VHF converter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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Reception of signals from VHF broadcasting stations operating in the range of 64,5-74 MHz to a VHF receiver having a range of 87,5-108 MHz can be carried out either by restructuring the input and heterodyne circuits of the radio receiver, or using a special device that converts the range signal 64,5-74 MHz to the desired part of the range 87,5-108 MHz.

To implement the first method, certain measuring instruments are needed (FM generator, millivoltmeter, frequency meter, oscilloscope). Yes, and experience with a specific type of radio is important, since there is always a danger that a radio amateur, unwittingly, can disable the device. More often this happens if there is also no circuit diagram of a custom receiver. That is why we propose to assemble a simple transducer that does not require intervention in the "organism" of the VHF receiver.

The converter (see Fig. 2) contains a K174PS1 chip, an inductor and several radio elements. The device works reliably when the supply voltage changes from 3 to 12 V.

IC K174PS1 is a balanced mixer. In our case, it is used as a balanced analog multiplier (BAM), the principle of operation of which is clear from Figure 1.

If a signal voltage is applied to one input of the BAU, and a local oscillator voltage is applied to the other, at the output of the BAU we get the sum and difference signals. The use of the K174PS1 chip can significantly reduce the level of parasitic harmonics. This IC also has the advantage that the connection between the inputs of the signal and the local oscillator of the converter is very weak. Therefore, input signals even around 3 V produce an extremely small detuning of the local oscillator frequency (less than 10 kHz).

For conversion, you can use both the sum and difference components of the output signal - it all depends on the selected local oscillator frequency. If it is in the range of 23-34 MHz, then the total component fc + fg is used. And if the local oscillator frequency is chosen equal to 162 MHz, the difference component fg-fc is used.

Specifications of the inverter

• Supply voltage, V.....3,75
• Consumption current (at Upit=3,75 V), mA............. 3,5
• Local oscillator frequency, MHz... 25±0,5
• Input signal range, MHz .......... 64,5-74
• Range of output signals, MHz .......... 89,5-99
• Output voltage with a receiving antenna length of 150 mm,. not less than 100 µV
• The duration of continuous operation without recharging the batteries, h ...... not less than 32

Schematic diagram

The converter is made according to the scheme with a combined local oscillator, and since the K174PS1 chip generates better at lower frequencies, the local oscillator frequency was chosen to be approximately 25 MHz. It is determined by the elements L1, C1, C4, C5 (Fig. 2). Moreover, the local oscillator does not need to be tuned to a specific frequency, it is only important that it lies in the range of 23-34 MHz and does not change over time.

The input signal from the WA1 antenna through the coupling capacitor C2 is fed to the signal input of the DA1 microcircuit, where the signals are mixed: the input and the local oscillator. The converted signal from the load - resistor R3 - is fed through the decoupling capacitor C6 to the VHF radio antenna. Capacitor C7 eliminates self-excitation of the microcircuit during partial discharge of the power source.

The prefix uses the total component of the output signal of the converter. The difference component (30-50 MHz) lies outside the operating band of the receiver and is filtered out by its input circuits. Since the local oscillator frequency is chosen fixed, this greatly simplifies the design of the converter, since it excludes tuning elements (KPI, varicaps): it is produced by the receiver itself.

Construction and details

All parts, except for batteries, are located on a printed circuit board 28x20 mm in size, made of one-sided foil textolite or getinaks 1-1,5 mm thick (Fig. 4).

MLT-0,125 resistors, KM capacitors. Instead of the K174PS1 chip, you can use the K174PS4. Power switch type PD9-5. The coil is wound on a tuning core Ø 0 mm made of carbonyl iron from armored cores SB-4a or SB-1.

The antenna is a piece of steel or copper wire Ø 0 mm, approximately 2,5 mm long, bent at a distance of 150 mm from the edge at a right angle. (In this case, a bicycle spoke was used.) With the curved end, the antenna is inserted into the hole in the board and fixed by soldering.

The assembled board is placed in the battery case 7D-0.1. from which four elements have previously been removed. The remaining three batteries are used to power the set-top box. Holes for the antenna and the power switch are pre-drilled in the bottom of the case, and a clamp with a clamp made of sheet bronze or other springy material is put on the side of it. Using a clamp, the transducer is attached to the telescopic antenna of the receiver. A wire is soldered to the clamp, passed through a hole in the side wall of the battery case. The "+" and "-" terminals are used to recharge the batteries from the charger for 7D-0.1, which can be purchased at the store or made by yourself

Establishment

A properly assembled transducer does not require adjustment. Before the final installation of the board in the case, it is advisable to check the current consumption with a milliammeter - its value should be within 2,6 -3,4 mA. After the end of the reception, do not forget to turn off the set-top box, otherwise you will often have to recharge the batteries. In conclusion, it should be noted that the converter can also work in conjunction with a receiver operating in the range of 64,5-74 MHz, then it can also receive VHF radio stations operating in the range of 87,5-108 MHz. In this case, the radio receiver will use not the total, but the difference component of the input signal of the converter.

Authors: A. Boyko, V. Krapivin; Publication: N. Bolshakov, rf.atnn.ru

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