Hi,

One way around the isolation challenge with semiconductor switches and PIN switches is to use them in a double pole configuration where you have a common "well decoupled from each other - as in series ferrite bead, series resistor, and parallel capacitor" on the RF switches. Each control line on the switch has this isolation filter. The output of one is fed to the input of the other arm of the switch, think of it as a DPST switch configuration from 2 SPST switches. Don't count on 2X isolation though, more like 35 dB + 35 dB = 55 to 60 dB maybe.


Some of the common design problems with semiconductor type of switches that result in much leakage and isolation degradation occurs due to:

1) Mis-managing the switch control decoupling (as FOLSOM mentioned)

2) Poor layout isolation (I've seen the RF input couple to the switch control line and lose 20 dB of isolation).

These switches need to be grounded VERY WELL. This includes the ground pad on the bottom of the IC package (if provided). If there is one, a via field (25 mil vias) to multiple ground planes works well. The device ground pins must go directly to a plane.

Place the decoupling capacitors within 10s of mils of the device. The RF traces take precedence though.

Try to make the traces that enter and exit the device are the same width as the pin to minimize VSWR losses.

If the frequency warrants it, usually above 700 MHz, use straight traces and curved bends (sometimes 45 degree and chamfered corners can be used depending on frequency).

Make sure you have the proper trace width to give 50 Ohm unless your target is 75 or 93 Ohms.

There is always discussion about FR-4, Rogers 4000 series products, G-Tek, Gore Speedboard, etc... Understand the material properties of the PWB, cost, CTE, Er, Loss Tangent, availability. FR-4 may or may not work for your application depending on your environment. The IPC and vendors have worked cooperatively to improve specifying & measuring critical parameters (see IPC-6018).

3) Whether the switch is reflective or absorbtive in operation and what works for your application. Be careful with layout of trans-impedance converters that accompany some RF switches and attenuators too.

4) Poor support from vendors and using their test circuits as "gospel". Many times these boards and stack-up are optimized and when the design is integrated onto a design with other circuit elements like 10MHz PLL Reference Clocks, mixers, attenuators, etc, "well what do you know, it doesn't operate as expected" becuase of other design considerations or circuitry partitioning. Breadboarding more of the circuit such as FOLSOM recommended is a very informed recommendation.



In summary, semiconductor switches can be used with proper attention to design & layout parameters.

Ross