RF attenuators - pulsed power how to guess? - Page 1
Forum section: electronics / rf-microwave

author: daqq
author_rank: Super Contributor
timestamp: February 10, 2020, 09:07:21 am
content: Hi guys,In RF attenuators (such ashttps://www.mouser.sk/datasheet/2/609/hmc656lp2-hmc658lp2-586875.pdforhttps://www.mouser.sk/datasheet/2/392/Susumu_01192017_PAT-1206544.pdf) I noticed that no one really mentionspulsed powerratings. The average values are there, but no mention of pulse power.For a particular application I need to get some 40 to 50 dB off a pulsed signal. The pulse is 1-3us wide, with a peak of around 30W (~+45dBm). The repetition rate is 500 pps max. So the max average power would be 0.045W (~+17dBm), well within the specs of both mentioned devices. But there is no data on how well they handle pulses.In non RF devices they generally have a pulse characterization (you can get away with several watts with a short enough pulse with a 0402 device).Is this one of those characteristics that are well known to RF guys? How much can I abuse these attenuators in pulse terms?I am planning on stringing a few attenuators together in any case, say, first 3dB, then 20dB, then 20dB, or something along those lines, so the power dissipation would not be those 30W in any case on any one device.Thanks,David

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2020, 11:15:18 am
content: Hmmm...... I am in no way an expert in the field, but here's my common sense opinion:With only a few us pulse width, and only 5 dBm above the max CW power rating of the first one, heating of even thin film resistance elements is unlikely to be a problem. Considering voltage into 50R, 25 dBm is 4V RMS, 5.6V peak, and 30 dBm is 7.1V RMS, 10V peak, which IMHO is low enough that there is no risk of flashover or insulation breakdown, especially as it has a DC withstand rating of around 5V (depending on specific model), so if there's no DC on it, a 4.4V increase of the peak voltage can be ignored.

author: daqq
author_rank: Super Contributor
timestamp: February 10, 2020, 12:11:21 pm
content: Quote from: Ian.M on February 10, 2020, 11:15:18 amThanks for the reply, but the input power is 30W, which is ~45dBm, which is 20dB above the nominal power rating for the Analog Devices one (+25dBm), so 100x. My guess is that it should be OK, given the fact that it's a glorified resistor, but I'm not sure. I've contacted tech support of a few companies, will post replies here later.edit: Oh, thanks, I just noticed the maximum voltage rating for the first device! Damn. Must be going blind. So that rules the first one out, as 30W @ 50 Ohm is ~40V.

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2020, 12:25:22 pm
content: AAAgh!! sorry about that goof. 30W is two orders of magnitude greater than the CW rating, which *may* be OK power wise at a very low duty cycle, but gives nearly 55V peak into 50 ohms. Flashover is unlikely, but, depending on the method of construction and feature size of the module, insulation breakdown would be a significant risk.Can you use some other method of tapping off the signal source? e.g. a slotted feeder or stripline or similar properly terminated with a matched load that can handle the pulsed power, with a pick-up loop weakly coupled to it to get the input power to the attenuator down to what can reasonably be handled.Obviously that would be no good if you *really* need a flat response over a very wide frequency range.

author: daqq
author_rank: Super Contributor
timestamp: February 10, 2020, 01:13:51 pm
content: QuoteSadly, this is already after some heavy attenuation from the signal tap. The actual main RF power is in the tens of kW region. Unfortunately, the power has to be this large, due to other requirements.QuoteThe resistive ones could be OK since it's just a resistive layer on ceramics, but the Analog Devices parts are apparently an GaAs die with some limitations.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 10, 2020, 01:48:13 pm
content: No real experience with RF attenuators, by my two cents based on experience with precision film resistors:Slight overvoltage doesn't lead to insulation breakdown or resistive element vaporization in an instant, but slowly degrades the resistive material over time. So if the attenuator isn't blown instantly by the pulse power, it still may lose its properties over time if the pulses are repetitively applied. So I'd go for a nominally 30W rated attenuator ...

author: daqq
author_rank: Super Contributor
timestamp: February 10, 2020, 07:45:05 pm
content: Damn.Well, so far the best I've found is this series:https://od.lk/f/MzBfMTQ3NDcyNjRfwith a nominal power rating of 20W and a pulse rating of 200W @ 1us/1% duty cycle. And of course, like everything microwave, it's absolute unobtainium at normal distributors

author: TheMG
author_rank: Frequent Contributor
timestamp: February 11, 2020, 01:35:17 am
content: I regularly use attenuators rated for 20W CW on systems putting out 1kW peak pulse power with pulse widths and repetition rates very similar to what you're dealing with.They are however N-type pass-through attenuators not tiny surface mount attenuators so I'm not sure how those would handle it.

author: radiolistener
author_rank: Super Contributor
timestamp: February 11, 2020, 02:45:50 am
content: sometimes, if you can accept equipment damage risk, you can try to use low rated attenuator for high power pulse and often low power attenuator will survive, but not always. Sometimes it works, sometimes your expensive equipment will be damaged. This is game of chance.But if you need for a reliable solution, or if you're worried about your very expensive equipment, then you're needs to use properly rated attenuator, even you're using it in pulse mode.

author: daqq
author_rank: Super Contributor
timestamp: February 20, 2020, 09:36:35 am
content: Update: I got a reply from minicircuits, according to their tech support, their KAT-x+ series (https://www.minicircuits.com/WebStore/dashboard.html?model=KAT-3%2B) should have no issues with the my requirements.I'm going with them as I am being ignored by Smiths Interconnect. Which seems a pretty normal approach from many of the companies I've contacted so far.Is this a normal approach, to ignore potential customers? How does one approach a company properly? Am I missing some bit of commonly known corporate etiquette?

author: capt bullshot
author_rank: Super Contributor
timestamp: February 20, 2020, 11:07:41 am
content: Quote from: daqq on February 20, 2020, 09:36:35 amFrom my experience, one gets easily ignored if your company isn't known as a large enough (substantial amout of money already done) customer. So you might have to first send someone from purchasing winking with enough money for them to notice you.

author: daqq
author_rank: Super Contributor
timestamp: February 20, 2020, 12:32:24 pm
content: QuoteI'm guessing that this is the same kind of company who complains about people using the cheaper Chinese alternatives when they turn down business without even politely telling me to bugger off. I'm getting this a lot.

author: eevjohn
author_rank: Newbie
timestamp: February 21, 2020, 02:46:34 am
content: The answer to your original question is that the “Mouser type” attenuator, a small signals device, is normally never used in your type of application, and so the manufacturers don’t include a peak power specification.Your choice of the higher power 10 dB chip type device as first attenuator is reasonable for your application and lower cost tabbed flange mount hybrid devices are readily available on the web. Be aware that these types of attenuator are not intended to be precision attenuators. If you are using them to set up a measuring circuit, then you should measure the attenuation of your actual devices.In general, medium to higher power resistive attenuators do have a peak power spec, and this is nominally 10 to 100 times the average power rating, and well below the flash-over voltage of the device and associated connectors.I don’t quite understand your comment “For a particular application I need to get some 40 to 50 dB off a pulsed signal”. You don’t say if you want to end up with a level of 50 dB below 10+ KW, or 50 dB below +45dBm.From your parameters in your first post, it sounds like your application is primary radar or similar.If I interpret your application correctly, how this is done in normal low power radar practice, is to use a 20 dB directional coupler as a first sampler, as it is matched, has little through loss, and is robust and can easily handle the peak power and voltages. This will normally have “N” type connectors.Then fixed coaxial attenuators are used on the sample ports, one for forward power, and the other (if fitted) for possible indication of VSWR.You don’t mention the frequency involved. Be cautious about the frequency limits of your attenuators and connectors.If it is 1030 MHz (SSR), or 3 GHZ (PSR), then there should be little problem with most hybrid or fixed attenuators.But, if it is 10 GHZ (marine radar), observe limitations before purchase.Finally, be cautious about the calibration of your attenuators, as they may be slightly off nominal value but within specification.Good luck.