UHF converter with SAW filter for narrowband AM/FM. Scheme, description

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UHF converter with SAW filter for narrowband AM/FM. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In transmitting and receiving devices in the UHF range, a master oscillator with a quartz resonator and subsequent frequency multiplication is often used. This complicates the design and its adjustment due to the presence of harmonic components, so such a transmitter can "contaminate" the air.

Good results were obtained using local oscillators, where surface acoustic wave (SAW) resonators are used as a stabilizing element. A diagram of one of these options is shown in Fig. 1.

UHF converter with SAW filter for narrowband AM/FM

The first stage on the transistor VT1 - a low-noise amplifier (LNA) with a resonant load Z1 - has a bandwidth of 6 ... 10 MHz (at a level of -3 dB). The high-pass filter C1L1C2 at the input of the cascade serves to cut off the frequencies of the meter range. The cascade on the VT3 transistor is a local oscillator, where a SAW resonator (Z2) at 418,325 MHz is used as a stabilizing element. The tightly coupled circuit L6C15L7C14 serves to provide the necessary rotation of the oscillation phase in the feedback circuit.

The mixer is made on a double-gate field-effect transistor VT3 with a Schottky barrier AP328A. Its input capacitance at the second gate is negligible, and this made it possible to connect directly to the local oscillator load circuit, while eliminating the buffer stage and without degrading the quality factor of the circuit, which is very important for obtaining a stable frequency and amplitude of oscillations. The load of the mixing stage is the L5C9 resonant circuit, tuned to an IF frequency of 10,7 MHz. The measured converter noise figure is 2,4...3 dB. Temperature instability - 2,6 10^-6.

The device was mounted using chip elements (mostly foreign-made) on a 25x42 mm board. Coil L1 has 5 turns and is carried out with a wire with a diameter of 0,2 mm on a mandrel with a diameter of 4 mm. The coils L2 - L4 of the bandpass filter Z1 are made in the form of pieces of copper wire with a diameter of 0,7 and a length of 12 mm. Their mutual arrangement is shown in fig. 2. The distance between the segments can be within 0,1 ... 0,3 mm.

UHF converter with SAW filter for narrowband AM/FM

The phase-shifting transformer L6L7 is made with two twisted wires 0,3 mm on the "ring" of a piece of PVC tube with a diameter of 5 and a height of 7 mm, its windings have 2 turns. Coil L5 is made on a ring with a diameter of 5 mm from M20VCh ferrite and has 7 turns of 0,15 mm wire.

Zener diode VD1 - any for a voltage of 3,8 ... 4,2 V.

When installing the mixer, it must be remembered that field-effect transistors with a Schottky barrier are sensitive to static electricity. Therefore, when working with them, it is necessary to ground the soldering iron, and for the transistor, close all the conclusions to each other during soldering.

Note that the LNA stage with a resonant load can be excited due to the presence of parasitic collector-base capacitance VT1, so you should carefully monitor this when tuning. Self-excitation can be easily eliminated by detuning the Z1 filter circuits, for example, by putting a ferrite ring (M20VCh) on the output of the VT1 collector. The stage gain does not make sense to do more than 7 ... 10 dB.

If generation does not occur during operation of the local oscillator or it is unstable, the tube height should be reduced to 5 mm, and the diameter to 3 mm. In this case, the parameters of the elements of the local oscillator remain unchanged. More "fine" adjustment is made by changing the distance between the turns.

This device has been successfully applied in a burglar alarm receiver operating at a frequency of 407,625 MHz. With a transmitter power of only 10 mW, reliable communication was carried out at a distance of 600 ... 700 m (directional antenna).

It should be noted that the SAW filter may have a "multihump" frequency response and generation occurs on one of the "humps". As a result of external influences (changes in temperature, supply voltage, etc.), "jumps" of the generation frequency within the filter passband are not excluded.

Author: Yu.Vladykin, Moscow

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