Prefix-GKCh for the ranges of 300 ... 900 and 800 ... 1950 MHz. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Adjustment of radio-electronic equipment with a visual display of the amplitude-frequency characteristics is always of great interest to radio amateurs and specialists, as it allows you to quickly see the results of the impact on the screen of the measuring device when you change any parameter or element of the adjusted product. The only drawback of this control method is the relatively high cost of industrial samples of frequency response meters. But radio amateurs found a worthy way out here too - the creation of simple attachments to an oscilloscope that has already become familiar. In this case, the frequency response of the oscilloscope itself does not play a special role. In the magazine "Radio" 1994, No. 1, p. 26, a description of such a set-top box for adjusting television equipment was given, indicating the possibility of expanding its functionality.

Today we provide recommendations for finalizing this set-top box in order to use it to adjust devices operating in the UHF and microwave bands (UHF channel selectors, tuners for satellite television broadcasting systems, etc.).

(click to enlarge)

The publication in the named journal of a description of a prefix for measuring frequency characteristics and subsequent responses from radio amateurs forced the development of recommendations for the mass repetition of a device operating in higher frequency bands. Below are descriptions of two options for improvements to the set-top box with generators for 300 ... 900 and 800 ... 1950 MHz. At the same time, it turned out that the modification of the prefix does not require its complete alteration, it is enough just to change the design of the high-frequency generator. Therefore, the numbering of elements on the given schematic diagrams continues the numbering of the main scheme of the attachment.

Figure 1 shows the GKCh scheme, it is the same for both high-frequency ranges. On transistors VT2 and VT3, a generator is made, the frequency of which changes within the required limits when the current through these transistors changes, and the cascade on transistor VT4 acts as a buffer amplifier.

The generators of each of the ranges differ in their design. Inductors for the indicated ranges, due to their small inductance, must be made in the form of strip lines. Figure 2 shows a variant of the GKCh for the range of 300 ... 900 MHz, and in Fig. 3 - for the range of 800 ... 1950 MHz. The configuration of the tracks that act as inductors L1 and L2 should be repeated as accurately as possible according to the figure. For the manufacture of generator boards, a double-sided foil fiberglass 1 mm thick was used. The side free from elements is left metallized and connected along the perimeter of the board to a common power bus with strips of thin copper foil.

When mounting generator boards, ceramic and glass-ceramic types of capacitors KM-5v, K10-9, K10-17V or KM-5, KD-1, resistors of the MLT-0,125 type should be used. All elements, including transistors, must have a minimum lead length that ensures reliable installation. The types of transistors indicated in the diagram can be replaced with KT3101A-2 and KT3132A-2.

The adjustment of devices begins with the setting of the voltage across the capacitor C15 by the resistor R8 within 5 ... 7 V. Then, according to the method described in the description of the basic version of the set-top box, the generator frequency range is set. In this case, it is possible to shorten the coils L5, L1 somewhat (no more than 2 mm), and since the frequency overlap ratio decreases, it may also be necessary to reduce the voltage across the resistors R2 and R4 of the basic design. To do this, in series with these resistors, between the cathode of the zener diode VD1 and the upper terminals of the variable resistors according to the circuit, additional resistors of the required size should be installed.

The uneven frequency response can be corrected by changing the value of the resistor R17 up to 150 ohms or by shorting part of the coil line L4.

In the design of the generator board for the range of 800 ... 1950 MHz, the capacitances of capacitors C7 and C9 can be halved and capacitors of the K10-42, K10-17, K10-9 types can be used mainly. If you use capacitors with leads, then the uneven frequency response of the device may increase. In addition, it is desirable to use resistors of the C19-20 type with leads of the minimum length as resistors R2 and R10 of the output voltage divider, and to perform the detector head circuit in accordance with Fig. 4.

Author: I. Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru

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