 Armstrong's solution was to set up an oscillator tube that would create a frequency near the incoming signal, and mix it with the incoming signal in a 'mixer' tube, creating a 'heterodyne' or signal at the lower difference frequency, where it could be amplified easily.For example, to pick up a signal at 1500 kHz the local oscillator would be tuned to 1450 kHz.Mixing the two created an intermediate frequency of 50 kHz, which was well within the capability of the tubes.