Highly efficient frequency converter on electronic keys. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In our time, it is difficult to surprise readers with any new circuit solutions - it seems that everything has long been invented. And yet the amazing thing is there. This time, the surprise was presented by a simple and well-known to many radio amateurs 74HC4066 chip containing high-speed electronic keys. On the basis of this microcircuit, the author developed an original frequency converter, the description of which is offered to the attention of readers.

Currently, high-speed key elements, usually made on field-effect transistors, are widely used in the mixer units of the receiving-transmitting equipment. The use of such keys can significantly improve the dynamic parameters of mixers.

However, as it turned out, the possibilities of high-speed electronic keys are by no means limited to switching analog and digital signals. On electronic keys, you can perform not only a mixer, but also a local oscillator. Moreover, 4 analog high-speed keys included in the 74HC4066 microcircuit. with the utmost simplicity, the circuits make it possible to create a high-quality frequency converter, i.e. node containing both the mixer and the local oscillator.

The block diagram of such a frequency converter, which is used in the direct conversion receiver, is shown in Fig.1.

The main feature of the circuit is that the conversion occurs at a frequency that is 2 times higher than the local oscillator frequency. A similar conversion principle is used in a mixer based on anti-parallel diodes, proposed by V.T. Polyakov [1].

Consider the operation of the converter on electronic keys. The local oscillator is made on the elements DD1.3 and DD1.4, which are part of the 74NS4066 microcircuit. When the ratio of the resistances of the resistors R1 and R2 to R3 is about 18:1, the constant component of the voltage on the capacitors C1 and C2, which are part of the heterodyne circuit, is about 1,7 V, and the amplitude value of the variable component of the local oscillator voltage is about 1,3 V .

From the graphs in Fig. 2 it can be seen that 2 it can be seen that the voltage on the capacitors C1 and C2, to which the control inputs of the keys DD1.1 and DD1.2 are connected, reaches the opening threshold of 2.5 V at an AC voltage level of about 0,7 of the amplitude value . With such a ratio of the variable and constant components of the voltage on the circuit, the duration of the open state of the key is ¼ "of the oscillation period of the local oscillator.

Since the local oscillator voltage on capacitors C1 and C2 is in antiphase, TODD1.1 and DD1.2 open in turn for % of the local oscillator period with an interval also of ½ of the local oscillator oscillation period. Thus, the time of the open and closed states of the key formed by the parallel connection of DD1.1 and DD1.2 is ½ of the oscillation period with a frequency 2 times higher than the local oscillator frequency and is optimal in terms of maximum conversion efficiency at a frequency that is 2 times higher local oscillator frequency.

It should be noted that the mixer circuit is completely reversible, and when a low-frequency signal is applied to one of the inputs, a high-frequency DSB signal is formed on the other. Local oscillator made on the keys of the 74NS4066 microcircuit. works stably at frequencies up to 11 MHz (with a supply voltage of 5-8) and 18 MHz (with a supply voltage of 10 V), while the conversion frequency is 22 and 36 MHz, respectively.

Obviously, the use of modern microcircuits of high-speed electronic switches (for example, FST3126) in a frequency converter will improve the parameters of the converter - increase the maximum conversion frequency and reduce losses in the mixer.

The use of a frequency converter whose local oscillator operates at a frequency 2 times lower than the receive frequency provides several advantages. First, at a lower frequency it is easier to obtain the required frequency stability. Secondly, the level of the local oscillator signal penetrating the antenna is reduced, which provides a significant reduction in the likelihood of interference in the form of a multiplicative background. Thirdly, given that the input and local oscillator circuits are tuned to different frequencies, these circuits can be located in close proximity to each other without fear of increased penetration of the local oscillator signal into the input circuits of the receiver and unbalance of the mixer. Consequently, the design of the receiver is simplified and its dimensions are reduced.

In addition, the converter has low power consumption, which is especially important for self-powered receivers. For example, the current consumed by the frequency converter when operating in the range of 80 m (3,6 MHz) is only 3,2 mA at a supply voltage of 5 V. At the same time, the high dynamic parameters inherent in mixers assembled on 74HC4066 switches are preserved.

This frequency converter can be successfully used in a superheterodyne receiver or transceiver. Since the received frequency is separated from the local oscillator frequency by the value of the IF. in some cases, it is convenient to carry out the conversion at the local oscillator frequency, and perform the mixer according to a balanced circuit. Such an implementation of the mixer is shown in Fig.3. In this circuit, with a ratio of the resistances of resistors R1 and R2 to R2 of about 1: 1, the constant component of the voltage across capacitors C2 and C2,5 will be close to 4 V, and, as can be seen from Fig. 1.1, the open time of the keys DD1.2 and DD0.5 .XNUMX will be approximately XNUMX LO voltage period.

Based on this frequency converter, a synchronous AM signal detector can be made. The scheme of the detector is shown in Fig.5. The local oscillator operates at a frequency that is 2 times lower than the frequency of the received signal. Synchronization of the local oscillator frequency with the carrier frequency of the received signal is carried out by direct capture.

The operation of the synchronous detector was tested when receiving radio stations of the 41 m broadcast band. The reception quality of radio stations received at medium volume was high. It should be noted that with this inclusion of a field-effect transistor, the range of input signal levels received without distortion is limited. To expand it, you need to add a tuned resistor with a resistance of 680 ohms to the circuit. One of the extreme outputs is connected instead of the drain of the field effect transistor to the connection point of the keys, the drain, in turn, is connected to the output of the movable contact of the resistor. The other extreme output is connected to a common wire. With a variable resistor, it is now possible to adjust the local oscillator frequency capture band depending on the signal level at the detector input, achieving the best selectivity.

In the practical manufacture of frequency converters according to the above schemes, the inductance of the coil And and the capacitance of the capacitors C1-C2 of the local oscillator circuit are calculated by the formulas:

C1 \u2d C3618 \uXNUMXd XNUMX / Fg,

where L is the inductance in microhenries; C is the capacitance in picofarads; Fg - local oscillator frequency in megahertz.

Literature

1. V.T.Polyakov. Radio amateurs about the direct conversion technique. - M.: Patriot, 1990.

Author: O. Shipilov, Kobrin.

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