Converting a 75 Ohm 8753E to 50 Ohm - Page 1
Forum section: electronics / projects

author: rogersstuart
author_rank: Contributor
timestamp: February 03, 2021, 12:37:18 am
content: I recently purchased a fairly beat up 8753E. I just got it in today. Turns out it's the 75 ohm model. I was pleased to find out that the only issue was a loose cable and gunked up keypad. I've read that the 75 ohm matching is partially done in the reflection bridges? and in software. Has anyone attempted to convert one of these to 50 ohms? I'm also curious if it's possible to easily (as far as these things go) add the 6 GHz option.Here's an alternative question since the mods might not be possible. I have one of those cheap VNA test boards that you can buy on Amazon. Not thinking, I had already attached a 50 ohm N to SMA adapter. So, since I messed that up I might as well try things. I'm not an expert on the theory behind this. I can read the s-parameters but I can't manipulate them. Anyway, so I set the cal kit to N type 50 ohm whatever and used the open load and short on the board to complete the cal. I was surprised to see that 50 ohms after that cal is right on the smith chart where 50 ohms should be. So it looks matched. Is this a legit way of doing things? What's the downside?

author: mankan
author_rank: Regular Contributor
timestamp: February 03, 2021, 06:59:56 pm
content: Note that the following is for 8753D. But since D model can be upgraded with the CPU board from E model this might be interesting for you.During my adventures with a corrupt EEPROM I had my 50Ohm 8753D thinking its a 8753D 75 Ohm at one stage.This might help:https://groups.io/g/HP-Agilent-Keysight-equipment/message/100895or maybe this:https://groups.io/g/HP-Agilent-Keysight-equipment/message/100895. However my notes says its one of the bits in address 0xB8 of the EEPROM that decides 50/75 Ohm.I do not if the directional couplers are impedance specific or not.

author: rogersstuart
author_rank: Contributor
timestamp: February 04, 2021, 05:40:06 pm
content: After doing some digging I think I have this figured out. It's possible to reprogram the instrument to make it think it's a 50 ohm device. I read on the Keysight forum that the 75 ohm model incurs only 1 dB of loss due to the additional hardware and that there are some corrections in software to make up for stuff. It would definitely need to be tested to see if that's true or not.In regard to the 6 GHz conversion. Other than flipping a few dip switches and pressing some buttons, the only part that would need to be replaced is the source unit. 08753-60146 is the part number. Unfortunately, it looks like it's nearly impossible to get ahold of one. The only available units are salvaged and are going for rare salvage prices.So, that leaves me wondering if there's another way to go about this. The 3 GHz source and 6 GHz source look very similar. There appear to be a few submodules connected with rigid coax. Can anyone here tell me which (or if all) of the submodules are different and what the part number(s) are?

author: mankan
author_rank: Regular Contributor
timestamp: February 05, 2021, 05:42:16 pm
content: Re 6GHz, search Keysight site for 08753-90500, its upgrade instructions for 8753ET. According to that its only source that needs replacing.

author: virtualparticles
author_rank: Regular Contributor
timestamp: February 05, 2021, 08:44:35 pm
content: As long as you tell the VNA that you're using a 50 ohm kit and calibrate with it, you'll get a good calibration to 50 ohms. The raw source and load match will be awful but those are calibrated out. See the fun video attached for more info.https://coppermountaintech.com/video-making-75-ohm-measurements-with-a-50-ohm-vna/Best,Brian

author: rogersstuart
author_rank: Contributor
timestamp: February 06, 2021, 04:11:35 am
content: I kind of got a bit too worked up and decided to remove a port coupler and test it. I couldn't find any evidence that there's anything 75 about it other than the 75 ohm N connectors. I thought I found a 35ish ohm resistor and foolishly decided to remove it. Turns out that it was a capacitor. Looking back, it was really obvious but when I saw the number on my meter I was convinced. I almost damaged it beyond repair (the gold gets eaten away by solder) but I managed to put a new 5.6n cap in there and wire it back up. I can't see much of a difference but then again, I don't have the right experimental setup to push this to its limit. After I got it back up and running I removed the software option for 75 ohm. I'm pretty much done with it now. I wish I could find an affordable 6 GHz source but it's not something I have to have immediately. My main reason for getting this is experimenting with microwave filters. At 2 GHz they're not exactly the smallest but they're also not impractically large.

author: rogersstuart
author_rank: Contributor
timestamp: February 07, 2021, 02:51:57 am
content: I got a reply from the original designer of the coupler. This is what he said on the Keysight forum. His name is Dr. Joel."Well you came to the right place. I designed that bridge (it is a like a Wheatstone bridge but it with un-even legs, there's an old RF Design article that describes it you can google, or look on page 88 of the second edition of my book , it's also somewhere in the first edition, to see the 50 ohm version) back in the late 80's. You might find my initials and the date on one of the substrates.You did indeed break off a capacitor. The coax cable with the ferrite beads forms a balun, which makes it act like a 1:1 transformer at the clear saphire circuit. Underneath that coax is a 10 ohm think film resistor from the coax outer conductor to ground. On the top circuit, right next to the outer conductor is a 50 ohm resistor between the outer conductor and the capacitor. The other side is connected to an about 375 ohm resistor to the center line. The 5600 pF cap is used as a DC block to keep DC on the center line from going though these resistors to ground. The other side of the coax is connected to a DC bias tee that allows you put DC bias on the center line. Theres a little filament wire that runs back and forth through the ferrite beads which is needed to counteract the DC current in the center conductor to keep the ferrite beads from saturating when DC goes through them. The capacitor is sized to cancel the parasitic inductance of the balun at low freqs.It's not really a true 75 ohm bridge because the coupled port is still 50 ohms so the resistor values are adjusted for that. The long, serpentine resistor is about 375 ohms. The coax is 75 ohms and the other end is terminated at the bias tee through a 25 ohm resistor because the input to the bridge is connected to the 50 ohms source. In fact it might not be exactly 25 ohms to compensate for some other loss effects.So: let me say that it will be a bit difficult to re-work this to turn it into 50 ohms. You could drop a SMT resistor across the 375 to bring it down to about 265 ohms (like a 900 ohm or so) which will improve the directivity, but the match will still be off as it is 75 ohms looking into. Which is not a terrible thing, just 14 dB return loss and can be corrected for with the vector cal. But maybe simpler is just changing out the N connector on the front, or using a 50-75 ohm adapter (you could use a min-loss pad too, our just a TypeN with 50 on one side and 75 on the other) to use it. The 75 ohm type N on that port will actually accept a 50 ohm M pin a few times (50 is bigger diameter than 75 ohms) before it breaks off. or you could even consider a lower value R1 resistor (like 190 ohms) to make it matched to 37.5 ohms, then at the connector right at the input make a 150 ohm SMT to ground at input pin to the body so looking out from the bridge, when the test port is terminated in 50 ohms, it would look like 37.5 ohms and looking back in from the test port would be 150 || 75 = 50 ohms. But that's a lot of work.An external min-loss pad will match it, but reduce the residual directivity and dynamic range by 12 dB. Simply using it as a 50 ohm tool without using the min-loss pad will keep the dynamic range and residual directivity but hurt the raw directivity and raw source match.Do be careful of the sapphire (looks like glass) circuit: if you crack it you'll be done for good (well maybe not, if the crack runs through the gold line on top you can solder across the crack). "

author: virtualparticles
author_rank: Regular Contributor
timestamp: March 09, 2021, 09:58:23 pm
content: Quote from: rogersstuart on February 07, 2021, 02:51:57 amJoel Dunsmore is the king of VNAs. I doubt anyone in the world knows more than he does.It's cool that he answered you on this.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2021, 10:01:33 pm
content: Do you know the title, etc. of the book that he cited?

author: Bud
author_rank: Super Contributor
timestamp: March 09, 2021, 10:25:18 pm
content: https://www.amazon.com/Handbook-Microwave-Component-Measurements-Techniques/dp/1119477131

author: bson
author_rank: Unknown
timestamp: March 09, 2021, 10:34:23 pm
content: Quote from: Bud on March 09, 2021, 10:25:18 pmOne less left in stock now.Must. Have.