation%20of%20the%20Tektronix%20012- 0482-00%20cable%20for%20the%20SG503.pdf Dennis Tillman W7PF |
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Craig Sawyers
Those are interesting measurements Dennis. And particularly interesting that although the
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Pomona/Belden the impedance is not 50 ohms, although the variation seems to be very low. And it will be more than an order of magnitude cheaper than the double shield Tek cable Craig -----Original Message----- |
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Very timely: this weekend I picked up a Tek 1502B, and it came with a 012-0482-00 cable and a 011-0123-00 BNC 50ohm terminator. Apart from that I only have "generic" connectors and terminators, of variable quality.
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Caveat: I don't understand the expected performance of the 1502B, and haven't calibrated it at all. Having said that, the only significant difference from your report is the velocity factor. In order for the 1502B to show the cable's length as 3ft, I have to set the velocity factor to 0.67. On 29/04/19 01:19, Dennis Tillman W7PF wrote:
I uploaded the results of my testing the SG503 012-0482-00 cable. In |
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Where does it show the speed of light in a vacuum? The TDR should be calibrated to the speed of light (electricity) in the cable. For 50 Ohm cable its about 66% of c. Varies with the impedance of the cable. If you know the physical length of the cable you can determine both its impedance and the velocity from it.
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On 4/29/2019 1:26 AM, Albert Otten wrote:
Interesting stuff Dennis! I have to look at it in more detail. I consider repeating these tests since I have both Tek cables, though I don't have quality SMA-BNC adapters. --
Richard Knoppow dickburk@... WB6KBL |
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On Mon, Apr 29, 2019 at 10:50 AM, Richard Knoppow wrote:
The document says: "The speed of light in a vacuum is 224844343.5 m/Sec= 0.2248m/nSec. " I guess Dennis already used some kind of velocity factor here. But this makes his following derivation invalid: "The Velocity Factor (the speed the signal travels in the dielectric of this cable) is VF = 2* 36.0in * 0.0254m/in * 4.4475nSec/m/9.0nSec ~ 0.90. " Albert |
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Hi Tom,
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Wikipedia is your friend when it comes to Velocity Factors. Scroll down this web page Or in your web browser type "wikipedia coax" and it is usually the first hit you get. Wikipedia lists about 40 or 50 types of cable with all their parameters including velocity factors. It is my favorite, quickest way to find any information about different coax cables. Dennis Tillman W7PF -----Original Message----- Dennis Tillman W7PF -- Dennis Tillman W7PF TekScopes Moderator |
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Hi Richard,
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The speed in a cable is mostly a function of the Dielectric Constant of the filling between the inner conductor and the outer shield. The dielectric constant of the filling along with the diameter of the inner wire and the distance to the outer shield determines the impedance. But there are many, many other considerations manufacturers have to consider. Many of those are not listed here. What is here is signal attenuation, how well shielded from interference. Other things to consider are triboelectric noise from flexing the braided shield if the cable will be carrying very small signals, group velocity, cutoff frequency, maximum power it can handle, and on and on. Take a look at this web page to get an idea of what factors are taken into account in design of a coax cable: Dennis Tillman W7PF -----Original Message----- -- Dennis Tillman W7PF TekScopes Moderator |
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You need to get the specifications (using the part number on the cable) which you are going to use (or test) and plug-in the Velocity factor into the 1502 for the distance reading to be correct.
On 2019-Apr-29 4:03 AM, Tom Gardner wrote: Very timely: this weekend I picked up a Tek 1502B, and it came with a 012-0482-00 cable and a 011-0123-00 BNC 50ohm terminator. Apart from that I only have "generic" connectors and terminators, of variable quality. --- This email has been checked for viruses by AVG. |
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I know that. I was pointing out that the velocity of propagation in a cable is not the same as in space and that it depends on the physical nature of the cable, including its dielectric constant but you are right that it's mostly the dielectric constant of the core.
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the OP was as On 4/29/2019 9:49 AM, Dennis Tillman W7PF wrote:
Hi Richard, --
Richard Knoppow dickburk@... WB6KBL |
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Hi Albert,
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I think my derivation is valid and I know the VF can't be that high. Did I leave something out in the way I calculated it? My purpose in calculating it (regardless of whether I got a perfect answer) was to show how to do it so everyone else with a TDR could do it. I wanted to illustrate how Velocity Factor was calculated from a TDR display. Instead of looking up a standard Velocity Factor, I was attempting to determine the velocity factor of the Tek cable based on its physical length and the TDR's "time distance" electrical length I measured on the screen. So I measured the time between the discontinuity at the start and at the end of the trace on the screen. Take a look at Figure 2. I marked off the starting point at "A" and the ending point of the cable at "B" and measured 9.0nSec between points A and B on the screen. Then I calculated the velocity factor based on that measurement of the transit time of the pulse through a 36.0" cable. The result I got, VF = 0.9 was very high and I commented on that in my results. The only cables that have such a high velocity factor are very expensive and usually very fat (large diameter). I know the velocity factor can't be 0.9 and that figure is only a guess because my TDR has not been calibrated in over 20 years. The VF is probably more likely around 0.8 which is still a general indication of a high quality cable. I also converted the mRho measurements to VSWR figures in a few places because that is also a consideration for some applications. For instance, the VSWR is listed in the specs for the 7A29 plugin as 1.4:1 at 10mV/Div or 1.2:1 on all other settings. The VSWR of the 012-0482-00 cable may have been a consideration of the engineering staff that determined what properties it had to have for the SG503. This raises another question which none of us may ever have the answer to: What are the unique qualities the Tek SG503 designer wanted this cable to have to be the ideal match to the SG503. It may be something as silly, in retrospect, as the stiffness of the cable. Maybe he/she like stiff cables. Why is it exactly 36in? Maybe he/she had a scrap piece of stiff coax exactly that length in his drawer and he/she said "Use this" to the production people when they asked what kind of cable should ship with the SG503. Meanwhile we are all guessing at whether using this cable with an SG503 is really important. Dennis Tillman W7PF -----Original Message----- -- Dennis Tillman W7PF TekScopes Moderator |
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Hi Merchison,
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I don't know about the 1502, but the 7D12 which I used has the ability to preset any velocity factor on the POLY scale between 0.67 and Air (which is one of the other distance scales). VF for air is around 0.99. Dennis Tillman W7PF -----Original Message----- -- Dennis Tillman W7PF TekScopes Moderator |
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On Mon, Apr 29, 2019 at 1:40 PM Dennis Tillman W7PF <dennis@...>
wrote: Hi Albert,You misstated the sped of light in vacuum, which is 299792458m/s per Wikipedia, whereas your doc states "The speed of light in a vacuum is 224844343.5m/Sec = 0.2248m/nSec.", which is a 25% error :). |
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A follow up bit of information which may matter with the SG503 cable:
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Within Tek it came as a surprise to at least one very senior engineer I know when he discovered that BNC connectors were NOT 50ohms. For an interesting explanation on how we arrive at 50 and 75 ohms for common cables read the paragraph titles "Choice of Impedance". In a few special cases where the need for exactly 50ohms was necessary Tek had their own connectors made. The difference is so slight in mechanical dimensions that you would never notice it, but Tek was a stickler for details, and this detail mattered when you are designing high performance equipment to laboratory standards. I was not surprised when the Pomona cable I tested showed up on the TDR as having an impedance of 50.6ohms. I expected it to measure 51 ohms before I tested it. I check every cable I get at hamfests or swap meets when I bring them home on a TDR before I will add them to my pile of coax cables. At first I was always puzzled by the random variation in impedance on coax I saw on the TDR within the same type of common cable designators. There are several variations of RG-58 for example: Belden makes RG58 in 50 52, and 53.5 ohm versions. The make RG58A/U in 54 and 50 ohm versions. RG58, RG58A/U, RG58C/U are common but other RG58 versions also exist. With this much variation in cable impedance it is to be expected that the connector manufacturers will come up with versions of their BNC to match almost any impedance close to 50ohms. That is the problem Tek had that forced them to do their own BNCs for special situations. The hardest part of doing this evaluation was getting to the absolute minimum Rho with the adapters and terminators I had to use. Almost all the measurements were within 5mRho -10mRho. That is approaching the "noise level" for the 7S12 which is just under 1mRho. Another thing to watch out for are the transitions in the connector. The last photo shows what happens to the signal when it traverses a poor quality adapter. The resulting swing indicated a very large shunt capacitance facing the signal before it reaches the end of its journey. Moral of this part of my results: Don't rely on unknown quality adapters. Also, buy terminators from reputable companies you trust, and if you have a TDR use it to check the parts you buy. Dennis Tillman W7PF -----Original Message----- -- Dennis Tillman W7PF TekScopes Moderator |
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Dennis,
You have an error in the free space velocity which Albert mentioned indirectly. Multiple reflection in the cable produces one or more poles in the transfer function. The SG503 may compensate for reflections from the ends of a 3 ft cable. However, something as basic as physical robustness of the cable may be the primary factor. Just making wild guesses. I'm not at all familiar with the SG503, but I have been doing a lot of TDR of cables and connectors with my 11801. I see a lot of not quite 50 ohm cables from reputable vendors. It's a bit disappointing. It also leaves the question of how close are the connectors to 50 ohms and over what BW. I suspect that the cable with the reflection in the middle was partially crushed or kinked at some point. I'd really like to see the no cable case at a faster sweep rate, say no more than 100 ps/div. You can see some of what I've been doing here: 170 in particular shows the reflection from a BNC-F to BNC-M connection between two cables which otherwise match. I'm fairly certain they came from the same spool of cable. The two cables are BNC to SMA jumpers a friend made up for me when I was singing the "Chinese Connector Blues". Have Fun! Reg |
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I know the length, and from that can calculate the velocity factor. Or
rather, let the 1502b calculate the factor for me. On 29/04/2019, Merchison Burke via Groups.Io <merchison@...> wrote: You need to get the specifications (using the part number on the cable) |
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On 4/29/19 1:40 PM, Dennis Tillman W7PF wrote:
This raises another question which none of us may ever have the answer to: What are the unique qualities the Tek SG503 designer wanted this cable to have to be the ideal match to the SG503. It may be something as silly, in retrospect, as the stiffness of the cable. Maybe he/she like stiff cables. Why is it exactly 36in? Maybe he/she had a scrap piece of stiff coax exactly that length in his drawer and he/she said "Use this" to the production people when they asked what kind of cable should ship with the SG503. Meanwhile we are all guessing at whether using this cable with an SG503 is really important. Dennis,Thank you so much for this wonderful informative tests and discussion of these cables. And it would be interesting to kn ow what motivated Tek to build this particular cable to this spec, while wondering what part of the spec, or unwritten assumptions about the ultimate cable assembly's properties were important. I blanched at the cost of the real thing, and under advice from one of our members, bought good BNC (not inexpensive) connectors, the rg223 cable, and built my own. But not having a TDR or any other way to test the result, I don't know how good mine are. Thanks again, john |
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