Agilent 8753ES A21 Test port coupler repair - Page 1
Forum section: electronics / testgear

author: inaxeon
author_rank: Regular Contributor
timestamp: February 07, 2022, 05:56:47 pm
content: Hello helloThis is a continuation of my attempt to repair (multiple) problems with my 8753ES. This time it's the input coupler on port 1.Previous thread:https://www.eevblog.com/forum/testgear/agilent-8753es-a24-microwave-transfer-switch-substitute/Attached are some pictures of the requested measurements, and a shot of the hybrid its self. Ideas appreciated!

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 09:09:39 am
content: I've made another really interesting observation to add to the mix.S11 "open" looks completely different when a 50cm cable is attached to the portWhereas S22 "open" is barely affected by attaching the same cable. Really interesting. Would love to know what's going on here...

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 10:02:31 am
content: And here is the proof that port 1 behaves when it has port 2's couplerattached: Open (with cable / without cable) and with load

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 04:05:51 pm
content: Have you measured the resistors in the good/bad bridge? The same thing that killed the diode might have messed them up. If so, I bet you could get them working again by scratching them off and replacing them with small surface mounts. Conductive side down, of course, and if you're a real keener you can sometimes find "bad" SMDs that let you peel off the contacts and resistive element.Back when I was diagnosing a similar issue in the analogous Anritsu bridge (which is, uhh, very analogous), I found this writeup helpful:http://www.ke5fx.com/Broadband_Coupler_Dunsmore.pdfYou might recognize the author!

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 04:20:29 pm
content: Pretty Pictures

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 04:28:30 pm
content: I was only able to identify one resistor, across the receiver output. I don't physically know where it is however.Attached a measurement of it. We're getting way beyond my area of expertise here...

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 05:22:13 pm
content: Look on the side near the front panel port. I think you might find two resistors hiding under goop + foam and a single sneaky resistor between the coax outer conductor and ground. In general, focus your attention on Figure 6 in the Dunsmore paper, and ignore everything behind the coax+ferrites.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 05:38:04 pm
content: That should not be there! ( circle in Blue). Whatever is on the top of the bridge circuit near the capacitor, it should not be there.The resistor from the mainline to one side of the capacitor should be 265 ohms, the resistor from the other side of the capacitor to the outer conductor of the coax should be 50 ohms. You can test them with an ohm-meter. Over powering the bridge sometimes burns out the 265 ohm resistor.Edit: I correct myself, that looks a bit like a piece of "poly iron" absorber. I guess sometimes the techs would place it on a spot on the circuit to improve raw directivity. In the original design it wasn't there but over time production techs sometimes find a bit of this or that helps them meet a spec so it gets included in their bag of tricks for tuning.Look also for any micro-cracks in the main-line. Test continuity on the main-line from the test port center conductor to the coax center conductor and on through to the bias tee.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 05:40:28 pm
content: Quote from: jjoonathan on February 08, 2022, 04:20:29 pmLooks very familiar to me. I wonder if my initials are also on that circuit?

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 05:48:16 pm
content: Well well. I think you've found it. On my good bridge I can indeed measure the 265 ohm, however on the bad bridge it's 3.8 mOhmOn a side note, both my good and bad bridge have the thing you say shouldn't be there, have have completely different serial numbers and appearance.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 05:57:04 pm
content: I measured that again on my Keithley 2002 which is actually calibrated, I'm getting 267 ohm to the mainline.So I'm scratching that off and putting a chip SMD in there then? Any recommendations on the part? any old resistor or do I need something exotic.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:09:53 pm
content: 267 is nearly exactly right, so don't scratch it off! Something else is wrong. The bridge will work OK any where from maybe 250 ohms to 270 ohms. But look t see if the capacitor is well connected (it is connected with conductive epoxy), look for a crack in the circuit that might open main line. It seems to me you have a DC open (low-frequency open) somewhere in the structure. Test continuity from each external coax to a place on the inside, for example test port center pin to coax center pin. Test contituity from DC torroid connection to bias circuit to the test port output (I've seen the coax center conductor become disconnected from the input or output main-line especially if the unit was dropped).

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:16:17 pm
content: Test coupled port center to case-ground, should be 50 ohms.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:21:24 pm
content: Sorry I've thrown you off there:Good coupler: 267 OhmBad coupler: 3.8 Mega OhmsSo that resistor is burned to a crisp.What is the value of that capacitor? Might order another in case I break it while operating on this thing...

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 07:29:00 pm
content: First be sure you've identified the bad componentIf it's an intermittent problem you may need to wire up sturdy connections to the ports and then start mechanically poking different components to find the culprit.----------------Re: replacing resistors, if you want to see a principled discussion, Joel's book has one, Handbook of Microwave Component Measurements, page 50. I have the 1st edition, but it looks like 2nd edition has similar placement. Unfortunately it's just a few pages beyond the kindle preview.In general, though: smaller is better (subject to constraints on actually fitting), maybe put two in parallel, conductive side down (the black side conducts, usually). Given that this is a one-off, buying specifically rated RF resistors might be in the cards. They'll probably still be cheaper than the shipping. What you're buying is laser trimming not in a pattern that's highly suboptimal for RF performance. Buying "special" RF resistors eliminates that possibility, but so does rolling the dice and getting lucky, or buying too-cheap-to-laser-trim resistors and DIY binning or trimming. In any case, frequency is low enough (1/10 wavelength rule) that you can probably get away with a lot. I'd probably give it a go with whatever SMD I had on hand and revisit if the resulting performance was really trash.To be clear, the performance will not be as good as the originals. There's a reason why thin/thick films are preferred, and it reeeallly helps to have a VNA for this type of task. Oruboros debugging only goes so far. Still, you should be able to measure the raw directivity if you have a decent load. I reckon you stand a very good chance of getting a workable result, only a handful of dB in raw directivity worse for the wear, and only then at the top of the frequency bandRe: JD initials on the Anritsu ceramic board, no, they changed up the homework before they turned it in. They added optional tuning pads everywhere -- it doesn't look like they used any of them, thoughSpeaking of which, the Anritsu bridge has notably fewer magnetics. At the 70kHz low end rating, it only has 4.4dB of raw directivity. IIRC the 8753 bridge is rated to 30kHz, I wonder if the extra magnetics just barely balance that out or more than balance that out?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:36:50 pm
content: The capacitor is about 10,000 pF as I recall, but if you don't plan to run bias you can just short across the contacts. It is used to block DC from going down the 265 ohm to the 50 ohm to ground. But, with the 265 gone, that's for sure the issue and consistent with your switch trouble too. Almost for sure it was an RF overload on port 1 and it took out R1 (that's the what we call the 265 ohm) and the protection diode. If you can sneak an something like an 0402 or 0201 in the spot it should work well enough. Of course the raw directivity will probably degrade quite a bit at high frequency but error correction will sort that out. The 8753 was the last VNA designed to be used without any error correction so the hard-specs on the bridge were quite tight. Something like 40 dB directivity to 1.3 GHz and 35 dB to 3 GHz.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:44:47 pm
content: Quote from: jjoonathan on February 08, 2022, 07:29:00 pmAh, that's the tricky bit. In fact, that was my first patent (for those interested in details: 4,962,359). Turns out you carefully choose the value of the capacitor to optimize the low-frequency directivity. That bridge should hold 20-30 dB directivity down to 30 kHz or lower.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 07:49:30 pm
content: Quote from: jjoonathan on February 08, 2022, 07:29:00 pmLooks pretty likely we have.Quote from: jjoonathan on February 08, 2022, 07:29:00 pmIt's not intermittent.Quote from: jjoonathan on February 08, 2022, 07:29:00 pmThat sounds too interesting not to buy ;-)Quote from: jjoonathan on February 08, 2022, 07:29:00 pmI'll buy some RF resistors. I can get 264 ohm with 2x 132 ohm parts in series. Not really worried about the cost. I got this thing very cheap. 3 faults in and I am yet to spend anything much on it...Quote from: jjoonathan on February 08, 2022, 07:29:00 pmDo I sound like a person who needs Keysight levels of performance? ;-)

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 08:08:37 pm
content: Quote from: inaxeon on February 08, 2022, 07:49:30 pmstandard 5% value of 270 should work well enough (nominal directivity of about 43 dB). If you can choose one physically that can span from the capacitor to the center pin of the coax, you could solder tack it at each end. the gold mainline is 1.25 mm (1250 microns) for reference. Careful or the solder will eat up the gold on the mainline trace. or a conductive epoxy would work well but then you would have to bake it and it doesn't like being baked above about 85C (that black epoxy on the beads is special low-temp epoxy; the conductive epoxy which is silver loaded is usually baked at 125 C).

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 08:16:26 pm
content: Ooh, that's a nifty trick with the capacitor!This is the dual directional bridge from the patent?https://www.ebay.com/itm/284439786962I ask because I recently bought an Anritsu MN4697B 4-port extender (from Keysight, lol) with the intention of cobbling together a VNA from the parts and a source. Thearchitecturediplexes out the low-band, so I have some freedom to swap it out. My plan was to try my hand at making a bridge with better magnetics and maybe active differential->single ended circuitry (leverage those new semiconductors!), but there are so many other parts of the project demanding attention that I'm tempted to just buy a couple of these and call it a day.EDIT: hmm, there's a 1.3GHz version of the 8752, but the Anritsu low band goes up to 2.4GHz or so. Maybe I should just put up with the couplers I have for now until I can give this proper attention.Semi-related: I saw R&S bragging about active bridges the other day, would you happen to know what that's about? Augmenting bridge magnetics with silicon CMRR?

author: inaxeon
author_rank: Regular Contributor
timestamp: February 08, 2022, 08:48:40 pm
content: Would just any old chip SMD resistor be OK then?If so, then I can buy either a 264 or 270 ohm no problem. It doesn't look like I have either to handEDIT: I'll stick with a decent quality thin film resistor, those are available at 267 ohm, the exact same as the existing coupler...Attached are images of what I believe you're implying for the repair? If so, then it looks like 0603 would have to be the size as 0402 does not bridge that gap.Also, are you saying I leave the husk of the old resistor in place?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 09:15:59 pm
content: I'd try to make the resistor as small as possible, even if I needed a bit of solder bridge, and I'd press the conducting part right up against the substrate, scrubbing away the original resistive element and flipping the new one upside down. Of course, we're altering the structure enough that I'm not sure general parasitic reduction technique is perfectly optimal, but the principles of A. make it small, B. mimic the original are probably sound. Frequencies are low enough that this is probably splitting hairs.Eating up the original tracks with solder is also a concern, so that could be a reason to lift up the whole structure and try going right from the center conductor to the capacitor pad. I've never had much trouble with this so I'd probably try dead-bugging it first, to minimize that series inductance.EDIT: I'd definitely get rid of the goop and absorption pad, though. If Joel didn't know about it, the degree of tuning represented by its placement is long out the window and now it's just in the way.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 08, 2022, 09:39:20 pm
content: Quote from: inaxeon on February 08, 2022, 08:48:40 pmYup, just like your picture. Just reach in there with the lightest tack of a tinned solder tip and see if you can make it contact across. If it sticks, test it out.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 08, 2022, 09:46:56 pm
content: Ah, right, trying the simplest thing first is always good advice

author: inaxeon
author_rank: Regular Contributor
timestamp: February 09, 2022, 09:55:23 am
content: The parts have been ordered. I'll report back here when they arrive, likely next week.In the meantime, could you tell me a bit about the poly iron ? in particular, why did the production engineers choose that material, and what effect does it have on the circuit?

author: inaxeon
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:10:36 pm
content: Ah the fun never ends.So it turns out that while I was testing the resistors in my GOOD coupler, I broke it. It looks like I pressed a bit hard on the end of the coax and cracked the bond. I was able to restore it back to life by soldering the joint, so all good now.An unwanted trial run for the repair of the bad coupler. It turns out my hand isn't as steady as it once was.PS. What is that little gold flap on the transmission line?

author: AJ3G
author_rank: Regular Contributor
timestamp: February 09, 2022, 02:36:17 pm
content: This looks like a frequency compensation capacitive element to me.Rich

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 09, 2022, 07:54:41 pm
content: Quote from: inaxeon on February 09, 2022, 01:10:36 pmWe would call that a tuning ribbon. Turns out the transition from the gold-line to the coax tends to be slightly inductive and we need a little extra capacitance to ground, sometimes, to optimize the return loss and directivity. The tuning ribbon is lifted or lowered by a tech while measuring the directivity of the bridge. If a bit too much solder was used on the connection it might not need any ribbon so it will be ripped off. Same with placing the bit of polyiron and with moving the beads back and forth a bit. Something like that can move the directivity from the -30 dB range to the -40 dB range; The connection for the coax to gold-line is hard to control so some adjustment was needed to optimize. In more modern bridges used in more modern instruments, where factory and user error correction are always on, this kind of tuning is no longer done. Just error correct any residual. But, back in the day raw hardware performance had to be better as error correction was not as often done.I'm very curious to see how the bad bridge repair goes.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 09, 2022, 07:59:39 pm
content: Quote from: inaxeon on February 09, 2022, 09:55:23 amI'm not sure of the exact ingredients, but it's an iron-loaded rubber or epoxy (both kinds are use) which is very lossy. A bit like absorber material in an antenna chamber. It might even be a trade name. We put it in cavities of RF housings to suppress modes, us it as tuning elements of baluns for RF mixers, using it to slope down an upward sloping RF path; it's the equivalent of Duct-Tape to the RF engineer.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 09, 2022, 08:45:05 pm
content: Quote from: Joel_Dunsmore on February 09, 2022, 07:54:41 pmThat is interesting, so with the repair I just did on the good coupler, ideally some kind of adjustment would be done to the ribbon to compensate (assuming my massive dollop of solder can be compensated for). From what you are saying - fiddle with it until directivity is highest.Quote from: Joel_Dunsmore on February 09, 2022, 07:54:41 pmI will be reporting it. I've ordered a range of 267 ohm resistors from Digi-Key, and I'll be having a little resistor pageant once they arrive. The idea is to fit the "purest" resistor of the lot.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 09, 2022, 09:15:06 pm
content: Another question - all these components, do the techs optimise them all for 6ghz use, even if they are destined for an instrument without option 006?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 10, 2022, 01:19:02 am
content: Quote from: inaxeon on February 09, 2022, 09:15:06 pmYes, there might be a slight difference but this bridge was originally designed for the HP85047 6 GHz test set (which also had a switched frequency doubler built in). It was repurposed for the 8753D and carried on to the E and the ES. So at what we call the "micro assembly" level, that part will have a 6 GHz test line limit. Back in the day we used to "bin out" parts (select better ones for higher frequency) but that is almost never done, because what happens if you get a lot of orders for high frequency and not enough bin out, so in today's world all designs are supposed to be 100% yield if all the parts going into it all meet their spec. No binning and no tuning (well, no mechanical tuning, you are allowed to do electrical adjustments). Thus, no more variable resistors: if something has to be adjusted, it has to us a DAC to set the adjustment programmatically. That being said you can have tuning that allows you to compensate for assembly differences but the tuning range has to be wide enough so that any assembly that meets the assembly document can be adjusted to meet the spec. For example (and don't F with it) there is a tiny tuning screw under the center pin of the main bridge that was put there to compensate for the variation of height above circuit that can happen when the RF pin comes through the package wall. It was intended to allow a tech to adjust the directivity without opening the bridge. Unfortunately, too many techs pushed it too far and cracked the sapphire circuit so we started setting it to a nominal value and had them adjust the tuning ribbon inside. Then only if at final test it wouldn't pass directivity spec, as a last resort they could try tweaking that tuning screw.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 10, 2022, 07:35:16 am
content: From what I could see the 85047 is typically specified from 300 KHz, whereas 8753ES is 30 KHz. Was the design of the bridge (I'll stop calling it a coupler) altered or is this the properties of other components in the test set?

author: khach
author_rank: Contributor
timestamp: February 10, 2022, 12:02:13 pm
content: Is that bridge classical HP double substrate design or new one single substrate according tohttps://patents.google.com/patent/US20070252660A1/en?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 10, 2022, 05:58:03 pm
content: Quote from: khach on February 10, 2022, 12:02:13 pmThe newer design isn't used in the 8753, it went into a version of the PNA, with extended frequency.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 10, 2022, 06:01:36 pm
content: Quote from: inaxeon on February 10, 2022, 07:35:16 ammain difference is the blocking capacitor in the DC feed was increased to a larger value (I think it went from 0.047 uf to 1.2 uF, as I recall. I don't remember changing the bias tee except to add snubbing diodes across the coils to avoid a kickback voltage, although those might have made it into the 85047 version as well.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 10, 2022, 07:38:31 pm
content: Quote from: Joel_Dunsmore on February 10, 2022, 06:01:36 pmThat's interesting, so there's potentially another repair option for those on a tight budget. These bridges appear to be a lot more common and cheaper than the 30 KHz units. I guess modification of that end of the bridge would affect its directivity less too? If one was to swap the capacitor for example...

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 12:57:06 pm
content: I've attempted the repair now. It didn't go as well as I'd hoped it would. Trying to solder the resistor across the capacitor and end of coax was basically impossible because the solder wouldn't take to either end. I had to scrape everything out and replace it all, including the capacitor.The results? Well I suppose it was as expected. Attached is the (relatively) good bridge doing S11 into a terminator. I think it's pretty good.Also attached is S22 from port 2 (the repaired bridge). It looks almost as expected, i.e. loss of directivity however with a weird trough at a few hundred MHz. I found that I broke the connection from the input connector to the stripline while I was screwing around in there so had to add a dab of solder to fix it, I think that may have introduced it.Additionally attached is S21/S12 of a bandpass filter I built and It's pretty much right on the money.Overall verdict however is that the instrument is usable once again, probably good enough for my purposes. I've been playing around with it for a bit it seems to be otherwise doing what I want.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 14, 2022, 01:37:12 pm
content: Nice workRe: the dip, Joel will have better calibrated intuition here, but I'm personally more suspicious of the new capacitor and its SRF than the dab of solder. Fortunately, a VNA is just the tool to test if a capacitor is still a capacitor at a given frequency -- it would just be a matter of setting up and calibrating a fixture, which happens to be a good intro-to-VNA project in its own right

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 01:57:56 pm
content: I'm thinking I re-do this with 0402 or 0201 components instead, and yes. I can now test stuff properly ;-)

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 02:08:20 pm
content: Oh Oh. I got another 4dB of directivity back by mopping up flux. hahaha.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 03:47:13 pm
content: I can see the big advantage of Joel's original technique. One of the issues I'm having trying to put components flat on the substrate is that I end up trapping flux under them which hurts the performance significantly.I had a few more goes replacing with clean components and I've got at least 34 dB directivity at 2.4 GHz now but can't get much better than 22 dB at 3 GHz. Definitely good enough for me...

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 14, 2022, 04:15:53 pm
content: Yep, the struggle is real. Even with a preheating pad, nano iron, good flux, and syringe with small tip. Was the big dip largely due to the flux?

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 04:22:25 pm
content: The "big dip" is still there. I actually swapped for an 0402 capacitor, made no difference. The effect of the flux was actually quite linear.Playing around with it, the dip isn't really affecting usage much as far as I can see. I can't see it at all when I'm testing antennas oddly.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 14, 2022, 06:17:27 pm
content: Well done, the dip simply says that's the frequency where the impedance balance of the bridge exactly matches the test load you are using; negative dips in return loss mean almost nothing (literally and figuratively). The rise as you go up in frequency is likely due to parasitic inductance in the 265 ohm resistor, or even "loss spreading" where the resistors effective termination is spread across the width of the resistor making the through line more lossy (one old AFRTG paper showed how a dot of resistor material on the end of an open line made a very good load termination above a few GHz.). That's why the original resistor was as thin as possible, to make the main-line contact area as small as possible. But this will be a completely serviceable bridge, for practically any purpose once you do a calibration on it. So there you go, you fixed up a broken box in a very fine manner.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 14, 2022, 07:59:52 pm
content: Quote from: Joel_Dunsmore on February 14, 2022, 06:17:27 pmIt could have been finer. It would be nice to wind back the clock and have another go with what I know now. I bought this thing to learn and I have to say the learning has been outstanding, even if the performance of it isn't anymoreNever would have expected an industry veteran and designer of the components to come to my rescue. Much appreciated.EDIT: attached another S22, where I managed to improve it a little mostly just by cleaning flux out of it.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 15, 2022, 12:08:39 am
content: Here's the spec from the 86205 which is the same on the inside: directivity at 30 dB to 0.3 MHz, 40 dB to 2 GHz, 30 dB to 3 GHz, 20 dB to 5 GHz, 16 dB to 6 GHzAnd I found in the bottom of my drawer, the original breadboard bridge for the bridge detector (designed by Wayne Fredrick), which design I stole (I guess Wayne stole it from Wheatstone so I don't feel so bad) and modified to move it up in frequency, and added bias (you can see the first bias board) and then the prototype of the HP 86205 before industrial design put a pretty package around it.PS: Prototypes are not number, Lab Pilots are numbered starting at 50, and production pilots start at 100; real shipping units typically start at 200. So this is the very first lab pilot unit.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 16, 2022, 09:13:06 am
content: I don't suppose you have a 75Ω version you could show us the inside of? Does that just have a matching pad at the input with the loss correction elsewhere i.e. in software?

author: TurboTom
author_rank: Super Contributor
timestamp: February 16, 2022, 04:05:26 pm
content: Here I posted a few photosof a 75 Ohms RLB from an 85046B test set. It's only rated to 2GHz IIRC, so obviously the circuitry could be kept a little less delicate, but still sufficiently "nice"... The "glass part" of the coupler shows less wide traces to match the higher impedance (obviously) and at the signal input, there's a resistive matching pad. At the output, there's some "discrete" LC matching network as it appears, but there may also be some additional impedance matching taking place at the transistion from the glass to the FR4 substrate that's not visible in the photos.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 16, 2022, 05:40:01 pm
content: I'm also intrigued about these:https://www.ebay.com/itm/384731461292?hash=item5993c836ac:g:KxsAAOSwb1piDA~j(5087-7054)They seem to be quite common (more so than 5087-7007) and have the more desirable N connector. Apparently from a three-dollar-bill variation of the 8753ES "H39" ?Would I be correct to assume (certainly if the H39 is for testing diplexers) that there's no biasing connections on the underside of these? I cannot find any picture of the bottom of one of these bridges frustratingly. Just wondering, another potential repair option for those who don't need bias.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 17, 2022, 07:07:05 am
content: Quote from: inaxeon on February 16, 2022, 09:13:06 amI think I still have one. It was a simple change, the main R1 resistor changes to560395 ohms as I recall, to change the balance point to 75 ohms. The main line was narrowed to be 75 ohm microstrip over airl; a 75 ohm cable was used in the balun and a 25 ohm back match resistor was used on the backside of the bias tee. The receiver and source were still 50 ohms so the coupled arm didn't change. I'll take a look tomorrow and see if I can find it again.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 17, 2022, 07:09:34 am
content: yes, before the 4 port ENA, literally ever cell phone had a diplexer that hand to be hand tuned, and a 3 port VNA (well, a switched 2 port VNA in fact) was extremely needed by the filter manufacturers. No one else ever bought one. the 4 port ENA stopped the sales of the k39 in it's tracks, as it should be.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 17, 2022, 07:57:06 pm
content: Quote from: Joel_Dunsmore on February 17, 2022, 07:09:34 amSo there is probably like 5 of them out there. That would explain why I can't find a picture of one

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 18, 2022, 05:34:17 am
content: Here's a picture of the 75 ohm bridge. Note the line is about half as wide as 50 ohm, the R1 resistor is 395 ohms (the formula is R1/75=50/10, but the 10 is really 9.5 when considering the lossyness of the ferrite beads. 75 ohm coax and a back matching resistor between the bias coil and the cap (tiny dark patch)

author: inaxeon
author_rank: Regular Contributor
timestamp: February 18, 2022, 12:12:22 pm
content: Thanks for that. By the way, where is R2 physically?I have a couple of more notes to add about my repair: I noticed that I had a big loss of directivity at 30 KHz on the repaired bridge. This turned out to be because I had used 0.1uF a capacitor, not 0.01uF. Changing that over fixed that problem.Second note: While I was back inside the bridge, I swapped out the 0603 267Ω resistor for an 0201 sized part. The difference? Bupkis.EDIT: I did get another 5dB at around 2.4 GHz by cleaning microscopic debris from the area where I believe R2 might be - between the end of the coax and the stripline to the receiver connection.

author: inaxeon
author_rank: Regular Contributor
timestamp: February 19, 2022, 08:57:47 pm
content: I took a few more macro shots, I think I found R2. For some reason I had failed to spot it up until this point!

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 20, 2022, 06:54:03 am
content: yup. R2. 50 ohm (the black resist material is 50 ohms per square so you can see the value of the resistor just by looking at the size).

author: 1audio
author_rank: Frequent Contributor
timestamp: February 20, 2022, 06:53:56 pm
content: I have a 8753E 75 Ohm. I have long wondered why the 75 Ohm world stops at 3 GHz. Is there something physical that makes 75 Ohm systems less useful above 2 GHz? Or is it a market driven issue?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 20, 2022, 07:37:23 pm
content: 75 ohms is principally used in CATV applications as the loss of 75 ohm cable is less than 50 ohm cable. In the past, the maximum frequency used with CATV cable was about 900 MHz, so no one would buy something with higher frequency. That being said, we did tune up a set of my 75 ohm bridges to 12 GHz so our own calibration lab could verify 75 ohm calkits to higher frequency if requested. The mode for 75 ohm type N is nearly the same as 50 ohm (both use identical 7 mm inside dimension of outer conductor) and in fact the 75 ohm female "long-slot" can survive a few insertions of 50 ohm type N-m without damage, though not recommended. Older 75 ohm systems used a short-slot female pin and those would break off; the long slot was designed for the 8753 bridge . It has poorer repeatability but much more robust.

author: zach05
author_rank: Newbie
timestamp: March 05, 2022, 08:56:49 pm
content: I guess there is a wire go through the ferrite along with the coaxis? What is this wire doing? Is it for balance out the (pulsed) bias current?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 06, 2022, 07:53:07 am
content: Quote from: zach05 on March 05, 2022, 08:56:49 pmKind-of. The balun has two different mu-factor beads on it, the first 2 are high frequency beads, and the next 4 or 5 are high-mu beads to pick up the low frequency. Unfortunately, when testing them, I found the inductive-beads saturated at quite low current so the low-end directivity of the bridge went south with bias. I figured out that by passing a filament through the outside of the coax could allow the a cancelation of the current through the center conductor; you can't tell from the picture but the bias current passes through the filament in one direction, goes out though a DC feed, is bypassed, and then goes back in through the adjacent DC feed to feed the center line. This bias cancelation allowed the bridge to maintain specified directivity up to 500 mA bias current. I did a very low frequency version of this bridge to test out the idea (using through-hole resistors). I think I still have it tucked away so next time I'm in the office I'll see if I can take a picture of it.

author: AJ3G
author_rank: Regular Contributor
timestamp: March 15, 2022, 10:28:16 pm
content: I am going to give this repair a try myself. I acquired a bridge assembly on ebay which was known to be blown (and it is). My thought was to use it as a standard RLB for my Scalar Network Analyzer. If I can achieve similar directivity as have been shown here, then I may just put it back together and keep it as a spare for my 8753E.Rich

author: inaxeon
author_rank: Regular Contributor
timestamp: March 16, 2022, 01:37:36 pm
content: Quote from: AJ3G on March 15, 2022, 10:28:16 pmI was eyeing that up. I'm in the market for a JPD bridge for my SA ;-) I wasn't going to plug my website - but - before attempting scroll down the the "Practical lessons learned" section at the bottom of this page:http://www.mattmillman.com/agilent-8753es-repair-part-3-a21-test-port-coupler/

author: AJ3G
author_rank: Regular Contributor
timestamp: March 16, 2022, 01:52:40 pm
content: Hi Matt:Thanks, I will check that out. Mine is acting like it has a blown resistor. I need to poke around the the area where the two resistors are mounted. One question I did have, would it not be better to use a conductive epoxy for this type of repair. As I look at it with magnification, it does not look to me like these were soldered by traditional means.I am trying to find a decent connector mate for the portion of the bridge which correlates to PORT 1/2 on the 8753E. It seems like this is a SMA, as one does thread in there nicely. I would like to find something which I can screw into the threaded holes in the bridge enclosure.Rich

author: inaxeon
author_rank: Regular Contributor
timestamp: March 16, 2022, 02:06:21 pm
content: Refer back to post #18 - JPD explains that epoxy is probably not practical due the excessive heat needed to cure. However it was what was used originally, but that was with that piece assembled in isolation hence the temperature considerations not being relevant.I don't think even the "solder" on the end of the coax is typical solder. Whatever it is, it appears to have a higher melting temperature and is quite brittle.

author: inaxeon
author_rank: Regular Contributor
timestamp: March 16, 2022, 02:20:43 pm
content: A few extra notes I didn't mention on my website:1) I ordered and tested quite a range of different resistors, 0201, 0402, 0603, thick film, thin film etc. I wasn't able to see any significant difference between any of them. The 0201's are maybe 2dB better approaching 3GHz but the same at 2.4 GHz and less. 0201s are very difficult to place and bond within such a delicate assembly in such a confined space. I'd figure that perhaps not worth it for many.2) Another commenter suggested that mounting resistors upside down would improve directivity. This wasn't my observation. I couldn't see any measurable difference however the orientation.3) I did not try "RF" resistors as availability is ridiculously limited at present, and there aren't even any types of a suitable value anyway.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 16, 2022, 07:06:03 pm
content: Quote from: inaxeon on March 16, 2022, 02:06:21 pmQuite correct. The capacitor (and other components on the bias side) are attached with epoxy (conductive or other depending on need) before the circuits are inserted. A wide gold-ribbon is attached to the other end of R2, and when the coax is put in, there is a little spring holder that fits into the two semi-circle groves in the package to hold the coax in place during cure. It is silver epoxied to the R3 pad underneath and the R2 ribbon is attached to the coax outer conductor with epoxy. The center pin is attached by attaching a ribbon across the main line and folding over the center pin, then it is tacked in place with a hot wedge and backed with silver epoxy. This gives a little compliance for temperature changes; if we hard fix it at each end temperature changes will cause the package to expend differently than the coax and break the connection. In fact the package is brass, not aluminum, to better match the coax expansion (our temperature test requirements are 10 cycles from +125 to -50). So the business end of the coupler doesn't use any solder. The center pin on the bias end is soldered to the circuit.

author: AJ3G
author_rank: Regular Contributor
timestamp: March 16, 2022, 07:16:08 pm
content: That is good information. I am not sure what has happened to this bridge but the center conductor near the two resistors is flapping around in the breeze. I lightly touched it to remove what I thought was debris. As I lightly tapped upon the center conductor, it is clear it is no longer affixed to the PCB.I am going to take another look under the microscope, but I think R2 has been completely blown. It certainly looks like it got very hot, and is measuring open.The blocking capacitor came off with almost no pressure. I was simply probing each side of the capacitor with spring loaded probes and I was using a very light touch. I think something bad has happened here.

author: inaxeon
author_rank: Regular Contributor
timestamp: March 16, 2022, 08:21:23 pm
content: Quote from: AJ3G on March 16, 2022, 07:16:08 pmWell that doesn't matter. I lost most of mine after copious amounts of mucking around trying out different components. The present situation is that it's replaced with a small strip of copper tape soldered to the remains of it. It doesn't degrade the performance.Quote from: AJ3G on March 16, 2022, 07:16:08 pmWould be interesting to hear what impact the replacement of that has.Quote from: AJ3G on March 16, 2022, 07:16:08 pmI also lost my blocking cap. Once again there wasn't any noticeable difference after it was replaced (certainly not with my measurement capabilities).Would be interesting to see pictures of this effort...

author: inaxeon
author_rank: Regular Contributor
timestamp: March 16, 2022, 08:24:58 pm
content: Quote from: Joel_Dunsmore on March 16, 2022, 07:06:03 pmWell that is interesting. I have to say I did find that particular arrangement to be an eyebrow raiser. Do you mean temperature changes during, or post production?

author: AJ3G
author_rank: Regular Contributor
timestamp: March 16, 2022, 09:25:22 pm
content: I would have to think this is Post production. I do not have a camera on my microscope, or I would have submitted some photo’s. Can you tell me, what type of RF Connector fits in to the port which the APC7 would normally go in. A SMA will thread, but I think the pin depth and gauge is different.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 17, 2022, 04:02:57 am
content: Quote from: inaxeon on March 16, 2022, 08:24:58 pmWe have instrument level temperature reliability test levels, we call them survival levels (something like =-40 to +70) but separately we have microcircuit levels that are -50 to +125C, cycle 10x. The idea is you want to be able to re-work a microcircuit without damage. Turns out the bridge has a more limited cycle after the coax is in place, +80 max, as the teflon in the coax will extrude and cause the coax to pop up. If a bridge was really over-powered, it could cause such an issue. Also, if it were dropped (outside the box, like it was taken out for testing) then it might be weight of the beads could pull the coax loose, but we also strife test that (bench drop, 1 meter) to make sure it won't break a bridge. But if it was overpowered for long enough the heat can loosen the epoxy and the end might pop out.When qualifying the bridge, I found the damage level to be onset of damage at +37 dBm; where onset was noticable non-recoverable change in directivity when driven at +37 dBm for 24 hours at 55C. power to destruction happened at about +42 dBm. So we back off 3 dB from +37 to say damage level is below +34 and another 3 dB to account for mismatch condition, and round down to +30 dBm.P.S. all the survival levels were modified when spinning HDD's were added to instruments because they have a much lower survival temperature.

author: AJ3G
author_rank: Regular Contributor
timestamp: March 17, 2022, 06:39:15 am
content: Hi Joel:This seems to explain what happened here. Center conductor has pulled up, and both R1/R2 appear open. It must have gotten pretty hot, because the debris I think I was attempting to remove looks like carbon. I also noted the conductive epoxy seems to have melted, and is some of it began to migrate from the center conductor solder joint down into the cavity behind the joint where there is a cavity/recess. If this was an severe over power event, then this may explain why C1 which I measure at about 7300pF @ 100KHz was so inclined to come loose with my ginger probing of it.Still working on identifying and obtaining what connector I can screw into the threaded area for testing when I get these parts sorted out and reattached to their appropriate positions. Ideally, I would like find a screw in connector that coverts to SMA Female. This way, if it can no longer be used a a coupler for the HP8753, I may repurpose it as a decent RLB with something better than 30dB of Directivity to 3GHz.Rich

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 17, 2022, 05:21:30 pm
content: Polyiron (I think it is trademark) is a powered iron that is mixed with epoxy or rubber, and forms an RF absorber. So in something like a bridge that depends on RF signals being balanced, you can put a little absorber on different legs (R1 or R2) to see if it improves the balance and directivity. We've also used it in things like balanced mixers to get better 2nd order suppression. And very often used in the lids of packages to suppress package modes or, for example, to prevent feedback from output to input through the air of an amplifier to prevent oscillation. That's an interesting problem where an amplifier might measure as being unconditionally stable but a package mode can provide a unwanted feedback path (look at the lid in the 8753 source package above the amplifier) .

author: AJ3G
author_rank: Regular Contributor
timestamp: March 25, 2022, 09:43:52 pm
content: Is there by chance another resistor located somewhere besides R1/R2. I have those installed, but I am getting awful RL Response. Seems like something else has gone whackyRich

author: jjoonathan
author_rank: Frequent Contributor
timestamp: March 26, 2022, 01:08:44 am
content: Yes, there is a third resistor R3 near the other two that connects the outer conductor of the beaded coax to ground. I believe it is directly beneath the coax, although some designs (the newer Keysight patent and the Anritsu design) place it on the same plane as R1, R2 but on the opposite side of the coax landing pad. It should be about 9.417 ohms.

author: AJ3G
author_rank: Regular Contributor
timestamp: March 26, 2022, 03:52:07 am
content: Good Info - Let me put it back under the microscope and see if I can locate it. Considering the other two were blown wide open, I suspect this one too is a problem. Now I just need to find it.

author: AJ3G
author_rank: Regular Contributor
timestamp: March 28, 2022, 09:50:24 pm
content: I found the final resistor. It too was wide open. I have included the RL response of the bridge. For R3, I have tacked in a 10 Ohm resistor for the moment. I am hopeful when I put in 9.41 Ohm in place of R3 I get a better low frequency response. I think this is a pretty good result.Rich