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measuring cable loss
Richard Clark wrote:
On Mon, 13 Aug 2007 13:09:09 -0700, Jim Lux wrote: Or, something like a 50k resistor into a 50 ohm load will be about 60 dB down, Hi Jim, Unlikely. With parasitic capacitance at a meager 1pF across the 50K, its Z at 10MHz would compromise the attenuation presenting closer to 50 dB down. At 1Ghz it would plunge like a rock. This, of course, presumes a 1/4 watt resistor. A better solution is to use surface mount resistors where the parasitics are down at 100aF - but then you will have a frequency dependant divider unless you can guarantee that the parasitic capacitance of the 50 Ohm resistor is 100pF (sort of casts us back into using a 1/4 watt resistor with a padding cap). At 1GHz, it is not going to look like a trivial 50K load anymore. A Pi attenuator will do it better. A resistive 30dB tap into a 30 dB pi attenuator, or something like that? That would get the resistor in the tap down to a reasonable value.. and, as you point out, at 1 GHz layout and component selection would be critical. I suppose if you're building a circuit board, a small parallel line coupler would work just as well, and probably be easier. in any case, the use of those nifty parts from AD does open up a lot of interesting applications. 73's Richard Clark, KB7QHC |
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