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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: 2465B Parametric Freq Measurement Error
Chuck Harris
Hi John,
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Best would be to swap the trigger hybrid from one of your 2465 family scopes that does work into the one that doesn't work, as a test. (as long as you restore the original to the calibrated scope, there will be no affect on calibration.) The trigger hybrid is crucial to everything in the parametric functions. An additional place to look would be to follow the signal from the DAC to the trigger hybrid threshold inputs (TLA, TLB). The 4051's and the TL072's and their associated sample and hold gate parts go bad sometimes (U2521, U2520, U2420, C2421, C2621). One repair/calibration blurs into the next. If you think I didn't repair something correctly, send it back, and I will fix it gratis. I know I tested the parametric functions on the unit I calibrated. It is a standard part of the tektronix calibration routines. -Chuck Harris John Ferguson via Groups.Io wrote:
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Re: 475A with High +110 rail
With 134v on Q1496 collector, you should see +61v at Q1490's base. This will completely shut down the regulator by reverse biasing Q1494 b-e. That said it only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483 would have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible, but not likely to cause these symptoms), leaving a leaky or shorted Q1496 or CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently fail. Also, check that CR1489 has not been installed backward.
Please forgive the caps, not shouting, just no other way to highlight for emphasis. Dave |
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Re: 2465B Parametric Freq Measurement Error
Hi Chuck,
The parametric functions do not work on one of my 2445Bs, the one you didn't calibrate.? It also doesn't sync on signal on start-up but does work well manually.? I chased this around? on EVblog a few years back, but never arrived at what to look at. Is it possible that cleaning up the contacts and gently re-seating a hybrid could resolve this? Would it make sense to buy a replacement hybrid just in case the clean-up doesn't produce the desired result? best regards john |
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2465 repair attempt with (horizontal?) issues, help needed
Hi all,
I got (yet another for my collection) 2465, it was stated it had problems before I got it, so no problem there. What have I done so far: Recapped the Power Supply, cleaned the Siemens fan and powered the scope up. Power supply is OK, all voltages are well within range in terms of value and ripple, set the reference voltage for 10V. Control DAC and reference are checked and set for a 2.5V range. After Power-On I have a straight trace, not really focussed perfectly, but more interesting: I got no proper readout, I have two blurry lines at the top and bottom of the screen. When I apply a calibration signal from the scope itself when on Auto Lvl the triggered light goes on, but the image looks like if it is not triggered, because I see just two horizontal lines. They seem to be the high and low level of the calibration signal. When I switch on the Voltages cursors they seem OK, a little vague, but the respond to the delta-settings knobs, but when I switch to the vertical (Time Cursors) I see some vague light in the tube as if the beam is hitting the sides? The trace stays where it is. Now the weird thing: as I am looking for causes, I tried the following: When I put the scope on X-Y mode using Ch2 I get a line instead of a dot?? When I check the outputs of the U800 I have clean static DC voltages (cursors and readout switched off) approx. between 20 and 60V, depending on the Ch1 position, so that should be a dot, but it is a horizontal line......???? Vertical position of this line (that should be a dot) is nicely controlled by the Ch2 position. When not in X-Y mode, the horizontal trace appears to be way to wide as well, so it looks like something other that the U800 is causing a horizontal deflection? Any ideas where this line / horizontal deflection is coming from? It's a line, so it should be some changing signal, but now I could use some tips and tricks. Thank and un saludo, Leo I |
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Re: 2465B Parametric Freq Measurement Error
Hi Chuck,
Exactly!Nope. I found others for sale, but these were cheaper. Tom
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Re: 2465B Parametric Freq Measurement Error
Chuck Harris
Exactly!
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Have you tried their product? -Chuck Harris Tom B wrote: Hi Chuck,I use one of the red plastic nutstarters that would be familiar to oldLike this: |
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Re: 475A with High +110 rail
tom jobe
On the 474A L.V. schematic it shows the expected voltage at the emitter of Q1496 as being 135.6 volts, which is quite close to your 134.66 reading at your +110 test point.
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It shows the expected voltage at the +110 test point as being 111 volts with only R1488 and Q1496 directly between the +110 test point and the expected 135.6 volts at the emitter of Q1496. Maybe you can find the problem by probing around right in that area? tom jobe... On 4/28/2019 10:00 PM, Mlynch001 wrote:
I have searched for a previous thread that was referred to in a post about a "high" 110 voltage rail on a 475 from back in 2012. I failed to find that post or any other applicable answer, perhaps I am not searching for the correct words? So I will ask for some help. I have a 475A with DM44 s/n B012228. After a thorough cleaning, I turned it on and find that for the most part it works great, no triggering issues, bright trace, all controls respond as expected, just dirty pots and switches. I supply various signals to the A and B channels and the scope displays a good trace with with amplitude matching the output of my signal generator, so calibration seems to be close enough for hobby work. The scope simply does not act as if there is anything wrong. For safety sake, I pulled the case and tested the various system voltages on the A9 interface voltage test points. I found everything to be spot on, except the +110 volt rail, Voltages as follows: |
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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: 2465B Parametric Freq Measurement Error
Chuck Harris
All members of the 2465 family use interchangeable trigger hybrids.
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To remove them, remove the four nuts at the corners of the module. Tektronix considered them all to be "selected"... which I think meant they tried them to see if they would work. the "-0X" suffix meant changes in production usually... stuff like Tek vs Maxtek, or this or that revision on their fabrication line... I would suggest that your first attempted fix be to remove the module, and clean the gold contacts with a Q-tip and IPA. When you put the module back, seat it with finger pressure, and hold it there as you finger tighten a pair of nuts on diagonal corners. Then install the other pair of nuts. Tighten the nuts 1/4 turn past where they just make contact. The tightness doesn't make or improve electrical contact, so just 1/4 turn past the nut touching the hybrid when it is touching the PCB. I use one of the red plastic nutstarters that would be familiar to old guys that built Heathkits. I wish I could find them new somewhere, but I have enough to last my lifetime of used. If cleaning the existing hybrid's contacts makes no improvement, then swap in a hybrid from a working scope, and that should fix the problem. -Chuck Harris Tony Fleming wrote: Is my Tektronix 2465 DMS using the same part number or do I need a |
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Re: 2465B Parametric Freq Measurement Error
Chuck Harris
of course, but only cal03.
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-Chuck Harris joe@... wrote: If I replace the trigger hybrid, do I have to do the whole calibration again? Joseph Suste |
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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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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Re: My TDR Evaluation of the SG503 012-0482-00 Cable has been uploaded
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.
Most remarkable is the slow speed of light in vacuum in the USA ;-) Albert |
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