ground current - what does it mean?

[chihhao]

ground current??

What does ground current mean ?
I saw an application which is said
"Controlling ground currents is important to maximize the SAW filter's
out of band rejection"
Please advise and thanks.

 

[biff44]

Re: ground current??

I guess you have to understand what a saw filter is to understand that comment.

A SAW device has a 2 terminal input, those 2 input terminals connect to a elctro/mechanical transducer that changes voltage into mechanical vibration in the SAW. Far at the other end of the SAW is another transducer that changes the mechanical vibration back into electricity, and outputs the electricity on another 2 output terminals. The terminals on either input or output are often "balanced", ie neither is connected to "ground".

In practical use, you come into the saw filter with unbalanced line, like microstrip. So at the input you somehow have to go from unbalanced line to the balanced 2 termial input.

As you might guess, there are microstrip discontinuity effects when you go from unbalanced to balanced line at the input. What you want if for 100% of the input current to go into the saw filter, for 100% of the return current to travel back thru the ground plane, and for none of the input current to go anywhere else (like in an evanescent mode in the ground plane). If any of the current goes somewhere else, it is likely to be picked up by the microstrip line at the far end of the saw filter. If that happens, then your 40 dB rejection saw filter all of a sudden looks like it only has 20 dB rejection!

One trick people used to do is to etch away the ground plane below the saw filter so that input microstrip ground currents do not have an easy path to the output microstrip ground currents.

This all gets a little more complicated today, as modern low cost saw filters are often packaged with a common ground and only 1 input and one output terminal! Not as useful as the older style.

Also, some saw filters have one of the input terminals and one of the output terminals welded to the metal case, further confusing the issue.

In general, you have to experiment around and follow the manufacturers mounting suggestions pretty religiously to get the specified performance.