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U2510B Datasheet(PDF) 10 Page - ATMEL Corporation |
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U2510B Datasheet(HTML) 10 Page - ATMEL Corporation |
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10 / 15 page  U2510B Rev. A3, 23-Feb-01 10 (15) 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 k W 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. |
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