element microwave filter with high side steepness, and a mixer with infinite ... Block diagram of an optical multi-subcarrier communication link is shown in Fig. 1.
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Circuit Theoretical Aspects of Optical Communication Links János Ladvánszky Ericsson Telecom Hungary, H1117 Budapest, Irinyi J. u. 4-20, Hungary on leave at Ericsson AB,16480 Färögatan 6, Kista, Stockholm, Sweden ABSTRACT Three topics are addressed: Optimum frequency allocation for optical communication channels, a lumped element microwave filter with high side steepness, and a mixer with infinite third order intercept point. Keywords: circuit theory, circuit design, optical communications, microwaves; channel allocation, microwave filters, mixers, intercept point, radio over fibre. 1. INTRODUCTION Block diagram of an optical multi-subcarrier communication link is shown in Fig. 1.
Figure 1. Optical communication link. In this talk, we concentrate on the microwave signal processing blocks. In Section 2, optimum channel allocation is considered. Optimum in the sense, that that with a minimum occupied bandwidth, the impairment effects of third order intermodulation and harmonics are prevented. In Section 3, a lumped element filter for the lower edge of the microwave band is shown. High side steepness has been achieved, applying the same circuit trick as in ladder quartz filters. Section 4 is about an infinite IP3 mixer. 2. OPTIMUM CHANNEL ALLOCATION Due to the nonlinearities in the laser and photo diodes, all intermodulation products and harmonics are present and we have to mitigate them. A possible way is, to select channel frequencies and gaps between them in a way that none of them fall in another channel. We present here a constructive approach to realize this. A sketch of the procedure is shown in Fig. 2.
Figure 2. Determination of the gap between the 1st and 2nd channels so that 3rd order intermodulation does not fall in other transmission bands.
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The most disturbing is the third order intermodulation product because it is the nearest in frequency to fundamental signals (Fig. 2). In order to determine the gap, fundamentals are placed at the edges of the first channel. In Fig. 2 we show a situation when the gap is too small and a third order intermodulation product falls into the third channel. The minimum value of the gap is the bandwidth B of the first channel. We can repeat the procedure as shown in Fig. 3.
Figure 3. Optimum channel allocation procedure, k is the step number To complete the procedure, we have to stop when some channel frequencies are greater than or equal to twice the left-side edge of the first channel. These channels cannot be considered. The procedure was tried in practice and patented [1]. 3. A LUMPED ELEMENT FILTER WITH HIGH SIDE STEEPNESS The 500 MHz – 3 GHz frequency range is the overlap spectrum where both lumped and distributed element filters are realizable. However, distributed filters in this frequency range are of large geometrical size. This is the main reason why we considered lumped element filters. If channels are relatively narrow and near to each other, high side steepness is necessary. From among the available realizations, the highest side steepness is provided by quartz ladder filters. Therefore the structure of our filter (Fig. 4) is motivated by the principles of quartz ladder filters.
Figure 4. Schematics of a lumped element filter for the 1195 – 1235 MHz frequency range. Our filter consists of four resonators. The first two and the second two are lower capacitive coupled. The two resonator pairs are interconnected by a capacitive T structure. Resonators and couplings are tuneable by trimmer caps. A special feature of our filter is that we exploit the parasitic series resonance of the coupling trimmers. Notches are tuned by these trimmers while the optimum pass band behaviour is adjusted by the trimmers in the resonators. Measured transfer function and reflection coefficient are shown in Fig. 5. Also patented [2]. 4. A MIXER WITH INFINITE IP3 Third order intercept point IP3 is a commonly accepted measure of large signal behaviour of mixers. In order to start, we have to clearly distinguish the interpolated and the real IP3. Interpolated IP3 is the crossing of straight lines fit to fundamental and third order products over a certain range of input level. Real IP3 is the crossing of fundamental and third order product curves. Usually interpolated IP3 is used, but the exact limits of the corresponding range of input power are forgotten. This is the motivation of our contribution.
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Figure 5. Measured S12 and S22 in dB as a function of frequency. Horizontal scale: 700 – 1700 MHz, 100 MHz/div, vertical scale: -80 – +20 dB, 10 dB/div. The steepness of the third order product line is three times that of the fundamental. At mid-level signals, the real curves deviate from the lines. It is interesting if the two lines become parallel because that means infinite IP3 and this can be achieved in practice (Figs. 6, 7, 8). A critical input signal level range can be shifted by applying gate DC bias. Real IP3 can never be infinite, but as shown below, interpolated IP3 can. If a mixer contains such input range, then the real IP3 occurs at higher input level. That is the practical use of this interesting phenomenon.
Figure 6. Schematics of a mixer obtaining infinite IP3 over a range of input levels.
Figure 7. Fundamental and third order products of the mixer in Fig. 6. Horizontal: Input power in dBm, vertical: output power in dBm, diamonds: fundamental, crosses: third order product.
Figure 8. Fundamental and third order products in the critical range. Horizontal: Input power in dBm, vertical: output power in dBm, diamonds: fundamental, crosses: third order product.
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5. CONCLUSIONS Three circuit theoretical aspects are mentioned: Optimum channel allocation, a lumped element bandpass filter structure and an infinite IP3 mixer. Our channel allocation scheme is optimum in the sense that it requires the minimum possible occupied bandwidth while preventing the negative effects of intermodulation and harmonic distortion. The most interesting property of our filter structure is that for high side steepness, parasitic inductance of trimmer capacitors is exploited. Infinite IP3 mixer is a warning that for interpolated IP3 specification, the corresponding input level range should always be mentioned. ACKNOWLEDGEMENTS This research has been done together with B. Dortschy of Ericsson Research in Kista, Sweden, whose kind help in discussing the theoretical results, in realizing the circuits and in measurements is greatly acknowledged. For providing excellent conditions for research work, our managers in Kista, S. Albrecht and H. Almeida, and in Budapest, V. Beskid and Á. Vámos are greatly acknowledged. REFERENCES [1] J. Ladvánszky and B. Dortschy: Subcarrier allocation device and method for allocating N channels to carrier frequencies, Ericsson patent filed in 2014, PCT/SE2014/050964. [2] J. Ladvánszky and B. Dortschy: A bandpass filter structure, Ericsson patent filed in 2014, PCT/SE2014/ 051236.
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