![]() This results in improved sensitivity as well as selectivity. If the filter is placed further down the IF strip toward the Detector end, such noise can be reduced. Noise is normally generated within the IF amplifiers themselves. So placing the filter early in the IF strip means that it will work in all modes. Or they convert down a second time for AM and FM later in the circuit. Many CB rigs only use a single conversion process for both SSB and AM. Placing the filter very early in the IF strip, such as immediately following the first mixer, will help establish the overall receiver selectivity. There are two schools of thought about its location in the circuit path: 1. FILTER PLACEMENT The exact placement of any filter in the receiver IF chain is very important, and can make a difference in receiver performance. The result is a dramatic drop in adjacentchannel interference. The skirts are very narrow, and at ☘ KHz, the attenuation can be 60 db or better. ![]() Compare this broad curve to that of the crystal filter. Strong adjacent channel signals can easily cause bleedover interference to the desired signal. Even at ☘ KHz away from center, the attenuation is still minimum, about - 18 db. But the typical IF transformer is much more broadbanded. Both curves at the insertion point are about 8 KHz wide. However the Q (the Selectivity Factor) of such transformers isn t nearly as high as that of a crystal. All CBs have tuned transformers between the IF amplifier stages to help produce the receiver s overall selectivity. Our filter uses two very high-q quartz crystals and a compensating amplifier to achieve this result. They work by narrowing the skirt selectivity around the center of the radio s IF passband. 1 INSTALLATION INSTRUCTIONS CHANNEL GUARD High-Frequency Crystal IF Filters These crystal filters for 7.8 MHz and MHz CB receiver IF circuits are designed to reduce bleedover interference. ![]()
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