III. Gm-C Filtering - Epublications - Université de Limoges

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Context of the PhD thesis VI. Conclusion This PhD thesis work took place in the TV reception domain. Indeed, NXP Semiconductors currently designs TV tuners for different standards such as analog TV, digital TV or cable TV. These spectra cover a wide frequency range from 45MHz to 1002MHz, with channel widths of 6 to 8MHz according to the standard. To obtain a high quality reception, the TV tuner has to handle the RF off-air or cable signal. The role of the tuner is to amplify and to select the desired channel among all the received ones. This has to be performed with as little degradation and distortion as possible. Indeed, the wanted channel is then transmitted to the demodulator and the cleaner the signal, the fewer the demodulation errors. That is why the TV tuner should present high RF performances. In particular, it is required to be low noise, to be able to manage weak wanted signals, and highly linear, to be able to manage strong interferers which may degrade the quality of the reception. Within the tuner architecture, an RF filter is located between the front-end low noise amplifier and the mixer. This RF filter realizes the first selectivity step of the tuner. It allows rejecting harmonic frequencies due to the downconversion of the RF signal at LO odd harmonic frequencies when mixing. Furthermore, this RF filter also allows the rejection of adjacent channels for the international standard compliance (Nordig or ATSC A/74 for instance). It also enables to reject non-TV signals like the FM bands (88-108MHz) which act as interferers. Specifications have been set on these different rejections as well as on the tuning range of the filter, in order to quantify the requirements. Currently the RF filters of NXP tuners are realized by means of LC resonators with several off-chip inductors. Due to the technological trend which aims at integrating the whole tuner on-chip (the so called fully-integrated silicon tuner), alternative integrated solutions, and in particular active topologies, are looked for. Hence, the impact and the opportunities of technology on fully-active solution have to be quantified. Besides, a primary focus is set on low-VHF bands since the inductances at this frequency (~100nH) prevent from any integration on-chip. From these issues, the problematic of the present PhD thesis has been determined and set to: Limitations & Opportunities of Active Circuits for the Realization of a High Performance Frequency Tunable RF Selectivity for TV Tuners - 172 -
RF selectivity challenges In this thesis, several topologies have been assessed. The purpose is to find which one best suits the abacus created with the rejections specifications of the RF filter. It comes out that the second order bandpass topology is the best. It actually needs few components which make it an attractive structure for easy tunability. Moreover, it is also possible to tune the quality factor Q of such a filter. Q describes the selectivity of the filter and is defined as the ratio of the central frequency by the -3dB bandwidth. The higher the quality factor, the more selective the filter. Once the topology chosen, the literature has been assessed to find most appropriate solutions. First, LC passive filters have first been studied as a comparison basis to active solution studied further. A figure-of-merit has also been introduced to handle fair comparisons with current state-of-the-art. As far as fully-active structures are concerned, the literature mainly considers Gyrator-C and Gm-C filters. Gyrator-C filters consist in emulating an inductive behaviour at RF frequencies by a gyrator, allowing the replacement of inductors by active counterparts. Gm-C filters actually consist in synthesizing a transfer function by means of integrators, and so of Gm-cells. For simple topologies such as the second order bandpass, these two kinds of filter may lead to the same structure. Gm-C filters The studied Gm-C filter structure is based on an analogy with a parallel LC resonator where the inductor is replaced by a gyrator. Tuning the gyrator value and tuning the capacitance in parallel, a large tuning range is obtained compared to a passive LC filter. The theoretical study of noise and linearity limitations leads to the choice of a moderate quality factor since as soon as Q increases, RF performances are strongly degraded. This study also demonstrates that high RF performances are required on all Gm-cells. It has been chosen to use large transconductance values (2.5 and 10mS) in order to decrease the overall noise of the filter. However, such transconductances have to be linearized to reach the linearity specification set on the Gm-cell. Several transconductor linearization techniques from the literature have been assessed. Only two of them exhibit an interesting linearity versus noise trade-off: the Dynamic Source Degeneration (DSD) which is also used in the literature for a TV reception RF filter [VI.1], and Multiple Gated Transistors (MGTR) which show the most promising results at the Gmcell level. Gm-C filters based on Gm-cells designed with these linearization techniques have been simulated in 65nm CMOS technology. Table 33 summarizes the obtained performances compared to literature results. The filter with MGTR-based Gm-cells presents the best performances. For 6dB gain and a Q-factor close to 4, it exhibits an IIIP3 above 8dBm up to 450MHz, with a 16dB NF. However, this structure appears to be very sensitive to bias as well as process and mismatch variations, where a robust solution is required for our applications. That is why no test-chip has been realized based on this architecture. - 173 -
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UNIVERSITE DE LIMOGES ECOLE DOCTORA
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Acknowledgments En préambule à ce
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Summary ACKNOWLEDGMENTS ...........
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V. A PERSPECTIVE FOR FUTURE DEVELOP
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French Introduction Les signaux TV
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I. Introduction to TV tuners and to
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I.1.b.ii Digital modulations In dig
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makes the covering of a large terri
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I.1.c.iii Terrestrial spectrum Firs
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I.1.e Broadband Reception Systems F
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I.2.b TV Tuner High Performance Spe
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I.2.b.ii High linearity Although co
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I.3 RF Filter Specifications I.3.a
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27. This leads to the downconversio
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These adjacent channel rejection re
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Above 870MHz, there are no more ter
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Newest silicon tuners generations,
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I.6 References [I.1] B. Razavi, RF
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II. Challenges of RF Selectivity Fo
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Figure 39. H3 and N+5 rejections of
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Figure 43. Second order Low-pass Fi
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Figure 46 illustrates the transfer
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H HPF s ( s) = s 1 + ω This corres
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Topologies comparison for a same Q-
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Figure 56. Low-pass to Notch Transf
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II.2 Passive LC Second Order Bandpa
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A possible solution is to split the
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II.2.c Second Order Bandpass Filter
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Figure 69. Band Switching Figure 70
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Another paper [II.7] provides a FOM
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A parallel self resonating circuit
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This gives the following values for
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Vb - 63 - M1 M2 Zin Figure 76. Anot
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when given, reported noise figures
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In all publications dedicated to Gm
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II.4.c Second Order Bandpass Filter
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Figure 87. Biquad Schematic The Gm-
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Vin Rm - 73 - C Vout Figure 89. Der
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Figure 93 depicts the RF performanc
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A second advantage of advanced BiCM
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II.7 Conclusion As a conclusion, th
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[II.13] C. Andriesei, L. Goras, and
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III.I Theoretical Study III. Gm-C F
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g V m3 in III.1.b Linearity Conside
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Assuming an ideal input transconduc
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Figure 102. Linearity of the filter
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Furthermore, a Gm-C circuit is not
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In terms of noise, the Flicker nois
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Table 7 summarizes the specificatio
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Idiff (A) levels reached with this
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III.2.e Unbalanced Differential Pai
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Hence, the combination of the expon
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Technique Current Increase Source D
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Using this structure and considerin
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The dimensioning of the transistors
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III.3.c Gm-cells with MGTR III.3.c.
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Furthermore, the high sensitivity t
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Figure 136. Filter in-band IIP3 ver
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III.4 Comparison of the filters III
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III.5 Conclusion Once the Gm-C seco
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IV. Rauch Filtering IV.1 Sallen-Key
Page 125 and 126: Thus, an equation linking Q and Gai
Page 127 and 128: IV.1.c.ii Proposed positive feedbac
Page 129 and 130: For both Rauch and Sallen-Key filte
Page 131 and 132: The high sensitivity to passive com
Page 133 and 134: Furthermore, it should also be take
Page 135 and 136: IV.2.b Innovative Implementation of
Page 137 and 138: esulting gain K, constituted of the
Page 139 and 140: Table 25. Sum-up of the OA specific
Page 141 and 142: Figure 153. OA voltage gain versus
Page 143 and 144: Filtering in the mirror, by means o
Page 145 and 146: However, this feedback makes the fi
Page 147 and 148: Figure 160. OA Gain and phase versu
Page 149 and 150: IV.3.c Implemented Filter and Test
Page 151 and 152: Figure 165 depicts the layout of th
Page 153 and 154: Figure 169. PVT variations of the f
Page 155 and 156: IV.4 Rauch Filter Performances: Mea
Page 157 and 158: Figure 178. IIP3 versus input power
Page 159 and 160: IV.4.c Performances Sum-up The filt
Page 161 and 162: Table 30. Frequency Limitations of
Page 163 and 164: IV.7 References [IV.1] Y. Tsividis
Page 165 and 166: V. A Perspective for Future Develop
Page 167 and 168: V.2 4-path Filter Simulations V.2.a
Page 169 and 170: The frequency tunability is ensured
Page 171 and 172: V.3 State-of-the-Art V.3.a 4-path F
Page 173 and 174: V.4 Conclusion In this chapter, it
Page 175: IEEE International, 2009, pp. 222-2
Page 179 and 180: The positive feedback Rauch filter
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Page 183 and 184: étudiée. Il s’avère que la lin
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Page 189 and 190: A.5 Friis’ Formula In case of mul
Page 191 and 192: A.7 References [A.1] Friis, H.T., N
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Page 201 and 202: B.2.c IIP2 measurement To measure t
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Page 205 and 206: C.1 Gyrator-C filtering year Ref. f
Page 207 and 208: C.2 Gm-C Filtering year Ref. f0 (MH
Page 209 and 210: year C.3 Rm-C filtering Ref. f0 (MH
Page 211 and 212: C.5 References C.5.a Gyrator-C filt
Page 213 and 214: [C.23] J. De Lima and C. Dualibe,
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Page 217 and 218: this gives: I I I md d1 d 2 = I I =
Page 219 and 220: D.2 MOS Degenerated Common-Source C
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FIGURE 54. H3 AND N+5 REJECTIONS AC
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FIGURE 168. PVT VARIATIONS OF THE F
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