Frequency rating on SMD resistors

[geggi1]

Is there any frequency rating for SMD resistors.
I'm playing with some AD8307 Log amps to make RF field strength meter/ power meter.
On the meter I'm using 2 100 ohm resistors in paralell to convert the input impedance form 1,1K to 50 ohm.

In the future I'm planning to use AD8313 to make a meter up to about 2,5Ghz.

[retrolefty]

Should be fine. It's wirewound resistors that you have to look out for when terminating RF frequencies as the extra inductance will cause error. I too build a RF milliwatt meter using a AD8307, great chip for up to 500 Mhz or so. Are they still around $15 a pop? 

 Going up to >2gHz will be a challenge where PCB material and traces will effect accuracy as much as the active and passive components.

[babysitter]

Sounds like a no-brainer, but datasheets might contain impedance over frequency plots.

Without datasheet it is preferrable to use the smallest SMD resistors you can handle. To parallel, for example 2x100 Ohm to get 50 Ohm, which I did recently for a resistive divider, I stacked 2 SMD resistors on top of each other to keep the Trace at the right microstrippy size.

[rfbroadband]

if you go through the math (equivalent circuit of an SMD resistor) you will find that each size has an resistance value where the impedance remains mainly constant over frequency (thus neither inductive nor capacitive behavior). For 0402 this turns out to be ~ 100Ohm, thus using 2 100 Ohm in parallel to create a broadband 50 will work nicely beyond 10GHz. below that ZOpt the R is inductive, above ZOpt it capacitive. For 0603 sized SMD R ZOpt is higher than 100 Ohm, even more so for 0805.

I hope this helps.