50 ohm scope input hack - Page 1
Forum section: electronics / beginners

author: silly sausage
author_rank: Regular Contributor
timestamp: February 01, 2025, 08:25:49 pm
content: i have a tek 465 , it has no 50 ohm input, if i connect a bnc t piece with a bnc 50 test/ cal resistor from my vna cal set on one leg of the t will that be the same as a 50 ohm scope input?.

author: tggzzz
author_rank: Super Contributor
timestamp: February 01, 2025, 08:52:58 pm
content: Quote from: silly sausage on February 01, 2025, 08:25:49 pmSince you have a VNA... measure it and decide.Alternatively, basic RF theory of transmission lines will define the effects of the 15pF (or whatever the scope's input capacitance is) and short stub.An improvement would be to have an inline 50ohm terminator, which will remove the stub but leave the 15pF.Another improvement would be to have a 3dB or 6dB pad between the cable and inline terminator, since that will reduce the effects of the 15pF.

author: BillyO
author_rank: Super Contributor
timestamp: February 01, 2025, 08:59:16 pm
content: I think with a 100MHz max scope the little 0.5cm stub will make no measurable difference.I've done exactly what you are suggesting and had no issues. You should be just fine.Oh, be sure you do not kill your load. You probably don't want to put more power into it than it's rated for.

author: radiolistener
author_rank: Super Contributor
timestamp: February 02, 2025, 06:36:03 am
content: Quote from: silly sausage on February 01, 2025, 08:25:49 pmIt's almost the same, but not exactly. The reason is that the internal terminator is placed closer to the ADC input, whereas with a T-adapter, there is an additional path of several centimeters. The difference lies in the presence of a slightly unterminated line, which results in more noticeable echoes and ringing compared to using the oscilloscope's internal terminator.But if your oscilloscope don't have internal terminator, you have no choice, so this is the only way.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 02, 2025, 07:38:04 am
content: Adding a 50\$\Omega\$ BNC pass through or a T with a terminator might be good enough, but you'll have to use BNC cables instead of the oscilloscope probes. Using 50\$\Omega\$ coax instead of the oscilloscope's probes will add the input capacitance of the oscilloscope's channel in parallel with the 50\$\Omega\$ terminator. Those 10-20pF might be a considerable (and unwanted) capacitive load for the DUT.At low frequencies this might not matter, but for precise measurement the oscilloscope capacitance is far from negligible.Another way can be to add the 50\$\Omega\$ BNC terminators at the DUT side, then probe normally, with the x10 oscilloscope's probes connected to the BNC terminator. This will work much better at higher frequencies, and the capacitive load will be much smaller for the DUT.There are probe-tip adapters that will fit a normal probe into a BNC. They look like these:https://duckduckgo.com/?q=oscilloscope+probe+tip+to+BNC+adapter&t=ffab&iar=images&iax=images&ia=images

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 02, 2025, 09:50:33 am
content: https://www.picotech.com/accessories/bnc-cables-and-adaptors/50r-terminator-bncThat assumes you can navigate around the Pico web page better than you can around Simons...SJ

author: benj38
author_rank: Frequent Contributor
timestamp: February 02, 2025, 03:20:36 pm
content: They say a picture is worth a thousand words.All measurements are of a very fast pulse with a rise-time of ~ 50ps. Each measurement is done in two ways:"internal", i.e., with scope input set to 50Ohm;"external", i.e., with scope input set to 1MOhm, an external 50Ohm terminator attached to one leg of a BNC T-adapter, pulse signal attached to the other leg with a coax cable, and the T-adapter attached to the scope input.Tektronix 2465B (400MHz analog scope) internal:Tektronix 2465B (400MHz analog scope) external:Tektronix 2465B (20MHz bandwidth limit selected) internal:Tektronix 2465B (20MHz bandwidth limit selected) external:The following set of measurements were performed with a Tektronix TDS3064B Digital Scope (600MHz bandwidth, 5Gs/s) internal in white, external in cyan (slightly offset for easy viewing).TDS3064B full 600MHz bandwidth:TDS3064B 150MHz bandwidth limit selected:TDS3064B 20MHz bandwidth limit selected:Finally, here is a comparison at full 600MHz bandwidth of the external terminator using a BNC T-adapter (cyan), and an external Tektronix inline 50Ohm terminator (white). There is no visible difference between the T-configuration and the inline configuration (this was true in all measurements, also with the 2465B Analog scope). I guess a difference will be visible only at much higher GHz bandwidths, in which case you would not use external termination anyway.In conclusion, for a 100MHz scope like the Tektronix 465, external termination (either inline or with a T-adapter) would do perfectly fine in my opinion.