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.
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:
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.
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.
W3JDR Bright Ideas 