Erroneous Leveled Sinewave Requirements in Tektronix TDS/TBS Service Manuals - Page 1
Forum section: electronics / general

author: niconiconi
author_rank: Frequent Contributor
timestamp: January 29, 2022, 08:37:33 am
content: Recently I became interested in leveled sinewave generators, especially whether it's possible to reach the amplitude accuracy required with inexpensive circuitry and equipment to perform a full oscilloscope calibration.Then I discovered what appears to be a serious error in multiple Tektronix TDS/TBS service manuals. In the Tektronix TDS 200-Series Service Manual , page 49, it calls for a "Leveled Sinewave Generator, 50 kHz and 100 MHz,±0.6% amplitude accuracy". This just doesn't make any sense.1. From experience, most leveled sinewave generators' amplitude accuracy is a single-digit percentage, 0.6% is an order of magnitude smaller - an error no more than 0.05 dB in power is extremely narrow.2. It's self-contradictory. It suggested two systems in the same table, the Wavetek 9100 and the Fluke 5500A, both equipped with an oscilloscope calibration option. But if we open the datasheet of Fluke 5500A , we see the 1-year absolute uncertainty is only 4%, and even the short-term amplitude accuracy, measured "within one hour after reference amplitude setting", is just 1%.3. If you review the service manuals for some newer Tektronix oscilloscopes with similar or better performance, such as the 3 Series MDO  or the TDS3000C , they only require a leveled sinewave generator with "±4% amplitude accuracy".Thus, I must conclude that the specification is erroneous. Worse, the TBS1000B (2014), a much newer (but still based on the same old TDS platform) model than the TDS 210 (1997), repeated the same error . This error was eventually corrected to 3% in the TBS1000C manuals - the next generation after the TBS1000B . I can't believe this error remained in the Tektronix manuals across multiple product generations, undetected for 20 years! The extremely poor specsmanship here is just frustrating.The takehome message is, beware of typos in the datasheets... TDS 200-Series Digital Real-Time Oscilloscope Service Manualhttps://download.tek.com/manual/071049203.pdf 3 Series MDO Specifications and Performance Verificationhttps://download.tek.com/manual/3-Series-MSO-Specification-Performance-Verification-077149902.pdf TDS3000C Specifications and Performance Verificationhttps://download.tek.com/manual/TBS1000C-Specification-Performance-Verification-Manual-077158300.pdf TBS1000C Series Specification and Performance Verificationhttps://download.tek.com/manual/TBS1000C-Specification-Performance-Verification-Manual-077158300.pdf Oscilloscope Calibration Options for 55XX Series Multi-Product Calibratorshttps://download.flukecal.com/pub/literature/1265775G_w.pdf TBS1000B and TBS1000B-EDU Series Digital Storage Oscilloscopes Service Manualhttps://download.tek.com/manual/TBS1000B-TBS1000BEDU-Digital-Storage-Oscilloscopes-Service-Manual_EN-US.pdf

author: alm
author_rank: Super Contributor
timestamp: January 29, 2022, 12:00:02 pm
content: Good catch! Even if you assume they were talking about amplitude flatness instead of accuracy, neither the Fluke 5500 scope calibrator option nor the older Tek SG503 / SG5030 leveled sine wave generators have a flatness specification better than 1%.

author: niconiconi
author_rank: Frequent Contributor
timestamp: January 29, 2022, 01:29:21 pm
content: Quote from: alm on January 29, 2022, 12:00:02 pmI also found the leveled sinewave requirements in all service manuals, even the correct ones, are vague. Only a single number in percentage, "amplitude accuracy" is specified, and this is problematic - it makes the requirements much stricter without justifications.For example, TBS1000C, an entry-level oscilloscope (100 MHz & 200 MHz), specifies "3% amplitude accuracy" and suggests a Fluke 5500A-SC calibrator. But even with its top-of-the-line option, Fluke 5520A-SC1100, it has just an amplitude accuracy of 2% at reference, and 1.5% flatness at 100 MHz, this is already a 3.5% error.Similar problems exist for 3-series MDO's 4% requirements. 3-series MDO is a middle-level oscilloscope (1 GHz). Even when we use the Fluke 9500B officially recommended by the manual, equipped with the best, top-of-the-line 9530 output head, under the most ideal conditions (VSWR 1.2)... The error is 1.5% at reference and the flatness is 3.0% at 1 GHz, giving an error of 4.54%, 0.54% higher than the official "minimum requirement".If the calibration lab doesn't use the top-of-the-line option, the error can even be 0.5% or 1% more in addition to that.Another interesting observation is how the performance of leveled sinewave generators changed over time. A vintage Tektronix SG503 (1974, 250 kHz to 250 MHz) has 3% error at reference and 3% flatness at 100 MHz in all ranges, a total error of 6% (but it's only 4% at X1 range). Now take a look at the modern Fluke 5520A-SC1100, the reference accuracy is 1.5%, and at 100 MHz the flatness is 2%, so the absolute error is 3.5%.SG504 (1974, 200 MHz - 1 GHz) has 3% error at reference, and 4% flatness across the spectrum, thus the absolute error is 7.12%. For a Fluke 5520A-SC1100, at 1 GHz the flatness is 5% - the absolute error is still 7% - the more things change the more they stay the same. The top-notch Fluke 9500B is better, 3.0% flatness at 1 GHz, 1.2 VSWR, the error is 4.54%.Conclusions:1. For leveled sinewave generators, 3.5% to 4% amplitude accuracy at 100 MHz is top-notch. However, even 7% is already pretty good.2. The "minimum requirement" for leveled sinewave generators in oscilloscope service manuals is poorly specified and safe to ignore, read the datasheets of the recommended equipment instead. Further, if you're a hobbyist with a 20-year-old oscilloscope, and you happen to have access to a 7% sinewave generator, but the service manual calls for a 4% one, just ignore the requirements and do the calibration, your oscilloscope will probably still be better.

author: alm
author_rank: Super Contributor
timestamp: January 29, 2022, 03:41:14 pm
content: I would think that flatness, which affects the shape of the signal, is more important than absolute accuracy. The latter just affects amplitude measurements, while the former may affect distortion, rise time, etc.Quote from: niconiconi on January 29, 2022, 01:29:21 pmWhy are you comparing a generator that starts at 200 MHz to a 5520A-SC100 at 100 MHz? If you compare the companion SG503 (250 kHz - 250 MHz) the reference accuracy is 3% (in X1 range). Flatness up to 50 MHz is 1% relative to reference, 3% from 100 MHz - 250 MHz. So the absolute error (if you add them) is 4% up to 50 MHz and 6% up to 250 MHz. Can't say I'm too impressed by the progress Fluke/Wavetek made in ~40 years.I'm not convinced adding up the errors is the correct thing to do here. It's of course the most conservative way, but it would also make sense to treat them as standard deviations of independent probability distributions and calculate the total error as the sum of squares (per GUM). Then the total error would be substantially lower, but still much more than 0.9%, obviously.Quote from: niconiconi on January 29, 2022, 01:29:21 pmBased on what do you conclude that a twenty year old scope is likely to drift this much? An analog scope I can see, but my impression is that digital scopes barely drift within their quite limited specs. What drift figures have you observed?

author: niconiconi
author_rank: Frequent Contributor
timestamp: January 29, 2022, 07:08:45 pm
content: Quote from: alm on January 29, 2022, 03:41:14 pmI edited my comment just minutes after I originally posted it after I realized the SG503 comparisons were missing. But strangely an hour later you were still getting the old comment? Was the webpage somewhat cached?

author: 2N3055
author_rank: Super Contributor
timestamp: January 29, 2022, 08:35:22 pm
content: That might have been a typo and it was meant to be +-0.6 dB flatness...

author: 1audio
author_rank: Frequent Contributor
timestamp: January 30, 2022, 07:42:04 pm
content: Validating flatness can be really vexing. Up to 100 MHz you can use thermal converters but those have a best case accuracy of 1% + 1% uncertainty at 100 MHz. Above 100 MHz the uncertainties from impedance mismatch etc. really limit the accuracy possible. Better understood as uncertainty because of all the interacting elements (SWR, native instrument and sensor limitations etc.) it seems 2-3% is what you get between 100 MHz and 1 GHz. Worse above 1 GHz. I have been chasing this since I started trying to calibrate my calibrator. Thermal converters that meet spec but don't agree is the starting point.I have an older Tek 191 cal generator that goes to 100 MHz but the cable is then the limit since the leveling is internal. Also it seems to have a level variation even with its internal leveling. Later models have the external leveling head but those are not going to be better than the specs above. Tek suggests in the 191 manual to use a sampling scope to verify the output. Which suggests that the sampling head won't have accuracy limits. I'm sure its good but not that good.FYI 1% is approx .1 dB (voltage) .05 dB (power). At RF the discussion shifts to power usually in dB re 1 mW into 50 Ohms. Another opportunity for confusion.I wound up getting an HP432 + HP478 for the HF but the gap between 1 MHz and 10 MHz I don't have a good solution for yet.

author: niconiconi
author_rank: Frequent Contributor
timestamp: January 31, 2022, 09:29:27 am
content: Quote from: 1audio on January 30, 2022, 07:42:04 pmYour comment makes me wonder if I get it right again... Just to confirm: If a leveled sinewave calibrator is putting 1 Vrms into a matched 50 Ω RF meter and the datasheet says the calibrator is 1% accurate, an RF power meter will see a numerical difference of 10 x log10(1.03 ^ 2), or 0.08 dB, is it correct?

author: 1audio
author_rank: Frequent Contributor
timestamp: February 02, 2022, 01:41:59 am
content: Being lazy I use the on line calculators (https://www.pasternack.com/t-calculator-power-conv.aspx) for that but the arithmetic looks correct. However you do need a very good match for that to be accurate. At lower frequencies (10 MHz and below) its less important but it becomes extremely important for high accuracy at higher frequencies. A diode detector should respond to the peak voltage but they are not perfectly flat response. Same is true of all the other detectors as well to varying degrees

author: Zenwizard
author_rank: Regular Contributor
timestamp: February 05, 2022, 04:39:33 am
content: The unit to check the flatness of the leveled sign wave generators SG503 and SG504 that Tek Made was a 067-0625-00 Pk-Pk detector. This ran all the way up to 1.050 Ghz for the SG504. I have some videos of repair and calibration of the SG504 and SG503 on You Tube Linked here :https://www.youtube.com/c/ZenwizardStudios/videosI have the 191 503 and 504 in the lab from Tek. The 191 needs nothing special. The Sg503 needs a 1% bnc cable to reach its rated accuracy's. Part number 012-0482-00. The SG504 has a leveling head that is external.Zen

author: alm
author_rank: Super Contributor
timestamp: February 06, 2022, 01:52:35 pm
content: What is a "1% BNC cable"? Since the SG503 leveling happens at the BNC output connector, I would think you need a decent quality cable, and calibrate the generator with this particular cable. And only use the same cable, or one of the same material/length. I don't think 012-0482-00 is particularly special, it's just a good quality cable that supposedly matches what it has been adjusted to at some point in the past (assuming it was Tektronix doing the adjustment).

author: analog1
author_rank: Newbie
timestamp: February 12, 2022, 07:07:52 am
content: I managed a busy cal lab for many years. I used to purchase 012-0482-00 cables 24 pieces at a time to avoid frequent failures experienced from heavy usage during on-site calibration jobs. The failure mode was usually due to poor crimping of the BNC center conductors. Even new cables were problematic. Ditto for the cables supplied by Fluke for the 552x series scope optioned caibrators.