Design and Research of L- and S-band SiGe Integrated Quadrature Signal Generators
Recent decades have witnessed a significant growth of the wireless communication market. There is a tendency to reduce the size and cost of the equipment used for wireless communication. Modern L-, S-, C-bands, transceivers are constructed using quadrature modulation based on integral quadrature signal generators [1–3].
The QSG functional block has a significant influence on the transceivers characteristics. The accuracy of the quadrature signals formation is directly related to Image Rejection Ratio (IRR). This parameter, in turn, determines the sensitivity of the receiver. Insufficient suppression of the mirror channel causes a degradation of the Error Vector Magnitude (EVM) of the receiver. This leads to an increase of Bit Error Rate (BER). Among most important tasks is ensuring stable operation of integral QSG in a wide range of operating frequencies while maintaining the relative simplicity of circuitry, and a small chip area.
Traditionally, four types of quadrature signal generators are distinguished: RC-CR circuits, RC-polyphase filters (PPFs), quadrature all-pass filters (QAFs), and digital quadrature phase splitters based on triggers. PPF are built on the basis of cascaded RC-CR circuits. However, the methods of their designing for Si and SiGe processes are significantly different. Thus, the design of the PPF is a separate area for the development of the QSG.
INTEGRAL RC-POLYPHASE FILTERS
Integral polyphase filters are used for splitting the signal phase into quadrature components in many applications (IQ modulators and demodulators, IQ signal generators, image rejection systems, polarization control, etc.) [4–6].
Polyphase filters are based on RC-CR circuits (Fig. 1). The PPF order is determined by the number of cascaded RC-CR circuits.
The RC-CR circuit is a combination of low-pass and high-pass filters. The disadvantage of RC-CR QSG is its narrow operating frequency band.
A detailed review of types, calculation methods and characteristics of passive polyphase filters is given in the sources [7, 8]. The authors analyze the influence of technological process mismatch and parasitic parameters of elements on the characteristics of the PPF.
Integral microwave PPFs have a number of advantages such as relative ease of implementation, a small area of topology, relatively high resistance of characteristics to technological mismatch. However, they are characterized by large insertion loss (around 3dB per stage). Buffer amplifiers are used to compensate for these losses. Moreover, the thermal noise of the resistors in the PPF stages has a negative effect on the noise figure (NF) of the receiver. The choice of the resistance of the first stage resistors in accordance with output resistance of the previous stage, and the resistance of the last stage resistors close to 50Ohms will allow getting an acceptable value of NF .
A passive polyphase phase filter can be connected to the signal source in two ways, which conditionally we call “Type A” and “Type B” connection. Each of them has its own advantages. Connecting the polyphase filter by “Type A” method shown in Fig. 2 (a) allows achieving a low phase error at the “pole” frequencies ωn = 1/RnCn. Fig. 2 (b) shows the connection of a polyphase filter by “Type B” method that, respectively, allows achieving a low amplitude error.
It was shown in  that a minimum of 4 stages of a polyphase filter are required to obtain a suppression of the mirror channel of 40dB in the 1.8 to 6GHz band. Passive high-order polyphase filters have high insertion loss.
Active polyphase filters are characterized by significantly lower signal attenuation [11–14]. However, with the increase in the transfer ratio, the stability is reduced due to the use of buffer amplifiers. The frequency characteristics of active polyphase filters are also limited by these amplifiers.
Polyphase filters with adjustable characteristics are included in a separate category. Adjustable PPFs are also called active, but they do not provide amplification of the signal. Frequency range tuning is carried out in two ways:
• using transistors as controlled resistors;
• using varicaps as controlled capacitors.
Table 1 presents parameters of the PPFs presented in periodicals and materials of the international conferences.
A review of the sources [7–13] makes it possible to determine a number of features of the design methodology for passive PPFs:
• it is preferable to select the “Type A” connection of the PPF to the signal source. The correction of the resulting amplitude error should be carried out by amplifier-limiters (AL);
• the separation of the “poles” of multi-stage PPFs is preferable;
• the optimal distance between the “poles” is determined by the relative bandwidth of the PPF;
• equal capacities in the PPF stages are preferable;
• the resistance of the resistors is calculated basing on the impedances of the signal source and the load of the QSG, and it must grow from stage to stage;
• the necessary and sufficient number of stages of the PPF is determined by the required value of the IRR and the mismatch of the technological process;
• the phase error correction can be made by an additional controlled phase shifter.
INTEGRAL QUADRATURE RLC FILTERS
In the integral version, especially for small-signal applications, quadrature filters based on RLC-circuitry are used [15, 16]. Fig. 3 shows a schematic diagram of the RLC filter.
The design of quadrature RLC filters has a number of features:
• Quality factor of the circuit must be close to 1 ( ). In this case, transmission ratio of the circuit for the voltage at the central frequency equals 3dB;
• the RLC filter bandwidth is determined by the quality factor of the integral inductance coils. Low-Q Si CMOS technology process coils provide a wide range of operating frequencies;
• the input impedance of the following cascade has a strong influence on RLC filter characteristics. High input impedance provides better performance.
In case of a capacitive load of RLC-quadrature filters, it is possible to correct the amplitude and phase errors by setting correcting resistors of the same value in series with the inductance coils and capacitors .
DIGITAL QUADRATURE PHASE SPLITTERS
Digital quadrature phase splitters based on D-type flip-flops are widely used in quadrature modulator and demodulator MMICs [17–19]. Fig. 4 shows functional diagram of the trigger QSG (a) and time diagrams of its operation (b).
The circuit is a combination of two frequency dividers. Table 2 shows the parameters of the trigger QSGs presented in periodicals and materials of international conferences.
It should be noted that the phase error in  was obtained upon correction by a first-order polyphase filter. Phase imbalance without correction is 3–4 degrees in the frequency range of 0.8–5.2GHz.
The advantage of trigger QSG is the opportunity to achieve a high level of matching in a wide frequency range .
Fig. 5 shows a schematic diagram of current-mode logic latch developed on the basis of 0.18µm SiGe BiCMOS process.
Direct current flows in such D-type flip-flop. This causes a low level of current emissions, which at a high switching speed can propagate to other sensitive elements of the circuit.
Amplifier-limiters are used to reduce the imbalance of the amplitudes at the outputs of the QSG. The AL also serves as a buffer amplifier, making sure that the output of the QSG matches a subsequent cascade.
The phase error in the trigger QSG depends on the duty cycle of the signal, which must be strictly equal to 50 %. Any deviation from this value results in a phase imbalance at the output of the QSG. It is possible to correct the phase error of the trigger phase splitter by using the AL at the input of the circuit.
Trigger circuits are sensitive to the quality of topology. The lengths of the conductors through which the LO signal flows must be strictly equal . Failure to comply with this condition leads to a change in the duty cycle and, accordingly, to the appearance of a phase error. A significant drawback of trigger circuits, limiting the possibility of their use, is the need to use an input signal with twice the operating frequency.
One, two and four stages polyphase filters, RLC-quadrature filter and trigger QSG on the basis of the 0.18µm SiGe BiCMOS technology process have been developed. To evaluate the possibility of using designed QSG in L-, S-, and C-band quadrature modulators and demodulators MMICs, their characteristics have been modeled taking into account temperature and technological mismatch.
Figs 6–7 show dependences of output signals relative amplitude and phase errors of the 1st and 2nd stages of PPFs and RLC-filter on frequency.
Fig. 8 shows the dependence of image rejection ratio when applying 1 and 2 stages PPFs and RLC-filter on frequency.
Fig. 9 shows the dependence of transfer ratio of 1 and 2 stages PPFs and RLC-filter on frequency.
Fig. 10 shows the dependence of input return losses of 1 and 2 stages PPFs and RLC-filter on frequency.
Figs 11–12 show the dependences of output signals relative amplitude and phase errors of 4 stages PPF and trigger QSG on frequency.
Fig. 13 shows the dependence of image rejection ratio on frequency when applying 4 stages PPF and trigger QSG.
Fig. 14 shows the dependence of transfer ratio of the 4 stages PPF and trigger QSG on frequency.
Fig 15 shows the dependence of input return losses of 4 stages PPF and trigger QSG on frequency.
Table 3 presents the resulting parameters of integral quadrature signal generators.
Digital phase splitter has best characteristics among the designed QSG. Besides, its parameters have shown better resistance to temperature and technological process mismatch. Using 4 stages PPF in quadrature modulator and demodulator MMICs is most promising. It is advisable to use controlled phase shifter in order to correct phase imbalance and respectively increase the value of the image rejection ratio to values comparable to the trigger QSG.
An overview of the types, methods and design features of microwave integral QSG that are widely used in the design of MMICs for transceiver modules has been presented, as well as design and simulation results of 1–4GHz quadrature signal generators based on SiGe BiCMOS technology. A comparative analysis of their characteristics with those of the nearest known prototypes has been performed and the possibility of QSG parameters correction has been shown.
The authors believe that the following terms and results in this study are novel: we have succeeded in surpassing by a range of technical characteristics the closest prototypes of the QSG developed earlier in Si technology processes.
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