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