IF generator for tuning receivers. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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IF generator for tuning receivers. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Tuning the IF path of a broadcasting or amateur radio receiver, especially if it uses lumped selection filters on LC circuits, is a rather laborious operation. This process can be greatly simplified by the swept frequency generator (GKCH), which is used in conjunction with an oscilloscope. The scheme of such a generator (it can also be used as a conventional generator of fixed frequencies) is shown in Fig. 1. It generates oscillations with center frequencies of 465 and 500 kHz and a swing band of up to ±20 kHz, as well as fixed frequencies of 465±25 and 500±25 kHz.

The sawtooth voltage generator is made on the elements DD1.1-DD1.3. It consists of an oscillator, which includes a Schmitt trigger on the elements DD1.1 and DD1.2 and an integrator on the element DD1.3. Due to the fact that the charge and discharge circuits of the capacitor C5 are different, a sawtooth voltage is formed at the output of the DD1.3 element, and short pulses are generated at the output of the DD1.2 element, which can be used to synchronize the sweep of the oscilloscope. Transistor VT1 acts as a buffer stage. From its output (resistor R18) sawtooth voltage is supplied to the output XS1.

(click to enlarge)

IF generator for tuning receivers

The IF generator is assembled on the element DD1.4. Its feedback circuit includes a circuit formed by an inductor, capacitors C1 - C4 and a varicap VD2. Two control voltages are applied to the varicap, one of which is constant (supplied through R1 - R4) and determines the center frequency of the generator, and the second is sawtooth (supplied through R17C6), it determines the swing band.

The center frequency is switched by changing the inductors L1 and L2 with the switch SA1. This is done in order to simplify the setup of the device and make a single scale of the resistor R17.

From the capacitive divider C2 and C3, part of the voltage of the IF generator is fed to the buffer stage on the transistor VT2, at the output of which smooth (R16) and step regulators (R19 - R21) of the output voltage are installed.

The following details can be used in the design: microcircuits - K176LE5, K561LA7, K176LA7; transistors - KT315, KT312, KT3102 with any letter indices; diode VD1 -KD509, KD521A, KD522B, D220, D223; varicap - KB104A-KB104E, KV119A; capacitor C9 - K50-3, K50-6, K53-1, the rest - KLS, KM, KT; power switch - P2K, MT1; resistors R2, R16-R18-SP, SPO, SP4-1, R5 -SP3-3, the rest - VS, MLT. The coils are wound on frames from the IF coils of the Alpinist-407 radio receiver and contain 350 (L1) and 310 (L2) turns of PEV-2 0,08 wire, multilayer winding.

Most of the generator parts are placed on a printed circuit board made of foil textolite. All variable resistors, fixed R19 - R21, capacitors C7 and C9, as well as output sockets and switch Q1 are located on the front panel.

Establishing the device comes down to grading the scales of resistors R2 and R17 and setting the required sawtooth voltage shape. To do this, first connect an oscilloscope (Rin = 1 MΩ) to the output of the element DD1.3 and resistor R5 achieve an undistorted "saw" shape. The change in its amplitude is performed by selecting the resistance of the resistor R9. The frequency of the "saw" can be changed by selecting the capacitance of the capacitor C5.

Then, a frequency meter is connected to the output of the IF generator, the resistor R2 is set to the middle position, and R17 to the lower position (according to the diagram). The magnetic circuit of the coil L1 sets the frequency to 465 kHz, and L2 to 500 kHz, then the scale of the resistor R2 is calibrated on both subranges and, if necessary, by selecting the resistors R1 and R3, the required tuning range and its symmetry relative to the central frequencies are achieved.

Then calibrate the scale of the resistor R17. To do this, the X input of the oscilloscope is supplied with a synchronization voltage from the XS1 socket of the IF generator, and the signal from the XS4 socket ("IF output" 1:10) of the IF generator and through a 100 Ω resistor from the exemplary high-frequency generator, which is used as reference. Resistor R18 sets the sweep length to the width of the entire oscilloscope screen. After that, by rotating the resistor R17 and changing the frequency of the exemplary oscillator, zero beats on the oscilloscope screen calibrate the scale of the resistor R17 "swing band" in kHz.

The inverter generator should be powered from a stabilized source with a current of at least 20 mA.

Author: I.Nechaev

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