Radio Scan Function

[dmascare]

Does anyone know how the scan function on a car radio works. Thanks

[Robert Reed]

I beleive most scans work by stepping the synthesiser of the local ocsillator frequency through its range, one channel at a time ( i.e.-10Khz for AM; 200 KHz for FM). When you tap the scan button again it stops the stepping action and stays on that station. The SEARCH function goes thru the same procedure at a faster rate, but with one hitch--its looking at the AGC voltage at the same time. When that sees an acceptable AGC voltage (based on received signal strength), it stops and locks on to that station (first one it sees). When you tap the search button again it repeats this process.<p>[ September 09, 2005: Message edited by: ROBERT REED ]</p>

[ezpcb]

usually a oscillator (or pll syntherizer) generate a step-up (or down) freq, and there's a signal detecting circuit. when the detector see a station there, it stops there (or remember the location)

[Dimbulb]

Reading about phase lock loops.
experimenting with a basic plls shows how a stable crystal reference can in effect be stepped.<p>in a different design:<p>a quiet dc voltage lets say from 1 volt to 12 volts from an opamp is supplied to a tuning diode that has the charecteristics of changing capacitance with a change in voltage. It is used in place of an air capacitor in the tuning circuit. If a recieved signal is found above a determined threshold the voltage increase stops.

[Robert Reed]

They are called varactors. And they also do more than local oscillator shift in these radios--they shift the front end resonant circuits (RF & mixer tuning circuits) at the same time.