A variable gain MOSFET amplifier is placed at the front of the IF strip to provide an IF gain control. The 100K potentiometer and LED is mounted on the front panel of the receiver. The brightness of the LED indicates the gain of the amplifier. The variable gain amplifier is used to set the noise level output of the audio. If band noise is very loud, which it can be on 20 meters, the gain can be reduced to minimize the abrasiveness of the band noise. For quiet band conditions it can be run wide open. The LED is used to show the operator its setting. I used a Super-Bright Blue LED here. The Blue LED reduces the gain of the amplifier because of its greater foward voltage drop (3.5 volts versus 1.5 for a regular LED). The full gain of this amplifier is not needed, so the Blue LED works fine. This was mainly done for looks, a regular red could be used. The 455kHz IF transformer can be any that are available. The connection to the center tap of the primary gave more gain. It is worth experimenting with the IF cans to see which connection works best, either at the center tap or at the end of the coil. All the 455kHz IF cans I have seen have a center tap on the primary side. The optoisolator has been removed and the S-Meter moved to the AGC circuit. The benefit of having a visible LED helps with diagnosis of the circuit. A Murata SFU455A filter can be used at the input of the strip, connected between the mixer and the input to the IF strip. The filter has a 6db loss. This helps remove some noise and cleans up the signal from the mixer, but in listening tests with/without the filter, no difference could be heard. |
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