A simple panoramic indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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The search for signals of stations operating away from the tuning frequency of the VHF transceiver can be facilitated by supplementing it with a simple panoramic indicator (see figure). It allows you to indicate the presence of stations in the ±40 kHz band. The information is displayed by the LED indicator on 15 light-emitting diodes. Each of them "controls" a bandwidth of about 5 kHz. When a signal appears in the overview strip, the corresponding LED lights up. By the brightness of its glow, you can roughly estimate the strength of the signal.

The indicator consists of a receiving part and a scanning unit. The receiving part contains an input amplifier, a local oscillator, a mixer, an IF amplifier, a low-frequency detector and amplifier, a sweep unit - a master oscillator, a pulse counter, a decoder and an LED indicator. The input amplifier is assembled on the transistor V1. Its load is the L1C4 circuit, tuned to the transceiver's IF frequency, in this case equal to 8750 kHz. The amplification of the cascade is regulated by the resistor R6.

(click to enlarge)

The local oscillator is made on the transistor V3. The RF voltage generated by it is higher in frequency than the input signal of the inverter. The frequency modulation of the local oscillator is carried out by the V22 varicap, which is supplied with a sawtooth voltage from the charge-discharge chain R28C35R30.

The mixer is assembled on a double-gate field-effect transistor V2. A signal from the input amplifier is supplied to its first gate, and the local oscillator voltage is applied to the second. The mixer load is a lumped selection filter tuned to the intermediate frequency of the indicator, which is chosen to be 460 kHz. The L7C26C27 circuit is also tuned to the same frequency.

The converted signal is fed to the IF amplifier and the indicator detector, assembled on the A1 chip. AGC circuits of microcircuits are not used. The rest of the inclusion is typical.

The low-frequency amplifier is made on transistors V4, V5. So that the loss of the constant component does not affect the operation of the indicator, the capacitance of the isolation capacitor C3З is chosen large enough.

The anodes of the V5-V27 LEDs are connected to the collector of the transistor V7 through the resistor R21. The cathodes of the LEDs are connected to the outputs "0" - "14" of the D2 chip.

The master oscillator is assembled on the A2 chip. It generates pulses with a repetition rate of 400 Hz. Having passed the divider by 16 on the D1 chip, the pulses in a parallel four-bit code are fed to the inputs "1", "2", "4", "8" of the D2 decoder, which switches the V7-V21 LEDs. Resistor R27 limits the pulsed current through the LEDs and the outputs of the D2 chip.

With every sixteenth pulse at the input of the D1 chip, a low logic level appears at the output "15" of the D2 chip, which opens the V29 transistor through the resistor R6. In this case, capacitor C28 is charged from the power source through resistor R35. With the arrival of the following pulses at the input of the D1 chip, logical 0 alternately appears at the outputs "0" - "14" of the D2 chip and, thus, the V7-V21 LEDs are connected in turn to the transistor V5. Capacitor C35 at this time is gradually discharged through resistor R30. The sawtooth voltage from it through resistors R16 and R15 is supplied to the varicap V22. This is how the periodic change in the frequency of the local oscillator and the synchronous sweep of the LED indicator are carried out. Since the capacitor C35 is discharged relatively slightly in each cycle, the frequency sweep is quite linear. With the selected parameters of the local oscillator circuit and a sawtooth voltage on the varicap, the swing of which is 2 V, the local oscillator frequency varies within ±40 kHz.

The input of the panoramic indicator is connected to the transceiver at a point through which a signal with a bandwidth of ±40 kHz passes. However, signals that fall into the passband on the mirror channel of the indicator must be suppressed here. In this case, this is a frequency of 9670 kHz (8750 + [2X460] = = 9670 kHz). If this condition is not met at the connection point, then a bandpass filter should be installed instead of the L1C4 circuit.

Establishing the block consists in tuning the circuits to the frequencies indicated above. The bandwidth of the indicator is determined by the tuning of the L3C16, L4C17, L5C18 and L7C26C27 circuits, and given the purpose of the panoramic indicator, it is chosen to be 3 kHz at a level of -3 dB.

The frequency swing range of the local oscillator within ±40 kHz is set by selecting the ratio of the capacitance of the capacitor C7 and the inductance of the coil L2. If it is necessary to reduce the sawtooth voltage on the varicap, you need to turn on the resistor R30 with a large resistance. The initial brightness of the LEDs is set by resistor R24.

Coils L1 and L2 are wound on frames with a diameter of 7,5 mm with an SCR-1 trimmer. L1 contains 15 turns of wire PEV-2, 0,27, L2 - 6 + 15 turns of wire PEV-2 0,2. The winding length of the coils is respectively 7 and 9 mm. As coils L3. L4, L7 and L5, L6 used, respectively, FSS III and FSS IV coils from the VEF-201 radio receiver.

The panoramic indicator is powered from a constant voltage source of 12 and 5 V. A voltage of 5 V is also supplied to the 24th output of the D2 chip and the 5th D1 chip. The 12th output of D2 and the 10th D1 are connected to a common wire.

Author: V. Tereshchun (UB50BJ), Uzhhorod; Publication: N. Bolshakov, rf.atnn.ru

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