TDS500 and TDS700 - SPC and FAS calibration - Page 1
Forum section: electronics / repair

author: Tantratron
author_rank: Frequent Contributor
timestamp: January 25, 2021, 06:33:44 pm
content: I've acquired lately a TDS784C which passes OK all SPC tests, so far so good.The only problem was the color CRT display which is failed and I'm trying to repair (another thread for those interested) so when I open the TDS, removing the blue cabinet I notice a strange thing.As some of you know, there has been a method to upgrade a TDS754C towards a TDS784C by slightly modifying the acquisition board. First changing the4 resistors so the software will choose 4GS/sthen second removing4 capacitors after the attenuator to have 1 GHz bandwidth.Ok what is strange, the 4 capacitors are installed so they should limit to 500 MHz and not offer 1 GHz analog bandwidth... furthermore the 4 resistors are in place to really choose TDS784C to offer 4GS/s instead of 2GS/s and honestly, I do not see any modification, trace of de-soldering reflow or any visual change so it seems legit done at the factory.Is it possible or a factory mistake by tektronix where this oscilloscope should have been a TDS754C and not a TDS784C ?It passes all the SPC self-test, do you know the concept of SPC, do they really check its maximum bandwidth performance ?In other words, say you hack or modify an acquisition board to have higher sampling, higher bandwidth, can we conclude that if SPC is PASS then no need for a full calibration ?Thank you, Albert

author: Nx-1997
author_rank: Regular Contributor
timestamp: January 26, 2021, 08:28:32 am
content: The SPC only corrects the offset present at low voltage ranges. To test the bandwidth you need a signal generator to measure the -3dB cutoff frequency. It is possible for SPC to pass and not have 1GHZ bandwidth.

author: Tantratron
author_rank: Frequent Contributor
timestamp: January 31, 2021, 09:13:41 am
content: Quote from: Nx-1997 on January 26, 2021, 08:28:32 amFirst please find attached 2 pictures showing respectively the ID jumpers setting (R1061 - R1062 - 1063 - 1064 set for 784C) and the CAPACITORS presence limiting the bandwidth (C1266 - C1267 - C1268 - C1269 set for 500MHz) on the TDS784C which I've purchased on eBay lately.I did remove the PSU cover which enables to see internally the model and part number of any TDSxxx which perfectly matches the P/N and S/N printed on the back end plate (GPIB, VGA, Option 13...). I confirm this because noticed in the past that some vendors would mix boards, metal plates, front panel so best way to see anycheatingis compare external S/N versus hidden PSU cabinet S/NVisually it is really hard to tell if the ID jumpers (4 resistors) were changed from TDS754C to TDS784C, I tend to say nope because no visible trace of flux, soldering or resin change. Which leads to my question or concern of why this acquisition board does leave the 4 capacitors known to limit the analog bandwidth to 500MHz.I do not have any GHz frequency generator neither knowledge, boards and equipment to run FAS for the moment.Would it be possible in theory that (C1266 - C1267 - C1268 - C1269) to be present but offers the 1GHz-3dB as hardware set from (R1061 - R1062 - 1063 - 1064) ?

author: Nx-1997
author_rank: Regular Contributor
timestamp: February 01, 2021, 04:46:47 am
content: See here:https://www.eevblog.com/forum/testgear/conversion-of-500mhz-tds744a-to-1ghz-tds784a/The capacitors physically limit the bandwidth, when I converted my tds754d to 784d (https://www.eevblog.com/forum/repair/conversion-tektronix-754d-to-784d-revision-4-acq-board/), removing the resistors was not enough I checked using a signal generator, the -3dB bandwidth was still at around 500MHz. I had to remove the capacitors to get the full bandwidth.I also had to remove capacitors when I converted my tds620b to tds684b (https://www.eevblog.com/forum/repair/(help)-conversion-tektronix-tds620b-to-tds680b-to-tds644btds684b/)So you have a 784c acquisition board with the capacitors still soldered, that is a mystery to me.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 01, 2021, 09:24:25 am
content: Quote from: Nx-1997 on February 01, 2021, 04:46:47 amThanks for the previous links describing the hack to convert from 754C to 784D, I already knew these as mentioned on my first post which is why I notice a contradiction (capacitors present versus resistor set).Either the vendor really was super smart by changing the resistors ID (I see no clear soldering change) then glued a TDS784C banner instead of TDS754C or Tektronix at manufacturing made a error...For the moment, I do not have 1GHz and above signal generator so cannot check the 3dB bandwidth. However do you know if the 1KHz output calibrator in the front panel offers to be very steep or fast rise to check my scope (I do have a P6243 active probe) ?What is really a problem, I do not have FAS equipment so I'm worried to find out if the calibration constant (EEPROM) were properly done... unless for some reason tekronix acquisition board could pass sometimes with the 4 capacitors installed the 1GHz complete test.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 01, 2021, 12:15:17 pm
content: Google for "fast pulser". It's not overly difficult to build a generator with a fast rising edge. If the edge is fast enough you should see a clear difference when the BW is limited to 500MHz vs 1GHz.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 01, 2021, 12:51:29 pm
content: Thanks@Ice-teawhere you give me an idea since i've two PG506. Its fast-rise output could do the job, rise time claimed to be less than 1ns for a 50% duty cycle.Another question comes to mind, if we take a sneaker vendor knowing the Acquisition board to be 500MHz (TDS754C or TDS540C so the 4 capacitors are there) but he manipulates the 4resistors to increase the sampling frequency from 2GS/s to 4GS/s. The firmware self-detects only the resistors to declare 1GHz or 500MHz (TDS784C or TDS754C) with no knowledge of the real analog bandwidth, no way to tell if the capacitors were removed or not. Can we consider safe and accurate from a calibration point of view that I revert or re-solder the resistors to declare 500MHz bandwidth ?Thanks, Albert

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 14, 2021, 10:13:20 am
content: I've been using again my old PG506 pulse generator, the fast rise output which was connected to my TDS794D then on my TDS540C and this crazy TDS784C (probably a TDS754C with cheating on ID resistor to pretend 1GHz bandwidth). My PG506 fast rise positive output works (less than 1ns) but its negative fast rise output is quite slow. I need to buy or find a higher range pulse and/or frequency generator because I know my TDS794D is proper calibrated so if it concludes to 1ns from my PG506, this is not enough to conclude.In theory 1ns rise time kind of call of a max 350 MHz bandwidth (Tr = 0.35/B) where both my good TDS540C, my good TDS794D and the wierd TDS784C show 1ns on their display. I've then tested pure sine using my old SG503, again the three oscilloscope shows a nice 270MHz which means impossible to tackle the 1GHz or 2GHz to really test the issue.I do have another technical and theoretical question... let's suppose you have a TDS540C or TDS754C good calibrated acquisition A10 board. If one cheats setting the ID resistor so the ASIC sampling goes to 4GS/s instead of 2GS/s but leaving the 4 limiting bandwidth capacitor (we keep 500 MHz). Do you know if the calibration remains correct so in other words, we just have a higher sampling rate while keeping the 500MHz oscilloscope bandwidth with no need to re-calibrate ?Albert

author: Jwalling
author_rank: Unknown
timestamp: February 14, 2021, 11:32:53 am
content: Quote from: Tantratron on February 14, 2021, 10:13:20 amI would recommend these:http://www.leobodnar.com/shop/index.php?main_page=index&cPath=124&zenid=67d747578742cf66182b35de03b147b0

author: madao
author_rank: Frequent Contributor
timestamp: February 14, 2021, 11:58:24 am
content: Yes, use leo bodnar pulser.Result with 1Ghz modell should look be like first attachment.Bodnar-Pulser is really fast. verified with fastest o'scope in my hand: 35psImportant: you must enable 50ohm terminator of o'scope, otherwise bad risetime and much overshoot.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 14, 2021, 01:28:30 pm
content: Thanks for letting me know of Leo Bodnar fast rise kit, that is very interesting for my lab.It seems ton only work with Windows but I only have Macintosh. However do you know if Leo's kit has standard fast pulse generation if say connected only to a USB plug or USB charger ?Say I buy one Leo Bodnar kit, I connect to my MacBook Air to get USB power only, what will be the default fast rise speed and amplitude since there is no compatible driver with MacOS ?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 14, 2021, 01:34:31 pm
content: Hi,The pulser needs only power to operate. There is no need to install any drivers or even connect it to USB host.It ships with optimal settings so most users don't even need to ever run any applications.You can plug it into your Mac, scope USB port, phone charger or power bank.CheersLeoQuote from: Tantratron on February 14, 2021, 01:28:30 pm