Power Amplifier - High frequency power loss. - Page 1
Forum section: electronics / repair

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 07:11:28 am
content: A RF PA is causing some grief at work. All frequencies from low range (can't recall of the top of my head sorry) up until ~27MHz give aforwardpower reading of 1kW, which meets the specs (tolerance of 50W). Once you go above this frequency the power starts to drop by a few 100W.Unfortunately I am not able to provide schematics as this is a military PA. Essentially, there is a pre-driver board, multiple amplifier boards (4 of them), a harmonic filter board and a few less significant boards. The only board/module that is frequency dependent is the harmonic filter board (pre-driver is but to a lesser extent). The harmonic filter board has been tested, swapped, and confirmed working with an in spec frequency response.Now, the thing I am having trouble wrapping my head around is that at half power, theforwardpower should obviously be half of a 1kW; 500W. This is true up until the >27MHz range. The odd thing is, at this frequency range there ismorethan half power, again by a few 100W.It's a shot in the dark by asking for any opinions on here seeing as I am unable to provide schematics, photos etc. But I figured it was worthy of a discussion as perhaps something is being overlooked.Edit: Original post made in a sleep deprivation state which had incorrect info and has now been corrected. My apologies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2015, 07:24:40 am
content: If you're lucky, someone's had a problem of similar description, perhaps with the same equipment. But barring that, yeah, without even the most basic description of topology or component values, we can't help much..Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: July 20, 2015, 07:38:10 am
content: A 1KW PA with 1KW reflected power?

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 07:43:12 am
content: Quote from: BennVenn on July 20, 2015, 07:38:10 amMy mistake, thanks for picking that up. Corrections have been made.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 08:15:08 am
content: Why should the reflected power be half? I'm confused, RF PA design is one of my areas of expertise and I have no idea what you are on about.They are also very very difficult things to design well. Has this recently developed a fault or has it always had this issue?

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 08:30:19 am
content: Quote from: KJDS on July 20, 2015, 08:15:08 amSorry for any incorrect information. I am not the tech who is working/usually works on this gear so I am only going by what I have been told (by a recently finished trainee)If at full power, the reflected power is 1kW, then at half power the reflected would be 500W?I believe it was returned from service to be surveyed, which is when the fault was discovered. Beforehand I can't say how it performed sorry.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 08:45:33 am
content: Quote from: tec5c on July 20, 2015, 08:30:19 amWhere are you measuring the reflected power? I assume that you've got the amplifier running either into an antenna or a dummy load. One thing that HPAs hate is a bad load, so to truly measure the amp you'd want the load with a return loss of better than 30dB, so just 0.1% of the power is reflected back. If the load isn't that good, then the performance of the amplifier is affected by the load. If half the power is being reflected by the load then all bets are off, unless there's a very big isolator on the output of the amp.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 08:54:18 am
content: The amp is running into a dummy load, the return loss at the aforementioned frequency range is around 20dB, typical for this unit is 40dB.Would a bad load only effect certain frequencies though?I'm pretty sure that the dummy load is the same one that is always used for this gear, i.e., it's rigged up in the test rack setup.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 09:12:50 am
content: The load sounds good, so where are you measuring the reflected power? There should be very little reflected from the load.Am I missing something obvious? Any chance of a block diagram of how you're making the measurement and the issues you're seeing marked on it?

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 09:56:07 am
content: I will have to look closer at the setup tomorrow as I can't recall the total setup off the top of my head.I'll post up what I find tomorrow evening.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 20, 2015, 09:59:45 am
content: Like others have mentioned: If you have half the power reflected there is something very wrong with either the load, the cables or the output impedance of the PA.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 20, 2015, 10:58:51 am
content: Quote from: tec5c on July 20, 2015, 08:54:18 amIf you have 1kW reflected power for a return loss of 20dB,your amplifier must have an output of 100kW.For a 100kW output,a few 100W error shouldn't be a problem.Check the return loss of the test load alone,in case it is faulty.

author: PE1RKI
author_rank: Regular Contributor
timestamp: July 20, 2015, 12:31:39 pm
content: Quote from: tec5c on July 20, 2015, 07:11:28 amthis is totally faulty thinking. i dont understand where you get this from.perhaps mixing it up with amplifier efficiency of 50%?relfected is what comes back from the load, that should be very low. even 50 watt reflected is alot. think of a few watt if it is a good amplifier.perhaps in this regard you are mixing reflected with forward power measurement, the power that comes out of the amplifier.how are you measuring this? with a bridge that only has a reflected output and you are told to mount this in reverse to actualy measure the forward ?at this point you should not stick any soldering iron into the amplifier until you understand forward and reflected power measurements.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2015, 12:35:39 pm
content: Quote from: tec5c on July 20, 2015, 07:11:28 amAre you using a Bird model 43 wattmeter to make these measurements? If so, are you using the correct plug-in for the frequency/power range? Are you sure the dummy load is flat across the frequency range of interest (verify by measuring forward and reflected power with the Bird). If you aren't using a Bird and a good dummy load then you can't really trust any of your measurements.Also, as evidenced by the confusion in the other replies, you are using the wrong terminology here: reflected power is the power that is not absorbed by the load and which reflects (ahem!) back to the source. When operating an RF amp into a dummy load with the correct impedance value (usually 50 or 75 ohms) the reflected power should be close to 0W (ie - VSWR close to 1.0)Quote from: tec5c on July 20, 2015, 07:11:28 amGiven the other (sparse!) details provided it sounds like this is a push-pull MOSFET based final amplifier with broadband transmission line transformers for impedance matching. If so, a decline in power with frequency points to a problem either with the gate biasing network or the drain supply; there is precious little else to go wrong here.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 01:38:35 pm
content: Quote from: MagicSmoker on July 20, 2015, 12:35:39 pmI've just looked up the specs, and a Bird 43 has an accuracy of +/- 5% full scale and a directivity of 25dB. Compare that with a decent power meter which is +/-3% of reading and 40dB for a measurement grade coupler then I know which option I'd choose. I agree about the dummy load.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2015, 05:13:10 pm
content: Quote from: KJDS on July 20, 2015, 01:38:35 pmI don't doubt there are power meters with better specs (on paper, anyway), but the Bird 43 is pretty much the industry standard. Having trust in your measurement (ie - good relative accuracy/consistency) is oftentimes more valuable than absolute/initial accuracy.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 06:41:00 pm
content: Quote from: MagicSmoker on July 20, 2015, 05:13:10 pmI have spent many of the last twenty years designing state of the art high power amplifiers for a variety of uses and state of the art real time antenna measurement systems. I've never seen a Bird in a design lab. I'm sure they are great for field measurements, but your statement that if you're not using a Bird then you can't trust your measurements is untrue.

author: PE1RKI
author_rank: Regular Contributor
timestamp: July 20, 2015, 07:18:17 pm
content: i have had several different bird meters here from different customers and they never showed anything close to my hp gear. sometimes half the amount, sometimes 2 times to much. just all over the place.i wont even take bird measurements close to serious.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2015, 07:52:27 pm
content: Quote from: KJDS on July 20, 2015, 06:41:00 pmThe OP is not *designing* an RF power amp, he is *servicing* one - I was keeping that context in mind when I recommended the Bird 43. That said, I'll admit it's been a long time since I've designed an RF power amp myself but most service techs then - and probably still now - use a Bird 43.If you are designing RF amps and antennas - especially for applications with strict power limits (e.g. - WiFi, cellular transceivers) then yes, you'll probably want to use something a bit more accurate that is current on its certified metrology lab calibration.

author: G0HZU
author_rank: Super Contributor
timestamp: July 20, 2015, 08:22:25 pm
content: I think I understand the phenomena the OP is seeing and I'll try and explain what I think the symptoms are:If power is fed to a linear amplifier and it shows 10kW FWD power and (x)W reflected power then you would expect the reflected power to fall to (x/2)W if the drive to the ampliier was reduced such that the FWD power fell by half to 5kW.But I think the OP is NOT seeing the expected (x/2)W reflected power at half the FWD power. If this is indeed the case then there are valid real world reasons why these symptoms can be observed.I won't list them all but you could have an issue with linearity at the higher power level where higher levels of harmonics are disrupting (corrupting?) the measurement system in one or more areas.eg the power meter may have different coupling/sensitivity to harmonics or its detector may be a crude type that suffers high uncertainty when harmonics are present.The load may be non linear and have poor performance at the harmonic frequencies or it may simply be changing its impedance at high power levels at certain frequencies as it gets hot.The coupler directivity (and coupling factor) may be different at harmonic frequencies.The amplifier could be going unstable at certain frequencies and power levels and this can produce all manner of strange symptoms on a basic power meter.QuoteI'm afraid that the Bird 43 'wattmeter' is not a proper power meter for various reasons. It's design is seriously compromised in favour of its versatility and it's arguable that it is technically inferior to a $5 CB power meter for some types of tests.Basically, don't trust any RF 'professional' who claims/argues that the Bird 43 is some kind of industry standard benchmark for RF power measurement. It most certainly isn't! However, the myths about this meter will go on and on because there is no shortage of gullible hams/owners/techs who 'believe' the folklore/hype about the capabilities of this crude and awful device.

author: G0HZU
author_rank: Super Contributor
timestamp: July 20, 2015, 08:54:30 pm
content: QuoteAgreed. The only time I've seen one in our labs (across a 25 year timeframe) is when a technician brought two of their own Bird 43 meters in to test along with various pairs of identical slugs.Unsurprisingly, the performance was pretty woeful because both meters were quite old and the slugs were erratic and even if they were cleaned and wiggled the meters and slugs didn't agree with each other let alone agree with our lab grade power meters.

author: BennVenn
author_rank: Regular Contributor
timestamp: July 20, 2015, 09:30:57 pm
content: Tec5c, are you a tech in the ADF?

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2015, 09:35:04 pm
content: Quote from: G0HZU on July 20, 2015, 08:22:25 pmLOL... fine, the Bird 43 sucks. I last designed an RF power amplifier c. 1997 so probably shouldn't have contributed to this thread. That said, most of you all have wasted your time either dithering over the OP's incorrect use of "reflected power" or my suggestion to use a Bird 43 to make the power measurements. Not helpful at all, in other words. You have a *specific* wattmeter recommendation for the OP to use or are you just here to make ad hominem attacks against me?

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 09:47:10 pm
content: Quote from: MagicSmoker on July 20, 2015, 09:35:04 pmYou stated that the only power meter that could be trusted is a Bird 43.I stated, having read the specs, that there were far better options available. HP and it's various incarnations, Marconi, Gigatronics and various other companies all produce bench power meters with very similar specs. Either use these at the far end of a high power attenuator on the output of the PA, or use an Agilent or Narda directional coupler, or a home made one if you've got some lab grade ones available.No-one has attacked you, merely your incorrect statement that the Bird 43 is the only trustworthy way to measure RF power. As statements go, that's wrong, but it's your statement that we're commenting on, it's not an attack about you, therefore not ad hominem.

author: G0HZU
author_rank: Super Contributor
timestamp: July 20, 2015, 10:03:57 pm
content: Quote from: MagicSmoker on July 20, 2015, 09:35:04 pmI'm really just questioning the design integrity of the Bird 43 wattmeter and I'm also warning anyone (including you) not to trust this meter or any of the claims made about it being an industry standard power meter.I can give you one example of why this meter can lie. It is designed to be used with a pure sine wave signal source as it uses a diode detector that operates linearly up at the FSD end of the meter.So it will be prone to huge measurement uncertainty if there are harmonics present. eg a single -20dBc harmonic can give a ballpark +/- 20% uncertainty in power measurement on this meter. A -30dBc harmonic gives approx +/- 6% uncertainty. The uncertainty depends on the phase of the harmonic.It also has an uncompensated detector that operates closer to square law at lower dial deflections and this means it suffers a significant error when used in cold conditions. Ironically it is sold as a field measuring device but if you read the small print on this meter it does not come with a temperature spec and they only specify performance up at the top end of the dial. This is because the accuracy is awful over temperature for lower dial readings. It's not a serious piece of test equipment.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 20, 2015, 10:05:24 pm
content: Okay, after some well needed rest I have realised that I have made a crucial error in my original post. The "reflected" power I mentioned should be theFORWARDpower, which was being measured on a powerreflectionmeter and in my sleep deprived stage, I wrote the wrong thing.More on this once I get to work and have a spare moment.My apologies for the silly mistake.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2015, 10:33:02 pm
content: Quote from: KJDS on July 20, 2015, 09:47:10 pmYeah, I sure did, and in retrospect that was a poor choice of words on my part. I made that recommendation both because I interpreted the OP's misuse of reflected power as a sign he might not normally work with RF PAs and therefore not have the best equipment at his disposal, but also because the Bird 43 is the only wattmeter I am familiar with that isn't total crap.Consequent to your and G0HZU's criticism of the Bird 43 I agree that it isn't quite the paragon of wattmeters, but merely dismissing my equipment suggestion without providing an alternative to the OP doesn't really help him.

author: KJDS
author_rank: Super Contributor
timestamp: July 20, 2015, 10:42:16 pm
content: OK, that's making a lot more sense.Most amplifiers droop at higher frequency, both in gain and maximum output power. Usually, on a broadband design that needs it, various compensation strategies are used to minimize the droop. Assuming that there didn't used to be a reduction in gain and output power and there is now, then something has changed. A big change, more than about a dB would indicate that there has been a failure somewhere, possibly on a matching or decoupling cap. If the gain is still ok at lower powers then that probably rules out a problem with the input side of the amp. If feedback is used, then it's also possible that something has gone wrong with the feedback circuit, but I hate using feedback unless the amp has to perform at very low frequencies. It's also possible that the active devices have degraded. One of the advantages of PA fault diagnosis is that when something goes wrong, it's usually very obvious where the fault lies, unless it's buried in the active device.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 21, 2015, 12:00:53 am
content: Quote from: MagicSmoker on July 20, 2015, 12:35:39 pmNo, a Rohde & Schwarz NRT Power Reflection Meter is being used to make the measurements.Quote from: BennVenn on July 20, 2015, 09:30:57 pmNo, I work for a Defence contractor.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:06:54 am
content: Right, so the trainee who was working on this PA was out running errands today so I thought I'd take a look at the faulty PA as I didn't have much else to do.I found the reason (at least my interpretation) for why the power was dropping at the higher frequency range. Within the diagnostic board there is a BIT test that monitors the collector voltage of the RF transistors on the 4 amplifier boards. It cycles through every ~1.5ms and checks that Vce is not >= 110v. If it is, then it backs off the power/reducing Vce, so come the next cycle (1.5ms) things appear fine so it tries to operate like normal, then thenextcycle Vce is back to being > 100v and the whole process is in a continuous loop giving a ringing effect.I monitored the collector voltage on a scope and sure enough, when you key the exciter you can clearly see the ringing which peaks above 110v.I suspected some degrading 47nF polycarbonate caps to be the cause (I can't recall their function without the schematic in front of me sorry) so I stuck 1000pF (all I had on hand at the time) in parallel across 1 of 8 suspected caps and sure enough, I noticed some improvement in the ringing.It was at this time the trainee returned so I left him to replace the caps and go from there. Hopefully this should fix the problem.Here is 1 of the 4 amplifier boards with the suspected bad caps. Image of the board taken from eBay, so my apologies for the quality.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 21, 2015, 10:51:52 am
content: Quote from: tec5c on July 21, 2015, 07:06:54 amLost in all the hullabaloo over my Bird 43 recommendation was what I suggested you check in the amplifier itself:Quote from: MagicSmoker on July 20, 2015, 12:35:39 pmP.S. - that R&S power meter looks a very nice.EDIT: corrected "R.S." to "P.S."

author: vk6zgo
author_rank: Super Contributor
timestamp: July 21, 2015, 12:52:45 pm
content: Quote from: G0HZU on July 20, 2015, 10:03:57 pmA Bird meter is all these things,but nevertheless,they have been very widely used for many years in circumstances where allowances can be made for their shortcomings.In many cases,if the "forward & reflected power"appears the same as that measured by the same instrument upon installation,all will be OK.In testing a complete transmitter with all the required filtering intact,the Bird is unlikely to be confronted by harmonics of such high levels as -20dB,or even -30dB.Back in the day,we used to check the matching of loads,antennas & amplifier input circuits with a Polyskop RF scanner/display,For output power measurement of lower power equipment,we used a HP probe section with a HP410C meter.High power stuff was tested for output power with a water cooled test load.

author: tec5c
author_rank: Frequent Contributor
timestamp: July 21, 2015, 09:51:10 pm
content: Quote from: vk6zgo on July 21, 2015, 12:52:45 pmAhh, there is an old R&S Polyskop in our test equipment storage area. I've never seen the thing even been moved let alone been turned on or better yet, been used! You just gave some insight to an application for it (which I have neglected to do any research on)

author: G0HZU
author_rank: Super Contributor
timestamp: July 22, 2015, 03:51:13 pm
content: QuoteIt would be confronted with harmonics that are higher than this if it were to be used to measure an agile broadband PA used for certain types of gov/mil service. When used with an antenna this type of amplifier doesn't have much in the way of filtering across several octaves of bandwidth. eg 20-1000MHz. Due to the frequency agility of the system there isn't time to switch any filters in and out so the amp runs 'dirty' apart from a basic roofing filter up at UHF.

author: BennVenn
author_rank: Regular Contributor
timestamp: July 22, 2015, 09:37:11 pm
content: There are many couplers and antenna tuners between the amp and the antenna. A great deal of filtering is done before it leaves the whip/array

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2015, 03:28:51 pm
content: Quote from: G0HZU on July 22, 2015, 03:51:13 pmIndeed,but that is more of a special case,which definitely supports your contention that the Bird is not a "top end" instrument.On the other hand,the vast majority of Transmitters which would constitute the "constituency" for the use of Bird meters,have harmonic suppression of 40dB or better,so that source of error would be minimal.