Active probe for Siglent VNA - Page 1
Forum section: electronics / testgear

author: euank
author_rank: Newbie
timestamp: January 08, 2025, 12:38:50 pm
content: I need to do some fault finding on an RF board, around 1.8Ghz. basically need to measure the RF around the signal path to see where it is being attenuated, or going missing. The only equipment we have at moment that can handle the frequency is a Siglent SNA5004. It has the signal analyser option enabled which would allow it to operate on one port and not generate any output in theory.I found this probe:https://www.picotech.com/accessories/active-single-ended-probes/tetris-2.5-ghz-active-probe.Would this work with the relevant N-type to BNC converter? Any other options (other than diy probes)?

author: Momchilo
author_rank: Regular Contributor
timestamp: January 08, 2025, 01:03:05 pm
content: There is this cheap probe as wellLD-ASP-2.7 Single Ended Active Probe – DC to 2.7GHz.There is already a discussion in this forum:https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/It started as an diy project but is quite professional now with a very comprehensive datasheet. He also has a partnership with a scope manufacturer who sells the probes with the scopes together.Right now the probes are out of stock. Not sure how long it will take to restock. The designer Lasmux is also active in this forum, maybe you can ask him.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2025, 01:42:47 pm
content: You don't need an active probe for that. A simple passive 'Low-Z' probe with 1:10 (20dB) or 1:20 (26dB) dampening works well for doing these kind of jobs. A golden oldie is the Tektronix P6156 but there are other, newer models as well. At the VNA / spectrum analyser side you'll want to use a DC-block and make sure to always have some attenuation enabled. When not sure, add an input level protector.

author: euank
author_rank: Newbie
timestamp: January 08, 2025, 03:32:05 pm
content: Quote from: Momchilo on January 08, 2025, 01:03:05 pmThanks I registered for an update on availability, looks interesting.

author: euank
author_rank: Newbie
timestamp: January 08, 2025, 03:36:56 pm
content: Quote from: nctnico on January 08, 2025, 01:42:47 pmThats interesting thanks, I thought one would need a high-Z probe so as not to mess with the signal path. I need to investigate that further. Interesting that the P6156 is a passive probe yet goes up to 3.5GHz! they are pretty cheap on ebay too. Finding resources / info on RF debugging is pretty hard it seems.

author: Kean
author_rank: Unknown
timestamp: January 08, 2025, 04:20:51 pm
content: You can sometime find an Agilent/HP 54701A at a reasonable price on ebay, but some I've seen listed are in rough condition and often missing probe tips or ground sticks.It is a 2.5GHz active probe with a 100k 0.6pF input, a 50 ohm male N output, and needs an external PSU. A PSU is not too hard to build, and info is available on this forum.Of course even a 0.6pF probe input will look like about 147 ohms impedance at 1.8GHz, so as nctnico mentions you can probably just use with a Low-Z/Z0 scope probe for the diagnosis you described.And I repeat his suggestion that you protect your SNA input with a DC block and some attenuation or a limiter just in case.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2025, 05:22:17 pm
content: Quote from: euank on January 08, 2025, 03:36:56 pmAt high frequencies (say >1GHz) the capacitance at the probe tip dominates the signal loading where it comes to measuring RF signals. The only advantage of an active probe is that it has onboard amplification but amplification can also be added seperately using an RF amplifier or the pre-amplifier in a spectrum analyser.

author: tautech
author_rank: Super Contributor
timestamp: January 08, 2025, 07:23:07 pm
content: Quote from: euank on January 08, 2025, 12:38:50 pmSiglent's TDR probes might work:https://siglentna.com/download/39641/?tmstv=1736363768

author: euank
author_rank: Newbie
timestamp: January 09, 2025, 02:19:56 am
content: Quote from: Kean on January 08, 2025, 04:20:51 pmThanks I had seen that before using the probe with external power, they all look a bit crusty on ebay though. The VNA has DC blocks permanently installed as our sacrificial connectors as we work mostly with DC on the RF to power our amps. Its easy to momentarily forget there is DC there and probe away or put a terminator on the DC...

author: euank
author_rank: Newbie
timestamp: January 09, 2025, 02:20:32 am
content: Quote from: tautech on January 08, 2025, 07:23:07 pmDon't know how I didn't see them on the website. :/

author: tautech
author_rank: Super Contributor
timestamp: January 09, 2025, 02:28:07 am
content: Quote from: euank on January 09, 2025, 02:20:32 amYeah, they're not listed amongst the ordinary probes but as Accessories/Options for the analyzers.