Nuts and Volts

OPEN COMMUNICATION

; FIGURE 4.

A popular analog-tuned, digital display (ATDD) portable AM/FM/SW radio from C Crane Company.

mixers as constant as possible, despite signal strength variations. There is also an automatic frequency control (AFC) circuit that keeps the local oscillator on frequency in spite of temperature or other undesired variations. An internal crystal oscillator is used as a reference for the frequency synthesized LO.

The block called control circuitry is used to implement the external frequency display and the tuning. The tuning is handled by an external potentiometer or step attenuator connected to a DC voltage. The varying DC voltage is applied to the TUNE1/2 pin, and converted into a digital number by the ADC and used to set the LO synthesizer on frequency. The whole chip is housed in a 16-pin small outline IC. Power comes from a DC source of 2.0 to 3. 6 volts. This is regulated to a constant value by the on-chip regulator.

As for antennas, a standard ferrite core inductor is used for the AM antenna, and the FM antenna is a small collapsible whip. The SW section uses the FM antenna.

The chip radio tunes the full worldwide AM and FM bands. AM range is 504 to 1750 kHz; FM covers 64 to 109 MHz; and SW covers 2. 3 to 28. 5 MHz.

TUNING AND DISPLAY OPTIONS

Modern radios come in three basic formats: analog-tuned, analog display (ATAD); analog-tuned, digital display (ATDD); and digital-tuned digital display (DTDD). The ATAD radios are the old familiar type using a knob or wheel for tuning, and a circular or linear dial to display the frequency. The ATDD mode uses a knob or wheel for tuning and a digital (LCD or LED) numerical frequency display.

The DTDD radios usually use pushbuttons for channel selection tuning and an LCD or LED display. Humans still seem to prefer a knob for tuning, so the wheel tuned ATDD form is still the most popular. Figure 4 shows an example of this form, although it does not use the Silicon Lab chips. The tuning circuitry inside the Si48xx chips implement the ATDD mode. The radio works with an external embedded microcontroller for the display. An external pot with knob or a detent-type click wheel is used to set the frequency.