adapter to add missing functions to usb scope - Page 1
Forum section: electronics / projects

author: diehard67
author_rank: Newbie
timestamp: January 05, 2024, 11:49:41 am
content: I got one of those hantek 6022bl usb scopes, and it doesn't have ac coupling or 50 ohm termination.I want to make an pair of adapters that can just plug onto the BNCs to ass those to the scope.I would rather not modify the internals of my scope.the questions are.....what type and value of cap to use for ac coupling?can this by itself mess up probe compensation?what about adding 50 ohm termination?would there be any point to 75 ohm termination? just a random thought.the thing I was thinking was just to put a cap inline in the signal wire with a switch in parallel, and to have a 50 ohm resister and switch inline between signal and ground, probably 2 100 ohm resisters in parallel just because that is easier to find in junk box parts.would making this literally be that simple or are there other design considerations?

author: tunk
author_rank: Super Contributor
timestamp: January 05, 2024, 12:42:46 pm
content: No expert, but for capacitor values you could look at scope schematics, e.g.:https://github.com/pecostm32/Hantek_DSO2000https://github.com/pecostm32/FNIRSI-1013D-Hack/tree/main/Schematicshttps://jyetech.com/wp-content/uploads/2018/07/dso150-shell-schematic-analog-ver-f.pdf

author: CaptDon
author_rank: Super Contributor
timestamp: January 05, 2024, 02:36:28 pm
content: One thing to remember about a.c. coupling is that if you touch the probe to the plate circuit of a tube type audio amplifier your scope input could briefly see the d.c. plate voltage until the capacitor stabilizes. On many scopes we switch the input selector lever to 'GND' which usually leaves the 10 meg resistor to ground on the scope side of the coupling capacitor on the BNC input jack and grounds the vertical amplifier input. This does not show up as a short across the input jack!! This allows the coupling capacitor to stabilize while the probe is attached to the circuit under test. Then after a second or two you flip the input selector to the active position which in this case would be A.C. If you build an A.C. coupling device please include a similar switch to protect your scope's input. It could be a D.P.D.T. wired when in the ground position the scope input is grounded and a 10 meg resistor is discharging the scope side of the coupling capacitor and in the other position the signal goes through the capacitor to your scope and disconnects the external 10 meg resistor remembering many scopes have a 10 meg input impedance although most are as low as 1 meg. The coupling capacitor will roll off your low frequency response so you may want another switch that would select between a .01uf and a .1uf coupling capacitor. Just food for thought. There are a lot of ways to do it and a lot of experienced 'Elmers' here to offer help. Cheers!!

author: tooki
author_rank: Super Contributor
timestamp: January 05, 2024, 04:51:26 pm
content: Quote from: CaptDon on January 05, 2024, 02:36:28 pmYou've just answered a question I’ve had for ages and never found a satisfactory answer to: what is the real purpose of the ground coupling setting on a scope! (“To show a baseline zero level” never seemed like a plausible explanation to me.)

author: moffy
author_rank: Super Contributor
timestamp: January 05, 2024, 10:23:51 pm
content: Quote from: tooki on January 05, 2024, 04:51:26 pmIn older analogue scopes the GND was important for finding the trace and adjusting the offset especially on the low voltage levels. You would switch to a low level and sometimes the trace would disappear, so you would select GND then manually adjust the offset(vertical displacement) until the trace appeared then you could switch to DC or AC and do your measurement.

author: diehard67
author_rank: Newbie
timestamp: January 17, 2024, 10:15:46 pm
content: ok so the attached pdf is what I have come up with so far.sooooo....should termination happen before or after coupling, or should I put in a switch??and I am not sure if R4 needs to be there or not, or for that matter what value it should be, R4 is meant to drain off the cap when switching to DC coupling, the theory is that it will prevent sparks in the switch.CaptDon mentioned a couple of values to the capacitor, are there advantages to have a selection here or is just the one good?feedback is appreciated.