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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING High-level mixer for direct conversion transceivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Direct conversion receivers and transceivers gained wide popularity, but their performance, achieved by the end of the 80s, has hardly improved since then. Significant progress in this direction is obtained, as the author of the published article shows, when using field-effect transistors in the mixer of the transceiver (receiver), switched on in the passive mode of controlled resistance. The advantages of heterodyne receivers (direct conversion) are widely known. This is simplicity, the almost complete absence of side reception channels, the high quality of the demodulated signal, etc. But they also have disadvantages. This is a two-signal reception and a small dynamic range, not exceeding 80 dB for receivers with diode mixers. It seems promising to use mixers based on field-effect transistors switched on in the controlled resistance mode. Such a mixer, made on a single field-effect transistor and described in [1], provided a heterodyne receiver sensitivity of 1 μV and a dynamic range of 65 dB. Here it is appropriate to say that the dynamic range of the heterodyne receiver mixer is limited from above not by third-order intermodulation distortion, as in high IF receivers, but by direct detection of interfering signals. The lower limit of the dynamic range is assumed to be equal to the sensitivity (for a given signal-to-noise ratio, usually 10 or 12 dB), and the upper limit is determined by applying to the input of the AM receiver a signal with a modulation factor of 30% (m = 0,3), detuned in frequency by 50 or 100 kHz, with an amplitude that provides the same 3-hour output as the sensitivity test. In American literature, the difference between the dynamic range limits of a direct conversion receiver is often referred to as AMRR - AM rejection ratio. The theory of radio circuits says that when switching from a single-cycle mixer circuit to a balanced one, the dynamic range expands by 30 ... 40 dB, which allows us to hope to get its value for a balanced mixer on field effect transistors of the order of 100 dB. One of the options for a balanced field-effect transistor mixer is described in [2], but it contains a balancing low-frequency transformer, which is laborious to implement and is subject to mains interference with a frequency of 50 Hz. Readers are offered a new version of the mixer. It was used in a heterodyne receiver for a range of 160 meters, the circuit of which is shown in the figure. Of course, nothing prevents the use of the mixer in other ranges, changing the data of the circuits and transformers accordingly. The input signal from the preselector (two-, three-circuit band-pass filter, not shown in the figure) is fed to the RF transformer T1 and then to the mixer, made on field-effect transistors VT1 - VT4.
The local oscillator of the receiver is assembled on a transistor VT5. Since the local oscillator is practically not loaded by the mixer, it is made single-stage according to the capacitive three-ton scheme. For the same reason, it turned out to be possible to abandon the buffer stage as well. The stability of the relatively low local oscillator frequency (1,8 MHz) turned out to be quite sufficient. The converted 3H signal passes through the C1L3C2 low-pass filter and enters the ultrasonic frequency converter, assembled on two bipolar transistors VT6 and VT7 in the usual way with a direct connection between the cascades. High-resistance sensitive phones can be connected to its output, or better, a terminal UMZCH, made according to any known scheme. The device works as follows: with a positive half-cycle of the local oscillator voltage at the gates of transistors VT2 and VT3, they open. In this case, the lower output of the secondary winding of the transformer T1 is connected to a common wire through the open channel of the transistor VT2, and the upper output of the same winding through the open channel of the transistor VT3 is connected to the input of the low-pass filter. Transistors VT1 and VT4 are closed at the same time, since the local oscillator voltage is supplied to their gates in anti-phase and a negative half-wave acts on them. In the next half-cycle of the heterodyne voltage, transistors VT1 and VT4 open, and transistors VT2 and VT3 close. In this case, the polarity of the connection of the secondary winding of the transformer T1 to the input of the low-pass filter is reversed. If the frequency and phase of the local oscillator and the signal are the same, then pulses of positive polarity appear at the output of the mixer. When the phase of the local oscillator is reversed at the output of the mixer, the pulses will be of negative polarity. Smoothed in the low-pass filter, they give a constant current at the output. In both cases, synchronous signal detection occurs. If the frequencies do not match, then a beat signal appears at the output. This mixer has the following features: - it does not have a balancing low-frequency transformer; - the winding of the high-frequency transformer does not contain a midpoint, which eliminates the influence of the asymmetry of the transformer windings; - parasitic drain-gate capacitances of transistors VT1 and VT3, as well as VT2 and VT4 are connected to the anti-phase terminals of the coupling coil with the local oscillator L2 and form a balanced bridge that does not allow the local oscillator voltage to enter the input circuit, which significantly reduces the radiation of the local oscillator through the antenna. The radiation of the local oscillator, in addition to the obvious harm - creating interference to nearby receivers - is fraught with parasitic reception of the same signal, but already modulated by the background of alternating current and other interference somewhere on the network wires or in extraneous power sources [2]. At the same time, a hard-to-eliminate growling sound is heard, which disappears when the antenna is turned off. A few words about the input and output impedances of mixers. As is known, the input and output resistances of a passive mixer depend on each other, but their values can be chosen to a large extent arbitrarily. The classic way to select the optimal load resistance of the mixer is to determine the geometric mean resistance of the open and closed mixer channels, while Rload = √Ropen Rclose. Determining the resistance of an open channel Rotp does not cause difficulties. It is tens of ohms. As for the resistance of the closed channel Rclose, it has an active-capacitive character. If we allow a parasitic capacitance of a closed channel of 1 pF, then its resistance decreases from 80 kOhm in the range of 160 m to 5 kOhm in the range of 10 m, not to mention the VHF bands. Assuming Ropen = 50 Ohm, we get Rload - 2 kOhm in the range of 160 m and Rload = 500 Ohm in the range of 10 m. In addition, high mixer load resistances in the heterodyne receiver require the installation of a low-pass filter with a high characteristic impedance. The inductance of such a low-pass filter contains many turns and is laborious to manufacture. Therefore, according to the author, it makes sense to reduce the load resistance of the mixer to a value of the order of 10Ropen, i.e., to approximately 500 Ohm. In this case, the additional losses in the mixer are 10%, the decrease in the mixer gain does not exceed 1 dB relative to the case of perfect matching, which seems to be quite acceptable. Let's go back to the receiver circuit. The KP305Zh transistors used in the mixer have a channel resistance of about 400 ohms at zero gate voltage, and about 25 ohms in the open state. In addition, they have a fairly large variation in resistance from instance to instance. When the heterodyne voltage passes through zero, the simultaneously open transistors VT1 and VT2, as well as VT3 and VT4, shunt the secondary winding of the transformer, reducing the transfer coefficient. Therefore, the maximum transfer coefficient of the mixer is achieved when a blocking voltage of -1,5 V is applied to the gates. It is better to use KP305 A or D transistors, which are practically closed at zero gate voltage and do not require constant gate bias. In the case of using better elements, we should expect an improvement in the parameters. On sale there are already key transistors with an open channel resistance of 1 ... 5 ohms. Unfortunately, with a decrease in the resistance (increase in conductivity) of the transistor channel, the parasitic gate-source capacitance also increases. Interestingly, the product of channel conductance and parasitic capacitance is approximately constant for different low-power transistors of the same generation. The level of the local oscillator signal leaking through the parasitic gate-source capacitance is approximately proportional to this product. However, all these considerations become insignificant when the mixer switches to the key mode. This is achieved by simply increasing the local oscillator voltage, because with an instantaneous gate voltage of more than +5 V, the transistors open completely. In the described receiver, after increasing the supply voltage from 9 to 15 V, the amplitude of the local oscillator voltage at the gates of the transistors also increased from 8 to 14 V. The transistors practically began to operate in the key mode, which favorably affected the linearity of the mixer, namely: the sensitivity of the receiver increased by 4 dB , and the upper limit of the dynamic range - by 6 dB. It is interesting to note that the mixer circuit exactly repeats the circuit of a diode bridge rectifier, only the channels of field-effect transistors are switched on instead of diodes. In addition, in the rectifier, the diodes are opened by the input alternating voltage from the transformer winding, and in the mixer - by the local oscillator voltage. Such devices can also be successfully used for synchronous rectification of the secondary voltage in high-frequency power supply converters, since the losses in high-power field-effect transistors are less than in diodes. The input transformer of the mixer T1 is wound on a K10x6x4 ring magnetic circuit made of ferrite with a magnetic permeability of 400. The primary winding contains 30, and the secondary - 100 turns of PELSHO 0,1 wire. The local oscillator coil is wound in bulk on a conventional plastic frame with cheeks 8 mm in diameter and 10 mm long. To adjust the inductance, a cylindrical threaded core (SCR) made of carbonyl iron is used. Winding is carried out with three PEL or PELSHO 0,2 ... 0,3 wires folded together. The number of turns is 30, it is specified depending on the size of the frame, when adjusting the frequency range of the local oscillator. Of the three resulting windings, one is used in the local oscillator circuit (L1), and the other two, connected in series, form a coupling coil (L2). The midpoint of the coil is obtained by connecting the beginning of one wire to the end of another. The L3 low-pass filter coil is wound on a K16x10x8 ring magnetic core made of 2000NM ferrite. It contains 200 turns of any thin insulated wire, PELSHO 0,1 is recommended. Establishing an ultrasonic frequency converter comes down to selecting a resistor R1 until the voltage on the VT7 collector is equal to half the supply voltage. When establishing a local oscillator, the capacitance of capacitor C8 is recommended to be selected as high as possible, at which stable generation still exists. Receiver tests showed the following results. When working on reception, the mixer provided a dynamic range limited by direct detection, equal to 100 dB at a sensitivity of 0,3 μV. In other words, an interfering AM signal with a detuning of 50 kHz, m = 0,3 and a level of 30 mV produced the same 3-hour voltage at the output as a useful CW signal with a level of 0,3 μV. The level of intrinsic noise of the receiver brought to the input was 0,1 μV. During experiments, turning off the local oscillator did not significantly reduce the overall receiver noise, which indicates the mixer's sensitivity reserves. It should be noted that during the experiments, the intrinsic noise of the transistor GSS was also heard, indicating the low quality of its output signal. The described mixer, like all passive mixers, can transmit a signal in any direction, i.e., it is reversible. When transmitting, when a 3H signal with a voltage of 2 V was applied to the low-frequency input of the mixer (at the connection point of the low-pass filter), the amplitude of the output voltage of the DSB signal was 1 V at a load of 50 Ohms. The unsuppressed carrier residual was found to be 5 mV. This means that carrier suppression without special balancing measures is up to 46 dB. Of course, in order not to worsen such a high carrier suppression, good shielding of the input circuits and the local oscillator is necessary. Literature
Author: M.Syrkin, UA3ATB
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