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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Mixer balancing method. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Mixers, frequency converters One of the most critical components of the receiver and transceiver is the mixer. The sensitivity and dynamic range of the receiver and the suppression of the carrier and other spurious emissions in the transmission mode depend on the quality of its work [1]. In communication equipment, double balanced mixers (DBS) are most often used on silicon diodes or diodes with a Schottky barrier. For DBS, an important characteristic is the degree of balance, i.e. carrier suppression. The studies conducted by the author indicate an increase in sensitivity and an expansion of the dynamic range with careful balancing of the DBS. In this regard, the author has developed and tested a new circuit solution that allows: - by 20 dB or more to increase the level of carrier suppression at low frequencies of the RF range (f<2 MHz); - during the balancing process, maintain the initial impedances of the mixer ports, including those with a significant change in the carrier frequency (within 0,1 ... 30 MHz or more). The use of the proposed circuit design also allows you to increase the sensitivity and dynamic range in the receive mode. Let's consider the process of balancing transceiver mixers in more detail. The main scheme of the DBS is shown in fig. 1. Such a circuit does not contain additional balancing elements and, due to the non-identity in the characteristics of the diodes, as a rule, does not allow obtaining carrier suppression of more than 30 dB even at frequencies below 10 MHz. At frequencies above 10 MHz, the carrier suppression is reduced. However, carrier suppression of 30 dB is insufficient.
Figure 2 shows a diagram of a DBS with balancing capacitors. The connection point of the capacitors is determined empirically by the maximum carrier suppression. In this case, it is possible to suppress the carrier by 40 ... 50 dB. Such values of carrier suppression are already acceptable for communication equipment.
However, this circuit does not make it possible to balance the mixer well at sufficiently low frequencies (0,5 ... 2 MHz). To eliminate this drawback of the balancing circuit, an additional balancing resistor is introduced (Fig. 3). But this scheme also has drawbacks. The most significant of them is a significant change in the input impedances of the mixer when a balancing resistor is introduced. In order to minimize the change in the mixer resistance, it is necessary to use a non-inductive low-resistance resistor.
The author proposed a new simple balancing method based on the substitution method [2]. The scheme (Fig. 4) combines the advantages of the above schemes (Fig. 2 and 3). This makes it possible to balance the mixers well at any frequency. In this case, during balancing, the resistance changes much less than in the circuit in Fig. 3. We select the capacitances of capacitors and the resistance of resistors empirically. However, this scheme does not eliminate the deterioration of the mixer balance when the carrier frequency changes significantly, for example, in the first mixer of a multi-band transceiver.
For the maximum possible and uniform suppression of the carrier when the carrier frequency changes over a very wide range, balancing elements switched when changing the ranges are introduced into the mixer. A variant of such a scheme is shown in Fig.5.
Here, RF switching is required, which is its disadvantage. The circuit presented in Fig. 6 is free from this drawback. The circuit in Fig. 6 allows to produce and maintain a high degree of balancing of the DBS even when the carrier frequency is tuned within the entire HF band.
The proposed circuit design of the DBS has proven itself well in the design of the author's multi-band transceiver. Literature 1. Artemenko V. On the methods of establishing direct conversion receivers and transceivers / Radioamator. - 1995. - N6. - P.24 - 25. Author: V. Artemenko (UT5UDJ), Kyiv; Publication: N. Bolshakov, rf.atnn.ru
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