For beginners radio Amateurs self-construction of a good receiver for monitoring of Amateur radio stations is associated with certain problems caused primarily by the lack of experience and the necessary measuring instruments. Widespread and low cost chips designed for home receiving equipment, allows you to create simple and easy device to repeat at home. We offer to you a dual-band receiver that provides a quite comfortable listening to stations on 80m and 20m. It does not require complicated adjustment, and contains only two coils for assembly. This is good KIT for the weekend.
- Ranges of operating frequencies: 3,5 and 14 MHz (80m and 20m)
- Crystal Quartz filter 4x - 5.2488 MHz
- Bandpass range of a receiving path (on the level of-6 dB) is 350…2700Hz
- Sensitivity from an antenna input, in case bandpass range 2,35 kHz, signal/noise 10 dB is 0,5 µV
- Level of intrinsic noises is not more than 18 mV
- Selectivity on the mirror channel is ~70 dB
- The AGC range to is ~60 dB
- PCB dimentions 95x50mm
- Power output (speakers 8 Ohms) is >50 mW
- DC 9-12 V, 18mA
Dual-Band Receiver designed as super heterodyne circuit with a single intermediate Frequency (IF offset) equal 5,2488 MHz. The core of the receiver is popular Motorola chip - MC3362. Among the important features of MC3362 we should note the high sensitivity (not less than 0.7 mV from the IC input (pin 1), but a relatively small dynamic range - about 80 dB blocking and 60 dB intermodulation.
The signal from the antenna comes to the adjustable attenuator – dual variable resistor 1-5K. Further, the signal through the contacts of the band switch goes to band-pass filters (BPF) – 80m band, which assembled on basis of small-sized inductors L1, L4, L5. When switching to the 20m band – BPF works on coils L2, L3, L6. In view of the fact that the receiver can be used antenna of any random length, the input of BPF has matching resistors.
The filtered signal from BPF comes to the input of the first mixer (pin 1, MC3362). From IF amplifier (pin 19 MC3362) IF signal passes through 4x ladder quartz filter (5.2488 MHz) and goes to the mixing detector (pin 17 MC3362) with reference oscillator and pre-AF gain. The VFO frequency tuning range is 8.74-9.11 MHz. The voltage on the varicap (23), and hence the Tuning of Receiver regulates by two variable resistors "TUNING" (coarse) and fine tuning. The frequency of the reference heterodyne is stabilized by a crystal 5.2488. Since the frequency of its generation must correspond to the lower slope of the quartz filter, it is shifted down from the nominal value by the inductor, connected in series with the crystal.
The audio signal from the second mixer, after pre-amplification comes to pin 5, to which has low-pass filter with a cut-off frequency ~3 kHz. Further, through the separation capacito the signal passes through another low-pass filter with a cut-off frequency ~3 kHz. Purified from spurious products conversion the signal goes to the AF amplifier, based on the popular LM386. The load of AF amplifier - volume control connects via additional simple low-pass filter with a cutoff frequency ~3 kHz, further reducing out-of-band noise, which significantly increases the comfort of listening to a broadcast on a modern broadband small-size low-resistance speakers or phones, such as computer and multimedia. The amplified signal is detected by diodes, and the control voltage of the AGC comes to the gate of the regulating FET. As soon as the value of the regulating voltage exceeds the threshold (approximately 1V), the transistor opens and the voltage divider formed by it together with the resistance, very effectively stabilizes the audio signal at the level of approximately 0.5-0.65V, which corresponds to the maximum output power of approximately 50 mW.