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HP 11660A Shunt for HP 8556A
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I may have missed it somewhere but I cannot find any details on how the insides of this shunt is made. I assume it is a pure resistive divider with something like a 550 ohm resistor in series with a 50 ohm resistor which then connects to ground, with the high impedance input connected to the 550 ohm resistor and ground (across the 550 and 50 ohm resistor) and the low impedance output connected to the junction of the 550 and 50 ohm resistors and ground (across the 50 ohm resistor).
Does anyone know if this is correct or what is correct? I need to build one. Steve, KJ5RV |
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I know what the technical definition of a shunt is. However this is an HP inline device that they call a "tracking generator shunt (50 ohm output)" It is used and came with the 8556A low frequency RF module as an accessory.
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I doubt it is a simple shunt. But I may be wrong. It is used with the tracking generator on the 8556A, which has a 600 ohm output impedance. When you connect the TG output to a 50 ohm input you are to use this shunt to properly match the two different impedances. They also specify and provide with the unit new a 600 ohm Feed Thru Termination and a 50 ohm Feed Thru Termination. But this device they refer to as a shunt with 50 ohm output. I am thinking it is something like a 550 ohm resistor with a 50 ohm resistor in series. The 550 ohm resistor input would be connected to the output of the 600 ohm TG. The 550 to 50 ohm junction would be connected to the input of the 50 ohm load, and the remaining end of the 50 ohm resistor would be connected to coax shield ground. That configuration would show a 600 ohm impedance for the TG and 50 ohms to the load and technically still be considered a shunt. I built such a device and used it for the performance test (Para 4-18) and got reading within specifications for the first 10 out of 12 measurements. The two that were outside of specifications were only outside by 0.278% and 0.827%. Which may be because my resistors were not precision enough (50.22 ohms and 552.58 ohms measured with Fluke 45). Ideal is a 11/1 ratio this is a 11.032/1 ratio so may be a contributing factor. Or it maybe that my design is not what HP used. All of this is measured between 1 KHz and 300 KHz with my outside the specifications measurements being at the 250 KHz and 300 KHz frequency. These external accessories seem to always disappear and never go with the used units. I was hopping someone might be familiar with them and give me some specific information as to exactly how they are built. At these frequencies construction is not a problem. Steve, KJ5RV br4av01 wrote:
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Hello Steve,
even using resistors of utmost precision you would neither obtain a 600 ohms input resistance (= load resistance for the TG) nor a 50 ohms output resistance (= source resistance for the 50 ohms load). You may verify this fact by terminating the output into 50 ohms and measuring the resulting input resistance (which will be 575 ohms). Terminating the input into 600 ohms (= source resistance of the TG) will yield a source resistance of approx. 47.916666 ohms for the load. Regards Martin |
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There was one on Ebay. I bought it. It has a shunt impedance of 50 ohms and a series impedance of zero ohms....Looks just like a 11048C to me...I don't understand.
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Dan in Chandler, AZ ================================================== --- In hp_agilent_equipment@..., "martin_u_fischer" <martin.u.fischer@...> wrote:
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Martin,
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Well that is the information I was looking for! Thank you, however that is not what I expected. That was option "2" for me. To me it seemed more logical to provide a 600 ohm load to the 600 ohm output of the TG and then provide a 50 ohm impedance to the 50 ohm load. But what do I know! That is why I asked! I will have to build one like that and see what difference it makes in my measurements. Thank you Martin. Good find by the way. But I am stumped, like you, as to what the difference is between the "11660A 50 ohm TG Shunt" and the "11048C 50 ohm Feed Thru Termination". Because the Feed Thru's I have measure exactly the same as you say this Shunt does. Maybe someone will know the difference. Steve, KJ5RV danaz.chandler wrote:
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Martin and all:
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I built a shunt conforming to the information Martin provided and reran the test. The results were approximately the same. The same 10 readings were within the specifications and the last 2 readings were just outside the specifications by less than 1%. So I guess this device is not too critical to the test, no matter how it is built. At less than 1% out of specifications, in a range I currently do not intend to use, I think I will leave well enough alone. Thanks again for the information Martin. Steve, KJ5RV --- In hp_agilent_equipment@..., Steve Vineyard <willltinker@...> wrote:
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Hello Steve,
today I got a PM from Steve [AZUR Electronics]; he shows a "HP 11660A Tracking Generator Shunt (home made version)" on his website and by courtesy he unveiled the secret around the component values of the 11660A. He figures out that the 11660A contains a single shunt resistor of 54.5 Ohms. Albeit this will constitute a gross mismatch for the 600 ohms output resistance of the TG, it will transform the effective source resistance into exactly 50 ohms. ³Õ´Ç¾±±ô¨¤! Regards Martin |
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That is interesting and it would now make sense why they did not use the 50 ohm pass thru, but created a unit specific shunt.
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I will have to change mine to that value and see if it makes any difference to my test results. Thank you Martin for your effort and thank you Steve (AZUR Electronics) for your courtesy of unveiling the secret. Steve, KJ5RV martin_u_fischer wrote:
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This does make a slight difference in the readings in case any one else is interested in the results.
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Thanks again to all that help resolve this issue. Steve, KJ5RV --- In hp_agilent_equipment@..., Steve Vineyard wrote:
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