U2510B Datasheet, PDF (10/15 Pages) ATMEL Corporation – All-Band AM/FM Receiver and Audio Amplifier

U2510B
Circuit Example
Figure 17 shows a circuit diagram for low end AM/AF
radios using the U2510B. Figure 18 shows a circuit
diagram of AM/AF radio for higher class designs using all
possible options of the U2510B. The layout of the PC
board, shown in figure 19, is suitable for both the circuit
example shown in figure 17 and the circuit example
shown in figure 18. The associated coil, varicon and filter
specifications are listed in the table: COIL DATA and
SPECIAL COMPONENT PARTS. The circuit diagram
(figure 18), has the following options compared to the
circuit diagram (figure 17) (the additional parts, which
have to be provided, are listed in parentheses):
a) Soft mute and high cut control in FM mode (1 cap.)
b) Electronic treble control in AM, FM and TAPE mode
(1 pot.)
c) On-chip mode control for TAPE application
d) RF AGC in FM mode (1 capacitor)
e) AFC, adjustable to the correct polarity and slope
(1 cap.)
f) Tuning indication using LED as an indicator
(1 LED, 1 cap.)
Option a) reduces the interstation noise by the two
functions: soft mute and HCC. Both are controlled by the
mute voltage (Pin 1). The soft mute reduces the loudness
only, while the HCC reduces the high-end audio cut-off
frequency of the audio preamplifier, when the signal level
falls below a given threshold. This signal level threshold
as well as the mute depth can be reduced by adding a
resistor (R₃) or by increasing the FM front–end gain.
Option b) allows the treble control for all operating modes
without the need of an additional capacitor. This concept
leads to a smooth and correct treble control behavior
which is an improvement compared to the controlled RC
network normally used.
Option c) is very useful for application in radio
cassette-recorders, for instance. In TAPE mode, the
AM/FM receiver blocks are completely switched off and
the signal from the tape recorder can be fed to the audio
amplifier ’s input directly. This saves quiescent current
and makes the TAPE switching easy. However, to
minimize switching noise by the mode switch, the
following switch sequence should be chosen: AM, FM,
TAPE.
Option d) improves the strong signal behavior by
protecting the FM mixer against overload. This is
provided by the integrated broad-band-width RF AGC. If
necessary, the AGC threshold can be decreased by a
resistor, loading Pin 11 to GND (not shown).
Option e) improves the tuning behavior substantially. The
special design of the on-chip AFC function means that
common disadvantages such as asymmetrical slope,
(chip-) temperature effects and unlimited holding range
are avoided. As mentioned in the “Pinning Description
Table”, the AFC slope has to be inverted when the local
oscillator (LO) frequency has to be below the receiving
frequency. This can be achieved by connecting Pin 21 to
the potential of Pin 8. In addition to the options described
above, the following proposals are implemented in the
circuit diagram (figure 18), too:
D An FM IFT is applied. This improves the channel
selectivity and minimizes substantially the spurious
responses caused by the FM ceramic filter (CF₂). With
the choice of the winding ratio of this IFT, the FM
front end gain can be matched to other values if neces-
sary.
D In the FM RF input section, the low cost antenna filter
(L₅, C₁₅) is replaced by a special band pass filter
(PFWE8). Such a BPF protects the FM front end
against the out-off-band interference signals (TV
channels, etc.) which could disturb the FM reception.
Design Hints
The value of the power supply blocking capacitor C₁₃
should not be below 470 mF. In addition, this capacitor
should be placed near Pin 26. This will help to avoid
unacceptable noise generated by noise-radiation from the
audio amplifier via the bar-antenna. In designs, where the
supply voltage goes below 2.5 V, the value of the blocking
capacitor (C₇) should be chosen as 47 mF or even higher.
To achieve a high rejection of short wave reception in
medium wave operation, the LO amplitude at Pin 5
should not exceed approximately 200 mV. This LO
amplitude depends on the LO transformer’s Q and its
turns ratio. For the LO transformer type described in the
“Coil Data Table”, a resistor R₄ (2.2 kW for example) in
parallel to the secondary side of the AM LO transformer
T₂ is recommended. To minimize feedback effects in the
RF/IF part in FM mode, the capacitor C₆ should be placed
as near to Pins 8 and 20 as possible.
As shown in the application circuit diagrams (figures 17
and 18), in FM mode ceramic filter devices are used for
channel selection (CF₂) while for FM, demodulation in
LC-discriminator circuit (T₄, C₂₄, C₂₅) is used instead of
a ceramic discriminator device.
Such an LC discriminator circuit can be easily matched
to the FM IF selectivity block by its alignment. The zero-
crossing of the discriminator can be detected at the
demodulator output (Pin 23). The zero-crossing voltage
is equal to half of the regulated voltage at Pin 8.
10 (15)
Rev. A3, 23-Feb-01