HP 85050-60006 broadband load toast? Measuring 120 ohms at DC  - Page 1
Forum section: electronics / rf-microwave

author: toastedcrumpets
author_rank: Regular Contributor
timestamp: December 05, 2019, 09:35:50 pm
content: I recently bought a APC-7 (7mm) mechanical calibration kit off ebay but the broadband load appears to be DOA as its registering 120 Ohms at DC (see below). This is not due to the frankenstein of adapters I'm using to go from APC-7->SMA->BNC->jacks as I've made a measurement using probes directly to the mating surfaces of the load (I normally wouldn't do this out of fear of damaging those surfaces, I was very gentle and used flats of the probes, not the tips).I'm sending the load back but a colleague remarked 120 ohm terminations are not unheard of; however, I can only find this in digital buses like CAN. I also believe the APC-7 connectors are 50Ohm by construction.I'm just looking for reassurance that the load is duff and I've not somehow found a unicorn mismatched load?I wanted to open the load up but unfortunately the seller wants it back.

author: TheSteve
author_rank: Unknown
timestamp: December 05, 2019, 11:08:25 pm
content: You're not wrong, it should be 50 ohms and has either been overloaded or dropped.

author: coppercone2
author_rank: Super Contributor
timestamp: December 06, 2019, 12:34:56 am
content: well, if they used parallel resistors.. 2x 120 would be 60 ohms. 3x 120 would be 40 ohms.if some stuff went out of circuit and other stuff deformed, it could happen I guess?

author: toastedcrumpets
author_rank: Regular Contributor
timestamp: December 06, 2019, 12:54:47 am
content: I think these loads use something slightly exotic for the termination like a sheet of sapphire (!?) Rather than discrete components. Certainly there's some RF magic going on to give it the frequency response it has.

author: toastedcrumpets
author_rank: Regular Contributor
timestamp: December 06, 2019, 01:13:41 am
content: Hmmm, reading a keysight application note "Specifying Calibration Standardsand Kits for Keysight VectorNetwork Analyzers" they have this to say:Arbitrary impedanceAn arbitrary impedance device is similar to a fixed load except that the load impedanceis NOT perfect. The previous generations of VNA, such as the Keysight 8510, 87xyseries and the early firmware releases of the PNA series, use a fixed resistance value.This suggests older systems, like my 8720A might have had different terminating resistance... I think I'll have to check the definition file for the kit next, including the default parameters on the VNA for the kit.

author: toastedcrumpets
author_rank: Regular Contributor
timestamp: December 06, 2019, 04:19:38 pm
content: I've checked the preprogrammed default offsets in my 8720A for the 7mm load and the default is 50Ohm. The three floppies that came with the cal kit aren't readable under linux or windows on two different floppy drives, so I'm calling it a broken load.

author: radiolistener
author_rank: Super Contributor
timestamp: December 06, 2019, 11:21:21 pm
content: probably it was burned out, may be someone connected it to transmitter as a dummy load

author: Tony_G
author_rank: Super Contributor
timestamp: December 06, 2019, 11:55:20 pm
content: Quote from: toastedcrumpets on December 06, 2019, 04:19:38 pmI don't know what kit you got but the disks may be in LIF format which HP used for their analyzers. Try reading them withLIFUTILTonyG

author: TheSteve
author_rank: Unknown
timestamp: December 07, 2019, 12:10:43 am
content: Quote from: toastedcrumpets on December 06, 2019, 04:19:38 pmDon't even question it, it is 100% damaged. It should be 50 ohms, end of story.