The project at hand is an airborne military service walkie-talkie with many different signal paths and isolation requirements, all in all, better than 100 dB isolation from anything else. Signal switching operations are involved, so we need to characterize the switch isolation characteristics when its signal path should be closed.
The on/off ratios (their isolation values) of four analog switches from three different suppliers were evaluated using the following boards as test fixtures:
Figure 1 This test fixture circuit board is used to test the isolation values of various quad analog switches.
The test fixture uses BNC connectors for input and output. A copper shielding wall is placed between the connectors to ensure as much as possible that the measured isolation value is due to the IC under test and not any artifacts from the test setup.
Figure 2 places a copper shield wall between the BNC connectors on the test fixture board.
Since the application is for speech frequencies up to 3 kHz, check the isolation value at frequencies one octave higher than 6 kHz. The 10kΩ resistor value is the expected circuit impedance using these switches.
The result is as follows:
Figure 3. Isolation values of switches under test relative to channel signal transmission.
We came to the conclusion that, while there are some differences between the different devices, all devices work well if operated as shown at the bottom of Figure 3 - that is, with both series sections open, Both shunt sections are closed.
