Fixed Attenuator Repair / Mods - Page 1
Forum section: electronics / rf-microwave

author: joeqsmith
author_rank: Super Contributor
timestamp: October 08, 2020, 02:08:00 am
content: Just another "working" eBay attenuator. Note the crater. Thought I would have a look. I assume these have BeO but was able to just warm the encapsulation with an iron set to fairly low temperature. The series section was good but the shunt had failed open basically right in the center of it.The edges look like a total hack job. Not at all what I was expecting.If you do ever have the chance to look at one, just keep in mind that BeO would have been fairly common. Don't go in there with dremel and start grinding away at it.I plan to rebuilt it using some better parts with different value that will make it useful around the lab.

author: Tony_G
author_rank: Super Contributor
timestamp: October 08, 2020, 06:47:33 pm
content: Any ideas on what caused the crater? Overpower event maybe?TonyG

author: SilverSolder
author_rank: Super Contributor
timestamp: October 08, 2020, 06:52:53 pm
content: Technically, the attenuator does work - It just attenuates rather a lot more than expected!

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 08, 2020, 10:00:50 pm
content: made similar experience:so called '100W' 40dB (3GHz) Attenuator got a crater from ~50W @ 432MHz, no idea why that happened; still waits for repair; indeed it still works - either as simple dummy or as Att with much, much higher attenuationa 10W 30dB Att I overloaded I got fixed with bigger SMD resistors

author: radiolistener
author_rank: Super Contributor
timestamp: October 08, 2020, 10:07:49 pm
content: I also have some experience with Chinese attenuators.30W 40 dB burned out from 15 W power.10W 40 dB burned out from 5 W power.Both burned out during test, just out of the box.Now I'm not sure if it worth to buy 100W versionOn the other hand, several years ago, I bought Chinese 10W dummy load:https://www.aliexpress.com/item/32771021842.htmlIt has 49.87 Ohm, good VSWR and pretty flat response. Sometimes I'm using it to terminate 15W power for 10-15 sec. It still alive and it has very nice good quality SMA connector with air insulator, it is very different from cheap "golden plated" Chinese SMA.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 08, 2020, 10:16:28 pm
content: But you are not using Chinese Watts, they are just the same as their Amp/Hour capacities of batteries.George G6HIG

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2020, 12:59:01 am
content: Quote from: Tony_G on October 08, 2020, 06:47:33 pmThe crater was due to the excessive heat/pressure when the shunt was damaged. It was open and I would think there was some arcing going on. I assume the PO over powered it.Quote from: SilverSolder on October 08, 2020, 06:52:53 pmIt certainly does "eBay work". Beause the shunt failed open, it attenuates much less than expected. The response is not very flat with that custom made stub hanging off it.Trying to find a source for flange mount resistors without a year lead time is the challenge.I've bought parts from this company in the past. They have a few in stock but nothing I can use.https://www.surplussales.com/RF/RFAtten-6.html

author: Grandchuck
author_rank: Super Contributor
timestamp: October 09, 2020, 11:18:20 am
content: Quote from: George Edmonds on October 08, 2020, 10:16:28 pmGood one George

author: joeqsmith
author_rank: Super Contributor
timestamp: October 10, 2020, 01:09:02 am
content: One of the connectors was damage and required replacement. Rather than just replace the part, I decided to remove the boards, sand the heatsink and paint it. I used some of the high temp paints I use with the bike engine parts.The new element is rated for a much higher frequency than the attenuator originally was. I'll make a label for it, once I have an idea how well it performs.So, I basically bought a decent heatsink with spreader, a couple of Teflon boards and a connector.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2020, 11:57:29 am
content: Some time ago I built this 50 ohm load. Basically, it's a 250W flange mount 50 ohm resistor followed with a 4.95K in series with the SMA (40dB). A bit of magnet wire to compensate it. It was made to test a current probe but I use it with the SA and VNAs to look at small amplifiers. The problem is the BW is limited to around 100MHz (limited by the tap) and the power it can dissipate is fairly low.Because the JFW's original network's shunt burned open, the plan is to modify it to provide about a Watt. A second attenuator is added to get down to the working range of the VNA. The new network provides about 50% derate.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 04, 2020, 01:12:26 am
content: The new parts arrived. Used DC to check the heatsink. I may end up mounting a couple of fans to it.It's fairly flat out to a GHz where the JFW was originally spec'ed. The new part has a +/-1dB flatness spec or to 3GHz. It's not a perfect mechanical fit and the launch isn't quite right. Very usable now.

author: SilverSolder
author_rank: Super Contributor
timestamp: November 04, 2020, 11:43:43 am
content: Impressive! - is there any contribution from the test gear in those charts, or is it really possible to measure RF this precisely?

author: joeqsmith
author_rank: Super Contributor
timestamp: November 04, 2020, 01:31:00 pm
content: Quote from: SilverSolder on November 04, 2020, 11:43:43 amHum... Well, that's a very good question. Yes, the equipment will have some contribution. This includes the standards, connectors, cables, VNA. Some error would be from the setup. Contrary to the belief that hand tightening connectors is good enough to 6HGz, I don't subscribe to that. However, the torque wrench would introduce some error. Then there is the untrained, unexperienced human (me).If you feel that the data is impressive, you should have a look at a post from member rf-messkopf. Follow their link to see a metrology grade 20dB attenuator.https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2964070/#msg2964070I don't have anything like that in my home hobby lab to try and verify the performance of my setup. I'm using home made SO standards and a cheap Mini-Circuits load. If you read through that thread, you will find I was getting some help from rf-messkopf.Yes, there's lot's of room for errors and the higher the frequency, the more critical everything gets.

author: SilverSolder
author_rank: Super Contributor
timestamp: November 04, 2020, 07:28:04 pm
content: LOL now that's what I call a flat response, on that 20dB attenuator!Even at 400Hz, I can detect sub-pF capacitance effects when playing around with bridge circuits, for example.I can barely imagine how controlled the "electrons' environment" has to be to contain the reaction at GHz frequencies...

author: joeqsmith
author_rank: Super Contributor
timestamp: November 05, 2020, 05:45:03 am
content: Within 0.03dBm to 4GHz, you know they weren't handing those out for free. For low frequency work (<GHz), I have a set of home made standards to get me in the ballpark. I wouldn't mind having a few high end standards to use above a GHz. I tried to make a T-check but it was a total waste of time. Similar to trying to make a load.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 05, 2020, 06:26:14 am
content: Quote from: joeqsmith on November 04, 2020, 01:31:00 pmreally? do you have to bring this notion to this thread? afaik there is only one "guy" in this forum swear by it. you are measuring S21 btw

author: SilverSolder
author_rank: Super Contributor
timestamp: November 05, 2020, 12:19:42 pm
content: Quote from: joeqsmith on November 05, 2020, 05:45:03 amThe physical dimensions and tolerances just get impossibly small at some point. Wonder how the high end devices are actually made... photolithography?

author: SilverSolder
author_rank: Super Contributor
timestamp: November 05, 2020, 12:26:47 pm
content: Quote from: Mechatrommer on November 05, 2020, 06:26:14 amI can get to sub-PPM resolution and sensitivity, but at that point things are so sensitive that you almost can't breathe in the same room without affecting something. So just like with high frequencies, the mechanical and electrical properties become a challenge to make stable over a range of temperature, humidity, etc. - the physical implementation of the ideal laws becomes a real issue!

author: joeqsmith
author_rank: Super Contributor
timestamp: November 05, 2020, 01:44:59 pm
content: Quote from: Mechatrommer on November 05, 2020, 06:26:14 amIt's actually very relevant to SilverSolder's question on sources of error. It also seems to be a very common opinion as I have had several people claim this after I posted the first video on the Nano. You are not alone but you are the most recent.You get five stars for knowing and making clear that was S21.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 05, 2020, 02:10:26 pm
content: Quote from: SilverSolder on November 05, 2020, 12:19:42 pmEspecially looking at the 3.5 standards I borrowed. The person that loaned them to me told me about when they first started their career in RF. Connector care was a mandatory lecture from the old German guy who managed their lab, before you were allowed to touch any equipment.I'm guessing a few of these would be sold on eBay as working, with an asking price of half of new.https://www.microwaves101.com/encyclopedias/how-to-not-trash-a-calibration-kitThe unknown thru is commonly used.http://ena.support.keysight.com/e5071c/manuals/webhelp/eng/measurement/calibration/advanced_calibrations/unknow_thru_calibiration.htm

author: Yansi
author_rank: Super Contributor
timestamp: November 05, 2020, 02:15:38 pm
content: Quote from: joeqsmith on October 09, 2020, 12:59:01 amShit ton of these on Aliexpress. Mostly, both new and used. Did not test any of them yet, but have a 10W 20dB attenuator on order.Aliexpress lead time is just about a month.

author: Bud
author_rank: Super Contributor
timestamp: November 05, 2020, 03:04:56 pm
content: This proves the validity of the 11th testament:"Thou shalt downscale all Chinese specifications by a factor of 2"

author: Yansi
author_rank: Super Contributor
timestamp: November 05, 2020, 05:48:53 pm
content: I am kind of surprised, that there would be any problems with the chinese power attenuators. The only thing I would direct concerns at, would be the flatness.Even 10W is a lot of heat and when improperly heatsinked it is possible to burn out even much higher specified part.If it burns out even after a very brief moment load, than oh well .. .

author: joeqsmith
author_rank: Super Contributor
timestamp: November 06, 2020, 01:29:37 pm
content: Quote from: Yansi on November 05, 2020, 05:48:53 pmI don't have any experience with them, which is why I asked about your testing when yours arrive. It's very possible that the parts mentioned above were not properly heatsunk. They don't provide any details.In my case, if the attenuator fails in such a way that more power is output than expected, it could damage the device it's attached to. Similar to how that original attenuator I show had failed. This particular attenuator will be followed with a second one.

author: Yansi
author_rank: Super Contributor
timestamp: November 06, 2020, 02:04:11 pm
content: Those chinese ready-made attenuators (have a bunch of N-connector type ones) are definitely quite small for the rated power, would guess the temperature rise would be extreme.I also use them mainly as a protection of equipment when probing unknown stuff and am aware of the bad heatsinking. To be honest, I have never disassembled any of those I have, so I am not even sure, what is lurking inside. 2512 SMD resistors are certainly not good enough for a "10W" attenuator, regardless of heatsinking.I can test them, but only with DC voltage, I do not have any suitable higher powered RF source.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 06, 2020, 07:22:41 pm
content: I don't see any reason why testing with DC would be a problem. Maybe just used a separate terminator rather than any test equipment to terminate the output. If you have a way to measure temperature, that may be interesting to know as well. Also mounting orientation and any sort of convection or forced air cooling. If you know where you purchased them from and part numbers, this may also be helpful.Should make for an interesting test.

author: Yansi
author_rank: Super Contributor
timestamp: November 06, 2020, 08:39:51 pm
content: RF current can flow only in thinner layer of the resistor and create more localized heating, than DC could, ... or... am I making this up?I have just checked what I have ordered, an seems I have only ordered 10W 50ohm terminators (flangemount resistor with the copper foil strip on the side). Did not buy any of the 10W flangemount attenuators, yet

author: joeqsmith
author_rank: Super Contributor
timestamp: November 06, 2020, 11:42:19 pm
content: As long as you apply the same power level, I would doubt you could detect a difference in the temperature profile. Actually, I doubt you could detect the profile just running it one way or the other. The film is not very thick, even for a thick film part. It would seem like if you ran some sort of extensive life test, you could maybe detect a difference in how you drive it. I guess if the part is going to break down at the peak voltage, we could also see some improvement using DC. A good study for your PHD but I don't see any practical use in this case. I would be far more concerned with how effective the heatsink is and want some general idea of the temperature.Too bad about the parts. If you decide to buy some and run the test, please post the results.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 07, 2020, 06:06:57 pm
content: The attenuator network is rated for 150W with proper mounting. I let it run for a about an hour in the office basically the way I plan to use it. Mini-Circuits VAT attached to the output is rated for 2W. 100W input power. There is a measurable difference between the two case halves. Majority of the VATs heating is conducted from the JFWs case.The heatsink is raised slightly but really this is pretty much a worse case condition. I'm not comfortable with the temps and will go ahead and make a bracket for the cooling fans.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 10, 2020, 03:57:38 am
content: Made up the bracket for the fans and added a label. Plan to run the fans from the small battery pack to the right.

author: Yansi
author_rank: Super Contributor
timestamp: November 10, 2020, 08:42:01 am
content: Awesome work with that cooling bracket.But would have added a standard barrel jack for the power supply input to the fans. (I am not used to having USB power banks all around). Just a different personal choice. The bracket looks almost like factory made.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 10, 2020, 12:35:27 pm
content: Thanks. Battery just meant less cables and the fans don't require much. I have a few USB chargers and an extension cable if I need to run it off AC.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 11, 2020, 01:37:05 am
content: Old Motorola MRF421 home made power amplifer (note the old Intel 486 heatsink) and my cheap power meter attached to the attenuator. 30Watts without fans.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 11, 2020, 01:58:11 am
content: Roughly 33 Watts is about it for this amplifier. Brymen's T2 showing the amplifer's heatsink temp with the PC fan running, T1 is the attenuator's heatsink. Attenuator's fans are running.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 11, 2020, 02:42:51 am
content: This amplifier was a kit I bought that was based on the MRF141G, rated for around 300W. Shown running at 150Watts. This is what the attenuator network is rated for.You can see the kit here:https://www.eevblog.com/forum/projects/home-made-pa-and-load/msg635226/#msg635226***Poor watt meter. Note the 0.5W MAX warning. They shouldn't include a 2W range if they don't want you to use it.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 12, 2020, 12:56:15 am
content: Running at roughly 150W with the added attenuators (40dB total or 15mW output), for about a half hour. Temperatures are stable and the heatsink/fans are having no problem keeping up. Idling along at 37MHz. Actually, if you saw the thread where I built the amplifier, you will notice I did not use a spreader. This is actually pretty much the limit of the amplifier as I built it.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 13, 2020, 02:43:19 am
content: Radiator and water pump off to the left. Putting a bit over 160Watts into the new attenuator. Seems to handle it just fine.