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#91
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Transmitter Output Impedance
On 6 mayo, 17:20, walt wrote:
On May 6, 6:30*am, Wimpie wrote: On 6 mayo, 03:19, walt wrote: On May 5, 5:26*pm, Wimpie wrote: On 5 mayo, 19:23, Jim Lux wrote: John KD5YI wrote: On 5/4/2011 7:54 PM, Jim Lux wrote: John KD5YI wrote: Acceptable is what the manufacturer recommends for his gear. What does this have to do with the device's output impedance? Absolutely nothing.. which is the point. Are we arguing the same point? but the summary is, That it is a bag of worms? I'm waiting with baited breath... Exactly.. In fact, as interesting as it would be to measure the output impedance of my radio, I started to think about what it would buy me, and came to the conclusion, almost nothing (other than satisfying curiosity). It *might* be interesting to look at (and write an article for QST/QEX or something) "optimizing radiated power". *Answering the question: do you really want a 50 ohm match on your antenna analyzer, or do you want maximum net power at the antenna feedpoint, and what that might mean for typical 100W solid state rigs, antennas, etc. (as a practical matter, this is what automatic antenna tuners actually adjust for: either minimum reflected power, or maximum fwd-ref) but it's possible that deliberately running a mismatch (as shown on your rig's SWR meter) might actually result in more radiated power. *e.g. if at 1:1 you have 100W fwd and 0 rev, but at 2:1 you have 150W fwd and 15W reflected, so you're actually net 135W vs 100W; assuming your rig doesn't otherwise have any problems. Hello Jim, I agree with the PA load mismatch issue. It is possible to get (some) more net power by applying mismatch to a PA stage (from experience).. But frequently it comes with too much increase in input power (so very hot heatsink). In case of high efficiency designs, the active devices may communicate to you by means of smoke or ejection of (hot) particles. Wim PA3DJSwww.tetech.nl In case of PM, tell the pigeon that abc is not in the address. Hi Wimpie and KD5YI, Will you please explain how it is possible to get more power delivered by applying a mismatch to the output of a PA? And for KD5YI specifically, I believe you have presented some inaccurate math calculations. You begin with delivering 100w into a matched load. Then you say you mismatch to 2:1 and get 135w forward and 15w reflected, leaving 120w delivered. You must be kidding!! No he is not kidding. I observed the same. At that time I was lucky to have full access to HP, Advantest and Rohde & Schwarz equipment to double-check everything (I first blamed my own gear). * You should leave the idea that all PA's have 50 Ohms output impedance, then it is easy to explain yourself. A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance. First, with a 2:1 mismatch and 100w delivered by the PA, the reflected power is 11.111w, which when added to 100w from the source, the forward power is 111.11 watts. When the reflected power that returns to the load is subtracted from the forward power, the result is 100w. You've heard the expression 'there is no free lunch'? So please explain to me, if you can, how you can deliver 120w when the source is 100w. Walt The extreme cases where you can get significantly more net output power by applying mismatch, are PA's with high efficiency (class-E, - D, -DE, etc). * I am currently designing a balanced class-E 500W stage. It can deliver 1 kW, but within very short time the mosfet's will explode (if the supervisory circuit doesn't act). Wim PA3DJSwww.tetech.nl Wim, are you saying that by using Class E amps you are able to violate the Laws of Physics pertaining to the Conjugate Matching Theorem and the Maximum Power Delivery Theorem? I cannot agree. Walt, Maybe you should familiarize yourself with class E (and other high efficient topologies). Set up a simulation (or measurement) and try to apply your "conjugated match" thing. You will find out that you can't make a class E PA that operates under conjugated match, unless you are going to play with the power supply's internal resistance (so the system becomes power supply limited). My current design outputs 500W into 4.5 Ohms, however the output impedance (load pulling) is 1 Ohm. The reason for non-conjugated matched operation is that in class-E the device is in voltage saturation for about 50% of the time. During that time the device has no gain, so a device used in class-E has less gain then the same device used in a non-saturated application. In other words: tuning is designed for highest efficiency, not highest output power. A load doesn't care what the source is. If the load impedance is the complex conjugate of the source, all available power will be delivered to the load. Quote from text above: "If the load impedance is the complex conjugate of the source.....". This If-statement has a "false" result for many PAs, try to broaden your view. Then, if the load impedance is either increased or decreased, the power delivered will decrease. Are you now saying that the concept I just stated above is no longer true? If you are, please explain in detail why this is so. How does 'high efficiency' overcome the requirement for impedance matching in the delivery of power? And are you agreeing with an earlier poster that with a 100w source and a mismatch of 2:1 the forward power will be 135w and 15w reflected, the power delivered to the load will be 130w? If so, will you please explain in detail how this can occur? Yes I agree with him, see my reaction to that statement My class-E PA design that I am doing now is designed for a 4.5 Ohms nominal load. If I change that load to 2.5 Ohms (VSWR=1.8), output increases to 700W, but it will be destroyed due too non-favourite combination of Vds and Id. Walt With kind regards, Wim PA3DJS www.tetech.nl |
#92
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Transmitter Output Impedance
On May 6, 11:43*am, Wimpie wrote:
You will find out that you can't make a class E PA that operates under conjugated match, unless you are going to play with the power supply's internal resistance ... As I understand it, Walt's approach is to pick a point inside the source at which the output becomes linear through filtering and call the impedance at that point the linear "source impedance" (including some boundary conditions). It is akin to the motion of a pendulum in a clock being linear even though the sustaining energy comes in non- linear pulses. -- 73, Cecil, w5dxp.com "Halitosis is better than no breath at all.", Don, KE6AJH/SK |
#93
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Transmitter Output Impedance
On May 6, 12:43*pm, Wimpie wrote:
On 6 mayo, 17:20, walt wrote: On May 6, 6:30*am, Wimpie wrote: On 6 mayo, 03:19, walt wrote: On May 5, 5:26*pm, Wimpie wrote: On 5 mayo, 19:23, Jim Lux wrote: John KD5YI wrote: On 5/4/2011 7:54 PM, Jim Lux wrote: John KD5YI wrote: Acceptable is what the manufacturer recommends for his gear.. What does this have to do with the device's output impedance? Absolutely nothing.. which is the point. Are we arguing the same point? but the summary is, That it is a bag of worms? I'm waiting with baited breath... Exactly.. In fact, as interesting as it would be to measure the output impedance of my radio, I started to think about what it would buy me, and came to the conclusion, almost nothing (other than satisfying curiosity). It *might* be interesting to look at (and write an article for QST/QEX or something) "optimizing radiated power". *Answering the question: do you really want a 50 ohm match on your antenna analyzer, or do you want maximum net power at the antenna feedpoint, and what that might mean for typical 100W solid state rigs, antennas, etc. (as a practical matter, this is what automatic antenna tuners actually adjust for: either minimum reflected power, or maximum fwd-ref) but it's possible that deliberately running a mismatch (as shown on your rig's SWR meter) might actually result in more radiated power. *e.g. if at 1:1 you have 100W fwd and 0 rev, but at 2:1 you have 150W fwd and 15W reflected, so you're actually net 135W vs 100W; assuming your rig doesn't otherwise have any problems. Hello Jim, I agree with the PA load mismatch issue. It is possible to get (some) more net power by applying mismatch to a PA stage (from experience). But frequently it comes with too much increase in input power (so very hot heatsink). In case of high efficiency designs, the active devices may communicate to you by means of smoke or ejection of (hot) particles. Wim PA3DJSwww.tetech.nl In case of PM, tell the pigeon that abc is not in the address. Hi Wimpie and KD5YI, Will you please explain how it is possible to get more power delivered by applying a mismatch to the output of a PA? And for KD5YI specifically, I believe you have presented some inaccurate math calculations. You begin with delivering 100w into a matched load. Then you say you mismatch to 2:1 and get 135w forward and 15w reflected, leaving 120w delivered. You must be kidding!! No he is not kidding. I observed the same. At that time I was lucky to have full access to HP, Advantest and Rohde & Schwarz equipment to double-check everything (I first blamed my own gear). * You should leave the idea that all PA's have 50 Ohms output impedance, then it is easy to explain yourself. A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance. First, with a 2:1 mismatch and 100w delivered by the PA, the reflected power is 11.111w, which when added to 100w from the source, the forward power is 111.11 watts. When the reflected power that returns to the load is subtracted from the forward power, the result is 100w. You've heard the expression 'there is no free lunch'? So please explain to me, if you can, how you can deliver 120w when the source is 100w. Walt The extreme cases where you can get significantly more net output power by applying mismatch, are PA's with high efficiency (class-E, - D, -DE, etc). * I am currently designing a balanced class-E 500W stage. It can deliver 1 kW, but within very short time the mosfet's will explode (if the supervisory circuit doesn't act). Wim PA3DJSwww.tetech.nl Wim, are you saying that by using Class E amps you are able to violate the Laws of Physics pertaining to the Conjugate Matching Theorem and the Maximum Power Delivery Theorem? I cannot agree. Walt, Maybe you should familiarize yourself with class E (and other high efficient topologies). Set up a simulation (or measurement) and try to apply your "conjugated match" thing. You will find out that you can't make a class E PA that operates under conjugated match, unless you are going to play with the power supply's internal resistance (so the system becomes power supply limited). *My current design outputs 500W into 4.5 Ohms, however the output impedance (load pulling) is 1 Ohm. The reason for non-conjugated matched operation is that in class-E the device is in voltage saturation for about 50% of the time. During that time the device has no gain, so a device used in class-E has less gain then the same device used in a non-saturated application. In other words: tuning is designed for highest efficiency, not highest output power. A load doesn't care what the source is. If the load impedance is the complex conjugate of the source, all available power will be delivered to the load. Quote from text above: "If the load impedance is the complex conjugate of the source.....". This If-statement has a "false" result for many PAs, try to broaden your view. Then, if the load impedance is either increased or decreased, the power delivered will decrease. Are you now saying that the concept I just stated above is no longer true? If you are, please explain in detail why this is so. How does 'high efficiency' overcome the requirement for impedance matching in the delivery of power? And are you agreeing with an earlier poster that with a 100w source and a mismatch of 2:1 the forward power will be 135w and 15w reflected, the power delivered to the load will be 130w? If so, will you please explain in detail how this can occur? Yes I agree with him, see my reaction to that statement My class-E PA design that I am doing now *is designed for a 4.5 Ohms nominal load. If I change that load to 2.5 Ohms (VSWR=1.8), output increases to 700W, but it will be destroyed due too non-favourite combination of Vds and Id. Walt With kind regards, Wim PA3DJSwww.tetech.nl Well, Wim, I still don't see how a 2:1 mismatch can achieve 135w forward and 15w reflected with a 100w source. How can a 2:1 mismatch achieve 130w forward power, when that mismatch can only reflect 0.111 x 100w? Also, how can that mismatch achieve 15w of reflected power when that mismatch can only reflect 0.111 x 100w? And how can 11.111 watts of forward power add to 100w to achieve 130w? I don't see how the power reflected at a mismatched load can be affected by the nature of the source. But I do understand how with a 4.5-ohm load can deliver 500w and a 2.5- ohm load can deliver 700w. I now understand what you mean by a mismatch can increase the power delivery. (Actually, a change in mismatch) But Wim, you pulled a fast one one us!!! Until now you didn't tell us that the source resistance of the source was 1 ohm!!! You also didn't tell us what the power would be if the load was 1 ohm (thus matching the load to the source) if the power supply was sufficiently large so the power delivery would not be limited by the power supply. So in reality, you really DON'T get an increase in power delivery by mismatching, but actually a decrease. What you're really doing is obtaining an increase in power delivery by decreasing the amount of mismatch from 4.5:1 to 2.5:1. So now I understand that you haven't violated any laws of physics, but IMHO, you have been misleading when you say you obtain a increase in power by mismatching, because that statement isn't really true, is it? take care, Walt |
#94
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Transmitter Output Impedance
On 6 mayo, 22:18, walt wrote:
On May 6, 12:43*pm, Wimpie wrote: On 6 mayo, 17:20, walt wrote: On May 6, 6:30*am, Wimpie wrote: On 6 mayo, 03:19, walt wrote: On May 5, 5:26*pm, Wimpie wrote: On 5 mayo, 19:23, Jim Lux wrote: John KD5YI wrote: On 5/4/2011 7:54 PM, Jim Lux wrote: John KD5YI wrote: Acceptable is what the manufacturer recommends for his gear. What does this have to do with the device's output impedance? Absolutely nothing.. which is the point. Are we arguing the same point? but the summary is, That it is a bag of worms? I'm waiting with baited breath.... Exactly.. In fact, as interesting as it would be to measure the output impedance of my radio, I started to think about what it would buy me, and came to the conclusion, almost nothing (other than satisfying curiosity). It *might* be interesting to look at (and write an article for QST/QEX or something) "optimizing radiated power". *Answering the question: do you really want a 50 ohm match on your antenna analyzer, or do you want maximum net power at the antenna feedpoint, and what that might mean for typical 100W solid state rigs, antennas, etc. (as a practical matter, this is what automatic antenna tuners actually adjust for: either minimum reflected power, or maximum fwd-ref) but it's possible that deliberately running a mismatch (as shown on your rig's SWR meter) might actually result in more radiated power.. *e.g. if at 1:1 you have 100W fwd and 0 rev, but at 2:1 you have 150W fwd and 15W reflected, so you're actually net 135W vs 100W; assuming your rig doesn't otherwise have any problems. Hello Jim, I agree with the PA load mismatch issue. It is possible to get (some) more net power by applying mismatch to a PA stage (from experience). But frequently it comes with too much increase in input power (so very hot heatsink). In case of high efficiency designs, the active devices may communicate to you by means of smoke or ejection of (hot) particles. Wim PA3DJSwww.tetech.nl In case of PM, tell the pigeon that abc is not in the address. Hi Wimpie and KD5YI, Will you please explain how it is possible to get more power delivered by applying a mismatch to the output of a PA? And for KD5YI specifically, I believe you have presented some inaccurate math calculations. You begin with delivering 100w into a matched load. Then you say you mismatch to 2:1 and get 135w forward and 15w reflected, leaving 120w delivered. You must be kidding!! No he is not kidding. I observed the same. At that time I was lucky to have full access to HP, Advantest and Rohde & Schwarz equipment to double-check everything (I first blamed my own gear). * You should leave the idea that all PA's have 50 Ohms output impedance, then it is easy to explain yourself. A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance. First, with a 2:1 mismatch and 100w delivered by the PA, the reflected power is 11.111w, which when added to 100w from the source, the forward power is 111.11 watts. When the reflected power that returns to the load is subtracted from the forward power, the result is 100w. You've heard the expression 'there is no free lunch'? So please explain to me, if you can, how you can deliver 120w when the source is 100w. Walt The extreme cases where you can get significantly more net output power by applying mismatch, are PA's with high efficiency (class-E, - D, -DE, etc). * I am currently designing a balanced class-E 500W stage. It can deliver 1 kW, but within very short time the mosfet's will explode (if the supervisory circuit doesn't act). Wim PA3DJSwww.tetech.nl Wim, are you saying that by using Class E amps you are able to violate the Laws of Physics pertaining to the Conjugate Matching Theorem and the Maximum Power Delivery Theorem? I cannot agree. Walt, Maybe you should familiarize yourself with class E (and other high efficient topologies). Set up a simulation (or measurement) and try to apply your "conjugated match" thing. You will find out that you can't make a class E PA that operates under conjugated match, unless you are going to play with the power supply's internal resistance (so the system becomes power supply limited). *My current design outputs 500W into 4.5 Ohms, however the output impedance (load pulling) is 1 Ohm. The reason for non-conjugated matched operation is that in class-E the device is in voltage saturation for about 50% of the time. During that time the device has no gain, so a device used in class-E has less gain then the same device used in a non-saturated application. In other words: tuning is designed for highest efficiency, not highest output power. A load doesn't care what the source is. If the load impedance is the complex conjugate of the source, all available power will be delivered to the load. Quote from text above: "If the load impedance is the complex conjugate of the source.....". This If-statement has a "false" result for many PAs, try to broaden your view. Then, if the load impedance is either increased or decreased, the power delivered will decrease. Are you now saying that the concept I just stated above is no longer true? If you are, please explain in detail why this is so. How does 'high efficiency' overcome the requirement for impedance matching in the delivery of power? And are you agreeing with an earlier poster that with a 100w source and a mismatch of 2:1 the forward power will be 135w and 15w reflected, the power delivered to the load will be 130w? If so, will you please explain in detail how this can occur? Yes I agree with him, see my reaction to that statement My class-E PA design that I am doing now *is designed for a 4.5 Ohms nominal load. If I change that load to 2.5 Ohms (VSWR=1.8), output increases to 700W, but it will be destroyed due too non-favourite combination of Vds and Id. Walt With kind regards, Wim PA3DJSwww.tetech.nl Well, Wim, I still don't see how a 2:1 mismatch can achieve 135w forward and 15w reflected with a 100w source. How can a 2:1 mismatch achieve 130w forward power, when that mismatch can only reflect 0.111 x 100w? Also, how can that mismatch achieve 15w of reflected power when that mismatch can only reflect 0.111 x 100w? And how can 11.111 watts of forward power add to 100w to achieve 130w? I don't see how the power reflected at a mismatched load can be affected by the nature of the source. But I do understand how with a 4.5-ohm load can deliver 500w and a 2.5- ohm load can deliver 700w. I now understand what you mean by a mismatch can increase the power delivery. (Actually, a change in mismatch) But Wim, you pulled a fast one one us!!! Until now you didn't tell us that the source resistance of the source was 1 ohm!!! You also didn't tell us what the power would be if the load was 1 ohm (thus matching the load to the source) if the power supply was sufficiently large so the power delivery would not be limited by the power supply. So in reality, you really DON'T get an increase in power delivery by mismatching, but actually a decrease. What you're really doing is obtaining an increase in power delivery by decreasing the amount of mismatch from 4.5:1 to 2.5:1. So now I understand that you haven't violated any laws of physics, but IMHO, you have been misleading when you say *you obtain a increase in power by mismatching, because that statement isn't really true, is it? take care, Walt Hello Walt, Off course I agree with you that you can never have more power then the conjugated matched power, but many PAs don't operate in this regime (as my very extreme class-E case). It was the reason for mentioning: "A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance." The problem is in whether you define mismatch based on the ohmic value printed on the back of the PA, or on the actual output impedance of the PA (that you mostly don't know in case of many solid state PA's). As the actual output impedance of the PA may not be 50 Ohms (for example 100 Ohms), a 50 Ohms load (as mentioned on the back of the PA) will provide the stated power (for example 100W). By applying 100 Ohms (that is VSWR = 2 for a 50 Ohms reference), the net power will increase to 112 W. Actually it wasn't me to experience this mismatch isue first, but my father during the time that CB was very popular overhere. The above example assumes that the output impedance of the PA is independent of load. Of course this is only true when the actual voltage across the active device and current through it doesn't saturate the active device too much. I think the saturation issue is one of the advantages of a linear PA with accessible R an X tuning. You just tune for maximum output given a certain load and drive (you may use plate current as a guide also). As long as your SSB signal's PEP stays below that output power, your active device will not go into voltage saturation and IMD will likely be acceptable. In case of a wide band push-pull PA, VSWR = 2 (with inconvenient phase) may provide a load where the active devices (mosfet or BJT), will go into voltage saturation at a net output power below the rated power of the PA. For constant envelope modulation this isn't problem, but for SSB/AM it isn't good. With kind regards, Wim PA3DJS www.tetech.nl |
#95
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Transmitter Output Impedance
On Fri, 6 May 2011 13:18:16 -0700 (PDT), walt wrote:
Well, Wim, I still don't see how a 2:1 mismatch can achieve 135w forward and 15w reflected with a 100w source. Hi Walt, Irrespective of who the author is of this statement (I've lost track), I too would like to see how this is reconciled. But seeing that you've asked several times.... 73's Richard Clark, KB7QHC |
#96
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Transmitter Output Impedance
On May 6, 5:50*pm, Wimpie wrote:
On 6 mayo, 22:18, walt wrote: On May 6, 12:43*pm, Wimpie wrote: On 6 mayo, 17:20, walt wrote: On May 6, 6:30*am, Wimpie wrote: On 6 mayo, 03:19, walt wrote: On May 5, 5:26*pm, Wimpie wrote: On 5 mayo, 19:23, Jim Lux wrote: John KD5YI wrote: On 5/4/2011 7:54 PM, Jim Lux wrote: John KD5YI wrote: Acceptable is what the manufacturer recommends for his gear. What does this have to do with the device's output impedance? Absolutely nothing.. which is the point. Are we arguing the same point? but the summary is, That it is a bag of worms? I'm waiting with baited breath.... Exactly.. In fact, as interesting as it would be to measure the output impedance of my radio, I started to think about what it would buy me, and came to the conclusion, almost nothing (other than satisfying curiosity). It *might* be interesting to look at (and write an article for QST/QEX or something) "optimizing radiated power". *Answering the question: do you really want a 50 ohm match on your antenna analyzer, or do you want maximum net power at the antenna feedpoint, and what that might mean for typical 100W solid state rigs, antennas, etc. (as a practical matter, this is what automatic antenna tuners actually adjust for: either minimum reflected power, or maximum fwd-ref) but it's possible that deliberately running a mismatch (as shown on your rig's SWR meter) might actually result in more radiated power. *e.g. if at 1:1 you have 100W fwd and 0 rev, but at 2:1 you have 150W fwd and 15W reflected, so you're actually net 135W vs 100W; assuming your rig doesn't otherwise have any problems. Hello Jim, I agree with the PA load mismatch issue. It is possible to get (some) more net power by applying mismatch to a PA stage (from experience). But frequently it comes with too much increase in input power (so very hot heatsink). In case of high efficiency designs, the active devices may communicate to you by means of smoke or ejection of (hot) particles. Wim PA3DJSwww.tetech.nl In case of PM, tell the pigeon that abc is not in the address.. Hi Wimpie and KD5YI, Will you please explain how it is possible to get more power delivered by applying a mismatch to the output of a PA? And for KD5YI specifically, I believe you have presented some inaccurate math calculations. You begin with delivering 100w into a matched load. Then you say you mismatch to 2:1 and get 135w forward and 15w reflected, leaving 120w delivered. You must be kidding!! No he is not kidding. I observed the same. At that time I was lucky to have full access to HP, Advantest and Rohde & Schwarz equipment to double-check everything (I first blamed my own gear). * You should leave the idea that all PA's have 50 Ohms output impedance, then it is easy to explain yourself. A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance. First, with a 2:1 mismatch and 100w delivered by the PA, the reflected power is 11.111w, which when added to 100w from the source, the forward power is 111.11 watts. When the reflected power that returns to the load is subtracted from the forward power, the result is 100w. You've heard the expression 'there is no free lunch'? So please explain to me, if you can, how you can deliver 120w when the source is 100w. Walt The extreme cases where you can get significantly more net output power by applying mismatch, are PA's with high efficiency (class-E, - D, -DE, etc). * I am currently designing a balanced class-E 500W stage. It can deliver 1 kW, but within very short time the mosfet's will explode (if the supervisory circuit doesn't act). Wim PA3DJSwww.tetech.nl Wim, are you saying that by using Class E amps you are able to violate the Laws of Physics pertaining to the Conjugate Matching Theorem and the Maximum Power Delivery Theorem? I cannot agree. Walt, Maybe you should familiarize yourself with class E (and other high efficient topologies). Set up a simulation (or measurement) and try to apply your "conjugated match" thing. You will find out that you can't make a class E PA that operates under conjugated match, unless you are going to play with the power supply's internal resistance (so the system becomes power supply limited). *My current design outputs 500W into 4.5 Ohms, however the output impedance (load pulling) is 1 Ohm. The reason for non-conjugated matched operation is that in class-E the device is in voltage saturation for about 50% of the time. During that time the device has no gain, so a device used in class-E has less gain then the same device used in a non-saturated application. In other words: tuning is designed for highest efficiency, not highest output power. A load doesn't care what the source is. If the load impedance is the complex conjugate of the source, all available power will be delivered to the load. Quote from text above: "If the load impedance is the complex conjugate of the source.....". This If-statement has a "false" result for many PAs, try to broaden your view. Then, if the load impedance is either increased or decreased, the power delivered will decrease. Are you now saying that the concept I just stated above is no longer true? If you are, please explain in detail why this is so. How does 'high efficiency' overcome the requirement for impedance matching in the delivery of power? And are you agreeing with an earlier poster that with a 100w source and a mismatch of 2:1 the forward power will be 135w and 15w reflected, the power delivered to the load will be 130w? If so, will you please explain in detail how this can occur? Yes I agree with him, see my reaction to that statement My class-E PA design that I am doing now *is designed for a 4.5 Ohms nominal load. If I change that load to 2.5 Ohms (VSWR=1.8), output increases to 700W, but it will be destroyed due too non-favourite combination of Vds and Id. Walt With kind regards, Wim PA3DJSwww.tetech.nl Well, Wim, I still don't see how a 2:1 mismatch can achieve 135w forward and 15w reflected with a 100w source. How can a 2:1 mismatch achieve 130w forward power, when that mismatch can only reflect 0.111 x 100w? Also, how can that mismatch achieve 15w of reflected power when that mismatch can only reflect 0.111 x 100w? And how can 11.111 watts of forward power add to 100w to achieve 130w? I don't see how the power reflected at a mismatched load can be affected by the nature of the source. But I do understand how with a 4.5-ohm load can deliver 500w and a 2.5- ohm load can deliver 700w. I now understand what you mean by a mismatch can increase the power delivery. (Actually, a change in mismatch) But Wim, you pulled a fast one one us!!! Until now you didn't tell us that the source resistance of the source was 1 ohm!!! You also didn't tell us what the power would be if the load was 1 ohm (thus matching the load to the source) if the power supply was sufficiently large so the power delivery would not be limited by the power supply. So in reality, you really DON'T get an increase in power delivery by mismatching, but actually a decrease. What you're really doing is obtaining an increase in power delivery by decreasing the amount of mismatch from 4.5:1 to 2.5:1. So now I understand that you haven't violated any laws of physics, but IMHO, you have been misleading when you say *you obtain a increase in power by mismatching, because that statement isn't really true, is it? take care, Walt Hello Walt, Off course I agree with you that you can never have more power then the conjugated matched power, but many PAs don't operate in this regime (as my very extreme class-E case). *It was the reason for mentioning: "A certain load that has mismatch referenced to 50 Ohms may have a nice match to a system with non-50 Ohms output impedance." The problem is in whether you define mismatch based on the ohmic value printed on the back of the PA, or on the actual output impedance of the PA (that you mostly don't know in case of many solid state PA's). As the actual output impedance of the PA may not be 50 Ohms (for example 100 Ohms), a 50 Ohms load (as mentioned on the back of the PA) will provide the stated power (for example 100W). *By applying 100 Ohms (that is VSWR = 2 for a 50 Ohms reference), the net power will increase to 112 W. * Actually it wasn't me to experience this mismatch isue first, but my father during the time that CB was very popular overhere. The above example assumes that the output impedance of the PA is independent of load. *Of course this is only true when the actual voltage across the active device and current through it doesn't saturate the active device too much. I think the saturation issue is one of the advantages of a linear PA with accessible R an X tuning. You just tune for maximum output given a certain load and drive (you may use plate current as a guide also). As long as your SSB signal's PEP stays below that output power, your active device will not go into voltage saturation and IMD will likely be acceptable. In case of a wide band push-pull PA, VSWR = 2 (with inconvenient phase) may provide a load where the active devices (mosfet or BJT), will go into voltage saturation at a net output power below the rated power of the PA. For constant envelope modulation this isn't problem, but for SSB/AM it isn't good. With kind regards, Wim PA3DJSwww.tetech.nl Thanks for the reply, Wim, but you seem to be evading my questions and concerns by what appears to be going off on tangents unrelated to my questions. Perhaps I need to refresh Electronics 101, because I simply can't understand the example you provided. You have a 100-ohm source terminated with a 50-ohm load that provides 100w. But you say with the VSWR = 2 the net power will increase to 112w. How does this happen? Wim, you are making statements that seem to disagree with known principles, yet you give no explanation of how these statements can be justified in relation to the known principles. And please tell me, Wim, what RF amps do you know of that have the source impedance indicated on the back? And you refer to solid-state amps, when you know my discussion (and experience) is with tube amps with pi-network outputs. I am totally ignorant on the operation of solid-state amps. I would like to be learning something from my discussions with you, but I'm sorry, Wim, you're making me feel more stupid the more we continue. Walt |
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Transmitter Output Impedance
On 5/6/2011 5:04 PM, Richard Clark wrote:
On Fri, 6 May 2011 13:18:16 -0700 (PDT), wrote: Well, Wim, I still don't see how a 2:1 mismatch can achieve 135w forward and 15w reflected with a 100w source. Hi Walt, Irrespective of who the author is of this statement (I've lost track), I too would like to see how this is reconciled. But seeing that you've asked several times.... 73's Richard Clark, KB7QHC Here is the header from that message: From: Jim Lux Newsgroups: rec.radio.amateur.antenna Subject: Transmitter Output Impedance Date: Thu, 05 May 2011 10:23:10 -0700 Organization: JPL Information Services, InterNetNews Lines: 41 Message-ID: References: NNTP-Posting-Host: seraphic.jpl.nasa.gov Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: news.jpl.nasa.gov 1304616136 24458 137.79.6.96 (5 May 2011 17:22:16 GMT) X-Complaints-To: NNTP-Posting-Date: Thu, 5 May 2011 17:22:16 +0000 (UTC) User-Agent: Thunderbird 2.0.0.24 (Windows/20100228) In-Reply-To: |
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On May 6, 5:24*pm, walt wrote:
You have a 100-ohm source terminated with a 50-ohm load that provides 100w. But you say with the VSWR = 2 the net power will increase to 112w. How does this happen? Walt, here is probably what Wim means by that. Source------1/2WL 50 ohm lossless------RLoad Vsource = 212v, Rsource=100 ohms If Rload = 50 ohms, PLoad = 100w If Rload = 100 ohms, PLoad=112.5w Wim must be assuming a 50 ohm SWR in both cases. -- 73, Cecil, w5dxp.com "Halitosis is better than no breath at all.", Don, KE6AJH/SK |
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Transmitter Output Impedance
Dear Wimpie: Many thanks. I am always on the look-out for applied
electronic problems for a colleague who is particularly skilled at crafting practice problems for the professional engineering exam (chartered engineer in Europe). Your comments about filter response reminds me of the (analog) FM receivers that had wonderful suppression of the next channel over, but horrible performance in the presence of noise from ignition - while other receivers had great performance in both respects. As you know, it had to do with phase response. Chebyshev is not always desirable. Warm regards, Mac N8TT "Wimpie" wrote in message ... On 5 mayo, 17:06, "J. C. Mc Laughlin" wrote: Dear Wimpie: The content of the paragraph below may well rise above the noise of this thread. I expect to learn something of value from your comments about why twice the (apparent) output Z of an amplifier was of importance and what you did to have the amplifiers conform. 73, Mac N8TT -------------------"Wimpie" wrote in message ... Hello Dave and John, snip Regarding the "academic discussion" I also agree. In my professional career where I designed several RF PA's, only 2 times the output impedance of the amplifier was of importance. In one of these cases I couldn't meet the specs and had to insert attenuation (some waste of power…). snip With kind regards, Wim PA3DJSwww.tetech.nl Remove abc first before setting free the pigeon. -------------- J. C. Mc Laughlin Michigan U.S.A. Home: Hello, Regarding the defined output impedance. The first time was during my thesis (third harmonic peaking PA). The stability margin was not very large (expected), and it could be improved by keeping the impedance seen from the base within certain limits. My teacher said, be careful, you only have two devices (BLW76). I tried to make an exciter (based on 2SC1307 BJT) with defined output impedance, but without success. So in the end I increased the output power of the exciter and inserted a 3 dB attenuator, not elegant, but it did the job. The second time was for H-field generation where wider bandwidth was achieved by adding a second resonator. When driving from a 50 Ohms source, it had a nice Chebyshev type pass band. Because of the ripple, it shows reflection in the pass band (this happens with Chebyshev response). However when driving from a PA (that was flat within the pass band when loaded with 50 Ohms), everything went wrong. I redesigned the filter/coil combination, designed a switching PA (half bridge in class DE operation) and skipped the 50 Ohms (I designed around 8 Ohms). The PA drives the filter directly in such away that most of the time the PA sees a nice (mismatched) load (that is inductive for harmonics). This resulted in the desired pass band with significantly increased efficiency. The strength of the H-field is controlled by varying the supply voltage (PWM circuit). Leaving out the 50 Ohms in between, saved several capacitors and inductors. With kind regards, Wim PA3DJS www.tetech.nl J. C. Mc Laughlin Michigan U.S.A. Home: |
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Transmitter Output Impedance
Dear Walt: I am giving a lecture on firearm law on Saturday and so can not
attend. Sorry not to see you and your bride. Warm regards, Mac N8TT "walt" wrote in message ... Hey Mac, are you planning on attending the Chapter 10 QCWA meeting in Cadillac Saturday? I'm planning on attending, so hope to see you there!!! Walt, W2DU J. C. Mc Laughlin Michigan U.S.A. Home: |
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