50 Ohm BNC Feed-Thru Impedance Adapter or Attenuator - Page 1
Forum section: electronics / beginners

author: HoracioDos
author_rank: Frequent Contributor
timestamp: May 05, 2017, 09:48:22 pm
content: A few months ago I bought a 50 Ohm BNC ended cable to connect a DSG to the DSO.https://www.aliexpress.com/store/product/J1005-BNC-BNC-oscilloscope-probe-Q9-signal-source-test-line-Q9-straight-line-with-the-standard/1810103_32352905000.htmlBy watching Dave's video #652 about coax cable reflections and how it should be terminated I bought this BNC Feed-Thru Terminatorhttps://www.aliexpress.com/item/P7001-2PCS-High-quality-BNC-to-BNC-Female-seat-50KY-50-ohm-matching-device-Q9-adapter/32661228102.htmlToday I received the impedance adapter and connected to the scope and I found that I must set the probe value to 2x to get the correct amplitude when I've always set the probe value to 1x.Trying to understand what's going on I found this reply in other thread.Quote from: c4757p on June 14, 2013, 07:59:11 pmDid I received an attenuator instead of an impedance adapter? That's why I get 1/2 Vpp. Am I confusing all these concepts? I would like to understand the basics.Thanks!PS: I guess I should buy this one but the cheapest shipping cost is too high.($67)http://www.tequipment.net/RigolADAPTOR-50-OHM.html

author: pigrew
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:08:22 pm
content: The feed-thru terminators are 50 ohm resistors between the signal and shield. If you measure its resistance (using an ohmmeter) while disconnected, it will read 50 ohm.An attenuator will have an attenuation value associated with it (such as 3 dB, 20 dB, etc). Its resistance will be larger than 50 ohms when measured while disconnected, though the larger value attenuators will approach 50 ohms. They are usually formed by a pi-network or a tee-network of resistors.The divide-by-two thing is likely due to the signal source being a 50 ohm source. Once you connect the termination, a voltage-divider is formed between 50 ohm and 50 ohm, which reduces the voltage by half.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:15:11 pm
content: Quote from: pigrew on May 05, 2017, 10:08:22 pm50 Ohm from DSO input and 50 Ohm from the adapter. Ok I got it. Vout = Vin /2 if R1 = R2.Thanks!

author: mmagin
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:37:11 pm
content: Measure between the center pin and ground on each side. In both cases you should get 50 ohms.Then measure from one center pin to the other center pin. If it's a feed-through terminator you will have 0 ohms, if it is an attenuator you will have some larger resistance.Any resistive attenuator like that is going to be either a pi-network or t-network of three resistors (doesn't matter which), you can model what you're observing either way.(Edit: perhaps this explains betterhttps://www.microwaves101.com/encyclopedias/attenuators)

author: HoracioDos
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:47:35 pm
content: Quote from: mmagin on May 05, 2017, 10:37:11 pmIt's a feed-through for sure. 0 Ohm from pin to pin and 49.9 Ohm from pin to shell.

author: w2aew
author_rank: Super Contributor
timestamp: May 06, 2017, 12:36:22 am
content: This video might help you to visualize what's happening.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: May 06, 2017, 05:57:38 pm
content: Quote from: w2aew on May 06, 2017, 12:36:22 amHello Alan!I'm already a subscriber. Your videos encourage me to learn as much as I can to get the most of them. I don't know how I missed this one.Thanks!!