Radio station CHU in Ottawa, Canada broadcasts standard time and frequency references on three shortwave radio frequencies, 24 hours a day. CHU transmits using amplitude modulation (AM) consisting of an RF carrier and a single upper sideband containing audio information. The beginning of each second is represented by a tone burst having an audio frequency of exactly 1000 Hz. In this video, two minutes of CHU audio are plotted on the Y-axis, while a local 1000 Hz sinusoidal reference is plotted on the X-axis, thereby producing a Lissajous pattern. The 1000 Hz reference is derived from WWVB’s 60 kHz carrier, and in this application is accurate to less than one part per billion. We would expect to see the 1000 Hz tone bursts from CHU produce a stationary pattern if the received frequency of the tones measured exactly 1000 Hz. What we see, however, is a slowly rotating ellipse, especially in and around the time of deep signal fades. This rotation is indicative of a shift in phase that is taking place over time. A change in signal phase over time implies that a change in frequency is also taking place. Since the audio recovered from the receiver is a product of the received carrier and upper sideband frequencies, it can be concluded from this exercise that the carrier and sideband signals are undergoing separate and distinct changes in path length, Doppler shift, polarization, and/or multipath propagation effects, even though they originated from a single transmitter, and are …
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