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Why Match ?
Hi All,
I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. I know there must be a good reason to put the matching pad in line for the sprectrum analyizer but I don't under why. Thanks, de KJ4UO |
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I was reviewing a 75 to 50 ohm resistive matching network using two
resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. I know there must be a good reason to put the matching pad in line for the sprectrum analyizer but I don't under why. Thanks, de KJ4UO In test devices the losses are not usually a problem as long as the levels are accounted for if absolute numbers are needed. The impedances must match if any tuning is done. As teh impedance of a receiver is not usaully 50 ohms over a wide frequency range , you use a 6 db pad or so to isolate the generator from the receiver . If tuned circuits are involved, the impedance missmatch (swr) can cause many problems . You tune a device for a 50 ohm source or load and then replace it with a 70 ohm device and the tuning will usually change . By using the pads, it is an attempt to keep the impedance changes at a minimum when the test instruments are removed and the device put back into normal operation. |
On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:
Hi All, I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. I know there must be a good reason to put the matching pad in line for the sprectrum analyizer but I don't under why. Thanks, de KJ4UO Dear KJ4UO, Some additional help with your L-pad - The minimum loss L-Pad for matching 75 ohms to 50 ohms would indeed have a loss of 5.7 dB. This is from the ITT handbook, pages 10-4 through 10-8. The design values for such an L-Pad would give the 5.7 dB loss based upon the following: Let P1 be the power delivered to a matched load (50 ohms) from the 50 ohm source. Let P2 be the power delivered to a 75 ohm load when fed through the 75 to 50 resistive matching network. Then P1 / P2 will be approximately 3.715. 10 x Log(3.7) = 5.7 dB. You can work that out from the design values for the L-Pad, which are 43 ohms in the series arm and 86.7 ohms in the shunt arm. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
PDRUNEN wrote:
Hi All, I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. It appears that you forgot to put the resistive matching network into the circuit. There must be a series resistor in there between the source impedance and the load impedance to obtain that 5.7 dB of isolation. -- 73, Cecil http://www.qsl.net/w5dxp -----------== Posted via Newsfeed.Com - Uncensored Usenet News ==---------- http://www.newsfeed.com The #1 Newsgroup Service in the World! -----= Over 100,000 Newsgroups - Unlimited Fast Downloads - 19 Servers =----- |
In message , Richard Clark
writes On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote: Hi All, I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. Hi OM, Snips However, this is not the best design for a matching network, they are usually three resistors in either a PI or T configuration. Several manufacturers of precision matching pads might disagree! Two resistors is enough, although you have to remember to add a rather odd number (unless the measuring equipment can add it for you). If you add a third resistor, you can make the correction a straight 6dB. That's probably the only virtue. Ian. -- |
On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:
Hi All, I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. I know there must be a good reason to put the matching pad in line for the sprectrum analyizer but I don't under why. Thanks, de KJ4UO Dear KJ4UO, I have fundamental objections to responding to respondants, so I will continue to address this thread directly. It has been suggested that a virtue of the T pad would give a clean 6 dB loss instead of some "not-so-nice" loss, like 5.7 dB. Well, that's not a problem because the L-Pad can also be designed for exactly 6 dB. In fact, it can be designed to provide ANY loss you want, so long as it's AT LEAST 5.7 dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design for that particular mismatch. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson
wrote: However, this is not the best design for a matching network, they are usually three resistors in either a PI or T configuration. Several manufacturers of precision matching pads might disagree! Hi Ian, I seriously doubt it, but you are free to offer examples. 73's Richard Clark, KB7QHC |
On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:
Hi All, I was reviewing a 75 to 50 ohm resistive matching network using two resistors, the insertion lost was 5.7 db. If we have a 100Vrms source with 50 ohm source impedance and it is driving a matched 50 ohm load then the load takes 1A and the power in the load is 50 watts. If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is 48 watts. (I*I*R) == (0.8)*(0.8)*75. I guess I must be not be taking something in account, but 2 watts does not equal 5.7 db. I know there must be a good reason to put the matching pad in line for the sprectrum analyizer but I don't under why. Thanks, de KJ4UO Dear KJ4UO, First, I must apologize for not noticing that your posting DOES mention that the application is a Spectrum Analyzer. That being the case, I can be much more enlightening about WHY we want a flat line (no standing waves). A spectrum analyzer is of use primarily in showing WHAT frequencies are present in the specturm AND what their relative levels are. Unless the line is flat, the relative levels will be distorted or inaccurate. This comes about due to the fact that a mismatched line becomes an impedance transformer that is very frequency dependant. That is, some frequencies will be artificially enhanced and others artificially subdued due to the varying impedance impedance seen by the source. Conversely, when the source sees a perfect match across its acceptance spectrum, only the built-in non-linearities will degrade the accuracy. In short, the line must be properly matched in order that the spectrum analyzer give the accuracy that it is capable of. BTW, it should be understood that the matching L-Pad, in order to be effective, must be physically located at the Spectrum Analyzer end of the line - not at the source end of the line. Interestingly enough, hams do not usually take this into account when tuning an antenna system, because they are only interested in a flat line for that short little piece of coax between the tuner and the transmitter. They don't normally do anything about the horrible SWR that exists on the feedline - Hi! Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Tue, 21 Dec 2004 21:16:03 GMT, (Robert
Lay) wrote: It has been suggested that a virtue of the T pad would give a clean 6 dB loss instead of some "not-so-nice" loss, like 5.7 dB. Well, that's not a problem because the L-Pad can also be designed for exactly 6 dB. In fact, it can be designed to provide ANY loss you want, so long as it's AT LEAST 5.7 dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design for that particular mismatch. Hi Bob, The difference between 5.7 and 6dB is immaterial if neither is calibrated. As for the desire for a nominal 6dB pad, that too is hardly of great merit for quality measurements with a good Spectrum Analyzer. Just like directional coupler design, precision applications focus on directivity and sacrifice round numbers in coupling to achieve better separation of ports. A 6dB attenuator will isolate your precision gear from the unknown better than a 3dB attenuator, but not as well as a 10 or 20dB attenuator. Now, as to the term isolation. It has a variety of meanings which in this case means that your measurement is less perturbed by the literal unknowns of your proverbial unknown being measured. That is, in your attempt to find a value (the proverbial unknown) your accuracy can be upset by variables whose magnitude can affect that accurate determination. Large attenuators obviously de-sense the instrumentation, but if you have sufficient dynamic range, then that is not a debit, but actually an asset. Hence de-sense or isolation is benign. When this large attenuator is placed on the source, it reduces the load's influence to pull or mismatch there too. This says nothing of actual mismatches, it simply presents what is called swamping. That is, you introduced known and controlled losses to buffer the measurement. Later you can subtract out the losses to find your proverbial unknown. I already alluded to the virtue of using attenuators to increase the power tolerance to the input of a Spectrum Analyzer, aside from this, the only practical use of attenuators is to introduce controlled loss to isolate the unknowns' influence. To answer "Why Match?" returns us to isolation. Once this is achieved, the measurement can be trusted to be faithful in proportion to that isolation. With this example of a simple 50 to 75 Ohm conversion, that measurement's faithful accuracy is fairly good. As for it being a ~6dB attenuator, by placing it into another test with an unknown, it will offer mixed results - a T or PI configuration at a higher attenuation would be far more flexible, and faithful. 73's Richard Clark, KB7QHC |
In message , Richard Clark
writes On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson wrote: However, this is not the best design for a matching network, they are usually three resistors in either a PI or T configuration. Several manufacturers of precision matching pads might disagree! Hi Ian, I seriously doubt it, but you are free to offer examples. 73's Richard Clark, KB7QHC http://www.maxim-ic.com/appnotes.cfm...mber/972/ln/en http://www.maxim-ic.com/appnotes.cfm...te_number/3250 http://www.maxim-ic.com/appnotes.cfm...mber/972/ln/en http://www.testmart.com/estore/produ...Fsearch%2Fspec. cfm~~MICCOM~~AGILEN~~11852B~~%20~~%20|1.html http://www.g4fgq.regp.btinternet.co.uk/padmatch.pas http://www.g4fgq.regp.btinternet.co.uk/padmatch.pas http://used-line.com/b2544p1pr0-Used-pads.htm http://www.minicircuits.com/dg03-159.pdf#search='minimum%20loss%20pad' + many, many more! Ian. -- |
On Tue, 21 Dec 2004 22:51:30 +0000, Ian Jackson
wrote: In message , Richard Clark writes On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson wrote: However, this is not the best design for a matching network, they are usually three resistors in either a PI or T configuration. Several manufacturers of precision matching pads might disagree! Hi Ian, I seriously doubt it, but you are free to offer examples. 73's Richard Clark, KB7QHC + many, many more! Ian. Hi Ian, Perhaps you can share from those many, many more, those which disagree? 73's Richard Clark, KB7QHC |
In message , Richard Clark
writes On Tue, 21 Dec 2004 22:51:30 +0000, Ian Jackson wrote: In message , Richard Clark writes On Tue, 21 Dec 2004 20:27:54 +0000, Ian Jackson wrote: However, this is not the best design for a matching network, they are usually three resistors in either a PI or T configuration. Several manufacturers of precision matching pads might disagree! Hi Ian, I seriously doubt it, but you are free to offer examples. 73's Richard Clark, KB7QHC + many, many more! Ian. Hi Ian, Perhaps you can share from those many, many more, those which disagree? 73's Richard Clark, KB7QHC The point I was trying to make was, if the L-pad was 'not the best design for a matching network', why are there so many about? It gives the minimum loss for a match in both directions (with the 43and 86 ohm resistors). What works better? Ian. -- |
On Tue, 21 Dec 2004 21:54:30 GMT, Richard Clark
wrote: On Tue, 21 Dec 2004 21:16:03 GMT, (Robert Lay) wrote: It has been suggested that a virtue of the T pad would give a clean 6 dB loss instead of some "not-so-nice" loss, like 5.7 dB. Well, that's not a problem because the L-Pad can also be designed for exactly 6 dB. In fact, it can be designed to provide ANY loss you want, so long as it's AT LEAST 5.7 dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design for that particular mismatch. Hi Bob, The difference between 5.7 and 6dB is immaterial if neither is calibrated. As for the desire for a nominal 6dB pad, that too is hardly of great merit for quality measurements with a good Spectrum Analyzer. Just like directional coupler design, precision applications focus on directivity and sacrifice round numbers in coupling to achieve better separation of ports. A 6dB attenuator will isolate your precision gear from the unknown better than a 3dB attenuator, but not as well as a 10 or 20dB attenuator. Now, as to the term isolation. It has a variety of meanings which in this case means that your measurement is less perturbed by the literal unknowns of your proverbial unknown being measured. That is, in your attempt to find a value (the proverbial unknown) your accuracy can be upset by variables whose magnitude can affect that accurate determination. Large attenuators obviously de-sense the instrumentation, but if you have sufficient dynamic range, then that is not a debit, but actually an asset. Hence de-sense or isolation is benign. When this large attenuator is placed on the source, it reduces the load's influence to pull or mismatch there too. This says nothing of actual mismatches, it simply presents what is called swamping. That is, you introduced known and controlled losses to buffer the measurement. Later you can subtract out the losses to find your proverbial unknown. I already alluded to the virtue of using attenuators to increase the power tolerance to the input of a Spectrum Analyzer, aside from this, the only practical use of attenuators is to introduce controlled loss to isolate the unknowns' influence. To answer "Why Match?" returns us to isolation. Once this is achieved, the measurement can be trusted to be faithful in proportion to that isolation. With this example of a simple 50 to 75 Ohm conversion, that measurement's faithful accuracy is fairly good. As for it being a ~6dB attenuator, by placing it into another test with an unknown, it will offer mixed results - a T or PI configuration at a higher attenuation would be far more flexible, and faithful. 73's Richard Clark, KB7QHC Dear Richard, OK - you're not getting any argument from me. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
On Wed, 22 Dec 2004 00:19:12 +0000, Ian Jackson
wrote: The point I was trying to make was, if the L-pad was 'not the best design for a matching network', why are there so many about? It gives the minimum loss for a match in both directions (with the 43and 86 ohm resistors). What works better? Ian. Hi Ian, The topic is Why Match? and the context is with bench gear, specifically a Spectrum Analyzer. It is pleasing that no sardonic quips as to this device's suitability for antenna matching has sallied forth. Attenuators serve a limited purpose - Isolation. They serve this well when they don't introduce uncontrolled error. They also serve as range extenders as do directional couplers, often with attenuators paired with them. In the case of using them with couplers, they insure port isolation by insuring port loading. They buffer any deviance from an expected 50 Ohm load that may be presented by monitoring gear attached through them to that port. This is by and large the context of the mini circuits links you offered. The extent of this "insurance" can be observed by computing how much load is presented to the affected instrument when the attenuator is left open, or shorted. T or PI configurations show a higher tolerance. An L Pad is a special case (less general form) of either T or PI, being that one of the three elements is replaced with either a short or an open. 73's Richard Clark, KB7QHC |
On Wed, 22 Dec 2004 00:19:12 +0000, Ian Jackson
wrote: What works better? Hi Ian, In following up the links that you provided, there is a superior reference found at the same site: http://www.minicircuits.com/appnote/an70001.pdf which in turn leads to a treasure trove on the topic that supports my favorite discussion on Mismatch Uncertainty. By following the links, they offer you articles and software to compute the accumulation of errors (and loss) found in mismatched sources looking at mismatched loads. A notable quote: "A fixed attenuator can help to lower the VSWR of cascaded (connected) components by providing isolation between the impedances, effectively masking the impedance mismatches." For both mismatched source and load, one handy shortcut offered is that the system suffers a SWR that is not the aggregation of the two, but the multiple of the two. This is not particularly significant for a source SWR of 2 seeing a load SWR of 2 (same result of 4 for either addition or multiplication), but above this value and loss begins to climb dramatically. 73's Richard Clark, KB7QHC |
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