Answers to questions on the designs of MMDS converters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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I will put here questions and answers on the designs of my 3 converters for TV MMDS - the simplest simple, с symmetrical entrance.

- I ask you to clarify how best to apply a bias to the bases VT1, VT2 of MMDS converters, from collectors or from feed-through capacitors?

There is practically no difference how to apply the offset. For the latest converters, I took the smallest MLT resistors, bit off the leads, removed the paint and the rest of the leads at the "zero", tinned the heads, placed them on top of the transistors and soldered their leads to the ends of the resistor heads. And already connected other elements to them.

- please indicate the approximate currents through the transistors of this circuit?

Transistor modes (except VT4) are set automatically. The currents are approximately the following: VT1 - 4 mA, VT2 - 7 mA, VT3 - 15 mA, VT4 + VT5 - 15 mA. Offset VT4 must be selected according to the maximum mixer transfer coefficient (near 0 current).

- if possible, describe in more detail the procedure for setting up the converter.

I start setting up with UHF UHF (VT5, VT6). Using a frequency response meter, I select the position of the tap of the output coil of the IF, achieving maximum and uniform gain in the range of 470-670 MHz. I send the signal to the input of the mixer VT4. Capacitor C16, together with its terminals, should resonate in the middle of this range. I connect the VT3 local oscillator and set its frequency to zero beats with a 2030 MHz generator. I connect UHF VT1, VT2 and finally set up the converter by applying a signal of 2500-2700 MHz to the input. UHF coupling capacitors with leads should resonate in the middle of this range. The resonance in the circuit is "blurred", so I pre-adjust the length of their conclusions outside the circuit. I turn on the signal generator with modulation, which is visible on the TV as wide horizontal stripes. Gradually reducing the signal level, I adjust the circuit according to the maximum contrast of the bands. In the absence of devices, you can try to tune the IF using UHF television signals, and look for the local oscillator frequency before the MMDS transmissions "hit" the UHF channels. Of course, this is possible only in the zone of reliable reception.

- why did you use transistors of different conductivity?

- specify the dimensions of the elements: L1, C14.

Back in the Soviet Union, I bought 200 pcs. transistors KT3109A in the store Young Technician for 65 kopecks. piece and still happy with it. In many of his microwave designs, he used them wherever possible. It turned out that they give 16 dB of gain each, in the UHF. And they even generate at 2,5 GHz! Well, in the input circuits, I used transistors more suitable for 2600 MHz, relying on their low noise properties. So that different conductivity is not an end in itself, it's just cheaper.

The L1 line is made of tinned sheet metal (or copper or silver :-)) in the form of an L-shaped flag 9 mm long and 2 mm down, for soldering to the board, directly to it without leads soldered cakes of capacitors C1, C2 type KD- one. Above the board, the line is 1 mm apart, however, the rest are the same. Capacitor C2, also a tin box bent at a right angle and soldered to the VT14 base, and its capacitive pad adjoins R4, from the VT9 emitter side.

Drawings:

Fig. 1

Fig. 2

Fig. 3

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

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