The frequency range of long wave (LW) broadcast is 153kHz to 279kHz, which is narrow than the medium wave broadcast frequency range (535kHz ~ L605KHz), but it is not easy to receive long wave broadcasts, it is difficult to:
Length wave sets, weak signals, if there is no action, its signal will be overwhelmed by the medium wave signal of strong power. In order to suppress the interference of the medium wave, the medium wave is required to filter out the medium wave.
Noise generated by peripheral electronic devices. Its frequency is mostly in the LW band. To avoid these interference. The long wave radio is set to set an antenna. And try to use the electronic equipment.
In order to improve the selectivity, it is necessary to increase the Q value of the tuning loop, but the Q value will turn off the useful sideband. The sound of the playback is lacking a treble.
This machine focus is to improve reception sensitivity and remove medium wave interference to improve the reception.
The circuit uses a straight discharge of mirror interference. Add 150 kHz to 300 kHz band pass filter. The block diagram is shown in Figure 1, and the whole machine circuit is shown in Figure 2, mainly the tuning circuit, two high-frequency amplification and bandpass filters in it.
Due to the high frequency amplification of 2SK241-Y, when the gate source voltage VCS is 0V, the flue-source voltage Vds operating in a saturated state is 5V, and the DC load resistance is below 220 Ω or less. To prevent the battery voltage drop from moving the work area to the unsaturated zone. The power supply voltage is 6V.
The tuning circuit uses a high Q value to increase sensitivity and reduce noise. The tuning circuit L1 = 1.8Mh, and the variable Diode is parallel with two LSV149. The tuning voltage is approximately 1.8V to 4.6V, and the tuning voltage is stabilized with 5V low pressure difference regulator, and the potentiometer VR1 is adjusted.
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The Q value of the tuning circuit depends not only on the performance of the LC circuit, but also related to other components connected to the LC circuit. For example, C1, C2 should be used with low loss polypropylene film capacitors. The equivalent parallel resistance of each component of the tuning circuit is shown in Table 1.
The load of the first stage high-frequency amplifier is a band pass filter (BPF). After test, the filter is 7 LCπ-type filters. The components of the filters are shown in Table 2. To avoid introduction of noise and generate abnormal oscillation, the entire filter is loaded into the shielding disc. The DC resistance of the inductor L101 is 13 Ω. The power supply decoupling resistor R11 is 22Ω. A total of 35 N, the description of the aforementioned previously described. The gain of the high placement of this level depends on R101 = 2.7kΩ. Due to the forward transmission of 2Sk241-Y, the voltage gain is 2.7kΩ × 10 ms = 27 times.
The second stage high placed circuit still uses 2SK241-Y, which is a simple LC parallel resonant circuit, which has a gentle bandpass in the frequency band of the LW. The DC resistance of L21 is 130Ω. R23 is 22Ω. Total 152 Ω, also below 220Ω or less. The output impedance R102 = 2.7kΩ of the BPF becomes a fixed bias resistance of the magnifying tube. This grade gain switch SW21 is switched for two-stop switch, and the two gear differ from 14 dB.
After high placing, the AM radio integrated circuit LA1600 is used to simplify the circuit. The output detection signal is smooth, and the carrier residual component is removed by the RC low pass filter.
The audio amplifier uses TA7368P, which outputs it to the speaker or headset.
Receive long waves. Three medium wave magnetic bars, size 70 mm × 12 mm × 6 mm here are used. The tape is integrated, and the magnetic rod of 70 mm × 12 mm × 18 mm is constituted, and a single layer is wrapped in a polyurethane lacquered line with a diameter φ0.32 mm. Until the inductor is 1.9MH: then adjusted to 1.8 MH, resistance 2.3Ω. If there is no L, it is measured, and it can be wrapped around 170.
The entire circuit board (except antenna) is surrounded by an aluminum foil, and the aluminum foil is insulated from the circuit board. Aluminum foil grounded. Mid-wave interference can be effectively suppressed.
Frequency range adjustment: Use VR1 to the left-handed broadcast to receive 153 kHz broadcast, otherwise adjust the value of R2; to the right to vr1 to receive 279 kHz broadcast. Otherwise adjust the value of C3.
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