Power Amplifier - High frequency power loss.

Power Amplifier - High frequency power loss.
Posted by tec5c on 20 Jul, 2015 07:11
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 a forward power 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, the forward power 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 is more than 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.

#1 Reply
Posted by T3sl4co1l on 20 Jul, 2015 07:24
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

#2 Reply
Posted by BennVenn on 20 Jul, 2015 07:38
A 1KW PA with 1KW reflected power?

#3 Reply
Posted by tec5c on 20 Jul, 2015 07:43
Quote from: BennVenn on 20 Jul, 2015 07:38
A 1KW PA with 1KW reflected power?

My mistake, thanks for picking that up. Corrections have been made.

#4 Reply
Posted by KJDS on 20 Jul, 2015 08:15
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?

#5 Reply
Posted by tec5c on 20 Jul, 2015 08:30
Quote from: KJDS on 20 Jul, 2015 08:15
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?

Sorry 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.

#6 Reply
Posted by KJDS on 20 Jul, 2015 08:45
Quote from: tec5c on 20 Jul, 2015 08:30
Quote from: KJDS on 20 Jul, 2015 08:15
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?

Sorry 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.

Where 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.

#7 Reply
Posted by tec5c on 20 Jul, 2015 08:54
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.

#8 Reply
Posted by KJDS on 20 Jul, 2015 09:12
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?

#9 Reply
Posted by tec5c on 20 Jul, 2015 09:56
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.

#10 Reply
Posted by PA0PBZ on 20 Jul, 2015 09:59
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.

#11 Reply
Posted by vk6zgo on 20 Jul, 2015 10:58
Quote from: tec5c on 20 Jul, 2015 08:54
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.
If 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.

#12 Reply
Posted by PE1RKI on 20 Jul, 2015 12:31
Quote from: tec5c on 20 Jul, 2015 07:11
the reflected power should obviously be half of a 1kW; 500W.
this 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.

#13 Reply
Posted by MagicSmoker on 20 Jul, 2015 12:35
Quote from: tec5c on 20 Jul, 2015 07:11
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 a reflected power reading of 1kW, which meets the specs (tolerance of 50W).

Are 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 20 Jul, 2015 07:11
Once you go above this frequency the power starts to drop by a few 100W.

Given 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.

#14 Reply
Posted by KJDS on 20 Jul, 2015 13:38
Quote from: MagicSmoker on 20 Jul, 2015 12:35
Quote from: tec5c on 20 Jul, 2015 07:11
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 a reflected power reading of 1kW, which meets the specs (tolerance of 50W).

Are 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 20 Jul, 2015 07:11
Once you go above this frequency the power starts to drop by a few 100W.

Given 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.

I'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.

#15 Reply
Posted by MagicSmoker on 20 Jul, 2015 17:13
Quote from: KJDS on 20 Jul, 2015 13:38
....I'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.

I 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.

#16 Reply
Posted by KJDS on 20 Jul, 2015 18:41
Quote from: MagicSmoker on 20 Jul, 2015 17:13
Quote from: KJDS on 20 Jul, 2015 13:38
....I'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.

I 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.

I 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.

#17 Reply
Posted by PE1RKI on 20 Jul, 2015 19:18
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.

#18 Reply
Posted by MagicSmoker on 20 Jul, 2015 19:52
Quote from: KJDS on 20 Jul, 2015 18:41
...I 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....

The 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.

#19 Reply
Posted by G0HZU on 20 Jul, 2015 20:22
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.

Quote
Are 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.

I'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.

#20 Reply
Posted by G0HZU on 20 Jul, 2015 20:54
Quote
I 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.

Agreed. 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.

#21 Reply
Posted by BennVenn on 20 Jul, 2015 21:30
Tec5c, are you a tech in the ADF?

#22 Reply
Posted by MagicSmoker on 20 Jul, 2015 21:35
Quote from: G0HZU on 20 Jul, 2015 20:22
...
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.

LOL... 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?

#23 Reply
Posted by KJDS on 20 Jul, 2015 21:47
Quote from: MagicSmoker on 20 Jul, 2015 21:35
Quote from: G0HZU on 20 Jul, 2015 20:22
...
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.

LOL... 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?

You 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.

#24 Reply
Posted by G0HZU on 20 Jul, 2015 22:03
Quote from: MagicSmoker on 20 Jul, 2015 21:35
Quote from: G0HZU on 20 Jul, 2015 20:22
...
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.

LOL... 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?

I'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.