EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: meballar on November 27, 2022, 05:07:23 am
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We had a Tek 1502 time domain reflectometer (TDR) in the shop. I had little experience with it, but I soon found out how incredibly useful it was. I could shoot the RF cable at the base of these monstrous LPA antennas and see all the impedance transitions through the 10 connections in the 105' path.
The biggest benefit I found with the TDR is its ability to clearly display sections of water contamination and degraded connections. I don't have any other TDR-type equipment (I read that the Fieldfox has an optional TDR function, but mine only has the CAT). I also have an Anritsu S332D, and like the Distance-to-fault/VSWR function, which I used frequently, along with the TDR.
I encountered some DC voltage when I connected to a wrong port (which had the B+ running through the cable to power the antenna amplifier), which I assume smoked the "unobtainium" tunnel diodes. I think to fix that I'll have to gamble on some of the 1502's on eBay advertised as not working, and hope that there are some salvageable parts.
So I bought a 1503 TDR off eBay, but have found that it is nowhere-even-close to being as useful as my 1502. I don't know the condition of the eBay 1503, but I ran the 1503 diagnostics and it had no errors. The pulses at the high and low impedance parts of a test cable look like what's depicted in the user manual-all I see displayed is an upclock (open) that drops back down to the middle, or downclock (short) that rises back up to the middle .
This is nothing like the display of the 1502's "fingerprint" showing all the impedance deviations (such as the aforementioned LPA antenna sections) within a short distance. When there's an open, the 1502 shows a steep upclock and a high plateau. When there's a short, the 1502 shows a steep downclock and low plateau. But other than those extremes, the 1502 tells a great story: I see a variable impedance over time, indicative of the impedance changes as the cable transitions to the antenna hardware (load coils, connectors, changes in cable impedance (50 ohm cable, to metal airline, to 93 ohm cable, etc.).
Does this huge difference between the displays I'm getting from my 1502 and 1503 suggest that the 1503 has a problem? Would it be a correct statement that the 1503 is more suited for just looking for shorts and opens in a cable path? Thanks for any insights.
Mike
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The 1503B is for longer cable lengths and non 50 ohm impedances. It has 10 times less resolution. Attached a document describing the differences.
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I encountered some DC voltage when I connected to a wrong port (which had the B+ running through the cable to power the antenna amplifier), which I assume smoked the "unobtainium" tunnel diodes. I think to fix that I'll have to gamble on some of the 1502's on eBay advertised as not working, and hope that there are some salvageable parts.
The 1502s are pleasing instruments, aren't they :)
The TD is even vulnerable to static in the cables; that's why the BNC connector has a unique shorting bar to discharge the static before the core is connected.
I have seen some suggestions that the TD could be replaced by something like the guts of Leo Bodnar's pulse generator http://www.leobodnar.com/shop/index.php?main_page=index&cPath=124 (http://www.leobodnar.com/shop/index.php?main_page=index&cPath=124) but I haven't seen anybody actually try it.
I have just obtained two non-functional 1502s. With one there's no display and the intensity control shaft is snapped, but ISTR the TD is working. I have choices:
- use it as a parts mule, which might include selling the TD and launch assembly
- find a replacement for the intensity potentiometer
I suspect I won't be able to decide to do the first within a timescale acceptable to you. But if you do get a parts mule, I'm interested in buying that potentiometer.
You could also play with a NanoVNA. It uses an inverse FFT calculation to determine the time domain response.
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[attach=1]Hello -Wim-,
That is great information, thank you. I did not know anything about the 1503, other than there was an eBay seller that had one for sale, with a picture showing an LMR600 cable plugged into it, and it had the correct vF set up. So it gave me enough confidence that it was working well enough to buy it.
But what a letdown that Tektronix 1503 didn't have the 1502's capability to microscopically look at the RF path! I attached a word doc with a pic of a healthy LPA antenna and pics of degrading issues. Hopefully it will load, since there's no indication that it did, other than selecting the file.
Thanks for your comments.
Mike
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But if you do get a parts mule, I'm interested in buying that potentiometer.
I am interested in buying a display module from a parts mule as my display is turning dark... :D
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But if you do get a parts mule, I'm interested in buying that potentiometer.
I am interested in buying a display module from a parts mule as my display is turning dark... :D
It is a CRT device, so there is no "display module" per se. There are several possible causes, only one of which is a faulty CRT.
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But if you do get a parts mule, I'm interested in buying that potentiometer.
I am interested in buying a display module from a parts mule as my display is turning dark... :D
It is a CRT device, so there is no "display module" per se. There are several possible causes, only one of which is a faulty CRT.
That is not the case for the 150xB and C versions, only the original 1502 uses a CRT. My unit is the B version, and from what I have found online, B and C versions for both 1502 and 1503 use the same display module.
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But if you do get a parts mule, I'm interested in buying that potentiometer.
I am interested in buying a display module from a parts mule as my display is turning dark... :D
It is a CRT device, so there is no "display module" per se. There are several possible causes, only one of which is a faulty CRT.
That is not the case for the 150xB and C versions, only the original 1502 uses a CRT. My unit is the B version, and from what I have found online, B and C versions for both 1502 and 1503 use the same display module.
Indeed, but you have only mentioned the 1502, not the 1502B.
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There was some experience that the TD in the 1502 could be replaced by a Russian TD (actually, there are 2, but the big unobtanium one is the one that gets fried). I had a lead on some of these but then the pandemic hit and everything went to hell. They were hard to find back then and, now of course, are completely impossible. Maybe someone in eastern Europe has some on the shelf, but the last quote I got was around $100 USD each.
It's a very handy device. The biggest problem I've had is with the self discharge of the battery pack. One of these days I'm going to modify it with an on/'off switch on the back of the pack.
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There was some experience that the TD in the 1502 could be replaced by a Russian TD (actually, there are 2, but the big unobtanium one is the one that gets fried). I had a lead on some of these but then the pandemic hit and everything went to hell. They were hard to find back then and, now of course, are completely impossible. Maybe someone in eastern Europe has some on the shelf, but the last quote I got was around $100 USD each.
It's a very handy device. The biggest problem I've had is with the self discharge of the battery pack. One of these days I'm going to modify it with an on/'off switch on the back of the pack.
Two easy alternatives: remove the pack, or remove the fuse from the pack. I do the former.
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Indeed, but you have only mentioned the 1502, not the 1502B.
I know, it was not very clear, but I think both the OP and me have a B-version (if I see the doc the OP posted), but I should have specified more clearly.
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Indeed, but you have only mentioned the 1502, not the 1502B.
I know, it was not very clear, but I think both the OP and me have a B-version (if I see the doc the OP posted), but I should have specified more clearly.
20:20 hindsight is a wonderful thing :)
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Hello,
A related topic. I have an 1502C with the Chart recorder. I see that the chart recorder is connected through a DB25. I wonder if i can connect the pcb directly through the DB25 (maybe with a DB25 to usb adapter) and get the print outs directly to the computer. That would be really helpful!
Has anyone tried this?
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Hello,
A related topic. I have an 1502C with the Chart recorder. I see that the chart recorder is connected through a DB25. I wonder if i can connect the pcb directly through the DB25 (maybe with a DB25 to usb adapter) and get the print outs directly to the computer. That would be really helpful!
Has anyone tried this?
There exists an SP232 module that fits into the 1502B/C unit (they are quite rare on Ebay however). This allows the computer to serially communicate with the 1502B/C. There are of course plenty of serial to USB converters available. Attached is the schematic, firmware image and user manual of the SP232 card. So it should be possible to build your own and write a computer interface software to display and control the 1502B/C. I once planned to do so, but got sidetracked with other projects.
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Attached a user manual of a software package where the TDR is used to monitor water content in the soil using such an SP232 module (to show what is possible)
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And the actual software package should it be of interest to anyone
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This is the Tektronix software that came originally together with an SP232 module.
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There exists an SP232 module that fits into the 1502B/C unit (they are quite rare on Ebay however). This allows the computer to serially communicate with the 1502B/C. There are of course plenty of serial to USB converters available. Attached is the schematic, firmware image and user manual of the SP232 card. So it should be possible to build your own and write a computer interface software to display and control the 1502B/C. I once planned to do so, but got sidetracked with other projects.
Oh this is great information! In particular the SP232 manual. I think it gives enough information to set the communication. Thank you so so much. I will work on this and see what i get.
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Oh this is great information! In particular the SP232 manual. I think it gives enough information to set the communication. Thank you so so much. I will work on this and see what i get.
Just a word of warning, if you are contemplating to wire a microcontroller directly to the parallel port without extra hardware to make a USB interface, some of the pulses put out by the 1502B/C are too fast to reliable catch (even with a hardware interrupt). I tried with an ESP32 and a STM32 blackpil, both were too slow to work reliable. Attached the Arduino code I had written for this, but after I realized I really needed the 82C55 to make it work, I lost interest in the project.
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I was definitely considering to use an Arduino to interface with the unit.
I've been reading the SP232 module user manual to understand how it works but it didn't occur to me that the arduino wasn't going to be fast enough.
This might be more complicated that I thought.
Thanks for sharing the arduino code. I will review it later.
Maybe it's a case for a Raspberry Pi... need to look into this. Might be an interesting project.
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I was definitely considering to use an Arduino to interface with the unit.
I've been reading the SP232 module user manual to understand how it works but it didn't occur to me that the arduino wasn't going to be fast enough.
This might be more complicated that I thought.
Thanks for sharing the arduino code. I will review it later.
Maybe it's a case for a Raspberry Pi... need to look into this. Might be an interesting project.
Yes, this also caught me by surprise. The mean reason was (if I remember correctly) that the strobe/ack (RD/WR) pulses were very short (<1µs). An Arduino together with an 82C55 interface chip will for sure be fast enough, as the strobe pulses are only used by the 82C55, and the Arduino only uses the bus. In that case even an Arduino Uno will be more than fast enough.
As to the Raspberry Pi, I would expect it to have a higher latency than an STM32 or ESP32, and it will also not be able to handle sub µs hardware interrupts also...
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Yes, this also caught me by surprise. The mean reason was (if I remember correctly) that the strobe/ack (RD/WR) pulses were very short (<1µs). An Arduino together with an 82C55 interface chip will for sure be fast enough, as the strobe pulses are only used by the 82C55, and the Arduino only uses the bus. In that case even an Arduino Uno will be more than fast enough.
As to the Raspberry Pi, I would expect it to have a higher latency than an STM32 or ESP32, and it will also not be able to handle sub µs hardware interrupts also...
I see.
I never used a 82C55 before. I will need to look into it.
Thanks for sharing all these comments. I'll let you know if i get somewhere with this.