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.
For more details on the Silicon
Labs chips, see www.silabs.com. NV
June 2013 65