Attachment with magnetic modulator. Scheme, description

Attachment with magnetic modulator. Encyclopedia of radio electronics and electrical engineering

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This describes an oscilloscope attachment in which the frequency generated by a tunnel diode is swept using a magnetic modulator.

The prefix provides a smooth overlap of the central frequencies within 20-100 MHz with a change in the deviation of these frequencies in the range from 0,5 to 10 MHz. With the help of such a set-top box, you can adjust the IF amplifier of the TV image, the TV channel switch on the first five TV channels, and also, using the harmonics of the oscillator, check the signal flow in 6-12 channels. The attachment scheme is shown in fig. one.

The generator coil L1 is wound on a toroidal ferrite core, which is placed in the air gap of the control choke Dr1. Through Dr1 flows direct and alternating current with a frequency of 50 Hz.

Fig.1 (click to enlarge)

By changing the value of the direct current using the potentiometer R3, the center frequency of the oscillating frequency generator is set, and by changing the value of the alternating current using the potentiometer R2, the required frequency deviation is set. To disrupt generation during the reverse path of the oscilloscope beam and obtain a zero line, a limiting amplifier circuit is used on transistors MP42 (T1, T2) and P213B (T3).

A voltage of 5 V is supplied to the input of the limiting amplifier through the phase-shifting circuit R1C6 R2 C6,3 from the filament winding of the power transformer. This circuit shifts the phase of the pulse on the divider R12 R13 R14 of the tunnel diode bias circuit with respect to the phase of the current flowing through Dr1, which ensures that the oscillator frequency changes in one direction during the forward path of the horizontal sweep of the oscilloscope.

To reduce the influence of the response of the tuned circuit on the frequency and linearity of the FM modulation of the generator, a buffer amplifier based on the GT313A (T4) transistor is used, assembled according to a common base circuit. To obtain a frequency mark, voltage is supplied to the detector through the capacitor C8 from the standard signal generator. Instead of the GSS, you can use a frequency label generator similar to that used in the prefix described in "Radio", 1968, No. 6.

The core of the inductor Dr1 (Fig. 2) is assembled from the plates of the output transformer of the radio receiver "Speedola". After disassembling the transformer, the plates are glued with BF-2 glue. The coil frame is shortened by 4-5 mm. and 2500 turns of PEL-0,14 wire are wound on it.

Attachment with magnetic modulator
Rice. 2. Magnetic modulator: 1 - choke core Dr1; 2 - throttle frame Dr1; 3 - ferrite ring-core coil L1.

The inductor L1 is wound on a 600NN ferrite ring with an outer diameter of 8,3 mm and an inner diameter of 3,6 mm. It contains 5 turns of PEL 0,3 wire. Winding L1 is performed as follows: two turns are wound on one side of the ring, and then, without changing the winding direction, three turns on the opposite side of the ring (see Fig. 2).

Located inside the screen, indicated on the circuit diagrams by a dotted line, they are mounted on printed circuit boards (modules) that are the same for both amplifiers. The adjustment of amplifiers begins, as usual, with checking the correct installation and measuring the operating modes of the lamps, which should not differ by more than ± 20% from those shown in the circuit diagrams. Then they proceed to tuning the contours in order to obtain the overall frequency response shown in Fig. 3.

Attachment with magnetic modulator
Ris.3

The following devices are required for tuning: oscillating frequency generator X1-7 (PNT-59) or PNT-3; VHF signal generator GZ-8 (GMV) or similar; tube voltmeter V7-2 (VLU-2) or VK7-3 (A4-M2) and a negative voltage source - 3 V.

First you need to set up the second cascade. To do this, the output cable of the PNT generator (attenuator position 1: 1) is connected to the control grid L2 through a 2200 pF capacitor, and the input cable of the PNT oscilloscope through a 33 kΩ resistor to the control grid of the video amplifier lamp. By adjusting the output voltage and amplification of the PNT with the appropriate knobs, an image of the frequency response of the cascade is convenient for observing on the screen of the cathode-ray tube of the device. During adjustment, make sure that the signal is not limited.

By rotating the L8 coil core, the best notch at a frequency of 31,5 MHz is selected according to the largest dip at the corresponding frequency of the curve visible on the screen. Then, by adjusting the position of the core of the L1 coil, and by tuning the L9 coil in the same way, the frequency response of the cascade is achieved as shown in Fig. 4b.

Attachment with magnetic modulator
Ris.4

Further, to adjust the differential-bridge filter, the output cable of the PNT generator is switched to the control grid L1, and the input cable of the PNT oscilloscope is switched to the cathode L2. The negative pole of the 3 V DC power supply is connected to the junction of R2 and capacitor C5, and the positive pole to the chassis. If the output of the AGC system was connected to the connection point of R2 and C5, then it must be turned off for the time of tuning. First, the rejector circuits L5C7 and L6C9 are tuned to 39,5 and 31,6 MHz, rotating the cores of the coils L5 and L6, respectively, until dips are obtained at the above frequencies. After that, by alternately adjusting the cores of the coils L1 and L4, the frequency response of the cascade is achieved, shown in Fig. 4, a. In this case, it should be borne in mind that the L1 setting determines the rise in the characteristic at a frequency of 33 MHz, and L4 - the rise at a frequency of 37 MHz and the best linearity of a gentle slope in the frequency range of 37-39 MHz.

After that, you should turn off the PNT, as well as the constant voltage source and connect the signal generator to the control grid of the L1 lamp, and the tube voltmeter to the control grid of the video amplifier lamp. The signal generator is set to a frequency of 39,5 MHz, an output voltage of 85 mV and a modulation factor of 50%. By rotating the core of the coil L5 and selecting the resistance of the resistors R5 R6, the minimum readings of the tube voltmeter are achieved. Then the same operations are performed with coils L6 and L8, rebuilding the signal generator, respectively, at 31,6 and 31,5 MHz.

Next, the signal generator and the tube voltmeter are disconnected from the amplifier and the PNT (output cable of the generator with an attenuator in position 1: 100) is again connected to the control grid L1, and the input cable of the oscilloscope through a 33 kΩ resistor to the control grid of the video amplifier lamp; a negative voltage source is also connected, as was done earlier when setting up the differential bridge filter. The core of the coil L7 sets the carrier frequency of the image (38 MHz} to the level of 0,5 of the slope of the frequency response, and the core of the coil L9 aligns the flat part of the characteristic at frequencies of 33-35 MHz. It should be ensured that the overall frequency response differs as little as possible from that shown in Fig. 3. This completes the tuning of the amplifier circuits.

During the final adjustment, do not rotate the cores of the coils L5, L6 and L8 of the notch circuits. Coils L1 and L4 of the differential bridge filter during this operation, as a rule, do not require adjustment.

Authors: V. Gorbenko, E. Gorbenko, V. Mironov; Publication: N. Bolshakov, rf.atnn.ru

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