Attenuators and scopes - Page 1
Forum section: electronics / projects

author: bitman
author_rank: Unknown
timestamp: August 09, 2020, 12:42:13 am
content: Hello - this is definitely a "noob" issue but I'm struggling to understand exactly what's going on. I bought a cheap attenuator on Ebay to allow me to use a more flexible probe attachment that doesn't offer any "x10" or similar (not sold as scope probes). I have a cheap Rigol DS1054Z scope which for my purposes is more than I need. The scope says there's a 1MOhm impedance on each channel which to my understanding means there's a voltage divider between the scope probe's built in and the scopes internal resistor. I was of the (wrong?) impression that the attenuator simply added a resistor in addition between the probe and the scope. Since the probes I want to use are "crocodile" type connectors that allows holding power, there's no resistor or anything in them (outside of the wire) - I simply assumed I could use the attenuator as a bit of safety catch and still allow me to set 5x on the scope channel.Problem: I'm not getting a precise result. It's off by "rather a lot". The used attenuator I got is a 5x 50ohm 2W from Tektronics (I think I could make the model number appear - it's kinda worn). When I use it, and put my scope at 5x without using anything but standard crocodile connectors (not the probe that came with the scope) what it measures is off by 0,5v or more volts depending on what I set the bench supply at. Nothing even close to what I know the supply is giving me.I am thinking my problem is that my scope is 1MOhm and the cheap attenuator is 50Ohm and hence they don't match. I am not able to understand enough of the connection to understand exactly what's going on, and in particular if there's something simple I can do to get much closer to the target. I deal mostly with 3.3->5v and not with milli-volts. I don't need a lot of precision (or I would have gone for a better probe). Am I on the right track? If so, how/where can I go to figure out WHY this is happening? Ie. it's not enough for me to know it doesn't work - I really want to understand why.

author: bob91343
author_rank: Super Contributor
timestamp: August 09, 2020, 01:37:24 am
content: The 1 megohm input resistance is standard for most oscilloscopes. If you want to attenuate the input, you generally use a probe with built in attenuation.For really high frequency work, the 1 megohm input becomes useless due to shunt capacitance and propagation delay via the probe cable. Instead, the oscilloscope will have a 50 Ohm input impedance which can be connected via a transmission line and thus have no inherent frequency limitation. If you want to attenuate the signal in this case, you use a 50 Ohm attenuator.Most decent oscilloscopes these days have the option of 1 megohm or 50 Ohms. In both cases one must respect the maximum rating. Typically for 1 Megohm, a few hundred volts or less is the highest safe value, which can be increased by using an appropriate probe. Typically for 50 Ohms it's around a few Volts, often expressed in dBm or milliwatts.

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 09, 2020, 01:48:39 am
content: Maybe this will help you to understand what’s inside a probe and why your 50ohm attenuator doesn't work correctly on a scope with a 1 meg input impedance.

author: TimFox
author_rank: Super Contributor
timestamp: August 09, 2020, 02:08:31 am
content: With a CRO that has only a high-Z (1 megohm) input, you need to put a 50 ohm terminator on the input and drive it through the 50 ohm attenuator for correct operation of the attenuator.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2020, 03:21:17 am
content: There are two problems:1. I know exactly what attenuator you are using; I have some of the exact same ones. But it is intended to be used with 50 ohm source and 50 ohm load impedances. The later can be solved by switching the oscilloscope input to 50 ohm mode or using a 50 ohm feed-through termination.2. The fact that you are getting any signal at all means that you are using a x1 oscilloscope probe. The problem is that x1 oscilloscope probes actually have a non-zero series resistance, usually a couple hundred ohms. That combined with the 50 ohm input resistance of the attenuator further attenuates the signal.What could work is a x5 low impedance probe, and this would work with your 50 ohm attenuator, however this would defect the purpose of having higher overload capability.