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#31
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On Fri, 25 Feb 2005 17:59:56 -0500, Tam/WB2TT wrote:
"gwhite" wrote in message ... Richard Clark wrote: On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote: RF transmitters are not .... Sorry OM, This was all nonsense. Nice articulation. I don't know who OM is, but RF transmitter power amps are not "impedance matched." Neither are audio power amps for that matter. My stereo amp has a spec on output impedance. As I recall, it was around 0.16 Ohms. Intended load is 4 - 16 Ohms. That works because the transmission line is less than 0.01 wavelength. So impedance matching becomes moot. If the speaker line were 1/4 wavelength long, there would be almost no signal transferred at all. Cheers! Rich |
#32
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Arguments about whether the power ammplifier is matched or not matched to 50
ohms arise due to misunderstandings about the meaning of "matched". Meaning 1. ------------- The PA has been designed for maximum, linear, undistorted power output when loaded with Ro ohms and the load reistance has actually been adjusted to equal Ro. Ro is usually 50 ohms. (There may be additional criteria to define what constitutes an optimum match.) Meaning 2. ------------- The load impedance Z = R+jX has been adjusted to equal the conjugate of the internal impedance resistance of the PA. (The internal impedance of the PA is usually unknown but the circuit is assumed to behave as if a conjugate match exists.) The two meanings are entirely different from each other. If there is danger of confusion then the meaning should be stated. Some people already use the descriptions "Zo match" and "Conjugate match". ---- Reg, G4FGQ |
#33
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In article ,
Rich Grise wrote: [... audio matching ..] That works because the transmission line is less than 0.01 wavelength. So impedance matching becomes moot. If the speaker line were 1/4 wavelength long, there would be almost no signal transferred at all. ( Unless you use the specially tapered euphonic cable! ) Actually, if you used a thick enough wire, you could go 1/4 wavelength at audio frequencies. If you connect a speaker straight onto the mains, chances are you are going further than that from the generator. -- -- forging knowledge |
#34
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In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote: Arguments about whether the power ammplifier is matched or not matched to 50 ohms arise due to misunderstandings about the meaning of "matched". Meaning 1. ------------- The PA has been designed for maximum, linear, undistorted power output when loaded with Ro ohms and the load reistance has actually been adjusted to equal Ro. Ro is usually 50 ohms. (There may be additional criteria to define what constitutes an optimum match.) Meaning 2. ------------- The load impedance Z = R+jX has been adjusted to equal the conjugate of the internal impedance resistance of the PA. (The internal impedance of the PA is usually unknown but the circuit is assumed to behave as if a conjugate match exists.) Meaning 3: The PA has been designed to deliver the maximum power at that load impedance and the distortion is not an issue. The two meanings are entirely different from each other. If there is danger of confusion then the meaning should be stated. Some people already use the descriptions "Zo match" and "Conjugate match". Actually meanings 2 and 3 are effectively equal in the case of the tuned system, if you define the Zo based on the change in output power vs connected impedance for small changes. Since a lot of such systems aren't linear, this is the way you end up having to define the impedance. You can't use open circuit voltage and short circuit current. Remember that this all started with the OP having a "transmitter". This would include any needed filtering. He was just connecting a 1/4 and 1/2 wave lengths of bent up wire. His output filter, I assume is just a bunch of LC sections. -- -- forging knowledge |
#35
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Ken Smith wrote:
"Motorola AN-721 takes on the theory. AN-758 gives a practical example matching 12.5 Ohms into 50 Ohms." As King, Mimno, and Wing said, a conjugate match is required to get maximum power out of the radio. Terman says the same on page 76 of his 1955 edition of "Electronic and Radio Engineering." There is also an Eimac application note on matching their tubes to a load on the final amplifier. If Larry, Moe, and Curly published an application note, we`d have a crowd of skeptics here. See Terman`s Fig. 3-21 on page 77 of his 1955 edition. Best regards, Richard Harrison, KB5WZI |
#36
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"Rich Grise" wrote in message news On Fri, 25 Feb 2005 17:59:56 -0500, Tam/WB2TT wrote: "gwhite" wrote in message ... Richard Clark wrote: On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote: RF transmitters are not .... Sorry OM, This was all nonsense. Nice articulation. I don't know who OM is, but RF transmitter power amps are not "impedance matched." Neither are audio power amps for that matter. My stereo amp has a spec on output impedance. As I recall, it was around 0.16 Ohms. Intended load is 4 - 16 Ohms. That works because the transmission line is less than 0.01 wavelength. So impedance matching becomes moot. If the speaker line were 1/4 wavelength long, there would be almost no signal transferred at all. Cheers! Rich There is nothing wrong with driving a transmission line/antenna from a zero impedance source. It does NOT change the SWR. The point is that an audio amplifier with a damping factor of 50 is NOT conjugate matched. Somebody mentioned Motorola Application note 721. This is what it says: ************************************************** ************************************** " ..the load, in first approximation, is not related to the device, except for VCE(sat). The load value is primarily dictated by the required output power and the peak voltage; it is not matched to the output impedance of the device. " ************************************************** ***************************************** When device people talk about "matching", they mean matching the load to what the transistor wants to see, which is not the conjugate of the output impedance. The way this is done is to build an amplifier, and vary the load until maximum output power is reached. The transistor is then removed, and the impedance looking into the coupling network is measured. The conjugate of this is sometimes listed as "output impedance" on data sheets. Newer data sheets will have an asterisk * next to that, and a note explaining what it means. If you look at Philips literature, you will see exactly the same explanation. This whole thing has been hashed out here about 5 times during the past year. Tam |
#37
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"Ken Smith" wrote in message ... In article , gwhite wrote: Ken Smith wrote: RF transmitter power amps are certainly "impedance matched" to the intended load. I'm sorry, but they are not. Nor are any power amps that I know of. Efficiency (and thus necessarily output swing) is what matters for power amps. To maximize swing requires load line matching, not impedance matching. I still say they are. Motorola AN-721 takes on the theory. AN-758 does a practical example matching 12.5 Ohms into 50 Ohms Ken, Motorola 721 specifically says the device IS NOT MATCHED. Tam -- -- forging knowledge |
#38
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If a conjugate match matters two hoots, or is involved in the slightest way
with the design of power amplifiers or any other sort of amplifier, why don't tube manufacturers state the internal resistance or impedance in tube date sheets? |
#39
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In article ,
Tam/WB2TT wrote: [...] Ken, Motorola 721 specifically says the device IS NOT MATCHED. It say " Strictly speaking..." is that the section you are refering to? Yes they do say that but then if you follow the design through, I think you will find that they try to cancel the reactive component. If you then put in the output device protection they didn't include, you end up with the matching as I explained elsewhere. As the impedance moves away from what the designer intended, the radiated power decreases. The point is that the OP was taking about hooking a "transmitter" to a length of wire. I was talking about the matching from a complete transmitter which I assume contains such protection to this length of wire. -- -- forging knowledge |
#40
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gwhite wrote:
The strongest argument for dropping the impedance matching concept is PA efficiency, and therefore maximum signal swing. Obtaining maximum swing is a load line issue. So what impedance does the reflected wave encounter? -- 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 =--- |
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