FST3253 Bus Switch as an RF Switch

The FST3253 is essentially a pair of 4:1 switches in a single 16-pin package. By connecting the two switch common ports together, biasing them at 1/2 Vcc and selecting the right combination of logic enables, one can create an 8:1 RF switch, ideal for selecting preselector filters in a multi-band receiver. I decided to build one and characterize it’s switching performance over the HF band up to 50 mHz. The results follow:

1st is the ON insertion loss with one output selected. The plot resolution is 2dB/div and is referenced to the center line of the display. You can see that the insertion loss is about 0.5-0.6 dB over the whole range. The sharp dropoff at the low end is due to the 0.1 uf coupling capacitors in the test circuit.

FST3253 ON Loss

Next is the OFF-state RF isolation. It’s plotted at 10 dB/div referenced to the top of the display and is 45dB or greater up to 50 mHz:

FST3253 Off Isolation

Finally comes the return loss looking into the common port, with the selected output terminated in 50 ohms. It’s plotted at 10 dB/div and is over 20 dB dB over most of the band. The sharp rise at the low end is due to the 0.01 uf coupling capacitors in the circuit.

FST3253 8_1 switch return loss

A pair of these switches with 8 bandpass filters in-between should produce a very nice switched preselector filter for my QSSR project, contributing about 1.5 dB excess loss above the filter losses.








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