Double balanced mixer SA612A. Reference data

Double balanced mixer SA612A

The active double balanced frequency mixer of the SA612A group (from Philips Semiconductors) is designed for use in radio receiving devices operating in a frequency band up to 500 MHz. In addition to the mixer itself, the microcircuit contains a built-in local oscillator and voltage stabilization circuits.

The basis of the mixer is a balanced (differential) amplifier, which provides an output signal that is proportional only to the difference between the signals at the inputs and does not depend on their absolute values, fluctuations in supply voltage, or changes in ambient temperature.

The device is housed in a plastic case of two design options: DIP8 (SA612AN) - for traditional installation (Fig. 1); S08 (SA612AD) - for surface (Fig. 2).

The block diagram of the SA612A balanced mixer is shown in Fig. 3. Pinout of the device: pins 1 and 2 - differential input of a balanced amplifier; pin 3 - common, negative power supply pin; pins 4 and 5 - differential mixer output; pins 6 and 7 - pins for connecting external local oscillator circuits: pin 8 - positive power pin.

As can be seen from the diagram, the device has two balanced inputs and outputs (hence the characteristic - double). This structure provides ample opportunities in constructing the input and output circuits of the mixer (see below). In particular, the use of a balanced mixer circuit allows you to get rid of conversion byproducts in the output signal.

Basic specifications at Tokr. av = 25 °C and supply voltage 6 V

Supply voltage, V......4.5...8
Current consumption, mA, maximum value......3
typical value......2.4
Maximum input signal frequency, MHz......500
Maximum frequency of the built-in local oscillator, MHz......200
Noise figure, dB (typical value), at an input signal frequency of 45 MHz......5
Conversion coefficient, dB, at an input signal frequency of 45 MHz, minimum value......14
typical value......17
Intersection point for third-order intermodulation IIРЗ*. dBm (typical), with input power -45 dBm.....-13
Input impedance of balanced inputs, kOhm (minimum value).......1.5
Output impedance, kOhm (typical value).......1.5
Input capacitance, pF......3
Operating range of ambient temperature, °C. -40...+85

* This is the name given to the conditional intersection point on the graph of the straight line characterizing the power of third-order intermodulation distortion with the continuation of the linear dynamic characteristics of the mixer. This parameter allows you to evaluate the dynamic range of the mixer using third-order intermodulation.

The indicated high-frequency parameters of the mixer were measured on a test bench, the diagram of which is shown in Fig. 4. It can actually be considered as a typical switching circuit.

Depending on the specific application of the chip, the input signal may be applied in different ways. In Fig. 5, a and b show resonant versions of the input circuit, and in Fig. 5,v - broadband (in this case, the unused pin must be “grounded” for alternating current with a capacitor with a capacity of 0.001...0.1 μF, depending on the operating frequency).

The mixer output signals (at pins 4 and 5) have opposite phases. The load can be switched on both between phases (Fig. 6,a) and single-phase (Fig. 6,b). The manufacturer allows the unused output to be left free; nevertheless, it is better to “ground” it too via alternating current through a capacitor.

As a frequency-setting element of the built-in local oscillator, you can use either an LC circuit (Fig. 7,a) or a quartz resonator (Fig. 7,6), operating at the fundamental frequency or harmonics. Paired with a harmonic resonator, it is necessary to use an additional LC circuit tuned to the frequency of the corresponding harmonic (L1C2C3, Fig. 7c). The ratings of external elements are determined from the same considerations as for a conventional local oscillator on a bipolar transistor. Pin 6 of the microcircuit is connected to the base of the internal transistor (VT1 in Fig. 7a).

The mixer can also operate with an external local oscillator (Fig. 7d). The input voltage amplitude at pin 6 of the mixer should be within 200...300 mV.

If necessary, the local oscillator signal can be supplied to an external amplifier stage through a coupling capacitor C5 (Fig. 7a) of small capacity. The oscillation amplitude of the local oscillator will be greater if pin 7 of the mixer is shunted with a resistor (R1) with a resistance of 1...10 kOhm.

In Fig. Figures 8 and 9 show the temperature dependences of the noise figure Ksh of the mixer at various values of the supply voltage and input power corresponding to the “intersection point for third-order intermodulation distortion” Pvx. from respectively, and in Fig. 10 - dependence of the same parameter Рвх. from the supply voltage.

Literature

Golovin O. V., Kubitsky A. A. Electronic amplifiers. - M.: Radio Isvyae, 1983, p. 87.
Polyakov V. T. About the real selectivity of HF receivers. - Radio, 1981, No. 3, p. 18-21; No. 4, p. 21,22.
Red E. T. Circuitry of radio receivers. - M.: Mir, 1989, p. 8.
SA612A. Double-balanced mixer and oscillator Data sheet. -

can be used in the high or intermediate frequency amplifier path of a radio receiver. The amplifier's transmission coefficient depends on the operating mode of the cascade on transistor VT1, which allows you to enter into the AGC with an adjustment depth of up to 40 dB.

The radio receiver (Fig. 39.9) can receive signals from amateur radio stations in the 14 MHz range (or 21 MHz when replacing circuits). consists of an input preamplifier based on transistor VT1 and two mixers with tunable (DA1) and quartz (DA2) local oscillators. The output signal with a frequency of 465 kHz is then supplied to AM/ and (not shown in the diagram).

The inductors of the radio receiver are made on frames with a diameter of 6-7 mm with tuning cores made of ferrite and contain: L2, L4-L9 - 18 turns of wire with a diameter of 0.3-0.4 mm turn to turn; LI, L3, L10 - 6 turns of the same wire, wound on top of the corresponding coils; L11 - 80 turns of wire with a diameter of 0.15 mm in bulk. The coils are made without screens. When using screens, the number of turns should be increased by 30-40%.

Rice. 39*17. Typical inclusion of the SA612A microcircuit

Rice. 39.18. Options for implementing the input circuits of a balanced mixer on the SA612A chip

Rice. 39.19. Options for the output circuits of the balanced mixer on the SA612A microcircuit,

Rice. 39.20. Options for the implementation of local oscillator circuits of a balanced mixer on the SA612A microcircuit

A typical microcircuit connection is shown in Fig. 39.17. Options for connecting input, output circuits and local oscillator circuits are shown in Fig. 39.18-39.20. Parameters of inductors, Fig. 39.17: L1 - 0.2-0.283 µH;

Rice. 39.21. on the ΝΕ612 chip

L2 - 0.5-1.3 µH; L3 - 5.5 µH,

L4 - 1.5-44 µH.

Using the ΝΕ612 microcircuit, a simple one can be made, Fig. 39.21. Interconnected oscillating circuits L1C5, L2C6 must be tuned to the frequency of the second harmonic of the input signal.

For CB radio stations operating on a frequency grid, digital synthesizers are usually used. Considering that when receiving signals, auto-tuning to the channel frequency is used, you can assemble a simple analog synthesizer of frequencies that can be smoothly adjusted across the range.

Rice. 39.22. frequency synthesizer based on SA612A chip

The frequency-modulated “analog” synthesizer shown in Fig. 39.22, is advantageously distinguished by the increased stability of the frequency of the generated signal, which is due to the use of a high-frequency quartz resonator at a frequency of 24 MHz. Smooth tuning is carried out in the frequency range 27.0-27.3 MHz. with electronic tuning it operates in the frequency range 3.0-3.3 MHz.

L1 contains 20 turns; L2 - 9; L3 - 2; L4 - 8; L5 - 3 (rebound); L6 35 turns of PEV-1 wire 0.23 mm, winding turn to turn. Coils L2 and L3, as well as L4 and L5, are located on common frames.

Rice. 39.23. Fragment of the receiving path on the SA612A chip

The radio receiving path (up to the circuits) on the SA612A chip is made with quartz

frequency stabilization, Fig. 39.23. The intermediate frequency signal is isolated by a piezoceramic filter at 10.7 MHz. Input circuit L1C2 is set to frequency 27.14 MHz.

Shustov M. A., Circuitry. 500 devices on analog chips. - St. Petersburg: Science and Technology, 2013. -352 p.

Any radio receiving device contains signal converters from HF to IF and IF to LF (there may be several intermediate frequencies). In the PPP there is only one such converter, from HF directly to LF. They are called mixers and are located immediately after the antenna and the DPF, or further - after the UHF, IF, thus “connecting” the main components of the receiver with the GPA, OG. Therefore, the parameters of the entire receiver largely depend on the efficiency and quality of signal conversion. There are two main types of mixers - passive and active. The former have a transmission coefficient of less than 1, and the latter provide a signal amplification greater than unity, however, to maintain the dynamic range, the amplification is not made large, usually no more than 10 times the voltage.

Any mixer, especially the very first one, in addition to the transmission coefficient, must also have a low noise level (to increase sensitivity). An equally important indicator is the ability to suppress powerful out-of-band signals, which can result in direct detection and “clogging” of the main signal.

Active type mixers will not be considered in this article, because This is a separate independent topic. The article is devoted to passive mixers made on passive elements - semiconductor diodes, as they are most widely used in various amateur radio designs. Passive mixer circuits based on field-effect, including high-power, transistors operating in key modes, as well as mixer circuits on electronic switches, have also become widespread. various types multiplexers/demultiplexers). However, this is also a topic for a separate article.

First of all, balanced mixers different types, represent symmetrical circuits, in which two signals (input RF and local oscillator) are mixed. Double balanced mixers are widely used in radio receiver circuits. They are balanced not only with respect to local oscillator oscillations, but also with respect to the input signal. This type of mixer attenuates both the local oscillator and input signals at the output. Naturally, the output also produces a lower level of conversion by-products compared to conventional balanced mixers.

At HF frequencies amateur radio bands(up to 30 MHz), ordinary high-frequency silicon diodes, for example, types KD503, KD509, KD514, KD521, KD522 and germanium type GD508, also have fairly good conversion properties.

In double balanced mixers, it is advisable to use Schottky diodes (for example, type KD922). A fairly common mistake is to consider KD514 silicon diodes to be Schottky diodes. These are not Schottky diodes, but according to some characteristics they are quite close to them. Sometimes this error occurs in old reference literature, because... According to the technology, a diode with a METAL-SEMICONDUCTOR contact was previously called a diode with a Schottky structure (according to the author of this technology). Its production technology is a cross between a conventional diode with a pn junction and a diode with a Schottky barrier. According to physics (not technology!), the forward voltage of silicon Schottky diodes is noticeably lower than that of conventional silicon diodes (using any other technology). In addition, there is a large ratio of reverse to forward resistance and insignificant capacitance at zero bias. Schottky diodes have a very short switching time, which expands the frequency range of their application (up to several hundred GHz).

The use of silicon, pulsed, epitaxial-planar, high-speed, short-recovery diodes KD514 (that’s what it’s correct to call them!) in high-speed switches, which include ring diode mixers, increases sensitivity by reducing the noise figure and, thus, can increase the gain of the IF path (and ultimately the sensitivity). Sometimes in practice, installing KD514 has a noticeable, audible effect, without selecting diodes, which cannot be said about KD503 and other types of diodes.

The amount of loss in a diode mixer is usually 6-10 dB. This is not much, but most designers want to have less losses. This suggests the need to use an active mixer in the receiver circuit. But the dynamic range (DR) of a receiver with a passive mixer is often greater than that of a receiver with an active mixer. In addition, DD is needed when the radio receiver is intended to work with powerful neighboring radio stations, or in the conditions of amateur radio contests, when in the general air dump, weak stations are adjacent to powerful neighbors. IN normal conditions this almost never happens. Thus, the magnitude of the receiver's dynamic range should not particularly concern us.

If the mixer is the first stage of the receiver, and this happens quite often, then all the main characteristics of the receiver practically depend on the quality of the mixer. The level of the mixer's own noise is important. The smaller it is, the higher the achievable sensitivity of the receiver becomes. From the above, it becomes clear that among diodes, preference should be given to those with the smallest direct internal p-n resistance transition. The smaller it is, the less noise is generated in the diode at the same current through the diode. It should be borne in mind that the stage following the mixer must also have a low noise figure. This is very important to realize the benefits of a passive mixer.

Figure 1 shows the circuits of a simple balanced mixer and a ring (double balanced) mixer made using diodes.

These mixers use balun transformers T1 and T2, wound on ring ferrite cores with a twist of three wires.

To achieve maximum sensitivity when setting up the mixer, you need to select the local oscillator voltage. Insufficient voltage reduces the transmission coefficient and increases the input resistance, and excessive voltage increases the noise of the mixer itself. In both cases, sensitivity decreases. The optimal voltage ranges from fractions of a volt to 1-1.5 V (amplitude value) and depends on the type of diode.

In mixers with back-to-back diodes (VPD), voltage is supplied simultaneously through the coupling coil - the signal from the input circuit and the local oscillator voltage (Fig. 2).

The local oscillator voltage is significantly greater than the signal voltage. For normal operation of such a mixer on silicon diodes, the local oscillator voltage should be 0.6-0.7 V (amplitude value). One of the diodes opens at the peaks of the positive half-waves of the local oscillator signal, and the other - at the peaks of the negative ones. As a result, the resistance of parallel-connected diodes decreases twice during the heterodyne voltage period. Hence such advantages of this mixer as the absence direct current(the mixer does not detect either the signal or the local oscillator voltage). And the local oscillator frequency is chosen to be half the signal frequency, which improves frequency stability and significantly reduces local oscillator interference to the input circuits of the mixer, because the emission of its signal is 30-60 dB lower (half the signal frequency) than with conventional mixers.

In a VPD mixer, it is best to use silicon diodes with a threshold voltage of about 0.5 V - they provide slightly greater noise immunity than germanium diodes. In any case, it is necessary to select the optimal local oscillator voltage for the maximum transmission coefficient. In general, all types of diode mixers require careful selection of the GPA voltage to obtain the best mixer parameters.

For more information about the operation of mixers, we also recommend that you refer to the works of V.T. Polyakov, G. Tyapichev, links to which are indicated at the end of the article.

Summarizing the above, it should be noted that in the above circuits of diode mixers it is required (except the right choice type of diode) both the symmetry (identical characteristics) of the diodes themselves, or their arms (in ring circuits), and the symmetry of the design. Thus, for the normal operation of diodes in mixer circuits, we can talk about the need for their correct selection and installation on circuit board(the design of installation of faucets on diodes will be discussed at the end of the article).

Without selecting diodes, it is difficult to ensure the required symmetry of the bridge, especially in those circuits where no balancing elements are provided, as in the circuits in Fig. 1 and 2. The required symmetry of the heterodyne voltage is achieved by the fact that the coupling coil (or broadband transformers) is wound simultaneously by two others twisted wires and placed on a ferrite ring strictly symmetrically. Failure to comply with this simple rule leads to the fact that some radio amateurs install modern types diodes are not selected during the initial debugging of the mixer design, considering that the asymmetry of the remaining home-made elements reduces the gain from their selection to zero. Naturally, the reasons for the asymmetry may be associated not only with the transformers themselves, so it is not recommended to rush to redo them.

When choosing diodes for a mixer based on reference materials, it should be noted that their capacitances should be the same (and as small as possible) at the same voltage. It is advisable to select a minimum switching (recovery) time. V.T.Polyakov, RA3AAE in his works indicates that preference should be given to diodes with a lower capacitance (no more than 1...3 pF) and the shortest reverse resistance recovery time (no more than 10...30 ns). This data can be found in reference books. When working on VHF, the requirements increase even more.

In many cases, the optimal choice may be to use ready-made diode microassemblies with selected characteristics. For example, the often recommended KDS523A, B, or diodes selected for the assembly (KDS523VR). However, in a number of cases, it is necessary to check these assemblies at least in the simplest way, since the permissible spread in them can reach 10% and this can negatively affect the operation of the mixers and will require adding balancing resistors and/or capacitors to the mixer circuit, which in generally useless, since it increases losses in the mixer. And this is always undesirable.

The selection of diodes based on direct resistance, which has recently become widespread, seems not so relevant, since an imperfect transformer (as mentioned above) will still introduce an imbalance in the arms of the bridge. Of course, if you are confident in the complete symmetry of the windings and their equality of total (complex) resistances, then using a conventional digital multimeter (in the “testing” mode) you can reject diodes with large deviations in direct resistances. There is a second reason, even more significant. The point is that the equality of direct resistances only means that with the same amplitude of the local oscillator, the same current will flow through the diode. But this is important for high voltages from the GPA, but for input signals, the amplitude of which is much smaller and lies at the microvolt level, the most important thing is the same I-V characteristics of the diodes precisely in the region of low voltages, i.e. at the very beginning of the current-voltage characteristic, and not in the region of high voltages.

Unfortunately, domestic diodes, even from the same batch, not to mention just the same type, have a very large spread of parameters, so simple selection by resistance (forward voltage) at one point of the current-voltage characteristic is ineffective. An explanation of why such a selection is not effective is given in the figure below. In fact, the spread of the I-V characteristics of diodes can be quite large, but by chance, at the measurement point, the internal resistance of the diodes will be the same with a fairly high accuracy. In fact, this is possible quite often. However, this is only the appearance of the identity of the current-voltage characteristics of the diodes. Selection using 2 points is more accurate. But such a selection is also only a check of the coincidence of static characteristics, and not dynamic ones.

Therefore, it is often recommended to use imported ones - the same 1N4148 (analogous to KD522). They have a significantly smaller spread, which guarantees Good work mixer even without selection. Although make a selection at one point of the current-voltage characteristic digital multimeter(in dialing mode) is very simple. It should be noted that in this circuit for selection (and in others too!) diodes must be connected using alligator clips or the like, but in no case by soldering. Even after connecting with clamps, you need to wait a while - heating the diodes by hand changes the measurement results (not to mention soldering). And they need to come to room temperature...

You can select diodes based on “direct voltage” by assembling the simplest scheme: from a stable source with a voltage of at least 10 V through a resistor, a forward current through a diode is set (for example, 1 mA). And they measure the voltage drop with any voltmeter with a high input impedance (tube, type VK7-9, or any digital, which is better). Select diodes that have the closest measured voltage values. You can check two points, for example, by setting currents of 1 mA and 0.1 mA.

A common technique is recommended for selecting diodes for a ring balanced mixer and is described B. Stepanov, RU3AX. It is used to compare the current-voltage characteristics of diodes in the forward direction. Since a semiconductor diode is a nonlinear element, direct measurement of its direct resistance with an ohmmeter does not allow such a comparison. This must be done at several (at least two) points current-voltage characteristics diode, measuring the voltage drop across the diode at fixed forward current values. A diagram of the simplest device that allows you to select diodes is shown in the figure.

For the selection of diodes, the exact values of the stabilized current are not important - all diodes will be compared at the same current values. It is only necessary that these values differ by about ten times... Details of the assembly and operation of this device are given .

There are also more serious approaches to selecting diodes for mixers. Experienced radio amateurs are sometimes skeptical about the methods outlined above and do not recommend selecting diodes for a forward current mixer, believing that such selection gives little benefit, especially for a highly dynamic mixer.

For example, developing the idea of measuring the voltage drop using stabilized currents (essentially comparing the current-voltage characteristics), it is proposed to supply an AC voltage of 12...24 V through a resistor that determines the current to anti-parallel diodes. Next, after the RC filter, the voltage is measured with a multimeter. Pairs are selected according to the minimum voltage spread at different currents (the lower the voltage and the smaller the spread, the better the pairs, the more complementary).

Evaluating this method, the conclusion suggests itself that the frequency AC voltage must correspond to the operating frequency, i.e., HF.

This selection scheme and methodology was tested V. Lifarem, RW3DKB, when developing its direct conversion transceiver and showed very good results. The functional diagram for selecting diodes is shown in Fig. 6.

A pair of diodes connected in back-to-back parallel mode is connected to the output of the GSS (from 0 to 1 V at a frequency of several MHz) through a resistor. The second end is connected to ground through a 30-50 µA microammeter with a MIDDLE POINT. Gradually increasing the voltage at the generator output to the maximum, observe the deviation of the indicator needle from zero.

Thus, when selecting a pair of diodes, the difference current is determined on a pointer device with a zero in the middle. Of course, it is ideal that the needle deviation is neither “plus nor minus”. A deviation of 1 µA is considered acceptable, although, with a certain persistence, it is possible to find perfectly matching pairs, fours and even eights.

Naturally, in this way “they kill at least two birds with one stone.” Here we observe a REAL coincidence of the parameters of the diodes at the OPERATING frequency and at operating voltages. At the same time, the equality of the throughput capacitances of the diodes is taken into account. This is the only way to select diodes for highly dynamic mixers.

And, secondly, with such a selection there can be no talk of any leakage of signals or direct detection, because a bridge made of perfectly matched diodes is perfectly symmetrical in ALL its parameters.

The author warns that the selection procedure is lengthy. In addition, diodes selected only by direct resistance (continuity) gave simply a poor result in the actual design of the TPP, which cannot be compared with the selection method described above and recommended, especially at HF. In the absence of a GSS, the role of the signal source can be performed by a GFO manufactured by a radio amateur for use in the same design. It should include an output signal level regulator, the role of which can easily be played by a low-impedance potentiometer.

Until now, we have talked about the selection of diodes for operation in mixers from the point of view of symmetry, determined by the uniformity (similarity, equality) of their parameters. But even one diode (like any other active and passive elements used in a receiver or transceiver circuit) can actively make noise.

The issue of noise in circuit elements has always been very relevant and all hardware developers, both professionals and amateurs, have to solve it. It’s easier for professionals, because... they are armed with special measuring equipment. Radio amateurs have to get rid of each in their own way. But every normal amateur designer has the opportunity to use simple low-frequency voltmeters for such purposes, which can be used to measure the noise level on the speaker (a kind of output meters). In theory, you need an RMS voltmeter, but in principle any will do. This, of course, is not an accurate device, but since your own ears are used in parallel, “working” on the same “more-less” scale, the noise is determined quite well.

The methodology used is, I hope, quite clear from the article. , only instead of the entire radio receiver, a part of it is used in the measurement - a sensitive low-noise ultrasonic sounder. V.T. Polyakov once wrote about this, proposing to evaluate the noise of the diode by connecting it through a separating capacitor with a capacity of several microfarads to the input of a sensitive ultrasonic frequency unit, which can be used as a low-frequency amplifier already assembled for the PPP. The diode was supplied with forward and reverse bias. A good diode should not noticeably increase the noise at the output of the ultrasonic amplifier at forward currents of up to several milliamps and reverse bias of up to several volts. According to the data from all the listed parameters, diodes of the KD514 type turned out to be the best. Several other types of diodes were compared in a heterodyne receiver with a balanced mixer at 20 MHz. The following values of the noise figure of the entire receiver (without RF frequency control) were obtained: KD503A - 32, D311 - 37, GD507A - 50, D9 - 200, D18 - 265. The last of the listed diodes should clearly not be used.

V.N. Lifar, RW3DKB, I connected a diode to the input of my ultrasonic sounder (the amplifier circuit using modern discrete elements can be taken from the article

) cathode to ground. A forward bias was applied to the anode through a 10 kOhm potentiometer, and the change in noise level with and without the bias was compared at the output. The offset could be changed using a potentiometer. Of course, there was also an oscilloscope at the output of the ultrasonic sounder to see what was happening with the noise track. The difference is visible. Since the noise is low-frequency, you can use a PC sound card by installing the appropriate program on the PC, taking it from the Internet.

By changing the amount of current flowing through the diode, the minimum noise of the diode is determined. It should be borne in mind that at very low currents the diodes make even more noise, because their internal resistance is also very high. And this is undesirable, because the noise voltage formula includes the resistance value.

As the current increases, the diode noise level first drops, then passes through an optimum trough, and then begins to rise again (with an increase in the forward current through the diode). That is why for diode mixers it is so important to correctly set the excitation amplitude so that the maximum current through the diode falls into this valley in order to ensure the minimum intrinsic noise of the diode mixer. In this case, it turns out to be a minimum-minimorum for a given type of diode and it can no longer be made smaller. Unless by replacing it with less noisy diodes of a different type.

The location of the diodes on the board must be strictly symmetrical relative to the surrounding elements and screens. This design provides the required balancing on the local oscillator side without installing additional elements. In general, to printed circuit board The mixer needs to be approached in the most serious manner. Installation should be carried out AS SYMMETRICALLY as possible, even at the expense of dimensions. You should not get carried away with microminiaturization of mixer circuits, because... At the same time, the parasitic capacitances of the installation increase noticeably. For example, in the TPP version V. Lifar, RW3DKB, the mixer diodes, connected back-to-back, were installed “stacked” one above the other horizontally, i.e. lay on the board, rather than standing next to each other, and their leads were inserted into ONE hole on the board. Naturally, the hole in the board was slightly larger than the thickness of one diode lead. Although it is probably acceptable to place them separately. However, unaccounted for mounting resistances and capacitances may appear, so the risk is not justified.

Due to their simplicity, high sensitivity and selectivity, and good reliability, direct conversion receivers and transceivers are popular among radio amateurs. But it is not always the case that a device, even one made according to a well-developed design, realizes the capabilities and parameters initially inherent in it.

As a result of many years of operation by the author of this article of this group of communication equipment, it turned out that low-frequency units (mainly low-frequency amplifiers) remain operational when the supply voltage is reduced to 2...6 V (at a nominal 9...12 V). At the same time, their gain, as a rule, decreases.

The main reason for the unsatisfactory performance of direct conversion receivers and transceivers is the suboptimal operating mode of the mixer. High parameters are achieved only with careful selection of the heterodyne high-frequency voltage on the mixer diodes. It should be within 0.6...0.75 V on silicon diodes and 0.15...0.25 on germanium diodes. At lower local oscillator voltages, the mixer transmission coefficient decreases. It also decreases at high voltages, since the diodes are open almost all the time. At the same time, the noise of the mixer increases.

Stability of the frequency and amplitude of the voltage supplied to the mixer from the local oscillator (especially at HF amateur bands), largely depends on the stability of the supply voltage.

In almost all circuits given in the literature, there is no circuit for regulating the heterodyne voltage on the mixer diodes. It is recommended to select a coupling capacitor between the local oscillator and the mixer or change the number of turns of the coupling coil. But this process is very labor-intensive and, moreover, does not give confidence that the device has been configured properly.

The disadvantage of this method is that during the setup process it is necessary to turn off the receiver (transceiver) and resolder the capacitor or rewind the coil. But during this time, the amateur station, whose reception volume is being adjusted, often stops working, and therefore it is impossible to know whether the sensitivity of the device being adjusted is increasing or decreasing. It is more expedient to carry out tuning using signals from a “weak” station during a stable passage of radio waves, i.e. when there are no noticeable fluctuations in the level of the received signal.

Due to lack of necessary measuring instruments Direct conversion receivers and transceivers are often tuned “by ear,” which does not have the best effect on their parameters.

Puc.1

In Fig. Figure 1 shows a diagram of a voltmeter-probe, modified in accordance with the recommendations given in. It allows you to fairly accurately measure the local oscillator voltage directly across the mixer diodes.

Let's consider simple ways adjustments and modifications of direct conversion receivers and transceivers, which eliminate the above design flaws.

Puc.2

First of all, during modification, a circuit for stabilizing the local oscillator supply voltage should be introduced. The stabilizer circuit is shown in Fig. 2. Zener diode VD1 is chosen with a stabilization voltage 1.5...2 times less rated voltage power supply for the receiver (transceiver). Resistor R 1 sets the optimal current through the zener diode. The resistance of resistor R1 must be such that the stabilization current of the zener diode VD1 does not exceed the maximum permissible value. Capacitor C1 reduces the “leakage” of zener diode noise, resulting in a reduction in noise modulation of the local oscillator voltage and a decrease in the overall noise of the receiver.

It is convenient to change the RF voltage on the mixer diodes with a tuning non-induction resistor connected in parallel or in series with the coupling coil (R1, respectively, in Fig. 3 and 4).

In the latter case, you can use both a transformer (Fig. 4,a) connection of the local oscillator with the mixer, and an autotransformer (Fig. 4,6). For more precise adjustment of the local oscillator voltage (for example, when receiving signals from low-audibility stations “by ear”), the RF voltmeter is turned off.

It should be noted that if the above modifications are applied, the number of turns of the communication coils should be increased slightly, since the introduction of a trimming resistor reduces output voltage local oscillator This especially applies to the option, the diagram of which is shown in Fig. 3. Taken together, the number of turns of the coupling coil, the resistance of resistor R1 and the capacitance of capacitor C2 must be such that the voltage on the silicon diodes of the mixer can be adjusted in the range from 0 to 1.2...2 V, on germanium diodes - from 0 to 0.5 ... 1 V. In this case, the optimal voltage is achieved approximately at the middle position of the resistor R1 slider.

You can regulate the output voltage of the local oscillator by changing the supply voltage, as was done, for example, in [3]. However, this is only suitable at frequencies up to 3...4 MHz. At higher frequencies (above 7 MHz), such adjustment can lead to a significant shift in the local oscillator frequency.

In Fig. Figure 5 shows a diagram of a local oscillator with a buffer node, into which an output voltage regulation circuit is introduced. When repeating, it should be taken into account that the emitter follower does not provide voltage gain, and therefore the high-frequency voltage on the coupling coil must be twice as high. than is required for normal operation of the mixer.

In amateur radio practice, diode balanced mixers are most widely used. Their main advantages are simplicity of design and configuration, lack of high-frequency switching when switching from reception to transmission. Balanced mixers for field and bipolar transistors are used much less frequently.

In simple balanced diode mixers, the local oscillator voltage and some output conversion by-products can be suppressed by 35 dB or more. But such results are achieved only in one direction: the one in which the mixer is balanced. In the original design of the transceiver, the mixer is balanced only towards the power amplifier. If a double balanced mixer is used, noise will decrease, sensitivity will increase, and noise immunity will improve.

Double balanced mixers are balanced on both inputs (outputs). They suppress not only local oscillator oscillations, but also the converted signal, leaving only the products of their mixing and thereby ensuring the purity of the spectrum. The use of such mixers makes it possible to reduce the requirements for the cleaning filter included at the mixer output, and even to abandon it altogether by connecting the mixer output directly to the IF amplifier, at the output of which there should be a main selection filter (for example, EMF or quartz filter). A significantly higher signal level can be supplied to the double mixer during reception, since it sharply weakens the effect of direct detection of a signal or interference, i.e. detection does not occur without the participation of local oscillatory oscillations, as happens in a conventional amplitude detector.

Most often in amateur radio designs a double balanced mixer is used, the diagram of which is shown in Fig. 6. It is also called ring-shaped, since the diodes in it are connected to the ring.

When operating in low-frequency ranges, high-frequency transformers are wound, as a rule, on ferrite rings of standard size K7x4x2 with a magnetic permeability of 600...1000 with three PELSHO 0.2 wires twisted together (3-4 twists per 1 cm of length). Approximately make about 25 turns (until the ring is completely filled). When installing a transformer, its windings are phased according to Fig. 6 and 7.

There are two main options for incorporating a dual balanced mixer into a transceiver. In the first, the signal passes both during reception and transmission in one direction from the input to the output of the mixers. This is, for example, done in the well-known Radio-76 and Radio-76M2 transceivers. Numerous experiments conducted by the author have revealed that with a heterodyne voltage less than the optimal one, sensitivity in the receiving mode deteriorates significantly, and with a higher voltage, the carrier suppression in the transmitting mode is significantly reduced (sensitivity also drops, but this is less noticeable to the ear than in previous case). The qualitative dependence of the main parameters of transceivers on the voltage level of the local oscillator supplied to the mixer is shown in Fig. 8 (curve 1 - sensitivity during reception, determined by ear, 2 - sensitivity, measured by instruments, 3 - carrier suppression during transmission).

In the second option, the signal in the receiving mode is fed to the input of the balanced mixer, and when transmitting, it is fed to the output. With this connection, the principle of reversibility of the mixer is used. This is how the RF path of the transceiver described in . Setting up a mixer in this case also comes down to setting the optimal heterodyne voltage and carefully balancing it. It should be especially noted that the setup operation does not depend on the principle of constructing the RF path of the transceiver.

First of all, you need to set up the mixers. The balancing resistor sliders in them are first set to the middle position. Next, connect the GSS to the antenna socket of the transceiver and gradually increase the heterodyne voltage at the mixers. The signal from the GSS is supplied at a level that exceeds the sensitivity of the receiving path by several times. It is necessary to achieve signal reception. There is no generator, the operation is performed by ear, receiving the signal from an amateur radio SSB radio station or a noise generator using a low-power zener diode.

Then each of the mixers is adjusted in turn. First, the optimal heterodyne voltage is selected. To do this, it is gradually increased and assessed by ear: whether the volume of reception of the GPS signal, radio station or noise generator is increasing. As the author noted, as the heterodyne voltage supplied to the mixer increases, the listening volume first increases, reaching a maximum, and then remains virtually unchanged (Fig. 8, curve 1). The heterodyne voltage should be set so that when it decreases slightly, the reception volume drops, and when it increases slightly, it does not increase. In practice, this is realized by moving within small limits the resistor slider that controls the output voltage level of the local oscillator. If the transceiver does not have this capability, then the device should be modified.

As a rule, an emitter follower is connected at the output of one or another local oscillator. In this case, the modification turns out to be very simple: the constant resistor in the emitter circuit of the transistor is replaced with a non-inductive trimming resistor of the same value as the constant one.

After optimizing the heterodyne voltage, you need to balance the mixers more carefully again. An RF millivoltmeter or oscilloscope is connected to the input or output (depending on the design of the transceiver) and, by moving the slider of resistor R1, and then adjusting capacitors C1 and C2 (see Fig. 7), we achieve a minimum reading. If devices with high input resistance are used, then resistors of similar resistance (within 50...100 Ohms) should be connected to the input and output of the mixer.

Preference should be given to balancing towards the output of the transmitting path. The difference in the balance between the input and output of the mixer should be small (a few decibels). If it reaches 10 dB or more, then this is, as a rule, a consequence of the fact that the heterodyne voltage supplied to the mixer is significantly higher than optimal.

To check and balance mixers, the author created simple devices. In Fig. 9, a shows a diagram of an RF amplifier, a mixer is connected to the input, and a mixer is connected to the output coarse settings high-frequency voltmeter (Fig. 9, b), for precision - HF probe (Fig. 9, c). In this case, there is no need to install additional resistors with a resistance of 50...100 Ohms in the mixer.

The mixers are finally adjusted after they are installed in the transceiver (it is switched to transmit mode). The device must first be set up in receive mode. To prevent microphone noise from interfering with balancing, the input of the microphone amplifier is short-circuited. The lowest-frequency mixer is balanced first, and then the others in the order in which the signal passes through them in transmission mode, achieving a minimum of RF readings at the load equivalent (Fig. 10) connected to the transceiver’s power amplifier. After this, the settings of the remaining nodes are adjusted. It is advisable to repeat this procedure two or three times.

Vladislav Artemenko (UT5UDJ) Kiev. Ukraine

LITERATURE

1. Polyakov V.T. Radio amateurs about direct conversion technology. - M.: Patriot, 1990, p. 264.
2. Stepanov B. Measurement of small RF voltages. - Radio, 1980, N 7, p. 55-56.
3. Artemenko V. Simple SSB mini-transceiver for 160 m. - Radio Amateur, 1994, N 1.c. 45, 46.
4. Artemenko V.A. A simple transceiver with EMF. - RadioAmator, 1995, N 2, p. 7-10.
5. Bunin S.G., Yaylenko L.P. A shortwave amateur's guide. - K.: Technology, 1984, p. 264.
6. Stepanov B., Shulgin G. Transceiver "Radio-76". - Radio, 1976, N 6, p. 17-19, N 7, p. 19-22.
7. Stepanov B., Shulgin G. Transceiver "Radio-76M2". - Radio, 1983, N 11, p. 21-23, N 12, p. 16-18.
8. Vasiliev V. Reversible path in a transceiver. - Radio, N 10, pp. 20,21.

A ring diode mixer, compared to a transistor one, has the advantage that it suppresses many conversion by-products and almost completely eliminates the direct passage of the signal in the IF amplifier and local oscillator circuit.

Schematic diagram

The signal to the input of the ring mixer (V2-V5) is supplied through an aperiodic cascade on transistor V1. The local oscillator voltage is supplied to the mixer through a symmetrical coupling coil L1 with an IF filter L2C4 tuned to a frequency of 465 kHz. The linearity of the converter at the signal input is maintained up to an amplitude equal to approximately 0.1 amplitude of the local oscillator voltage.

The optimal local oscillator voltage (taking into account losses on resistors R3 - R5) is 150...400 mV, the permissible signal voltage is 10...30 mV. This places restrictions on the gain of the RF amplifier - it must be the minimum necessary to obtain the required sensitivity of the receiver. In addition, the RF amplifier must be covered by an effective AGC.

Details

Coils L1 and L2 are wound on a unified three-section frame placed in ferrite (600НН) cups with an external diameter of 8.6 mm. Sub-constructor - size CC2.8 X 14 made of ferrite of the same brand. Coil L1 contains 3X6 turns of PELSHO wire - 0.1 (wound in two wires), coil L2 - 3 X 24 turns of PEV-2 wire - 0.1.