Upgraded VPA for YES-98M transceiver. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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It must be admitted that the initial implementation of the GPA was not entirely successful. Over time, the main disadvantages were revealed: poor repeatability and low frequency stability. The subsequent, more thorough study of this node, made it possible to completely eliminate the above disadvantages. Moreover, the new version of the GPA, shown in Fig. 1, can be recommended for use in almost any transceiver where problems arise, namely, with the implementation of frequency stability. The GPA works in conjunction with the DACH system - digital auto-tuning of frequency.

Fig.1 Scheme of the upgraded GPA transceiver "YES-98M"

A slightly modernized Kolpitz generator circuit is taken as a basis, which is distinguished by the possibility of implementing a higher quality factor of the oscillatory system than in the well-known high-frequency generator circuits. The active element of the GPA - the VT5 transistor is connected according to the emitter follower circuit, due to the high input resistance and the small capacitance of the capacitor C18, the shunting of the oscillatory circuit is insignificant. The generator, assembled according to the Kolpitz scheme, is known for its stable generation, and two branches of negative feedback: parallel (resistor R24) and serial (resistor R21) ensure the operation of the VT5 transistor in the mode of a constant (thermostable) current generator. The low capacitance of the emitter junction of the KT368A transistor (about 2 pF) and the low output impedance of the cascade create conditions for good decoupling of the oscillatory system as a whole from the subsequent load.

The collector capacitance VT5 (about 1,5 pF) is many times smaller than the capacitor C17, and does not affect the oscillatory system. The use of a low-noise transistor KT368A (with a normalized noise figure) and the above features contributes to the creation of a generator with good thermal stability and a low level of side (phase) noise.

After a series of experiments, for switching the ranges, a circuit was chosen for connecting loop capacitors using switching diodes KD409A and transistor switches. In this case, cheap and common KT315 transistors are used. By simultaneously turning on the diode and the transistor, a small differential resistance of the switching circuit is achieved (connection of the loop capacitor to the case). This maintains a high quality factor of the oscillatory system, which is directly related to the stability of the generated frequency. The capacitance and differential resistance of the switching circuit is not much larger than the same parameters of a conventional relay, but, undoubtedly, it is better in terms of thermal stability.

To ensure a good shutdown (with a closed transistor), as well as to obtain minimal transient capacitances of the switching circuit (for example, one VD1, VT1 circuit), a blocking voltage of +9 V is supplied through a high-resistance resistor R7. The required switching current through the diode is set by resistor R6. The use of sufficiently high-resistance resistors in the base circuit of the transistor (R11, R12, C8) creates the conditions for a good decoupling of the generator from the switching (range) voltage, which can be unstable (+ 9V).

The emitter follower on transistors VT6, VT7, which has a low output impedance, has a high load capacity and provides good decoupling from subsequent stages.

Elements D1.1 and D1.4 form a rectangular signal. Triggers D3 of the cascades are designed to divide the GPA frequency by 2 or 4. The encoder, assembled on diodes VD6 ... VD14 and microcircuit elements D1 and D2, when a range voltage is applied, selects the appropriate subrange. From the output of D1.3, the signal comes to the input of the push-pull cascade. The output signal level is set by resistor R36, and its symmetry by resistor R38. The step-up transformer Tr.1 provides an output voltage of 6...7 V at a load of 2 kOhm, which is sufficient for subsequent supply to the mixer of the "YES-98M" transceiver. By changing the transformer connection circuit to lower the voltage, the GPA can be used in conjunction with low-resistance mixers. The output stage provides a good shape and stable amplitude of the output signal on all ranges.

Frequency tuning (traditionally for the "YES-98M" transceiver is carried out with KVS111 varicaps and a twenty-turn 10 kΩ potentiometer, although the disadvantages of this tuning method are well known. The traditional tuning method with a variable capacitor is, of course, preferable, and its quality indicators are higher.

The generator itself operates in the frequency range from 15,82 to 25,2 MHz (for an intermediate frequency of 8820 kHz), which allows the use of a relatively small high-quality coil, as well as small capacitors. It should be noted that on the 10-meter band, the tuning interval is limited to 28,0 ... 29,0 MHz, so one more sub-band should be introduced for complete coverage.

Structural elements and details

The GPA is assembled on a single-sided printed circuit board 117x60 mm, 1,5 mm thick, and soldered into a box (35 mm high) made of tinplate with removable covers. The generator part is separated from the rest of the circuit by a partition. The inductor L is placed in the screen, which is used as a housing from the RES-6 relay.

The VT5 transistor is selected for maximum gain, at least 100. To select loop capacitors, capacitors with different TKEs are required: MPO, P33 and M47. The VD3 diode is composite - from two KD409A connected in parallel. Capacitors C6 and C13 must be of high quality with low TKE. It is desirable to power the GPA from a separate voltage stabilizer (KR142EN8A).

Setting

First of all, the manufacture and subsequent adjustment of the GPA is a very painstaking work that requires great accuracy and patience. It should begin with checking the modes for direct current. Then, starting from the low-frequency one, it is necessary to establish the boundaries of the tuning of each subband. Having applied a constant voltage of + 5V to the input of the DAC, you should check and, if necessary, set the required alternating voltages. Resistors R36 and R38 set the required amplitude and symmetry of the output voltage (signal).

The stability of the GPA frequency was initially tested on a mock-up, and then on a prototype installed directly in the transceiver. In the breadboard version (with the connected TsAPCh and loop capacitors with TKE M47), the frequency stability manifested itself as follows: after a 2-minute warm-up, the frequency overshoot was 500 Hz, and then during 8-hour operation, the frequency changed by ±5 Hz. The maximum inaccuracy of tuning to the correspondent is 40 Hz (depending on the applied CAFC scheme). In the working version of the GPA, in which the loop capacitors were made up of several with different TKE, there was practically no run-out after switching on, and during 8 hours of operation the output frequency remained practically unchanged (judging by the digital scale). When working on the air, frequency deviation is not observed. The spectrum analyzer did not check the output signal of the GPA.

Author: G.Bragin, RZ4HK, Chapaevsk; Publication: N. Bolshakov, rf.atnn.ru

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