2. Lab 2 - Bandpass Filter

posted Feb 2, 2010, 2:19 AM by Bill Bai   [ updated Feb 3, 2010, 11:45 PM ]
(PSPICE Simulation 2nd Order Bandpass Filter)

Lab 2 involved fabricating the bandpass filter that goes at the very front of the radio, right after the antenna.  This filter only lets the FM radio band (88MHz to 108MHz) through.  As you can see from the simulation circuit and frequency analysis, once you get outside the bandpass region, the gain drops very quickly.  It should be 40dB/decade for a 2nd order filter.  However, as you can see from my spectrum analyzer screen capture below, that is not the case with my circuit.  My solder joints are fine, but the roll-off rate and cut-off frequencies are nowhere near what they should be.  This needs to be fixed because the radio won't work correctly without the proper bandpass filter.  The likely culprit is a shoddy part, so I'm going to have to order some inductors.  

At least I got a 94% on my lab report. =D

(PSPICE Simulation of the Bandpass Filter - note the high roll-off rate)

(Spectrum Analyzer ScreenCap - nowhere near 40dB/decade roll-off)
Bill Bai,
Feb 2, 2010, 2:36 AM