Can I use a 50ohm scope as 1mohm by adding a series 1mohm resistor to the input? - Page 1
Forum section: electronics / beginners

author: Eth7an
author_rank: Contributor
timestamp: July 04, 2019, 02:27:53 pm
content: I just have a really nice 50ohm scope and I want to use it for general purpose.

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2019, 02:39:15 pm
content: Quote from: Eth7an on July 04, 2019, 02:27:53 pmIf you are asking this question, you are probably in danger of damaging your scope - and that sounds like it would be a shame.General purpose 1Mohm//15pF scopes are protected againstsomehigh voltages. Typically 50ohm ones aren't, and are fragile: just look at the legend by the input.I suggest you understand the info athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Oh, yes. To answer yourspecificquestion, you could put 1Mohm inserieswith the input to make it more "general purpose". But the 1Mohm and 50ohms will be a potential divider that will attenuate the input by a factor of 20000!

author: ArthurDent
author_rank: Super Contributor
timestamp: July 04, 2019, 03:21:00 pm
content: The previous post is a good answer but have you read the scope manual? It would strike me strange if a general purpose scope didn't have a way to switch the input impedance between 50 and 1M ohm.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 04, 2019, 03:36:57 pm
content: Some high BW scopes have dedicated 50 Ohms inputs. Beyond some 500 MHz the normal 1 M|| 20 pF input is no longer high impedance, because of the capacitance. In addition cable reflections get more and more important, so that passive probes are about limited to some 300-500 MHz.A dedicated 50 Ohms input can be lower noise than a high impedance input and is also easier to build for high BW.So some scopes made for high frequency skip on the high impedance input. If high impedance is needed, one adds an active probe.Instead of an expensive active probe one may as well by a cheap 200 MHz DSO instead as an alternative instrument.

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2019, 04:25:08 pm
content: Quote from: Kleinstein on July 04, 2019, 03:36:57 pmThat would be preferable for several reasons: cost (especially for >1 probe), robustness, diversity.As the OP is a beginner, I wouldn't suggest a cheap old analogue scope might be sufficient for their requirements.If the OP mentions the type of his existing scope, we may be able to offer more insights.

author: bdunham7
author_rank: Super Contributor
timestamp: July 04, 2019, 04:34:57 pm
content: Quote from: Eth7an on July 04, 2019, 02:27:53 pmWhat scope? A dedicated 50R scope is a pretty specialized instrument.The general answer is no, you need specialized probes and they won't be typical high impedance. Active FET and attenuated probes like the Tek P6156 are what you need. The Tek P6156 will have a 5K impedance with the 100X attenuator, if you can find one.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2019, 04:35:55 pm
content: Quote from: tggzzz on July 04, 2019, 02:39:15 pmThat is what I was going to say. Adding a series resistor to make the input impedance 1 megohm essentially produces a low-z x20,000 probe.Quote from: Kleinstein on July 04, 2019, 03:36:57 pmThere are external impedance buffers available to convert a 50 ohm input into a 1 megohm input with its associated input capacitance specification. Some active probes have 1 megohm inputs and can be used to do the same thing. These are more often used with RF instruments like spectrum analyzers so that passive high impedance probes can be used with them.https://www.mathews-engineering.com/store/p1/High_Impedance_Buffer_Amplifier.htmlhttps://www.tek.com/datasheet/tca-1meghttps://teledynelecroy.com/probes/differential-amplifiers

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2019, 05:24:14 pm
content: Quote from: Eth7an on July 04, 2019, 02:27:53 pmTo stop wild guesswork, it would be really helpful if you could post the make and model of the scope.EDIT:Ah, I found it in your first post in April... It's a Tek 7104 with a pair of 7A29 1GHz plugins! In that case, your simplest (and probably cheapest option) would be to find a 2 channel, lower frequency plugin with standard 1Meg inputs.

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2019, 05:34:40 pm
content: Quote from: Gyro on July 04, 2019, 05:24:14 pmSigh.The OP would benefit directly from readinghttps://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/The rest of us would benefit indirectly.

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2019, 05:48:37 pm
content: I'm searching from the UK, so I can't see what the cheapest price + shipping would be for you but a 7A26 might be a reasonable bet (assuming that the rest of the scope is working ok)...https://www.ebay.com/itm/Tektronix-7A26-Dual-Trace-Amplifier/222980140441If not, tell us what sort of thing you're hoping to do with the scope - it might be that you would be better selling that one and buying something more appropriate.

author: Eth7an
author_rank: Contributor
timestamp: July 05, 2019, 01:54:34 am
content: Thank you! That's a great idea!

author: tggzzz
author_rank: Super Contributor
timestamp: July 05, 2019, 06:12:56 am
content: Quote from: Eth7an on July 05, 2019, 01:54:34 amWhat's a great idea? Quite a few suggestions have been made.It helps everyone if you use the "quote" button not the "reply" button"

author: radiolistener
author_rank: Super Contributor
timestamp: July 05, 2019, 07:06:59 am
content: Quote from: Eth7an on July 04, 2019, 02:27:53 pmShort answer - No. You cannot use it in such way.