Understanding Step Attenuators in VNAs - Page 1
Forum section: electronics / testgear

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 01:58:32 am
content: I am trying to understand the "Step Atten b1" and "Step Atten b2" menu options in the the Rohde&Schwarz VNA model ZVL3.Its manual is availablehere.It states that:QuoteI want to do an S21 measurement of an active DUT (a +55dB power amplifier) and I must set the VNA's internal signal source power (the stimulus) to 0dBm because the DUT expects that much at its input.The maximum signal level on the front panel of the instrument is stated as +27dBm, 30V DC for each VNA input, so 55dBm is waaayyyy to much for it.Do I risk damaging the VNA if I set the "Step Atten b2" to 30dB and apply 55dBm (316W) of RF power to the VNA's input (port2)? After all 55dBm - 30dB = 25dBm which is below the maximum allowable 27dBm.Despite this reassuring little math, I suspect that this is too much power to apply to the VNA's input because my VNA is not big enough to contain an attenuator which can attenuate a 57dBm (500W) signal by -30dB*down to the acceptable 27dBm (500mW).Also, when I change this "Step Atten b2" value, I do not hear any RF relays clicking inside the VNA, which I expected to hear if the internal attenuators were switched in. I realize that silent electronic attenuators exist, but I do not think that they can dissipate 500W (57dBm) of power.The manual does not list the maximum dissipated power of these "Step Attenuators" anywhere.*The 30dB is the maximum value that I can enter into the "Step Atten b2" menu.All of the above leads me to believe, that I do not understand what this "Step Attenuator" setting really does.The manual does not make it any clearer, so I am hoping one of you will help me understand it.

author: 0culus
author_rank: Super Contributor
timestamp: October 06, 2020, 02:06:33 am
content: 316W is a LOT foranytest instrument. I would NOT recommend trying to put that directly into a VNA. I don't know the specifics of what your measurement needs are, but you need a really large external attenuator (and maybe a suitable directional coupler too, just to knock 20 or 30 dB off right away) rated for the frequencies you need, that can take 400W or more, which you probably can characterize beforehand and then correct for it (your VNA might even be able to do this already).  or at least characterize so you can deconvolve the data later. after reading the OP more closely, if you can get a suitable 30 dB directional coupler that can handle your power levels, you should be home free.

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 02:28:52 am
content: Quote from: 0culus on October 06, 2020, 02:06:33 amYes, I know. That's why I think I do not understand what the "Step Attenuator" setting really does.Quote from: 0culus on October 06, 2020, 02:06:33 amI described it.It is an S21 measurement of a +55dB power amplifier with 0dBm signal at its input (which means +55dBm at its output).Quote from: 0culus on October 06, 2020, 02:06:33 amYes, I know that I can do this S21 measurement with e.g. a -30dB 500W external attenuator connected to Port2 of my VNA, but that does not help me understand what these internal "Step Attenuators" settings are really for.

author: 0culus
author_rank: Super Contributor
timestamp: October 06, 2020, 02:30:09 am
content: Quote from: GonzoTheGreat on October 06, 2020, 02:20:31 amSure, I was just making it super clear for anyone less knowledgeable that applying 316 watts to any RF test instrument is a BAD idea!  what I meant about your measurement is that I didn't know if there was some reason you couldn't do the the measurement with a big attenuator or a directional coupler + dummy load.If I had to venture a guess, the step attenuators are likely for a similar purpose as they are in a spectrum analyzer, to control the level going into the front end and keep it at an optimal level for components further down the chain.

author: TheSteve
author_rank: Unknown
timestamp: October 06, 2020, 03:01:15 am
content: The attenuator is to do exactly as it says - attenuate a signal, but it can only handle up to the maximum power input it is specified for(+27 dBm in this case). You likely don't ever want to run right up to that power level either. It would risking too much when you think of the cost of the VNA. You also don't want any compression or ADC clipping on the input which is likely to happen if you drive the input too hard.

author: Bud
author_rank: Super Contributor
timestamp: October 06, 2020, 03:14:26 am
content: Quote from: GonzoTheGreat on October 06, 2020, 02:28:52 amIt creates a loss (attenuation) in series with the test port.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 06, 2020, 03:40:40 am
content: A source attenuator reduces the signal output at the test port, it being a step attenuator means that it operates in set steps rather than fully variable output power - as it's probably a mechanical device switching in attenuation steps - but the output can typically be fully variable within the operating range because the electronic attenuators normally part of the source (not an option) can be variably controlled at a much finer step size.This all said, by the sound of things you probably want to read up on the basics and the instrument more before you use it. As you probably know, VNAs are expensive, precision pieces of equipment where even improperly connecting connectors can effect calibration and performance in the long term. I highly doubt your 55dB amplifier can output anything close to 300W of RF power - that kind of power level requires specialty devices and anything with that much amplification and ends that high would need several stages and a beefy power supply - if your DUT doesn't have a massive heatsink on it, it will not be approaching that output power level even under ideal conditions. You do have to be careful of power going into the VNA, it is sensitive and not designed for field use so it's not well suited to measure high output powers (and any DC bias on the input port can be deadly to the VNA!), but you can always include an attenuator on the input port, or, if the VNA has the capability, switch in attenuation on the input port as well, provided the signal at the port does not exceed the instrument's stated maximum (and I would knock 10dB off of that to be safe).You can also compensate for any lack of internal attenuation with an external attenuator, you just attach it to the port going into the input of the amplifier, then establish your calibration plane at the ends of the cable that will be attaching to the DUT, so that the attenuation of the external attenuator and its RF peculiarities are nulled out by the calibration algorithm. For this kind of amplifier measurement, it's common for VNAs to have an attenuator option installed only on a single port, so they have one low level output available for high gain amplifiers to remain within the safe input range.Whether your attenuation is added at the input to the amplifier or the output (or both) depends on your measurement. An attenuator at the DUT's output lets you push it harder and measure the performance near the high end of its capability, whereas an attenuator at its input lets you better evaluate noise floor and small signal performance. With especially high gain, it's probably advisable to have at least some on both, and depending on what's available, you'll always need a lower power rated attenuator if its on the input to the DUT in the case of amplifiers.

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:32:37 am
content: Quote from: DaJMasta on October 06, 2020, 03:40:40 amIt does have several stages and 1kW power supply with a beefy heatsink and can burn out a 200W incandescent light bulb.

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 09:51:32 am
content: Quote from: DaJMasta on October 06, 2020, 03:40:40 amIt is not a source attenuator because the stimulus signal amplitude stays constant as I vary the "Step Atten b1" menu option. I have verified the stimulus amplitude with a separate Spectrum Analyzer.Quote from: DaJMasta on October 06, 2020, 03:40:40 amThen I should hear it switching, shouldn't I ?I even stopped the fan and used a stethoscope but I cannot hear anything switching mechanically. I am young so my ears are still pretty good.Quote from: DaJMasta on October 06, 2020, 03:40:40 amAnd the stimulus power is fully variable in 0.1dBm steps using aseparate menu item. I can also see it vary on the separate Spectrum Analyzer connected to the VNA.The "Step Atten b?" setting doesn't seem to have anything to do with the stimulus power and amplitude.Quote from: DaJMasta on October 06, 2020, 03:40:40 amThat's exactly what I did and it did not leave me any wiser. That's why I posted this question on this forum.Quote from: DaJMasta on October 06, 2020, 03:40:40 amI know. I tried an external -15dB , 25W, DC-6GHz attenuator with a lower output power active DUT (35dBm output with 0dBm input). This should apply maximum of 20dBm to the VNA's input port #2, because 35dBm - 15dB = 20dBmHowever, I had intermittent "IF overload" alarms and was experiencing something that looked like compression or ADC clipping, see the screenshot attached below.However when I set the "Step Atten b2" to "15dB" the readings improved, the ripple disapeared, the DUT's max gainincreased by +11dB(despite being driven by the same 0dBm stimulus), see the screenshot attached below.Now I am really confused because the -15dB attenuation of the external attenuator and the "15dB" of the "Step Atten b2" setting add up to -30dB, don't they? So the maximum power reaching the VNA's receiver should be 5dBm, because 35dBm - 15dB - 15dB = 5dBm....BUT, BUT, BUT I know from experience that my VNA works reliably up to +20dBm of input signal level without any clipping, front-end overload and distortion. The maximum stimulus signal power can even be set to +20dBm at a touch of the button and yield reliable results with a 0dB "through" without any "IF overload" alarms, clipping and other distortions.So now I am even more confused.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2020, 11:24:17 am
content: Quote from: GonzoTheGreat on October 06, 2020, 01:58:32 amRisk damaging? More like guaranteed success.You didn't by chance own an Agilent prior this this?? I rescued one that some idiot had done something similar to. She was hot enough to blue the steel.Personally, I would spend the $50 on one of the pocket VNAs and learn some basics. When you smoke it, worse case, you're out $50. Buy whatever accessories you need to make the measurement with the Nano and you will be ahead of the game when you are finally ready to play with something more expensive.

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 11:44:57 am
content: Quote from: joeqsmith on October 06, 2020, 11:24:17 amI am trying to learn and I did not smoke the R&S nor the Agilent.The NanoVNA does not have the "Step Atten b?" setting which I am trying to understand and which is thesubject of my Original Post.

author: switchabl
author_rank: Frequent Contributor
timestamp: October 06, 2020, 11:48:45 am
content: Quote from: GonzoTheGreat on October 06, 2020, 09:51:32 amThe specifications seem to be relatively clear (page 7)https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/ZVL_dat-sw_en_5213-8150-22_v1200.pdfMax. (nominal) input level is -10dBm @0dB attenuation and 20dBm @30dB attenuation, 1dB compression point 0dBm @0dB attenuation. 27dBm CW damage threshold inpendent of attenuator seeting.This would seem to be in line with what you see with your amplifier and in fact suggests that you need even more attenuation if you want reliable results.Are you sure you had 20dBm input at 0dB attenuation without overload? Have you actually verified that the output is +20dBm with a power meter? The ZVL max. output power is specified at 0dBm/10dBm typ.Depending on the settings during calibration, you may still see a flat trace with a through even when you drive the receiver into compression, but that doesn't mean that you get valid measurements for anything else.

author: TurboTom
author_rank: Super Contributor
timestamp: October 06, 2020, 12:08:10 pm
content: I think themanualis pretty clear on page 217 what "Step Attenuator b2" means: It's the attenuator in front of the the receive branch of port 2. So if you want to measure S21, this would be the correct attenuator to adjust in order to reduce input signal strength to the linear range of the receiver. That it's not a good idea to connect such a powerful RF amp directly to a VNA has been explained enough I guess...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2020, 02:39:09 pm
content: Quote from: GonzoTheGreat on October 06, 2020, 11:44:57 amI did not suggest you did, only that your chances of damaging it by exceeding the maximum input level are pretty good. The fact you would even ask tells me the risks are high which is why I suggested you learn the basics on one of the very low cost units.When I wrote:QuoteThis would include step and fixed attenuators, loads, samplers... everything you would need to do the job. Outside of the step attenuator, I would assume you need to buy everything else anyway. Personally, I use the manual step attenuator to make other measurements so you may find other uses for it as well.

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: October 06, 2020, 03:46:28 pm
content: Quote from: switchabl on October 06, 2020, 11:48:45 amI expected them to be in that manual which I linked in my Original Post, it has some specifications in it, but not all - that's why I could not find the answer.Quote from: switchabl on October 06, 2020, 11:48:45 amAn that is where my misunderstanding was rooted.I treated the 27dBm as the Maximal input level and was expecting the instrument to work correctly and accurately up to that level.The -10dBm Nominal Input Level (without attenuation)changes everything.Now I understand, that I should always strive to present a -10dBm level signal (without attenuation) to the input of this instrument in order to achieve the best accuracy. Of course, I can present more powerful signals with additional attenuation....and if I am willing to accept 1dB receiver compression, then I can even apply a 0dBm signal without attenuation.Quote from: switchabl on October 06, 2020, 11:48:45 amYes, but now I see why - when I enable the 20dBm stimulus output, the instrument automatically applies additional attenuation at the receiver. This is done silently.Quote from: switchabl on October 06, 2020, 11:48:45 amNot with a power meter but with a separate Spectrum Analyzer. Also, I observed the voltage amplitude across a 50Ω load with a scope and calculated the average power as P=VRMS2/50Ω.Thank you Switchabl.In the end it was just another case of RTFM ...or the wrong FM.