----- Original Message ----- From: Sean H. Breheny To: Sent: Sunday, October 17, 1999 12:53 AM Subject: RF High-Isolation Switches > Hi All, > > Does anyone know of some SPDT toggle switches that have a very low > capacitance between unconnected contacts? I tried a bunch of regular > miniature and subminiature toggle switches and they all seem to have about > 2 pF between the pole and contact when not connected. Since,in my > application, the switch needs to isolate a high-gain amplifier with 1Meg > ohm || 20pF input, 2pF is not insignificant,and is resulting in only about > 10 times attenuation In RF circles we say -10dB :) ,instead of isolation. How much isolation does your system need? -20dB, -30dB ?? The preffered method of RF switching is to use PIN diodes in a series/ shunt arrangement to effectively short the RF signal to ground. Forward biasing the diode make it appear RF short, reverse bias RF open. They can require a fairly high current (10mA per device is not unusual) and can be a little awkward to drive, but give good results over large (> octave)bandwidths. A reverse isolation figure of -20 dB is easily acheiveable. You might want to take a look at your layout also - are you sure that the capacitance is due to the switch? It might be your layout. Are you operating in a 50ohm environment - maybe you should be. You do not mention the frequency of the signal you are trying to isolate, which is of supreme importance. Mechanical RF switches are specailist and somewhat expensive. Regards Mick