[Moon-net] Parasitic capacitance for surface mount resistors in LNA's

[Chris Bartram]

Joe

> If one can choose between an 0402 resisitor and an 0603 resistor for his
> LNA and he wants to minimize parasitic capacitance, which would he
> choose?  I would expect the smaller part would be lower, but I am not
> sure, since the "plates" of the capacitor are closer together.  Googling
> around different manufacturer's datasheets has not yielded much
> information, other than one that says their 0603 resistor has about 2pF
> of parasitic capacitance.

2pF seems rather high. How is it measured? 

The capacitive strays around a SM resistor are effectively a pi section. 
Although the shunt capacitance at either end mainly depends on the exact 
geometry of the pads used on the PCB, and can be easily calculated,  
something like 0.3pF is more likely end-to-end. However, in the end, if you 
want to know what the exact figures are, you'll have to measure them - and 
that's not trivial. The chances of a SM resistor upsetting the performance of 
an LNA are pretty small.

In practice, there's not a huge difference between 0603 and 0402 resistors 
used as 50ohm loads, and one manufacturer of my acquaintance uses 0603 parts 
as loads to ~10GHz. If there are no size constraints, I'd go for 0603 on all 
bands below 10GHz.

73

Chris
GW4DGU