It's November 6. How fast time flew by.

     Let's discuss a bit about mixing frequencies. Two frequencies can be mixed in a MIXER  stage. This is evident in the ordinary BC AM radio. The first tranny is the mixer stage.  The antenna tunes to the desired station with a frequency F1. A local frequency F2 is generated and these two frequencies are mixed. The result is the sum and difference of F1 and F2, and the two individual frequencies. Usually, the difference is chosen and is amplified. In the AM band, this difference is chosen to be 455khz. So, the local osc is always 455Khz below the incoming signal. It could well be above too and the 455khz will appear. This is called the intermediate frequency. Thus, a powerful RF amp is fixed tuned to 455khz. All stations are converted to this frequency. The local oscillator can be a single tranny and is usually done in the SW band. For the BC band, the mixer can also be tasked to handle the local oscillator, thus the stage is called the MIXER-OSCILLATOR stage.

     BUILD AN AM BC BAND RADIO AND LEARN ALL THESE PRINCIPLES.

     The tuning capacitor in an AM radio has tqo sections. One is for the antenna and the other one for the osc. They are ganged together. As you tune to a different station, you are also tuning the local osc so that the difference will always be 45khz.