|MadSci Network: Physics|
An FM radio detects the FM signal by using a band pass filter and "Phase Locked Loop" (PLL). The band pass filter is tuned to focus only on one station at a time (like a spotlight), and the PLL is employed to extract the music. the PLL is basically a voltage controlled oscilator and a comparator used as an error detector, linked in a control loop. This control loop acts as a servo that constantly tries to keep the local oscillator synchronized (phase locked) to the incoming signal, by increasing or decreasing the frequency of the local oscillator as to minimize the error signal. Remember that the incoming signal is a varying frequency carier, and the actual music is encoded as the frequency variations (that's what Frequency Modulation means). By amplifying the local oscillator's control voltage, the modulator is extracted (which is the signal of interest, the music). This is because the control loop supplying the control voltage is trying to make the oscillator mimic the incoming signal. And the control voltage is proportional to the frequency, which (if close enough to the incoming signal) is proportional to the music itself. In order to scan, two things happen: the bandpass filter scans the spectrum, and at the same time, the characteristits of the PLL's control loop are changed as to make it capable of locking into the various frequencies being scanned. However, the control circuit for the PLL and the PLL itself are not perfectly linear. As a result, chances are the control system as a whole has a step response that exhibits hysteresis. Hysteresis is what happens when a non-linear system behaves differently depending on the direction its being asked to go. For example, when you adjust the rear view mirror in your car, its sometimes very anoying that it does not stop where you leave it, and you have to push just a little further so it settles at the desired position (or jiggle it). When moving the mirror in a different direction, you also have to push a bit further so it stops at the desired position. Notice that the final position *of motion* is different depending on the direction you move it, even if the position it ends up is the same. This is a perfect example of hysteresis. Bycicle shifter cables, camera tripods, and many others are good examples as well. The hysteresis of this step response would result in a scanning rate that is not always the same at all frequencies or in all directions. the scanner is probably moving linearly up or down the spectrum but the PLL control circuit does not just follow perfectly, it may lag behind and catch up, and it may even overshoot slightly after the catchup. And how it behaves in one direction does not need to match what happens in another because of nonlinearity. I believe that in order for the circuit to work properly and avoid mistaking noise for a radio station, the scanner only stops if the PLL has seen a minimum amplitude signal for a minimum length of time. Because of the histeresys and nonlinearity of the PLL and its control loop, its quite possible that the scanning rate in one direction may prolong locking time barely longer than minimum for detection, and that in the opposite direction locking time may be well bellow the required minimum. This would explain why a weak radio station may be easier to catch when going in one direction than the other. It is also possible that this phenomenon only occurs when the scan begins *near* the target station, and that there is no directional difference if the target station is far from the starting point. I have no reason to believe this would always work in the same direction in most radios, but that would be an interesting experiment. Your mad scientist, Aurelio R. Ramos
Try the links in the MadSci Library for more information on Physics.