|
On Thu, 03 Mar 2005 13:44:06 -0800, Richard Clark wrote:
On Thu, 3 Mar 2005 20:53:48 +0000, John Woodgate wrote: Doesn't everyone know that an audio amplifier that id designed to feed an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or less. Hi John, I hope that was a joke. Please! You know Mr. Woodgate _hates_ explaining his jokes: "Doesn't everyone know that an audio amplifier that [is] designed to feed an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or less[?] An output source impedance of 8 ohms would dramatically decrease the electromagnetic damping on the loudspeaker voice-coil - by the huge factor of .... two! (;-) [^^^^] Please notice the last sentence in that paragraph. ;-) 73's Best regardses? ;-) Cheers! Rich |
On Thu, 03 Mar 2005 22:47:00 GMT, gwhite wrote:
I see one line here with no content. Hi Forrest, Great! Now try with the other eye. :-) 73's Richard Clark, KB7QHC |
Rich Grise wrote:
On Thu, 03 Mar 2005 13:44:06 -0800, Richard Clark wrote: On Thu, 3 Mar 2005 20:53:48 +0000, John Woodgate wrote: Doesn't everyone know that an audio amplifier that id designed to feed an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or less. Hi John, I hope that was a joke. Please! You know Mr. Woodgate _hates_ explaining his jokes: Mr. Clark _hates_ reading and comprehending. I forsee a clash royal. |
On Thu, 03 Mar 2005 23:07:14 GMT, Rich Grise
wrote: Please! You know Mr. Woodgate _hates_ explaining his jokes: Hi Rich, Some love explaining their jokes. I've gotten quite a bit of correspondence to that matter already. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
On Thu, 03 Mar 2005 22:47:00 GMT, gwhite wrote: I see one line here with no content. Hi Forrest, Great! Now try with the other eye. :-) At least your not even pretending to have an argument anymore. Ah, sweet progress. |
On Thu, 03 Mar 2005 23:22:07 GMT, gwhite wrote:
At least your not even pretending to have an argument anymore. Ah, sweet progress. Hi OM, I suppose this means you failed the eye exam with the other eye. 73's Richard Clark, KB7QHC |
Wow this is a long thread. Don't really know where I should put my two bits
in, but here it goes. I have designed several RF PA sections in the past. 500MHz at about 50W. Pretty easy stuff if you have the right tools and know how to use them. The tools I like using for matching the power output FET is two triple stub tuners. One on the input of the FET and one on the output. So it goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm dummy pad - spectrum analyzer. Then just tune the stubs for the performance you desire, these include: efficiency (thermal issues), harmonic content, spurious emissions, load VSWR considerations, cold start, ect. Then remove the FET and look into the triple stub tuners with the network analyzer. Model and duplicate the network out of discrete components that can handle the voltage/power, send the design off to the enviro test lab, and head home early for the day. Cheers, Thomas "gwhite" wrote in message ... Richard Clark wrote: As I've noted in the past, you can fill a library with negative assertions... The troublesome assertion is not the negative one. It is that RF PA's are conjugate matched. Neither you nor Ken has provided a single example of such a design that also extracts the maximum amount of "linear" power from a device and essentially its power supply (after all, that is what it is: a _power_ amp). Your example said nothing about output-Z, which suggests you have no clue, since you didn't even remotely address the issue. For Ken's part, he recently obfuscated by dismissing an example that was primarily intended to be illustrative, but yet holding the salient points. He completely ignored (or didn't understand) the clipping issue. Further obfuscation was provided by talking about "protection circuitry," which may or may not exist in a circuit, but adds zero to a discussion regarding how the PA is to be loaded. "Protection" is a non-stater because the PA is either off or impaired. Ken's argument is circular. He say's that if a design is done for conjugate match, then it will behave as if it is conjugately matched. Well of course (or at least sort of under specific test conditions and circuits)! It is self-fullfilling prophecy but it unfortunately makes no statement regarding obtaining the maximum power out of the circuit in the sense of turning DC power into RF power (yes, *extracting* power from the DC supply and transformed to RF). This is paramount to PA design. To use the device to maximum efficacy, as Cripps puts it, a load-line match is needed. Ken's "conjugate match" design won't do that, and that's why PA's aren't designed that way. The bottom line is that if I design an amp via load line techniques using the same device and power supply as Ken (him using conj-match), my amp will deliver higher unclipped PEP than his. That is the factual result you resist. Now if you want to pay for extra power and big devices, that's your business--go ahead and attempt to conj-match your amp--but engineers who design PA's don't do that. Another idealized and hypothetical example to elucidate the load-line principle is offered. Let's say we have a 10 W FET we'll build into a class A circuit. An RF choke is used to supply drain current. We DC bias it to Vd = 10 V and Id = 1 A. Just for argument sake, let's say it has a constant internal resistance of 110 ohms and the device will break down at 25 V. According to the most idealized and standard load-line theory, we should load it to rL = Vd/Id = 10 Ohms. This idealization includes the definition of positive and negative clipping -- whichever comes "first" -- of being the operational limit for output voltage swing. Clipping is associated with severe distortion. Since we need rL to be 10 ohms, and Ri = 110 ohms, we need to make the actual load resistor equal to: RL = 11 Ohms. Let's check that result and see if it meets the clipping constraint for maximum available power. positive swing = Id*rL = 1*10 = 10 V negative swing = Vd = 10 V Power delivered to RL: Pload = 10^2/(2*11) = 4.55 W The efficiency is a little under 50% because of the internal resistance. Note the Load resistance is decidely not the conjugate of the internal resistance. Let's spot check the load to see if it at least appears to be the peak available power, by testing two loads "immediately" on either side of our optimum 11 ohms. Let RL = 10 ohms positive swing = Id*rL = 1*9.17 = 9.17 V negative swing = Vd = 10 V Since we positive clip at 9.17 V, we are limited by our design clipping constraint to only driving the PA such that 2*9.17 V is the maximum available voltage swing. Power delivered to RL: Pload = 9.17^2/(2*10) = 4.20 W Let RL = 12 ohms positive swing = Id*rL = 1*10.82 = 10.82 V negative swing = Vd = 10 V Since we negative clip at 10 V, we are limited by our design clipping constraint to driving the PA such that 2*10 V is the maximum available voltage swing. Power delivered to RL: Pload = 10^2/(2*12) = 4.17 W Sure enough, the power peaked at a load of 11 ohms, just like load-line theory says it will. Now let's see what the available power hit of conjugate matching is. By definition, conj-match insists RL = Ri = 110 ohms. Again we are limited in our clipping constraint by static drain current, and supply voltage, specifically 10 V. Our negative swing limit is, as ever, 10 V (the drain voltage). positive swing = Id*rL = 1*55 = 55 V This would breakdown the device, but the lower negative swing will force us to back down the drive to meet the design defined clipping constraint. Pload = 10^2/(2*110) = 0.455 W Conjugate matching resulted in a 10*log(0.455/4.55) = 10 dB available power hit. Power amplifiers are not designed with conjugate matching in mind. You don't need to re-invent the wheel. Just follow well established principles when doing cookie cutter PA design. The list could go on,... LOL. Given your pattern, I am sure it will. You sighed with content at being offered a "relevent question/statement" Your re-iterative response contains the same (how could it be otherwise?) slack of precision that started this. Want to try again? Not really. The problem isn't precision, it is you can't, or refuse, to comprehend what is being said, which I presume is why you instead write with the most bizarre terms and phrasology that has nothing of import to the topic at hand. You could have as easily expressed what sense they ARE matched, For what seems like the billionth time now: they are load-line matched. ...but instead this time offer what Basis of Matching you are attempting to describe. I've given a didactic example (actually a couple), you just don't--or more likely won't--get it. If you don't like my example, you can refer to Cripps, who is considered one of the preeminant RF PA experts in the world. Even more simplistic is Malvino's discussion on pp177-185 of the first edition ((c) 1968) of "Transistor Circuit Approximations." It is basically a technician level description, so perhaps it is well-suited to you. In academics, load-line theory is presented down to tech level courses and up across to engineering. That some engineers and techs aren't clear on the load-line concept for PA's (or *any* circuit needing a wide symmetrical swing) is notwithstanding. This is the more rigorous approach that eliminates vague descriptions and uses standard terms. If you have to query about what "Basis" means (used by professionals - namely metrologists who can quantify Output Z of all sources) - then we can skip it as a topic out of the reach of amateur discussion. I see you still don't know what impedance is. In any case, it doesn't mean that looking into a properly designed PA output with a network analyzer confirms the conj-match precept, it doesn't. Impedance is a *linear* conception, a portion of linear theory, and again by definition: Z = V/I V and I are sinusoids (phasors). But with power amps, substantial non-linearity exists (destroying the linearity assumption of impedance), thus applying a linearly defined concept to a non-linear milieu is a misapplication. You are attempting, as is Ken, to stuff a square peg down a round hole. Why? The concept is even questionable for the most linear of the power amps: class A. In any case, given real devices with real supplies, the conj-match ideal is next to worthless. While I could agree that the borderline may be fuzzy regarding where and when to drop the impedance notion, it still stands that the concept is not useful in determining how to optimally load an RF PA. At this point you own the conj-match assertion as much as Ken. Prove it! You can't because it is fundamentally incorrect. Note: Again, RF PA's should be load-line matched. Does not qualify as a Basis. Load-line matching is such a basic electronic concept it is unbelievable how oblivious you are to the concept. Read a basic book. Don't rely on me: look it up and do your own design! It is suggestive of one, but because you indiscriminately mix several Basis within your discussions, it is your responsibility to be precise. You just like to hear yourself talk. I've been explicit and precise. You just don't know anything about the elementary electronics principle of load line matching. I presume this is why your comments have zero substantive responsiveness. If you can accomplish this, then we can proceed to review how little it all matters. If you keep ignoring what I've written, and that which is written in elementary electronics texts, you can remain happily ignorant of understanding the simple-basic-fundamental concept presented. Your choice. Barring resolving any of these issues of precise language,...] The guy ignorant of the definition of impedance and that s-domain theory *is* linear circuit theory (and more goodies) is talking about "precise language." Amusing. I notice that you rather enjoy... No, I don't enjoy it at all. Your lack of electronic understanding is dismal, especially given your tone. It would have been a lot easier for me if Ken hadn't made the erroneous statement in the first place and made a correct one instead. That would have been my preferance. ..fruitless jousting with them than challenging my support of Ken's (supposed) statement that you say is your focus: However, responding to the bald statement, I find nothing objectionable about it. That's because you don't understand the difference between impedance matching and ac load line matching. We will leave that as another dead-end. I suspect you will. I already understand it -- you're the one who doesn't. "One of the principal differences between linear RF amplifier design and PA design is that, for optimum power, the output of the device is not presented with the impedance required for a linear conjugate match. That causes much consternation and has been the subject of extensive controversy about the meaning and nature of conjugate matching. It is necessary, therefore, to swallow that apparently unpalatable result as early as possible (Section 1.5), before going on to give it more extended interpretation and analysis (Chapter 2)." -- Cripps, p1 The quote is on Page 1. Swallow it now. Learn something for a change. |
Richard Clark wrote:
I suppose this means you failed the eye exam with the other eye. There's no need for supposition. You don't know anything about PA design. You demonstrated that clearly enough for a blind person to see. |
Richard Clark wrote:
On Thu, 3 Mar 2005 20:53:48 +0000, John Woodgate wrote: Doesn't everyone know that an audio amplifier that id designed to feed an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or less. Hi John, I hope that was a joke. 73's Richard Clark, KB7QHC I think he just meant that damping factor is important in an audio amp. At least I hope that's what he meant. He forgot to mention that for that output impedance to be relevant, you need superconducting wire to the speakers as well as superconducting voice coils. tom K0TAR |
On Fri, 04 Mar 2005 01:57:06 GMT, gwhite wrote:
I suppose this means you failed the eye exam with the other eye. There's no need for supposition. Hi OM, I thought not. 73's Richard Clark, KB7QHC |
On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
wrote: Wow this is a long thread. Don't really know where I should put my two bits in, but here it goes. I have designed several RF PA sections in the past. 500MHz at about 50W. Pretty easy stuff if you have the right tools and know how to use them. The tools I like using for matching the power output FET is two triple stub tuners. One on the input of the FET and one on the output. So it goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm dummy pad - spectrum analyzer. Then just tune the stubs for the performance you desire, these include: efficiency (thermal issues), harmonic content, spurious emissions, load VSWR considerations, cold start, ect. Then remove the FET and look into the triple stub tuners with the network analyzer. Model and duplicate the network out of discrete components that can handle the voltage/power, send the design off to the enviro test lab, and head home early for the day. Cheers, Thomas Hi Thomas, Thanx, your two bits were worth more than the academic plug nickel. This is something that our original poster should hearken to as his needs were obviously production oriented. Bench experience will trump cut-and-paste theory in a heart-beat. However, triple stub is pretty aggressive. How long did it take you to flatten response? 73's Richard Clark, KB7QHC |
I read in sci.electronics.design that Richard Clark
wrote (in ) about '1/4 vs 1/2 wavelength antenna', on Thu, 3 Mar 2005: On Thu, 3 Mar 2005 20:53:48 +0000, John Woodgate wrote: Doesn't everyone know that an audio amplifier that id designed to feed an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or less. Hi John, I hope that was a joke. If you read the whole paragraph, you will see. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
I read in sci.electronics.design that Tom Ring
wrote (in ) about '1/4 vs 1/2 wavelength antenna', on Thu, 3 Mar 2005: He forgot to mention that for that output impedance to be relevant, you need superconducting wire to the speakers as well as superconducting voice coils. See the last sentence, about the effect of an **8 ohm** source impedance on damping. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
"Richard Harrison" wrote:
Radio transmitters don`t produce significant harmonics. It`s the law. They are linear power sources. We can and do tune them for all the power they will produce under their particular operating conditions of drive and d-c power supply. They operate at more than 50% efficiency which means that they don`t take power 100% of the time, but are switched-off during part of the r-f cycle. Output impedance is thus an average over the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the tank circuit and other filters. _______________ Note that without adjustment, modern, solid-state FM broadcast transmitters can (and do) provide 80% or better PA efficiency into a 50 ohm load across 20% bandwidth, with no tank circuit or other in-band filter(s). If this is done in a commercial service, certainly it could be done in amateur radio devices. Physics is not application-selective. Posters of various forms of "Absolute Truths" to the contrary might well do a bit more research. RF |
Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated using the HB-IP bus from the spectrum analyzer and a computer. The computer/analyzer also looks for harmonic content and spurious emissions during this procedure. Think it takes about ten seconds to do this. "Richard Clark" wrote in message ... On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma" wrote: Wow this is a long thread. Don't really know where I should put my two bits in, but here it goes. I have designed several RF PA sections in the past. 500MHz at about 50W. Pretty easy stuff if you have the right tools and know how to use them. The tools I like using for matching the power output FET is two triple stub tuners. One on the input of the FET and one on the output. So it goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm dummy pad - spectrum analyzer. Then just tune the stubs for the performance you desire, these include: efficiency (thermal issues), harmonic content, spurious emissions, load VSWR considerations, cold start, ect. Then remove the FET and look into the triple stub tuners with the network analyzer. Model and duplicate the network out of discrete components that can handle the voltage/power, send the design off to the enviro test lab, and head home early for the day. Cheers, Thomas Hi Thomas, Thanx, your two bits were worth more than the academic plug nickel. This is something that our original poster should hearken to as his needs were obviously production oriented. Bench experience will trump cut-and-paste theory in a heart-beat. However, triple stub is pretty aggressive. How long did it take you to flatten response? 73's Richard Clark, KB7QHC |
Richard Clark wrote:
On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma" wrote: Wow this is a long thread. Don't really know where I should put my two bits in, but here it goes. I have designed several RF PA sections in the past. 500MHz at about 50W. Pretty easy stuff if you have the right tools and know how to use them. The tools I like using for matching the power output FET is two triple stub tuners. One on the input of the FET and one on the output. So it goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm dummy pad - spectrum analyzer. Then just tune the stubs for the performance you desire, these include: efficiency (thermal issues), harmonic content, spurious emissions, load VSWR considerations, cold start, ect. Then remove the FET and look into the triple stub tuners with the network analyzer. Model and duplicate the network out of discrete components that can handle the voltage/power, send the design off to the enviro test lab, and head home early for the day. Cheers, Thomas Hi Thomas, Thanx, your two bits were worth more than the academic plug nickel. This is something that our original poster should hearken to as his needs were obviously production oriented. I doubt you understand what he wrote. I can't fathom why you would be concerned with the OP when your own difficulties are so acute. Bench experience will trump cut-and-paste theory in a heart-beat. How would you know? However, triple stub is pretty aggressive. How long did it take you to flatten response? How long will it take you to figure out that he wrote not a wisp of a word on what the "output-Z" of the amplifier is? He did write that he determines how the amp was loaded to acheive power, something I've been saying is the prime concern. |
On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma"
wrote: Response is flattened through gain controlling the pre-amp from a look-up table held in the micro's EEPROM. The alignment procedure is automated using the HB-IP bus from the spectrum analyzer and a computer. The computer/analyzer also looks for harmonic content and spurious emissions during this procedure. Think it takes about ten seconds to do this. Hi Thomas, 10 seconds to adjust all 6 stubs? 73's Richard Clark, KB7QHC |
Richard Fry wrote:
"Physics is not application-selsctive." True. The laws of physics are inviolable. The FM amplifier does not need linearity. Amplitude distortion is irrevelant. Severe clipping to remove amplitude variations is common practice. Phase/frequency shift is the modulation of interest. Clipping generates harmonics and FCC rules limit harmonic transmission in all services. Any manufacturer wants to require the fewest user adjustments. I`m not surprised that tuned frequency selective circuits are minimized. I would be surprised if some final filter were not used to guarantee compliance with the rules. Best regards, Richard Harrison, KB5WZI |
No the triple stub tuners are only for development. Production boards have
discrete components to form the match network. Power levelling or "flattening the response" is computer adjusting the output power to compensate for the reactive components to ensure a constant output power over the entire band of the radio. We also put in a small temperature compensation coefficient into the EEPROM because the PA tends to put out more power when it is cold. "Richard Clark" wrote in message ... On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma" wrote: Response is flattened through gain controlling the pre-amp from a look-up table held in the micro's EEPROM. The alignment procedure is automated using the HB-IP bus from the spectrum analyzer and a computer. The computer/analyzer also looks for harmonic content and spurious emissions during this procedure. Think it takes about ten seconds to do this. Hi Thomas, 10 seconds to adjust all 6 stubs? 73's Richard Clark, KB7QHC |
On Fri, 4 Mar 2005 11:11:32 -0600, (Richard
Harrison) wrote: "Physics is not application-selsctive." True. The laws of physics are inviolable. .... I would be surprised if some final filter were not used to guarantee compliance with the rules. Hi Richard, Yes, that would be the technical marvel of the ages, but just like our rigs, even the biggest FM transmitters bend to the necessity for output filtering: http://www.broadcast.harris.com/prod...%20Bro%2DB.pdf There is an amusing claim, however, for their power module(s) "Each module is conservatively rated to produce 850W of power into a system VSWR of 1.5:11." Not a very good copy editing job is my guess. Looking at the "efficiency" side of the equation is simple here too: Power Consumption (nominal) • Z2CD: 4.0kW at 2.2kW output power 55% • Z3.5CD: 6.1kW at 3.75kW output power 61% • Z5CD: 7.9kW at 5kW output power 63% • Z7.5CD:11.7kW at 7.5kW output power 64% • Z10CD: 15.3kW at 10kW output power 65% • ZD20CD:31kW at 20kW output power 65% 73's Richard Clark, KB7QHC |
Thomas Magma wrote: No the triple stub tuners are only for development. Production boards have discrete components to form the match network. Power levelling or "flattening the response" is computer adjusting the output power to compensate for the reactive components to ensure a constant output power over the entire band of the radio. We also put in a small temperature compensation coefficient into the EEPROM because the PA tends to put out more power when it is cold. Richard was asking how long it took you to tune the triple stub filters during devolpment. I am curious about the exact nature of the impedance transmformation these devices provided. jk "Richard Clark" wrote in message ... On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma" wrote: Response is flattened through gain controlling the pre-amp from a look-up table held in the micro's EEPROM. The alignment procedure is automated using the HB-IP bus from the spectrum analyzer and a computer. The computer/analyzer also looks for harmonic content and spurious emissions during this procedure. Think it takes about ten seconds to do this. Hi Thomas, 10 seconds to adjust all 6 stubs? 73's Richard Clark, KB7QHC |
On Fri, 04 Mar 2005 17:39:04 GMT, "Thomas Magma"
wrote: No the triple stub tuners are only for development. Hi Thomas, I thought 10 seconds was awful quick. How long would it take to flatten the response when manually adjusting the triple stub tuners? What merit did you find with triple that could not be found with double stub tuners? 73's Richard Clark, KB7QHC |
"Richard Harrison" wrote
The FM amplifier does not need linearity. Amplitude distortion is irrevelant. Severe clipping to remove amplitude variations is common practice. Not so. You confuse receivers with transmitters. Limiting is supplied by the IF strips of FM receivers to reduce/remove AM components on the incoming wave, but FM broadcast transmitters are operated well below any limiting/clipping level, and that is probably true of ham FM txs also. Synchronous and asynchcronous AM are low in broadcast FM tx RF stages (the FCC spec is -50dBc), but not because the FM amplifiers are "clipping." Broadcast FM txs easily can be adjusted over an output power range of ~25% to 105% or more simply by adjusting drive to the PA (keeping constant PA volts). This technique often is used for output power control/VSWR foldback, actually. Clipping generates harmonics and FCC rules limit harmonic transmission in all services. .. I would be surprised if some final filter were not used to guarantee compliance with the rules. You are confused again. I wrote that no "tank circuit or in-band filter(s)" were necessary to achieve the high efficiency I described. Your post I was responding to states that a "tank circuit and other filters" are necessary for high efficiency -- that is not true. Harmonics are present at the PA output of an FM transmitter, but "clipping" is not the process whereby they are generated, as I state above. They are reduced to legal values using a lowpass/harmonic filter. The FCC attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB below the unmodulated carrier. The lowpass/harmonic filter does not improve efficiency--it has a small amount of insertion loss in the FM band. RF |
Hi gwhite,
I would have to agree with you on most everything you have said through this thread. I once saw my boss (with his "PHD") try to model and match a power amp based on the small signal parameters off the datasheet. He insisted that the stated input and output impedances were characteristic parasitics of that amp and wouldn't change between a small or large signal. It was kind of pathetic to watch him struggle for over a month on the matching network, and I think he had resorted to guessing in the end. I've often questioned why manufactures put small signal parameters on their datasheets? Makes no sense to me. Even if they do publish some large signal parameters it is unlikely to be the exact same mode of operation that you need for your project. Playing with triple stub tuners on PA's has shown me that there are many combinations of input and output impedances that appear to give similar results at any one frequency, but give different results at others frequencies. So it's a matter of finding the input and output impedance that give you adequate performance over the entire scope of your project. Thomas "gwhite" wrote in message ... Richard Clark wrote: On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma" wrote: Wow this is a long thread. Don't really know where I should put my two bits in, but here it goes. I have designed several RF PA sections in the past. 500MHz at about 50W. Pretty easy stuff if you have the right tools and know how to use them. The tools I like using for matching the power output FET is two triple stub tuners. One on the input of the FET and one on the output. So it goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50 ohm dummy pad - spectrum analyzer. Then just tune the stubs for the performance you desire, these include: efficiency (thermal issues), harmonic content, spurious emissions, load VSWR considerations, cold start, ect. Then remove the FET and look into the triple stub tuners with the network analyzer. Model and duplicate the network out of discrete components that can handle the voltage/power, send the design off to the enviro test lab, and head home early for the day. Cheers, Thomas Hi Thomas, Thanx, your two bits were worth more than the academic plug nickel. This is something that our original poster should hearken to as his needs were obviously production oriented. I doubt you understand what he wrote. I can't fathom why you would be concerned with the OP when your own difficulties are so acute. Bench experience will trump cut-and-paste theory in a heart-beat. How would you know? However, triple stub is pretty aggressive. How long did it take you to flatten response? How long will it take you to figure out that he wrote not a wisp of a word on what the "output-Z" of the amplifier is? He did write that he determines how the amp was loaded to acheive power, something I've been saying is the prime concern. |
"Richard Clark" wrote:
Looking at the "efficiency" side of the equation is simple here too: Power Consumption (nominal) (clip) _________________ Another case of writing without knowing, I see. The power consumptions you cite are the TOTAL values for those transmitters, not of the RF power amplifiers alone. The total value includes the exciter, driver(s), power supply losses, control circuits, and RF combining losses, as well as power for the internal cooling fans. The PA modules have 80% or better efficiency, by themselves. The reason I know is that I was the author of those specs. RF |
If your amp has to operate over a wide frequency range it is not likely that
you can flatten the response just with stubs. Stubs should be looked at as more single frequency devices than broadband networks. But you can use the stubs to plot out the appropriate impedance curve on the Smith Chart to ensure a flat response when you model in the discretes. I usually just try to get the flatness of the response as close as possible and rely on a software calibration routine to flatten it off. Saves a lot of time. It's my understanding that a triple stub tuner of the right length can reach anywhere on the Smith Chart where as a double stub can not. "Richard Clark" wrote in message ... On Fri, 04 Mar 2005 17:39:04 GMT, "Thomas Magma" wrote: No the triple stub tuners are only for development. Hi Thomas, I thought 10 seconds was awful quick. How long would it take to flatten the response when manually adjusting the triple stub tuners? What merit did you find with triple that could not be found with double stub tuners? 73's Richard Clark, KB7QHC |
On Fri, 4 Mar 2005 12:42:47 -0600, "Richard Fry"
wrote: "Richard Clark" wrote: Looking at the "efficiency" side of the equation is simple here too: Power Consumption (nominal) (clip) _________________ Another case of writing without knowing, I see. Hi OM, Yes, I do recall your claims that contradicted Mendenhall's explicit efficiency computations. So I see no need to pursue undocumented claims you offer. Unless you can supply specific references from Harris about this 80% efficiency, then such comments remain as suspect as before. The reason I know is that I was the author of those specs. I am still wondering about the odd entry of: "Each module is conservatively rated to produce 850W of power into a system VSWR of 1.5:11." I notice you passed on discussion to this particular point of accuracy. 11s can be explained by hitting 1 too many times, or 80 by hitting an errant 0 too many. One of those things that escape the notice of a spell-checker. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
"There is an amusing claim, however, for their power module(s) "Each module is conservatively rated to produce 850W of power into a system VSWR of 1,5:11." Not a very good copy editing job is my guess." Richard must be right. I guess a finger was left too long on the no.1 key and nobody caught it in time. I admire Gates` scheme of paralleling many relatively low powered amplifiers. If one fails, you can continue almost as if nothing happened. Very nice. Best regards, Richard Harrison, KB5WZI |
On Fri, 04 Mar 2005 19:09:20 GMT, "Thomas Magma"
wrote: If your amp has to operate over a wide frequency range it is not likely that you can flatten the response just with stubs. Hi Thomas, Certainly not as conventional Triple Stubs. However, care to provide some of the cogent details of that particular project? Any interesting insights? 73's Richard Clark, KB7QHC |
"Richard Clark" wrote:
Unless you can supply specific references from Harris about this 80% efficiency, then such comments remain as suspect as before. You may take what I wrote as being "from Harris," because I was part of Harris FM Product Management for those transmitters before my retirement in 1999 (after 19 years there). I was responsible for documenting all performance features and parameters published for the product line, using numbers generated and approved by Engineering. If the PAs alone were as (in)efficient as you imply with your calculations, power consumption for the entire transmitter would be considerably higher. Common sense should tell you that PA module efficiency would have to be much higher than the efficiency calculations you posted in order for total power consumption to be as stated on the Harris spec sheets. I am still wondering about the odd entry of: "Each module is conservatively rated to produce 850W of power into a system VSWR of 1.5:11." Yes, that is a "typo," as you noted. Very good. It should read "...VSWR of 1.5:1." RF |
Richard Fry wrote:
"Not so. You confuse receivers and transmitters." FM transmitters often use Class C amplifiers and frequency multipliers on the modulated signal. An AM signal can not be amplified by a Class C amplifier because of severe distortion of the modulated signal. In FM, amplitude distortion is irrelevant no matter where it occurs, receiver or transmitter. The recovered audio will sound just fine. It`s one of the many advantages of FM. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
Richard Fry wrote: "Not so. You confuse receivers and transmitters." FM transmitters often use Class C amplifiers and frequency multipliers on the modulated signal. An AM signal can not be amplified by a Class C amplifier because of severe distortion of the modulated signal. In FM, amplitude distortion is irrelevant no matter where it occurs, receiver or transmitter. The recovered audio will sound just fine. It`s one of the many advantages of FM. The question seems to be: If an FM transmitter's output signal is not a reasonably pure sine wave, is a low-pass filter used between the transmitter and antenna to reduce the harmonics? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil Moore wrote:
Richard Harrison wrote: Richard Fry wrote: "Not so. You confuse receivers and transmitters." FM transmitters often use Class C amplifiers and frequency multipliers on the modulated signal. An AM signal can not be amplified by a Class C amplifier because of severe distortion of the modulated signal. In FM, amplitude distortion is irrelevant no matter where it occurs, receiver or transmitter. The recovered audio will sound just fine. It`s one of the many advantages of FM. The question seems to be: If an FM transmitter's output signal is not a reasonably pure sine wave, is a low-pass filter used between the transmitter and antenna to reduce the harmonics? How can a filter filter correctly when its input is terminated in an indeterminate impedance? 73, Tom Donaly, KA6RUH |
Richard Fry wrote:
"Note that without adjustment, modern solid-state FM broadcast transmitters can (and do) provide 80% or better PA efficiency into a 50 ohm load across 20% bandwidth, with no tank circuit or other in-band filter(s)." Well, Richard Fry didn`t say there were no out-of-band filters or traps. One could have a low-pass filter that cut-off above 108 MHz, but below 176 MHz, and no harmonic would get through the filter. 80% or better efficiency isn`t coming from a Class A amplifier, so maybe it comes from a Class B amplifier. One fly in the ointment is found on page 354 of Terman`s 1955 edition: "The theoretical maximum possible plate efficiency that can be realized in a Class B amplifier is pi/4 or 78.5 per cent;---." Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote
In FM, amplitude distortion is irrelevant no matter where it occurs, receiver or transmitter. The recovered audio will sound just fine. It`s one of the many advantages of FM. ___________________ It may sound just fine to you, but carefully made performance measurements of a received FM signal having high AM show otherwise. AM on FM is far from irrelevant. RF |
"Cecil Moore" wrote:
The question seems to be: If an FM transmitter's output signal is not a reasonably pure sine wave, is a low-pass filter used between the transmitter and antenna to reduce the harmonics? ___________________ Yes. This question was answered in my post in this thread of 00:24UTC today, which I will paste below: "Harmonics are present at the PA output of an FM transmitter, but "clipping" is not the process whereby they are generated, as I state above. They are reduced to legal values using a lowpass/harmonic filter. The FCC attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB below the unmodulated carrier. The lowpass/harmonic filter does not improve efficiency--it has a small amount of insertion loss in the FM band." RF |
On Fri, 4 Mar 2005 13:41:50 -0600, "Richard Fry"
wrote: If the PAs alone were as (in)efficient as you imply with your calculations, power consumption for the entire transmitter would be considerably higher. The implication is drawn by and from your inertia. Common sense should tell you that PA module efficiency would have to be much higher than the efficiency calculations you posted in order for total power consumption to be as stated on the Harris spec sheets. Hi OM, It is tedious to have to carry your water for you. I had to chase down your Mendenhall references, this seems to be a consistent trait. Claims are generous in this group and heavily discounted due to the paucity of facts. Such facts as may be drawn out, but could have had been as easily offered by you: "For even greater reliability, any PA module can be used as an IPA module, with absolutely no modification." It is quite obvious that as an IPA, that in the lower wattage systems it represents overkill at 845W to generate drive to final PAs to 2.2 KW output. Hence the lower total efficiency. On the other hand, an IPA driving 845W to generate 22KW obviously makes better efficiency sense and is found in the overall 64.5% figure. NOW, if the PA finals, accounting for 22KW are 80% efficient, that must mean that they only consume 27.5KW of power to do so, and that with a power input rating of 31KW then leaves the IPA (an identical 80% efficiency module) and control circuitry to absorb 3.5KW to deliver the drive of .845KW. It follows that for an 80% efficient IPA, it accounts for 1KW power consumption. This remainder is easily attributable to power supply losses (if we simply assign an industrial average efficiency of 95% for power conversion) otherwise the system TTL circuits and LCD meters suck down 2.5KW on their own. This, as you put it (but fail to evidence), would quickly subdue suspicion. And an equal treatment to more conventional, retail Amateur Radio Transmitters also reveals efficiencies through the same exercise. It is quite evident that such transmitters are no where near these vaunted examples - but few dare venture into these dissections. 73's Richard Clark, KB7QHC |
"Richard Harrison"
80% or better efficiency isn`t coming from a Class A amplifier, so maybe it comes from a Class B amplifier. One fly in the ointment is found on page 354 of Terman`s 1955 edition: "The theoretical maximum possible plate efficiency that can be realized in a Class B amplifier is pi/4 or 78.5 per cent;---." ____________ No fly to those who really know this subject. They are neither Class A or B. Those wanting to comment on modern solid-state VHF amplifier designs and performance parameters really should research them before doing so. We are 60 years beyond the date of this citation from Terman. RF |
Thomas Magma wrote:
I've often questioned why manufactures put small signal parameters on their datasheets? Makes no sense to me. They might be of some use for specific cases. For example, if the PA is class A, is used well backed off because of high PEP-to-avg ratios of the signal, and you've managed to get the output load dialed in, s-params can be useful for a first cut at the amplifier *input* match. I've always still had to do some tweeking though. Also, with some work and considering the load-line match, they can give you an idea of what gain can be accomplished. This might already be in the data sheet though, as you mention. Even if they do publish some large signal parameters it is unlikely to be the exact same mode of operation that you need for your project. One of the large signal parameters I like best is how much power the device can dissipate. ;-) Voltage breakdowns and Imax are nice too. ;-) ;-) |
"Richard Clark" wrote regarding Harris "Z" FM broadcast transmitters:
"For even greater reliability, any PA module can be used as an IPA module, with absolutely no modification." It is quite obvious that as an IPA, that in the lower wattage systems it represents overkill at 845W to generate drive to final PAs to 2.2 KW output. Hence the lower total efficiency. On the other hand, an IPA driving 845W to generate 22KW obviously makes better efficiency sense and is found in the overall 64.5% figure. Yet another case where you write with guesswork, not knowing the facts. Obviously you do not understand the architecture of this line of transmitters, even though what I am about to write is available on the Harris website. The PA and IPA modules are the same, and consist of two, independent amps--each amp capable of 425W output. Their actual output power depends on the tx they are installed in, and the power level required from it. The only thing they have in common is a heat sink. An IPA at any power level uses only one of these amps per 5kW (or less) block of PA amps. The other amp of the IPA remains unpowered and in reserve, and autoswitches on line if the active one fails. The lower AC input to RF output efficiency of the lower powered transmitters arises from the fixed overhead in all units for losses OTHER than in the RF amplifiers, i.e., power supply losses, exciter and controller power, RF combiner and harmonic filter losses, and cooling power--the AC consumption for which in low power units is a larger proportion of the total. NOW, if the PA finals, accounting for 22KW are 80% efficient, that must mean that they only consume 27.5KW of power to do so, and that with a power input rating of 31KW then leaves the IPA (an identical 80% efficiency module) and control circuitry to absorb 3.5KW to deliver the drive of .845KW. It follows that for an 80% efficient IPA, it accounts for 1KW power consumption. This remainder is easily attributable to power supply losses (if we simply assign an industrial average efficiency of 95% for power conversion) otherwise the system TTL circuits and LCD meters suck down 2.5KW on their own. Your analytical skills are seriously wanting. Please re-read my response above. It is quite evident that such transmitters are no where near these vaunted examples - but few dare venture into these dissections. It is "evident" only to those who don't understand the subject. Others have not dared to venture into these dissections probably because THEY know better. RF |
| All times are GMT +1. The time now is 05:42 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com