Adjustment and configuration of electronic products. Purpose of adjustment and operating conditions of radio-electronic equipment and instruments

Adjustments in radio receivers .

In radio receiving devices, the required operating modes are established and maintained using adjustments individual elements circuits that provide both best conditions receiving a useful signal and converting it into information.

All types of adjustments can be divided into two main groups:

Adjustments that change the seme parameters, forming the frequency and phase characteristics of the receiver;

Adjustments that provide the required operating modes of the receiver elements.

The first group includes tuning to a given frequency or tuning to operating frequency within certain limits. Adjusting the selective properties of the receiver and its bandwidth, setting certain phase relationships.

The second group includes setting the specified electrical modes of active devices (transistors and lamps), setting the modes of individual components, adjusting the gain of the receiving path, and matching individual circuit elements. Depending on the intended purpose, the listed adjustments are divided into production, technological and operational. The first are carried out during the production process or during the repair process. These include adjusting the circuits with trimming capacitors or coil cores, adjusting filters, setting the required voltages on the electrodes, matching feeder lines, etc.

Operational adjustments can be either manual or automatic.

The main ones are:

Adjusting the receiver tuning frequency;

Selectivity adjustment;

Gain adjustment.

Frequency adjustment.

Frequency adjustment includes pre-tuning to the nominal frequency of the received signal and adjustment during operation.

The receiver can be tuned both by the reference generator and by the received useful signal. The number of tunable elements is determined by the receiver circuit and frequency range. Tuning to a given frequency can be either smooth within the operating range of the receiver, or fixed, ensuring the installation of a finite number of frequencies.

Tuning can be carried out either manually or using an electromechanical drive, with fixation of pre-set operating frequencies. In superheterodyne receivers of the centimeter and millimeter ranges, the preselector is in most cases wideband and the receiver is tuned by setting the local oscillator frequency. In a klystron local oscillator, this can be done by mechanically adjusting the resonator, or by changing the voltage on the reflector.

When using quartz local oscillator frequency stabilization in receivers, tuning is carried out either by changing quartz crystals or by using several quartz oscillators that provide a grid of stable frequencies in a given range.

In superheterodyne receivers with a tunable preselector, the tuning of the UHF and local oscillator circuits is coupled. Changing frequencies during tuning should ensure a constant intermediate frequency.

In most cases, circuit adjustment is carried out using variable capacitors, structurally combined into one unit. Depending on the type of receiver and its purpose, the capacitors can be air or film dielectric, discrete capacitors or varicaps.

Variable capacitors have a sufficient coefficient of coverage of the range of capacitances, high quality factor and linearity of capacitance change. The disadvantages are the rather large dimensions of the tuning unit, the complexity of the design with a large number of simultaneously tunable circuits, and the long tuning time.

When using a block of variable-capacitance capacitors, the parameters of the individual elements of the block are approximately the same; the overlap coefficients of the capacitance and, consequently, the frequency range will be approximately the same. However, these capacitors do not provide a constant frequency difference in the converters of superheterodyne receivers.

At intermediate frequency f etc=f G-f With the range overlap coefficients must be different.

With the same overlap coefficient, the difference between the tuning frequencies of the UHF and local oscillator circuits will be in range, since the UHF circuits will be detuned relative to the signal frequency. This will lead to a decrease in gain, which decreases the more the wider the amplifier bandwidth.

To eliminate this drawback, the circuit settings are paired. One of the pairing options is to introduce additional capacitors into the local oscillator circuit.

Inductance L G L is selected such that in the middle of the range both circuits have a difference in setting equal to f etc. Capacitors are selected as follows: C V» C min, and C A« C Max. In this case, on low frequencies operating range when C = C Max capacitor capacity C A does not matter, but the capacitance of the capacitor C V reducing the resulting capacitance of the oscillatory circuit increases its resonant frequency and, consequently, the local oscillator frequency, bringing the frequency difference closer to the value of the intermediate frequency.

A discrete capacitor is a store of constant-capacity capacitors with series-parallel connection of groups. The use of these capacitors reduces the tuning time, which is primarily determined by the speed of the control circuit and the switch itself. Displaced options are possible when discrete capacitors and discrete inductors are used simultaneously to rearrange oscillatory systems.

The main disadvantage of tuning using discrete capacitors is the limited number of settings and the complexity of the switching circuits.

In relatively low-power cascades, a varicap is used as a frequency-tuning element, which is practically inertia-free in changing capacitance and requires a low-power source of control voltage. The use of varicaps allows you to automate the setup process.

A significant disadvantage of a varicap is the significant nonlinearity of its characteristics, which improves the selective properties of the receiver. One option to reduce the influence of the nonlinearity of the characteristic is to increase the bias voltage applied to the diode. It is possible to include an additional linear capacitor in the capacitive part of the circuit, but this reduces the frequency range coverage coefficient.

The best result of compensating for the nonlinearity of the characteristic is obtained by the cross-current sequential inclusion of varicaps.

In this case, thanks to compensation of even current harmonics, the influence of nonlinearity of characteristics is reduced. In this case, it is necessary to ensure the symmetry of the shoulders by selecting varicaps according to the parameters.

Tuning by changing inductance is carried out using variometers or discrete inductors. In the first case, mechanical movement of the coil core inside its frame or closing of part of the turns using a current collector is used. In this case, the overlap coefficient is about 4÷5. However, it must be taken into account that simultaneously with a change in the inductance of the coil, its quality factor also changes, and the tuning mechanism itself is quite complex and cumbersome, which limits the number of simultaneously tunable circuits. The use of a discrete inductor allows for electronic tuning, which is similar to tuning with a discrete capacitor, but is even more cumbersome.

In professional microwave receivers, a non-tunable input and switched filters are used. With a non-tunable wideband preselector, the antenna, UHF and frequency converter are matched using wideband transformers, and tuning is achieved using local oscillator tuning.

In practice, the filter method of tuning a receiver is widely used, in which the entire range of operating frequencies is covered by a number of non-tunable filters, the bandwidth of which is selected with a margin for mutual overlap. The number of filters is determined by the selectivity requirement of the receiver and is limited by the complexity of the control circuit.

Thus, to receive signals in the frequency range, it is necessary to perform a number of operations, including switching the corresponding circuits, switching antennas, etc.

An important step in the operation of any receiving device is precise tuning to the operating frequency, which includes setting the required local oscillator frequencies (there may be several of them in professional receivers) and tuning the resonant preselector circuits to the signal frequency. When working using frequency synthesizers in the local oscillator, it is possible to tune relatively easily within a short period of time. However, it is more difficult to quickly adjust the preselector by switching on the desired sub-range and adjusting the resonant circuits. In this case, various switching circuits are used, the elements of which are required to have a high contact resistance for the switched current in the open state and a minimum in the closed state. They must also have a small throughput capacitance between the contacts at the operating frequency. In selective circuits, switching is carried out by mechanical or electrical elements.

Reed switches are sealed and magnetically controlled contacts made of a soft magnetic alloy. The capsule is filled with inert gas or evacuated. When the capsule is introduced into a magnetic field, the petals close, and when the field strength weakens, they open due to their own elasticity. The magnetic field is created by a special control coil.

Switching diodes with electronically controlled have high resistance at reverse bias voltage and have low differential resistance at forward bias current.

Adjusting the receiver bandwidth.

The selective properties of the receiver are usually ensured during its design, but in some cases such a need arises during operation. So, in receivers of connected radio links, this makes it possible to weaken the influence of interfering stations neighboring in frequency.

Adjustment can be carried out discretely or smoothly and, as a rule, manually. The adjustable elements can be selective systems of the linear part of the receiving path, mainly in the amplifier, as well as in low-frequency cascades.

To smoothly adjust the passband in the amplifier path, adjustable filters are used, which are a system of two tunable circuits connected to each other using a quartz resonator and are the load of one of the amplifier cascades. Thus, when changing the detuning of the circuits, you can adjust the passband, since when they are tuned to an intermediate frequency, the passband is maximum, and when detuned, it narrows. The limits of bandwidth adjustment are determined by the allowable gain losses.

In receivers that have concentrated selection filters in the IF path, selectivity is adjusted by switching filter elements while maintaining the rectangularity of the resonant characteristic within certain limits.

In the post-detector part of the receiver, the bandwidth is adjusted by changing the frequency response in the region of high and low frequencies (timbre control). Passive tone controls are included in the amplifier's input circuit. A regulator that reduces the gain in the high-frequency region is connected in parallel to the input circuit of the amplifier and is represented in the following form.

The values ​​of R p and C are chosen much larger than the similar input parameters of the amplifier. At R p =0, the decrease in frequency response is practically determined by the time constant τ = c R y. If R p ≠0 the decline will only be up to frequency f 1 , after which the resistance Χ c =1/ωc becomes significantly less than R p and does not affect the resulting resistance of the circuit with R p. The frequency response does not change until the frequency, after which it decreases due to the capacitance Cy. A passive tone control that increases the gain in the low-frequency region has the following form and works similarly to the R f C f circuit.

Gain adjustments in the RPU.

For a given amplification stage circuit, K 0 =p 1 p 2 SR e, where p 1 and p 2 are the corresponding switching coefficients, S is the slope of the collector characteristic of the transistor, R e is the equivalent load resistance, taking into account the shunting of the circuit by the transistor and the load. The gain can be adjusted by changing any value included in this expression. When choosing control methods, it is necessary to obtain a significant change in K 0 from the control voltage, a small control current, and a small dependence of other amplifier parameters when the gain changes.

Adjusting the gain by changing the slope of the characteristic.

This adjustment is carried out by changing the operating mode of the active element, so it can be considered modal. In this case, it is necessary to change the bias voltage on the control electrode, which will lead to a change in the slope at the operating point (in a bipolar transistor, in addition to S, q input and q output change). The regulating voltage can be supplied to both the base circuit and the emitter circuit.

In this circuit, the bias voltage at the E-B junction will be U eb =U 0 -E ρ. As E ρ U increases, the eb decreases, which will lead to a decrease in the collector current I k0 and S k, and as a consequence, a decrease in K 0. The gain control circuit must provide a current in this circuit approximately equal to I 0e, which means that I ρ must be relatively large. It is preferable to supply E ρ to the base circuit when U eb =U 0 -E ρ. The adjustment current I ρ =I g is I g ≈(5÷10)I 0b and is small.

This circuit provides less stability due to the absence of a resistor in the emitter circuit, because its presence will lead to a decrease in the adjustment effect. Otherwise, it is necessary to increase E ρ.

Adjustment by changing R e can be carried out in various ways.

By including a diode in the circuit.

When E ρ >U k the diode is closed and does not bypass the circuit. R e and K 0 are large.

At E ρ

Adjustment by changing switching factors.

The voltage from the circuit is supplied to the divider Z 1 Z 2. By changing one of the resistances you can change p 1. The adjustment circuit for p 2 is similar. Coils with variable inductance or capacitors with variable capacitance can be used as resistances. However, this cannot avoid contour detuning. The best results are obtained by using an attenuator with a variable gain connected between the stages. Adjustable dividers, capacitive dividers on varicaps, and bridge circuits are used as an attenuator.

When |E ρ |<|U 0 | диоды Д 1 и Д 2 открыты, а Д 3 закрыт. Коэффициент передачи максимален. По мере увеличения E ρ динамическое сопротивление диодов Д 1 и Д 2 увеличивается, а Д 3 – уменьшается, reducing the attenuator gain.

It is possible to use it as a controlled resistance field-effect transistor, when under the influence of E ρ the resistance of its channel changes.

Attenuators based on pin diodes, which have a large range of resistance changes and low capacitance, are widely used.

The operation of pin diodes is controlled by changing the bias in the transistor base circuit. At zero voltage, adjustments D 1 and D 2 are closed, and D 3 is open (attenuation is minimal). When E ρ is maximum, D 1 and D 2 are open, D 3 is closed (attenuation is maximum).

Adjustment K 0 using an adjustable OOS circuit.

The OOS is introduced into the emitter circuit of the transistor. The depth of feedback is adjusted by changing the capacitance of the varicap. As Ereg increases, the diode closes more strongly, while its capacitance decreases, and the feedback voltage increases, thereby decreasing K0.

In the post-detector part of the receiver, the methods for adjusting K 0 are similar to resonant amplifiers. Smooth potentiometric gain control is more often used, and in broadband amplifiers it is usually used in low-impedance circuits. In wideband stages, gain control is often used using adjustable feedback.

An adjustable voltage divider is used to change the constant voltage at the base.

Gain adjustment is carried out by changing the alternating current resistance in the emitter circuit, as a result of which the depth of feedback and the cascade gain change.

The voltage is supplied to the other stage through a controlled divider. Z 2 includes the input impedance of the subsequent stage.

Automatic gain control (AGC).

AGC is designed to maintain the output signal level of the receiving device or amplifier near a certain nominal value when the input signal level changes. The use of AGC is necessary because the input signal level can change quite quickly and chaotically, which cannot be responded to using manual adjustment.

There are many reasons for changes in the input signal level:

Changing the distance between the radiation source and the receiver;

Changes in radio wave propagation conditions;

Changing the receiver from one station to another;

Changing the mutual direction of the receiving and transmitting antennas; etc.

In radar receivers, to the listed reasons one can add fluctuations in the effective reflective surface of the target, changes in targets with different effective surfaces, and random changes in the polarization of received waves.

Ideally, the receiver output voltage should remain constant after reaching a certain output voltage value that ensures normal operation of the terminal device. In this case, the gain must change according to the law

K=U out min /U in at U in ≥ U in min

AGC circuits are built according to two principles: “backward” adjustment and “forward” adjustment. Otherwise, they are also called reverse and direct. Inverse AGC systems (systems with feedback) in them, the point at which the voltage that forms the regulating action is picked up is located further from the receiver input than the point at which the regulating action is applied.

In direct AGC systems, the point at which the AGC trigger voltage is picked up is located closer to the receiver input than the point at which the control voltage is applied.

Reverse AGC systems cannot ensure complete constancy of U out, since it is an input to the AGC system and must contain information for a corresponding change in the regulatory action. In addition, this system cannot simultaneously provide a large depth of adjustment at U out ≈const and high performance for reasons of stability. At the same time, this system protects from overload all cascades located further from the input than the point of application of the control action.

Direct AGC systems can, in principle, provide ideal control when U out ≈const with U in ≥ U in min and arbitrarily high speed. In reality, this is not feasible, since the degree of constancy of the output voltage is determined by the specific data of the elements of the AGC circuit and receiver circuits, subject to technological variations in parameters, time and mode changes. When using this AGC system, cascades located further than the point of application of the regulatory influence are protected from overloads.

The AGC system itself is exposed to a signal with a wide dynamic range, is subject to overload and must contain its own feedback. This system itself turns into a separate receiver channel with a rather complex circuit.

In practice, inverse AGC systems are more widely used, and it is possible to use combined AGC systems.

The block diagram of reverse AGC can be presented as follows

The control voltage is supplied to the amplifier from the output side. The AGC detector ensures that E ρ is proportional to the output voltage, i.e. E ρ =K d U out. The AGC filter filters out the components of the modulation frequencies. This scheme is called a simple AGC. Before or after the detector, an amplifier can be turned on in the AGC circuits, and then the AGC is considered amplified.

The block diagram of a direct simple AGC includes the same elements.

The functional diagram of the combined AGC includes the following elements.

The reverse AGC system is formed by the detector D ARU1, filter F 1 and all cascades of the main path located between the input point of the control voltage U ρ1 and the output of the high frequency unit (HFB).

The direct AGC circuit includes a detector D ARU2, a filter F 2 and a constant voltage amplifier U ARU2. The regulating voltage U ρ2 is introduced into the UHF and ULF, which may or may not be present. Filters Ф 1 and Ф 2 give the AGC circuits the necessary inertia, due to both the stability of AGC 1 and the lack of demodulation of amplitude-modulated signals in AGC 1 and AGC 2.

There is no need to reduce the gain of weak signals (Uin< U вх мин), не обеспечивающих номинального выходного напряжения при максимальном усилении всех каскадов. Для придания цепям АРУ пороговых свойств они запираются принудительным смещением и отпираются тогда, когда напряжение входного сигнала превысит напряжение запирания. Как правило напряжения запирания (задержки) подаются на детекторы или усилители (На схеме E 31 и E 32).

The delay can be entered based on the average value of the signal or the maximum. AGC circuit 1 does not have a special amplifier and is not an amplified system. AGC 2 system is strengthened, it has a greater depth of regulation and is capable of providing a smaller dynamic range of the output signal.

With a weak signal at the receiver input and maximum gain at its output, noise created by external interference and the receiver's own noise is heard. To eliminate this defect, silent AGC systems are used.