Quote:
Originally Posted by Cruisin Marine
I wonder if anyone has tried to reproduce this and test its viability?
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Switched resistance loading of LC resonators, and then the same thing with quartz crystals for the narrower bandwidth is the standard method of implementing the wide range of switched IF bandwidths in spectrum analysers.
So it works, and there's a lot of them around. Usually the filter is five synchronously tuned stages and they all switch together. As sync tuned filters they are bullet-shaped on top but the skirts are not as steep as Chebyshev or Butterworth tuning would give so their shape factor is poor as communications filters go, but their pulse response and lack of ringing is more important in this application.
In later analysers, the resistors aren't switched but done with PIN diodes whose resistance is set by individual DACs. The phasing/centring is done with varactors and individual DACs. The analyser software allows you to connect the cal signal to the input and the analyser jumps around calibrating the IF filters on all the offered bandwidth (1, 3, 10 etc series). It's fun to watch. The resulting DAC settings are then saved and evoked whenever that bandwidth is selected.
Care is needed with varactors. They exhibit capacitance controlled by the immediate and instantaneous voltage acros them. They are very fast and big signals will pump the capacitance thus pumping the filter characteristic and intermodulating other signals. Not what you want in an analyser or receiver. Operating levels have to be carefully planned to keep this below noticeability. Usually this compromise means the noise figure suffers.
David