The signal from the microphone WM, amplified by the transistor VT1, through the resistor R4 is applied to the varicap VD1 which is used to modulate the oscillator, built on VT2. Modulation is performed by delaying the frequency quartz ZQ1 varactors, the capacitance of which varies in time with the input signal. The working point of the variable capacitor is determined by the resistor R2. Coil L1 compensates for the capacitance of the varicap in the mode of absence of modulation.
The output circuit of the generator L2C3 is configured to a first harmonic of the quartz 54 MHz.
The cascade of doubling the frequency, assembled on the transistor VT3, works under the scheme with a common base and inductively connected to the generator through the coil L3. Oscillating circuit L4C6 in the collector circuit of a transistor tuned to the frequency of 108 MHz. The build-up transistor VT. can be adjusted using the podstroechnik coils L2L3.
This cascade also works as the terminals of the amplifier, working With, and the harmonic oscillation circuit L4C6 controls the operation of the output circuit, which multiplies the frequency sweeps up to 432 MHz.
Multiplication of frequency in the last cascade effect with the help of varicap VD2 operating at current ( parallel connection ), which is mounted in a matching chain. This circuit provides an efficiency of about 55 percent and does not require a rigid keeping of the values of the elements. A series resonant circuit C8L5, tuned to a frequency of 108 MHz, provides effective swinging of the varicap and thus increases the efficiency of the scheme. The resistance of the shunt resistor R10 determines the operating point of the varicap, the current comes through, straightened detection. The resistance of 30... 200 kω, choose empirically.
Using LC circuits L6C9 circuit Zeller, tuned to a frequency of 324 MHz, is consistent with the output of the cascade, where there is a mixture of frequencies, resulting in the summation and subtraction of the higher harmonics. As a result, in addition to higher harmonics component 4 * f2 = 432 MHz is formed an additional component f2 + 3f2 = 108 + 324 = 432 MHz, which further increases the efficiency of the output circuit. Necessary higher harmonic of 432 MHz is filtered by a chain of L7C10C11 and fed into the antenna. Configuring the transmitter requires a lot of patience. All the circuits of the output circuit is a mutual influence on the matching and the resonant frequency of each other. To optimally configure the transmitter, all capacitors must be variables, it is possible to use an absorption wavemeter, indicator bulb (2.5 V, 0.07 A) with the coupling coil (2 turns ) and field strength meter.
Configure terminal cascade should reveal the absence of any discontinuity ( current consumption, the field strength ), which are a symptom of the presence of unwanted oscillations. Resonances in all points must be sustainable. If during the configuration of the identified undesired resonances, it is possible to eliminate them in several ways:
- screening cascades to reduce parasitic relationships;
the change in capacitance bypass capacitors;
- reduction of the working q of the oscillating circuit;
- the use of links rather than capacitive, inductive.
By optimal tuning of the output circuit receives the maximum power of the higher harmonics. The varicap should not be overloaded thermally and voltage. Load the varicap should be a maximum of 30% of capacity saturation.
As the varicap VD2 is desirable to use devices like SW901, SW102, SW104, SW107, SW110. Antenna - a piece of stranded wire with a length of 170 mm.
The coil L1 has 15 turns of wire sew 0.25 mm, wound on a mandrel with a diameter of 4 mm.
The coil L2 has 5 turns of the same wire wound on the frame 6 mm in diameter, on top of it wound coil L3 - 2 turns of wire of 0.25 mm. is inserted into the frame ferrite core.
Coils L4, L5 are 3,5 and 7 turns respectively are wound silver plated wire with a diameter of 0,361 mm on the mandrel with a diameter of 6 mm. 3
Coils L6, L7 are 3.5 and 2 turns respectively are wound silver plated wire with a diameter of 0.56 mm for mandrels with a diameter of 6 mm.