The simplest TV converter MMDS to UHF. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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This simple converter is designed to convert a TV signal from the 2,5-2,7 GHz band to the 470-670 MHz band. For use in the vicinity of an MMDS transmitting station. The disadvantage of a simple circuit, the direct transmission of the UHF signal induced to the MMDS receiving antenna, is eliminated in the following versions of the converter. Modified feedthrough capacitors here they are used as reference, lying on the board from above.

The circuit is made by hinged mounting. The +12 volt supply is fed down the drop cable through a simple LC summation filter.

For this converter, I can add the following:

- the L2 line in the local oscillator is made of tinned 1,5 mm wire, which is soldered from feed-through capacitors during their refinement; - the length of the L2 line is indicated for a frequency of 1015 MHz, and the mixer operates at the second harmonic of the local oscillator frequency;

- capacitors from 4 pF to 2,2 pF were used as C4,4 and the required frequency was adjusted by a long line L2;

- type C4 is unknown to me. Structurally, this capacitor is made in the form of a cylinder 3 mm in diameter and 3 mm long, made of white VHF ceramics, and thick leads were soldered from the ends. During installation, the leads are soldered. Capacitors of the KM4 and KD-1 types were also successfully used, but they are much less durable;

- the capacitor C2 has one of its outputs 10-12 mm long, which is twisted into a coil L1. Orient this coil L1 perpendicular to the axis of the coil L3 to minimize their inductive coupling and place as far apart as possible. The resonance frequency of the series circuit C2,L1,C3 should be about 570 MHz, and its frequency response is wide, with a bandwidth of 200 MHz. This filter is designed to weaken the penetration of the heterodyne frequency at the input of the IF VT3, VT4;

- the circuit formed by L3 and the output capacitance of the transistor VT4 plus mounting capacitances is tuned by stretching-compressing the turns of the L3 coil to the middle of the 470-670 MHz range. For tuning, it is necessary to connect a load of 75 ohms and, by selecting the position of the tap L3, to achieve a stable maximum gain in a given band. To increase the stability of this node, in the following designs, I separated the L3 and L4 coils from the rest of the circuit with a shielding partition. The cascode circuit VT3, VT4 gives great gain and setting up this node is perhaps the most difficult procedure;

- all parts in this circuit are located on one side of the board directly on top of the tinned foil. Feed-through capacitors (the outer lining is soldered to the board) and other grounded parts are used as fixing and supporting elements for other parts;

- a 20 mm strip of tin-plated sheet is soldered around the perimeter of the board, the resulting box is covered with a lid. The cable entry is made through tubes rolled from tin and everything is soldered with low-melting solder POS61k for rigidity and tightness.

As I already wrote, this design has the following disadvantages:

- the absence of a filter at the input allows UHF TV signals to pass freely to the output and possibly interfere with the reception of some MMDS channels;

- the half frequency of the local oscillator is not only worse for conversion, but it is just as badly delayed by the filter C2, L1, C3 and gets to the output of the converter through the IF. Of course, TV channel filters effectively remove the local oscillator signal, but it is simply impossible to adjust the frequency response of the converter with the local oscillator turned on.

Therefore, I continued to improve this converter and made two more, more successful options - simple и с symmetrical entrance. And yet, it is this page on my site that remains the most visited. Apparently, the apparent simplicity of the design captivates, for the repetition of which even novice radio amateurs who do not have other measuring instruments other than a tester undertake to repeat. Enthusiasm is commendable, but personally I took up microwave technology only after 20 years of radio design. When developing my designs, I used generators, frequency meters, and frequency response meters of the corresponding ranges. Repeating, of course, is easier, but I probably wouldn’t be able to set up the microwave converter well with one tester.

However, I wish you all good luck!

Author: E.Shustikov, UO5OHX ex RO5OWG; Publication: shustikov.by.ru

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