How do I find out the capacitance of a BNC connector channel on an oscilloscope? - Page 1
Forum section: electronics / beginners

author: Bubble-Flow
author_rank: Newbie
timestamp: April 29, 2021, 11:53:02 am
content: I am using a piezo hydrophone connected to an oscilloscope. In order to convert the signal displayed in volt on the oscilloscope to bar, the manufacturer has provided the formula on the attached pic.According to the explanation it should be either 13 or 50 but on the tektronix tds 210 I am using, nothing is stated. Nowhere on the data sheet or manual can I find the capacity of the BNC connector either. Where do I get this information from or is this not relevant and I am somehow not understanding the formula. Please help!Thanks for reading!

author: CaptDon
author_rank: Super Contributor
timestamp: April 29, 2021, 01:14:11 pm
content: Most Tek stuff is marked right at the connector such as 10m/15pf meaning 10 megaohms shunted by 15 picofarads. The manual should most definitely state the input impedance. Some of those little handy scopes have probes attached that can't be removed and even those stated the resistance and capacitance right on the probe.Best wishes. Look at the page where the vertical channel frequency response and sensitivity settings are listed.

author: mindcrime
author_rank: Unknown
timestamp: April 29, 2021, 03:10:57 pm
content: I was confused as hell by this at first. So is "capacity" a synonym for "capacitance" then? I've never heard "capacity" used that way before...

author: TimFox
author_rank: Super Contributor
timestamp: April 29, 2021, 03:47:23 pm
content: In this context, “capacity” is a mistake. Tho only English technical term forCin the equation is “capacitance”, measured in Farads. “Capacity” is a perfectly good word for other concepts.

author: ebastler
author_rank: Super Contributor
timestamp: April 29, 2021, 03:53:46 pm
content: Quote from: mindcrime on April 29, 2021, 03:10:57 pmProbably just a translation error. Is that a German hydrophone by any chance? In German, the same word (Kapazität) is used for both, capacity and capacitance.

author: mindcrime
author_rank: Unknown
timestamp: April 29, 2021, 04:30:06 pm
content: Quote from: ebastler on April 29, 2021, 03:53:46 pmAh, that would make sense. Had me going there for a sec, because when I think "capacity" in an electrical / electronics context, my mind immediately goes to "current carrying capacity" which would be something quite different!

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 29, 2021, 10:40:08 pm
content: Here is a screen-shot from TDS200 series User Manual (Appendix A. Specifications):

author: David Hess
author_rank: Super Contributor
timestamp: April 29, 2021, 10:49:55 pm
content: You can also measure it with a normalizer and square wave source. Put a 1 megohm resistor in series with the input and a small 10 to 20 picofarad variable capacitor in parallel with the resistor. When driven with a low frequency square wave source, adjust the variable capacitor for a square waveform. Now the variable capacitor has the same value as the input capacitance and can be easily measured.

author: Bubble-Flow
author_rank: Newbie
timestamp: May 26, 2021, 01:34:00 pm
content: Quote from: Vovk_Z on April 29, 2021, 10:40:08 pmIs 20 pF the capacitance of the BNC connector?I am not sure becacuse I have a found a universal manual for the TDS 200 series, which states that the input capacitance on 10x and 1x position are 14,5 and 80 pF respectively. I have attached a scrrenshot from the maual. I am a bit confused. Which information is the recorder input capacitance?Also how do I know which position I am using? 10x or 1x? Is this a parameter in the settings I have to adjust?

author: Bubble-Flow
author_rank: Newbie
timestamp: May 26, 2021, 01:39:49 pm
content: Quote from: mindcrime on April 29, 2021, 03:10:57 pmQuote from: TimFox on April 29, 2021, 03:47:23 pmYes, apparently they mean capacitance by "capacity" here. Thanks for mentioning it. I was also confused and couldn't find any information about "capacity" anywhere. I corrected the title of the question.Quote from: ebastler on April 29, 2021, 03:53:46 pmYes, the hydrophone is by a german manufacturer. That's probably why there is a mistranslation in the manual!

author: Bubble-Flow
author_rank: Newbie
timestamp: May 26, 2021, 03:16:39 pm
content: Quote from: Bubble-Flow on May 26, 2021, 01:34:00 pmNevermind! I Just figured out I had posted the specs of the P2100 probe not the actual oscilloscope. 20 pF is correct!Thanks everyone

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 26, 2021, 07:22:28 pm
content: Quote from: Bubble-Flow on May 26, 2021, 01:34:00 pm1. 14.5pF is the property of a probe. It is clearly stated there.It means that for an external circuit a probe is like a 14.5 pF capacitor.Quote from: Bubble-Flow on May 26, 2021, 01:34:00 pmI am a bit confused here too :-). You switch it to either 10:1 (the most used) or 1:1. Nobody other. If somebody other works with it - you can look at a probe switch and find in which position it is.PS: we compensate the probe only in 10:1 position. There is nothing to compensate in 1:1.