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Band pass Filter

2017-02-21 18:18  
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Band pass Filter
A band pass filter was designed for low insertion loss to help maintain the receiver noise figure. In keeping with this, NP0 ceramic capacitors were used for the 68 pF and 5 pF fixed-value capacitors. The trimmer cap was a 5 -20 pF ceramic variable with a Qu of 300. (Digi-Key bottom-adjusted SG20016-ND). The leads were bent so that each trimmer cap could be adjusted from the top. The L1 and L2 inductors were wound using 27 turns of #26 AWG enamel coated wire on T50-6 powdered iron toroids. A tap was made four turns up from the grounded end. Qu is ~ 250 for these inductors. The center frequency is 10.125 MHz, the bandwidth is 0.88 MHz and the loaded Q of the resonators is 11.5. The easiest method to tune the resonators is to peak the trimmer caps for the greatest measured output voltage using an oscilloscope. I used the receiver VFO temporarily terminated with a -10dB, 50 ohm pad to obtain the correct filter input impedance and connected it to the input end of the filter. I temporarily terminated the output of the filter with a 51 ohm resistor to ground. The VFO was tuned to the center frequency by placing it next to a receiver set on 10.125 MHz. A frequency counter can also be used. The trimmers were adjusted on each resonator to obtain the highest measured voltage possible. The filter was then placed in the receiver after removing the temporary alterations used during calibration. If you do not have access to test equipment, tune the resonators at the center frequency while listening to the receiver in the headphones to obtain the greatest possible band noise. Confirm your adjustments by tweaking the trim caps while listening to a QSO as well.