50 ohm BNC cable VS 1GHz active probe (Lecroy AP020) - Page 1
Forum section: electronics / testgear

author: TK
author_rank: Super Contributor
timestamp: September 02, 2020, 10:11:37 pm
content: I just got a LeCroy DDA-120 (LC584AXL equivalent) 1GHz scope and wanted to measure USB 2.0 Full Speed D+ and D- signals.From wikipedia this is what I know about the electric characteristics of the bus:QuoteI tested a 50 ohm BNC cable soldered to GND and D+, and the AP020 active probe. I don't see any difference except for the direct cable being 50 ohms, it creates a voltage divider... the active probe is 1Mohms so I see the correct voltage levels.How far can I go with the direct cable method VS the active probe?

author: langwadt
author_rank: Super Contributor
timestamp: September 02, 2020, 10:46:40 pm
content: http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2020, 03:12:43 am
content: Quote from: TK on September 02, 2020, 10:11:37 pmIf attenuation is acceptable, then nothing prevents adding a series resistor to the front of the coaxial cable to make a low-z passive probe with GHz bandwidth. So a x10 probe would have a 500 ohm input resistance and a x100 probe would have 5000 ohm input resistance.The disadvantage of low-z probes is their low impedance compared to an active probe. There are also "bias-offset" low-z probes which can minimize circuit loading and were common in the past for measurements on ECL but could still be used today.

author: TK
author_rank: Super Contributor
timestamp: September 03, 2020, 05:06:45 pm
content: Quote from: David Hess on September 03, 2020, 03:12:43 amI tested with a 500 ohm resistor and it works great as a 10X probe. It can measure 0.9ns or lower on an STM32 SPI bus signal... my next question is: are these signals on the STM32 capable of doing 800-900ps rise time?

author: graybeard
author_rank: Frequent Contributor
timestamp: September 04, 2020, 04:10:49 am
content: https://emcesd.com/1ghzprob.htm

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2020, 04:29:24 am
content: Quote from: TK on September 03, 2020, 05:06:45 pmIf that is what you measured, then they sure can. Manufacturers often do not bother specifying rise and fall times since they are not important in most applications.For verifying the performance of your probe and oscilloscope, I suggest getting a fast edge generator:http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295Or make one from some LVC logic running at 5 volts.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 04, 2020, 09:30:24 am
content: or... get anEraSynth Micro RF Generatorwith free DHL (if you cant afford both).. Leo Bodnar's is a lot cheaper at first glance but i ended up paying (incl the not free shipping) about half the cost of EraSynth. EraSynth has many purposes, 12MHz - 6.4GHz generator which will become handy in your RF design such as clock generator, mixer etc, has modulation, sweep, 15dBm and voice control amplitude (i'm planning one day to try it out as a voice radio transmitter all the way of choosing 12MHz - 6GHz carrier). you are going to need it anyway if you digging into GHz rabbit hole region (much like a AWG/Function generator that everybody love in MHz region). Leo's is one very specific purpose only, to verify devices' bandwidth, or maybe as a TDR tool (need your own SW and manual processes and calculations) up to 10GHz BW, other than that, it can only be usefull as a 10MHz clock and nothing else. will be worthwhile if you have many HF/RF devices to verify or designing T&M up to that BW. step respond 0.35/Tr method is only half the story at verification, you can also do -3dB point verification as another half of the story with RF generator such as EraSynth (VNA or SA with track gen is another half) with its 300+ ps risetime (1GHz BW, see attached) is the right spot for what you currently need and have imho, ymmv.