Historically negative resistance oscillators were very alluring in the making of heterodyne receivers, because they had a considerably wide tuning range, worked stable across the tuning range without any corrections in the circuitry and offered great frequency stability (at the time). This made them a preferable choice for use as local oscillators in het receivers. Originally tetrode tubes were used for the anode current kink (or sometimes the second grid current kink) in their characteristics, which essentially made a small portion of their characteristics act in a negative resistance way. Nowadays you have special gunn diodes and whatnot, but back to basics, you can make a very simple NR Oscillator with just two transistors and a few components around them.
