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#21
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"Non-dissipative Source Resistance"
On Sun, 13 Jun 2010 11:04:50 -0700 (PDT), Richard Fry
wrote: I am waiting to see if Richard is willing to subscribe to his own reference's writings I responded to you some 20 minutes before you posted, and now await your response. So your subscription to your own authority is contingent upon me? I'm flattered. Also please comment on whether or not a Class C amplifier operating on a linear portion of its transfer curve will function as a linear amplifier. "If the amplifier is tuned exactly to resonance, the plate load impedance will be purely resistive and the load line will be linear." I would note that the bulk of his paper concerns frequency products - something the bears quite intimately upon linear operation. Your question is rather opaque in that the appearance of subtext is undoubtedly bound up in the term "linear" which Mendenhall has already employed within the norms of amplifier design. How linear is linear? Give me a technical criteria (subjectivity abounds and is not very informative except for statements of fashion). Express the least departure from a line response in percent that you accept as being linear. If it exceeds Mendenhall's design or commercial product, then I am afraid you won't be satisfied with any answer. 73's Richard Clark, KB7QHC |
#22
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"Non-dissipative Source Resistance"
On Jun 13, 1:20*pm, Richard Clark wrote:
How linear is linear? *Give me a technical criteria... A truly linear device does not produce r-f intermodulation products when signals of different frequencies are applied to it at the same time. For many practical uses a linear device does need to be perfectly linear. Let's say that the IM products need to be only - 20 dB w.r.t the lowest amplitude value of the various input frequencies. The Mendenhall paper I quoted conclusively shows that even such nominally linear performance is not an attribute of Class C vacuum tube r-f amplifiers -- especially at, and near the center frequency of the PA output tuning/matching network. That Mendenhall paper also shows that such r-f amplifiers do not present a functional, 50+j0 termination to r-f energy entering via the tx output connector. That is the reason why such energy is present in the PA plate circuit, giving rise to r-f intermodulation and/or changing the power dissipation there. RF |
#23
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"Non-dissipative Source Resistance"
On Sun, 13 Jun 2010 12:16:40 -0700 (PDT), Richard Fry
wrote: On Jun 13, 1:20*pm, Richard Clark wrote: How linear is linear? *Give me a technical criteria... Let's say that the IM products need to be only - 20 dB w.r.t the lowest amplitude value of the various input frequencies. Medenhall's paper shows "RF spectrum showing distortion products" that are 30dB down. His statement: "If the amplifier is tuned exactly to resonance, the plate load impedance will be purely resistive and the load line will be linear." is attached to that linearity which is greater than you expect. 73's Richard Clark, KB7QHC |
#24
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"Non-dissipative Source Resistance"
On Jun 13, 3:31*pm, Richard Clark wrote:
On Sun, 13 Jun 2010 12:16:40 -0700 (PDT), Richard Fry wrote: On Jun 13, 1:20*pm, Richard Clark wrote: How linear is linear? *Give me a technical criteria... Let's say that the IM products need to be only - 20 dB w.r.t the lowest amplitude value of the various input frequencies. * Medenhall's paper shows "RF spectrum showing distortion products" that are 30dB down. His statement: "If the amplifier is tuned exactly to resonance, the plate load impedance will be purely resistive and the load line will be linear." is attached to that linearity which is greater than you expect. 73's Richard Clark, KB7QHC To Richard Fry, Richard, it appears that you have ignored my post where I reported data that proves the output resistance of Class B and C RF amps is non- dissipative. Have you reviewed or ignored my reference to Chapter 19 in Reflections 3, or the combined sections of that chapter that appear in my web page at www.w2du.com that uses an example from Terman's Radio Engineers Handbook? If you haven't yet reviewed it I urge you to do it, and then if you disagree with my measured data I'd like for you to explain the reason for your disagreement. Walt, W2DU |
#25
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"Non-dissipative Source Resistance"
On Jun 13, 2:31*pm, Richard Clark wrote:
Medenhall's paper shows "RF spectrum showing distortion products" that are 30dB down. Not near the resonant frequency of the output tuning/matching network. He shows as little as 5 dB at 0.8 MHz offset of the external signal from the carrier of that tx. It would be less than 5 dB for lesser separations. The reason that the IM performance improves for greater frequency separations is related to the r-f bandwidth of the output network -- not the "linear" operating characteristics of the PA, or that a 50+j0 ohm load is seen by the external energy reaching the tx output connector. RF |
#26
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"Non-dissipative Source Resistance"
On Jun 13, 2:58*pm, walt wrote:
To Richard Fry, Richard, it appears that you have ignored my post where I reported data that proves the output resistance of Class B and C RF amps is non-dissipative. I know your conclusion, thanks Walt. The impedance of a perfect transmission line also is non-dissipative, but that does not change the power dissipated in a termination of that transmission line equaling its Zo. Do you agree that the Mendenhall paper I have quoted shows that the source impedance of an operating Class C vacuum tube PA and its output tuning/matching network designed for a 50 ohm load is not, by itself, a functional 50+j0 ohms? RF |
#27
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"Non-dissipative Source Resistance"
On Jun 13, 5:04*pm, Richard Fry wrote:
On Jun 13, 2:58*pm, walt wrote: To Richard Fry, Richard, it appears that you have ignored my post where I reported data that proves the output resistance of Class B and C RF amps is non-dissipative. I know your conclusion, thanks Walt. The impedance of a perfect transmission line also is non-dissipative, but that does not change the power dissipated in a termination of that transmission line equaling its Zo. Do you agree that the Mendenhall paper I have quoted shows that the source impedance of an operating Class C vacuum tube PA and its output tuning/matching network designed for a 50 ohm load is not, by itself, a functional 50+j0 ohms? RF No Richard, I've not made that inference. The point I'm emphasizing is that because the output resistance of these amps is non- dissipative, no reflected power from a mismatched load enters the amp, but is totally re-reflected in the forward direction. I make no comment on whether the output resistance is 50 +j0, because when all the available power is being delivered at given grid drive the output resistance equals the load resistance, which doesn't have to be 50 + j0. Proof of that is that when the load resistance is either increased or decreased the output power decreases. Walt, W2DU |
#28
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"Non-dissipative Source Resistance"
On Jun 13, 4:33*pm, walt wrote:
The point I'm emphasizing is that because the output resistance of these amps is non- dissipative, no reflected power from a mismatched load enters the amp, but is totally re-reflected in the forward direction. Walt, wouldn't that non-dissipative output resistance also reflect reverse power on other close frequencies, such as those coupled from a co-located transmission system within the bandwidth of the tx output network? If so, such external signals would never enter the transmitter, and could not cause r-f intermodulation by mixing with the main signal in the (non-linear) PA plate circuit of that transmitter to produce a 3rd order product at 2F1 - F2, and other combinations. But such r-f intermods are a fact of life, as shown in the Mendenhall paper, and in the existence of elaborate and expensive r-f filtering hardware needed to enable several stations to operate from one location while meeting a radiated r-f intermod spec of -80 dBc -- even from one antenna! RF |
#29
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"Non-dissipative Source Resistance"
On Jun 13, 6:09*pm, Richard Fry wrote:
On Jun 13, 4:33*pm, walt wrote: The point I'm emphasizing is that because the output resistance of these amps is non- dissipative, no reflected power from a mismatched load enters the amp, but is totally re-reflected in the forward direction. Walt, wouldn't that non-dissipative output resistance also reflect reverse power on other close frequencies, such as those coupled from a co-located transmission system within the bandwidth of the tx output network? If so, such external signals would never enter the transmitter, and could not cause r-f intermodulation by mixing with the main signal in the (non-linear) PA plate circuit of that transmitter to produce a 3rd order product *at *2F1 *- F2, and other combinations. But such r-f intermods are a fact of life, as shown in the Mendenhall paper, and in the existence of elaborate and expensive r-f filtering hardware needed to enable several stations to operate from one location while meeting a radiated r-f intermod spec of -80 dBc -- even from one antenna! RF Richard, I'm not qualified to answer your question about intermods, so I won't even try. However, interference to the amp from radiation from an antenna driven by another amp will not be phase or frequency coherent with the tx receiving the interference. Perhaps that's the reason the interfering signal gets through while the waves reflected from a mismatch don't? In other words, could the coherent relationship between the forward and reflected waves result in the total re- reflection at the non-dissipative output resistance of waves returning from the mismatched load? Walt |
#30
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"Non-dissipative Source Resistance"
On Jun 13, 5:25*pm, walt wrote:
However, interference to the amp from radiation from an antenna driven by another amp will not be phase or frequency coherent with the tx receiving the interference. Perhaps that's the reason the interfering signal gets through while the waves reflected from a mismatch don't? Measurements made of broadcast transmitters in the engineering test lab at Harris Broadcast Division show that power reflected from a mismatched load does indeed enter the PA plate output circuitry, and change the amount of power dissipation and cooling requirement for the transmitter. That is the reason why virtually every modern manufacturer of broadcast transmitters specifies a maximum value of load VSWR for which their maximum rated powers are permissible. For Harris FM transmitters that value is 1.7:1 (any phase angle). As the load VSWR exceeds that value the transmitter progressively "folds back" its output power to protect itself from component damage. RF |
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