Attenuator rf input power - Page 1
Forum section: electronics / rf-microwave

author: hf303909
author_rank: Newbie
timestamp: May 18, 2021, 06:03:03 am
content: Hi,I have a Narda rf attenuator (765A-20) which is rated as followsPower In Average: 50W max.Power In Peak: 2kW max.https://nardamiteq.com/docs/051-FIXEDATTENUATORS.PDFQuestion:Unfortunately"Peak" is not specified in the data sheet - so what exactly does "Peak" mean?Do you think this attenuator is capable of handling 100-150W rf input power for 1-2 seconds?ThanksMichael

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2021, 12:35:19 pm
content: This may be limited by the voltage brake down of the part. They may have some sort of overload rating. For example, you could run it at 1.5 times the rated power for 5 seconds. Maybe some duty cycle or cool down time would be defined. At 75C, looks like your part derates to about half. Have you tried to contact the manufacture yet and explain your application?I bought a custom modified attenuator off eBay that was basically just a series resistor. The previous owner must have overloaded it, which caused the ground to open. They may have modified more than just the attenuator.Maybe a lightning strike. I was testing a small power amplifier and needed a larger 20dB attenuator. Rather than buy one, I decided to make my own mods to it as it really had no value being dead.https://www.eevblog.com/forum/rf-microwave/fixed-attenuator-repair-mods/

author: hf303909
author_rank: Newbie
timestamp: May 19, 2021, 01:39:28 pm
content: Thanks for your feedback and for sharing the link to your very nice repair projectI have already contacted the manufacture but have not yet received a response...So I was hoping that there might be someone here who has practical experience/insight with the peak power rating.

author: radiolistener
author_rank: Super Contributor
timestamp: May 19, 2021, 03:14:10 pm
content: According to my experience, dummy load can survive after a short peak power which exceed their rating. Sometimes I put 15 W into 10W Chinese dummy load for 5 seconds and it survive.But it looks that attenuators are more sensitive to overload, because in my experience no one attenuator survived even when I applied power which fits their rating. I think most of them burned out because I didn't know that they are single direction (Chinese attenuators don't have any marking about that). But I don't recommend to exceed rating power for attenuator even for several seconds.

author: xmo
author_rank: Regular Contributor
timestamp: May 19, 2021, 04:36:25 pm
content: With respect to the peak power rating of attenuators, JFW says:"If no pulse width is specified, then 1 microsecond pulse width is assumed.To calculate the duty cycle, divide the average power rating of the unit by the peak power. For example, if a part is rated at 10 W average power and 1000 W peak power, the duty cycle would be (10/1000) = 1%."

author: TheMG
author_rank: Frequent Contributor
timestamp: May 20, 2021, 12:28:11 am
content: With regards to RF components such as attenuators, dummy loads, directional couplers, etc. "peak" usually means low duty cycle pulsed RF power in the microsecond range.When you're dealing with seconds (rather than microsecond pulses), there are too many unknowns to know for sure if it's safe for the attenuator.A lot depends on the internal construction of the attenuator. The thermal resistance between the resistive elements and the case, as well as the thermal mass of the resistive element itself, will dictate how much power it can handle for a few seconds without damage.Since such information is not usually available, to be on the safe side use an attenuator that has a sufficiently high continuous power rating. Unless of course you don't particularly care about the risk of damaging the attenuator, and the consequences that failure might have.

author: rfclown
author_rank: Frequent Contributor
timestamp: May 20, 2021, 04:53:15 am
content: Peak power is just what is says, the maximum power at any instance in time. Average power is just what is says, the average power over time.So... " 100-150W rf input power for 1-2 seconds"100 to 150 watts is less than the peak rating of 2000 watts, so this is ok for the peak rating of 2000W (you never put in more that 2000W).The average rating is 50W. For a 100W input, the duty cycle would have to be <= 50% (100*0.5=50). For a 150W input, the duty cycle would have to be 33.3% (150*.333=50).So if you put in 100W for 1 second, you'd have to wait 1 second before doing it again (and again).If you put in 100W for 2 seconds, you'd have to wait 2 seconds before doing it again (and again).If you put in 500W for 1 second, you'd have to wait 9 seconds before doing it again (and again). 500W * 10% duty cycle = 50 W (avg rating)

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2021, 10:37:02 am
content: Quote from: rfclown on May 20, 2021, 04:53:15 amAccordingly, OP could put 150W into the device for an 8 hour shift and turn it off to cool overnight and everything will be ready for the next day. That's only a 30% DC and by your loose definition, our peak is still only 150W. Should be fine.I did find where Narda defined peak for some of their other devices which was in the uS, I wasn't able to find any application notes for their attenuators.

author: rfclown
author_rank: Frequent Contributor
timestamp: May 20, 2021, 01:26:11 pm
content: Quote from: joeqsmith on May 20, 2021, 10:37:02 amYou're right, I'm wrong, to a degree. I realized after I sent this and went to bed that you can't extend the time indefinitely. I was using whole numbers to simplify the math for the explanation, and the OP's original question specified 1-2 seconds. If I was using an attenuator shown in OP's original post I wouldn't hesitate to put 150W into it for 2 seconds. I have attenuators just like that. But I wouldn't push the duty cycle like in my first reply to get all the way to 50W average, just like I wouldn't use a 1/4 watt resistor at 1/4 watts continuously (I'd use a 1/2W at least instead). I probably wouldn't go more that 25W average.50W is quite a bit of heat. If you actually dissipate 50W in an attenuator, it will get very hot. Think of touching a 60W incandescent bulb.TheMG's answer is on target.I worked at one company where our products put out 12W or 50W RF power. We didn't have any attenuators over 2W in the lab because our duty cycle was low.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2021, 04:39:01 pm
content: Quote from: rfclown on May 20, 2021, 01:26:11 pmIt's about a $1000 part.https://www.digikey.com/en/products/detail/l3-narda-miteq/765A-20/9092997I would want to make sure I understood the manufactures datasheets.

author: rfclown
author_rank: Frequent Contributor
timestamp: May 20, 2021, 05:50:59 pm
content: Quote from: joeqsmith on May 20, 2021, 04:39:01 pmYou are correct, and like the OP stated, the datasheet isn't specific on the peak; hence his question. I just now Googled and the only App Note I found on their attenuators didn't talk about about power ratings.https://nardamiteq.com/product-spec/037-AttenuatorAppNotes.pdfI happened to have two 50W attenuators at my desk right now. One is a Fairview FMAT4008-40. Datasheet specifies 50W CW input, and 5kW peak 5 usec 0.5% duty cycle. 5kW*0.5%=25Whttps://www.fairviewmicrowave.com/images/productPDF/FMAT4008-40.pdfThe other is a Meca 650-0301F4. Specs are 50W Input Power, 1000W Peak Power 1 usec pulse 5% duty cycle. 1kW*5%=50Whttp://www.e-meca.com/rf-attenuators/n-type/50-watts/650-3-1f4The peak specs are rated with a duty cycle that is on par with the CW input spec. This is what I've seen in the past, and why I answered the question as I did originally. I just didn't say originally that I would not normally run at maximum rated power, but would de-rate to give a safety margin.When I was Googling for the specs of the Narda, I hit an eBay sale for a 765-20 for Buy-it-now $99.95+$15.20 shipping. I've bought all my high power attenuators used, because as you point out, they are expensive new. $100 bucks is still something you don't want to blow up.

author: Miti
author_rank: Super Contributor
timestamp: May 21, 2021, 11:35:48 am
content: Quote from: radiolistener on May 19, 2021, 03:14:10 pmAre all the attenuators unidirectional? I have some test fixtures at work with some Minicircuits 20dB 20W and 10dB 5W and I couldn’t find anything about directionality in their data sheet. They are generously derated and work fine but I’d like to know what to expect.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 22, 2021, 12:23:50 am
content: Quote from: Miti on May 21, 2021, 11:35:48 amI've only ever run into uni-directional attenuators in the 500W+ power range. All small (<500W) attenuators I've ever use have been bi-directional (same power handling in both directions).That does not mean small attenuators that are uni-directional don't exist, but they are definitely uncommon.The reason big power attenuators are usually uni-directional is for cost and complexity reasons. An attenuator, whether it's a T-pad or a PI design, essentially has 3 resistive elements within it. Most of the power dissipation, depending on the dB value, occurs in 1 or 2 of the resistors, the third (near the output port) dissipates much less power. So in an attenuator handling big power, downsizing the third resistor can reduce the size and cost of the overall attenuator, but means the maximum power handling in that direction is significantly reduced, making a "uni-directional" attenuator.With small attenuators, sizing the resistive elements for equal power handling regardless of direction does not significantly add to the size or cost, so these smaller attenuators are virtually always bi-directional.

author: tkamiya
author_rank: Super Contributor
timestamp: May 22, 2021, 02:51:16 am
content: Narda's spec sheet for this attenuator is useless. Most of the time, the word PEAK is specified in duration and duty cycle. Also, there is another spec: "Average." What does it mean? Does it mean continuous at 100% duty cycle or what? As a fine print, it shows de-rating curve for temperature. It still doesn't explain what "average" means.Take a look at this datasheet from Mini-circuits. In particular, look at Maximum Ratings. It says Peak Power 1KW, and below that, it says 0.05% duty cycle 5 us (that's 5 micro second pulse).With that said, if you put a thermocouple on it, and measure the temp, cut the signal as soon as temp raises to 100C (taking into account internal element is hotter than the heat sink), you could probably use it at "peak". For 100 watts to 150 watts, you should be OK for few seconds. Make sure it cools down before you apply the signal for second time.

author: rfclown
author_rank: Frequent Contributor
timestamp: May 23, 2021, 05:00:46 pm
content: Quote from: Miti on May 21, 2021, 11:35:48 amYou have to read the markings and look at the datasheet. Most lower wattage attenuators are not specified and are assumed bi-directional. It's nice when the manufacturer clearly marks then, like in the attached pictures. One is a uni-directional Weinschel clearly marked as to what power is allowed on input and output. The other is a bi-directional Aeroflex/Weinschel. I find that most bi-directional attenuators are not marked like this one is. I've haven't used any 1W-2W attenuators that were specified. I've always assumed bi-directional for those, and I've never blow one up. I've used 50W 1.2% duty cycle signals (0.6 watts avg) with 1W attenuators in both directions safely (previous job. did this daily for 2.5 years). None of my 5W-20W attenuators are marked, and I assume bi-directional (I have several). 50W and above it's a mixed bag (but often uni-directional).

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2021, 05:50:11 pm
content: With respect to unidirectional: I recommend buying high-power unidirectional attenuators instead of equal-power-rating dummy loads. A simple termination on the output end makes it a good dummy load, but the output makes a good place to monitor the signal on a normal CRO, spectrum analyzer, or meter.

author: radiolistener
author_rank: Super Contributor
timestamp: May 23, 2021, 10:46:21 pm
content: Quote from: TimFox on May 23, 2021, 05:50:11 pmbut it looks that dummy load can keep much higher peak power than attenuators. At least in my experience all attenuators burned out from power near to it's rating value, while dummy load works several seconds with 300-400% power overload with no damage.Some burned out attenuators can still be used as dummy load, for example 40 dB attenuators can works as dummy load even without termination. But of course you're needs to use input which is not burned out.

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2021, 11:09:13 pm
content: My experience was with high-power unidirectional attenuators from Bird, which are built similarly to their dummy loads. Was your bad attenuator experience with uni- or bi-directional units?

author: radiolistener
author_rank: Super Contributor
timestamp: May 24, 2021, 01:13:17 am
content: Quote from: TimFox on May 23, 2021, 11:09:13 pmIt was all Chinese attenuators from different manufacturers. I don't know about all of them, but some of them turned out to be unidirectional. I found it too late, unfortunately Chinese attenuators are not marked and in most case even seller don't know if they are uni directional and which input is for high power.

author: hf303909
author_rank: Newbie
timestamp: May 31, 2021, 06:53:52 pm
content: Update:Still no feedback from Narda support.I also contacted 2 sales reps but they were not able to answer to my question and so they forwarded my request to Narda support. But even then I did not get a response from support....It seems that Narda is not willing to share more detailed technical specifications with their customers.

author: tkamiya
author_rank: Super Contributor
timestamp: May 31, 2021, 09:08:41 pm
content: Which is often the case for those of us who buy on eBay or second hand in other ways. If a user has not bought it from company's official distributors, we are really company's customers. Use conservative ratings and that's often the best course of action.