Need help to understand  oscilloscope input - Page 1
Forum section: electronics / beginners

author: peetDup
author_rank: Newbie
timestamp: July 13, 2020, 10:51:58 pm
content: Hi,I picked up an old analog oscilloscope (metrix OX712D), since it's my first scope i'm trying to learn how to use it and how it works.I found the schematics, and there is something that bother me : on both inputs there is a 51Ohm resistor which does not appear on the schematicsIMG_20200713_095722.jpg.What is the purpose of those resistors ?Thank you

author: TimFox
author_rank: Super Contributor
timestamp: July 13, 2020, 11:03:29 pm
content: Many modern oscilloscopes (both analog and digital) have a switchable 50 ohm resistor to terminate the input for use with coaxial cables. Perhaps the previous owner added them permanently for that purpose? The switch at the lower left is the normal switch to switch in a series capacitor for AC coupling, or short it out for DC coupling. Be careful about the 51 ohm resistor: the normal maximum voltage for your oscilloscope (5 V/cm times 8 cm or so) would fry that small resistor: 40 V into 50 ohms is 32 watts. 20 V is only 8 W, but the part you photographed is probably rated at 0.5 W.

author: peetDup
author_rank: Newbie
timestamp: July 13, 2020, 11:25:59 pm
content: That makes sense. But why 51ohm and not 50ohm?Since I have an 50ohm terminator, can I safely remove remove them ?Thank you

author: ArthurDent
author_rank: Super Contributor
timestamp: July 13, 2020, 11:38:55 pm
content: Quote from: peetDup on July 13, 2020, 11:25:59 pm51 ohms is a common value while 50 ohms is mainly available in precision resistors that are more expensive and harder to find. The difference is basically meaningless.If you're planning to use regular probes, the added resistors could be a big problem, especially with a X10 probe. You'd probably be better off removing the 51 ohm resistors.

author: cliffyk
author_rank: Frequent Contributor
timestamp: July 14, 2020, 03:29:31 am
content: Quote from: ArthurDent on July 13, 2020, 11:38:55 pm^This, 51 Ω is a standard value for ±5% and better tolerance resistors and are as stated quite commonly available. The resistors you have are ±2% tolerance so their actual value could be anywhere from 49.98 to 52.02 Ω.TimFox's caution re: power rating is spot on...

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 14, 2020, 04:43:33 am
content: The OP’s first photo link clearly shows the resistor is series, not in parallel with the bnc input pin. So, power dissipation, probe error, etc is all meaningless.Based on the schematic, it looks like an a owner modification. The red tubing used to insulate the lead - do you see that used elsewhere in the scope? If not, it supports the owner mod theory. The schematic does show R1, what appears to be a 10 ohm resistor in series with the input. But, that is after the ac/dc/gnd selector switch.

author: peetDup
author_rank: Newbie
timestamp: July 14, 2020, 08:21:01 am
content: I removed the resistors to match the schematics, everything seems to work. I'll take a closer look just in case there are other modsThank you all for your help

author: tggzzz
author_rank: Super Contributor
timestamp: July 14, 2020, 08:35:19 am
content: Quote from: ArthurDent on July 13, 2020, 11:38:55 pmOnly with a *10 "high" impedance probe (which isn't high at, say, 100MHz - do the calculation!).With a simple *10 resistive-divider probe (which has ahigherimpedance of 500ohms), then you would need the 50ohm termination at the scope.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2020, 11:37:09 am
content: Quote from: WattsThat on July 14, 2020, 04:43:33 amA physically small 51 ohm carbon composition resistor is often added in series directly after the BNC to suppress resonances and act as a fusible link.

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2020, 03:14:45 pm
content: Oops--that was my mistake about the 51 ohms shunting the input--I had a problem opening that photograph.With a 51 ohm resistor in series with the input, and < 20 pF input capacitance, that will limit the bandwidth to roughly 150 MHz, which is perfectly reasonable for such an oscilloscope, which I assume has a bandwidth of no more than 50 MHz.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 11:47:08 pm
content: Quote from: TimFox on July 15, 2020, 03:14:45 pmSome of the input capacitance is before the series resistor as part of the BNC connector.