Logic analyser probing pron - Page 1
Forum section: electronics / testgear

author: nctnico
author_rank: Super Contributor
timestamp: April 30, 2016, 08:46:43 pm
content: For one of the FPGA projects I'm working on I need to probe a whole bunch of signals between a carrier board and an FMC module using my Tektronix TLA715 logic analyser + TLA7AA4 acquisition module. I thought it would be interesting to show / share some of this to give people some ideas about what is out there.Some of the signals in the project are in the 200MHz-ish range and I want to collect around 60 signals. For an earlier version of the project which used much lower frequencies I got an FMC breakout board from Zebax.com and soldered a lot of wires to a 0.1" header to mictor converter board but that was a lot of work and I didn't want to do that again. Also things get 'interesting' when probing high frequency signals. The cheapest probes (on Ebay) for the TLA7AA4 module are the P6860 and these are intended to be mounted on a special footprint on the board in order to minimise influences on the signal. Each probe pin is an active FET probe which has a very low input capacitance and a very high bandwidth (>2GHz). With some haggling a complete probe cable (34 channels) costs around $40 each + shipping. The breakout board is a different story: it costs nearly $400 from Zebax.com but it has 12 layers and according to Zebax it can be used up to 10GHz.One of the advantages of the Tektronix TLA7AA4 module is that is has 4 analog outputs (2GHz bandwidth) which can carry any of the input signals. This means that the logic analyser probe also provides a very high quality analog probing capability for use with an oscilloscope.Below a picture of the setup:The TLA7AA4 acquisition module front view:

author: lukier
author_rank: Unknown
timestamp: May 01, 2016, 12:34:38 am
content: Nice write up. $400 for FMC breakout board - ouch!. Does one really need 12 layer PCB for 200 MHz signals? Is that board fully passive?BTW Do you have experience with P6880 differential probes or can you use P6860 and later the software can subtract captured signals?I see your P6860 are in a good shape. Some cheaply listed ones on ebay have exposed coax ground shield at the connector, like the insulation was pulled or heatshrunk somehow (not to mention missing elastomer). Do you have press-in nuts in the PCB or nut bars (pure unobtanium on ebay) to screw the probes?

author: D3f1ant
author_rank: Frequent Contributor
timestamp: May 01, 2016, 02:51:47 am
content: That is pretty cool, thanks for sharing.

author: nctnico
author_rank: Super Contributor
timestamp: May 01, 2016, 02:03:21 pm
content: Quote from: lukier on May 01, 2016, 12:34:38 amThe 12 layers aren't necessary for 200MHz but the manufacturer claims the board should work up to 10GHz. I would take that with a grain of salt but at least they went to great lengts to get the signal integrity as good as they can. The project I work on has some differential LVDS signals and 2.5 CMOS signals so I have to see how I can get the thresholds right. I can probably just ignore one channel of a differential pair.QuoteI got my P6850 probes (the ones I bought for this project) including the nut-bars, new elastomer contact pads and stickers for $40 (or less) each. It's just a matter of making an offer and doing some haggling.

author: bson
author_rank: Unknown
timestamp: May 01, 2016, 09:54:15 pm
content: What's more droolworthy than a big, fat, wad of LA cables? It's mad scientist stuff!