|All CBs have tuned transformers between the IF amplifier stages to help produce the receiver's overall selectivity. However the "Q" (Selectivity Factor) of such transformers isn' t nearly as high as that of a crystal - The graph shows this difference. Both curves at the insertion point are about 8 KHz wide. But the typical IF transformer is much more broadbanded. Even at +/- 8 KHz away from center, the attenuation is still minimum, about - 18 dB. Strong adjacent channel signals can easily cause bleedover interference to thedesired signal. Compare this broad curve to that of the crystal filter. The skirts are very narrow, and at the same +/- 8 KHz, the attenuation can be -60 dB or better. Theresult: a dramatic drop in adjacent channel interference.|
|TYPICAL BANDPASS FILTER|
Many rigs use 2 or 3 stage transformer coupling. Each connects with a small (2.5pF)capacitor. Remove this and install CG filter here.
|TYPICAL IF AMP STAGE|
Each IF stage that uses a transistor amplifier is usually coupled by a capacitor. Remove this and install CG fllter here.
|TYPICAL CHANNAL GUARD FILTER|
Use 2 pcs. standard 10.695MHz X-Tals
and a transistor for amplification.