CC BY
29
UDC 621.372.54=111
N.V. Ivanov, A.S. Korotkov
A 180-nm CMOS HIGH-LINEAR COMPLEx G -C FILTER
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FOR RECEIVERS OF SATELLITE NAVIGATION SYSTEMS
Realization of high-linear Gm-C complex filter in 180-nm CMOS technology to be used in satellite navigation system receivers is covered. The filter with 25 MHz bandwidth was designed with THD of about 80 dB.
G -C FILTERS; COMPLEX FILTERS; SATELLITE NAVIGATION SYSTEMS RECEIVERS;
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TRANSCONDUCTANCE AMPLIFIER; 180-nm CMOS.
Н.В. Иванов, А.С. Коротков
КОМПЛЕКСНЫЙ G -C ФИЛЬТР С НИЗКИМ УРОВНЕМ НЕЛИНЕЙНЫХ
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ИСКАЖЕНИЙ НА ОСНОВЕ КМОП-ТЕХНОЛОГИИ С РАЗРЕШЕНИЕМ 180 НМ ДЛЯ ПРИЕМНИКОВ СИСТЕМ СПУТНИКОВОЙ НАВИГАЦИИ
Рассмотрена реализация Gm- C комплексного фильтра с низким уровнем нелинейных искажений для применения в приемниках систем спутниковой навигации. Рассчитан фильтр с полосой пропускания 25 МГц с уровнем третьей гармоники порядка —80 дБ.
G -C ФИЛЬТРЫ; КОМПЛЕКСНЫЕ ФИЛЬТРЫ; ПРИЕМНИКИ СИСТЕМ СПУТНИКОВОЙ
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НАВИГАЦИИ; ТРАНСКОНДУКТИВНЫЙ УСИЛИТЕЛЬ; КМОП 180 НМ
Complex Gm-C filters are widely used for channel selection in the front-end of on-chip receivers. The advantages of Gm-C filters, as compared to conventional active-RC filters and switched capacitor filters, include a higher operating rate and lower power consumption. However, high linearity is a problem for this type of filter structures. It is mostly caused by the characteristics of a transconductance amplifier (TA). If the linearity problem is solved, the advantage of a high operating rate could be of use in satellite navigation system receivers, e. g. GPS and GLONASS systems, where the channel passband in a broadband mode is about 25 MHz.
This paper presents the implementation of a well-known technique which realizes the Gm- C complex filter, described in [1]. It was used for significantly low frequencies — Bluetooth and ZigBee applications. The technique is based on the frequency shift of low-pass filter amplitude response by implementing interconnections. It should be noted that there are just a few known TAs designed for a frequency range of satellite navigation systems. The first part of the paper
is devoted to the problem of high-linear TA realization suitable for the proposed application. The second part describes the proposed filter realization. The third part discusses the problem of filter tuning.
Transconductance Amplifier Realization
Linearization techniques for TAs were properly discussed in [2]. Most common are adaptive feedback technique, source degeneration technique and compensation technique. Even without considering the Gm/1 ratio, which is a factor of efficiency for TAs, it was shown that the compensation technique, first proposed by authors of [3], is optimal for the high-linear TA design in terms of THD level in equal conditions. Though lower THD could be achieved with a source degeneration technique, it is relevant in exchange for a lower Gm/I ratio, which leads to higher consumption for the same G value.
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The proposed TA structure is shown in Fig. 1 a. It includes saturation region amplifier section, M1-M4, triode region amplifier section M5-M8, feedforward circuit M9-M14
b)
Fig. 1. Transconductance amplifier structure (a); DC characteristic of proposed TA (b)
Table 1
Comparison of transconductance amplifiers
Transconductance amplifier [3], 2002 [4], 2010 [5], 2012 (source degeneration) This work
Technology, nm 350 180 180 180
Supply voltage, V 2.5 1.8 1.8 1.8
THD, dB >54 (1 1 MHz) 51 (1 Vpp 50 MHz) <50 (0.8 10 MHz) 59.22 (1 50 MHz)
Gm value, uA/V 68 120-287* 140-340* 159 (130-200)*
* Variation due to Vc change for the fixed linear input amplitude range.
and current mirror as dynamic load M15-M20. Using a parametric analysis in Cadence IC 6.15 for UMC_180 library transistors P_18_MM and N_18_MM, supply voltage Vdd of 1.8V and control voltage Vc of 1V, the TA with Gm value of 159 uA/V was designed. The transistor length was set to 240 nm for better operating performance. Resulting transistors sizes are the following: M1-M4, M11: W = 1.95 um M9-M10: W = 0.975 um, M7-M8, M14 W = 240 nm, M5-M6: W = 2 um, M12-M13 W = 1 um, M15-M20: W = 14.8 um.
In Fig. 1 b DC characteristicGm = f (VinDC), where VinDC is the applied voltage difference at the input of the TA, is shown. 1 % Gm variation range is ±672 mV, which is nearly 75 % of the supply voltage limit. Table 1 compares characteristics of the proposed TA versus familiar analogues. The GJI ratio, which was mentioned only for [3], is 1.15 for the proposed amplifier. In [3] it was less than 1.
In [2], no information was given for the frequency at which THD was simulated. As it is, for lesser input amplitude of 0.5 Vpp THD was certainly less than 59 dB. For better comparison the power consumption and Gm/I ratio should be analysed, but most publications do not provide necessary information.
Complex Filter
The proposed filter structure is shown in Fig. 2 a. Number 1 marks the input TA, which realizes voltage-to-current conversion; 2 marks TAs in a resistor mode; 3 marks a block of gyrators, which realizes inductance imitators;
4 marks blocks of grounded capacitors;
5 marks blocks of interconnection gyrators G11-G55 for complex reactance transform: each node of a frequency-dependent element, namely capacitor, is connected to its counterpart in other channel. The Frequency shift is defined by the equation:
®0 = Gii / Ci, where G.. — value of interconnection TA (in gyrator); C. — value of the node capacitor.
Calculated values for the capacitors are the following:
C1 = C5 = 2.50 pF, C2 = C4 = 6.55 pF, C3 = 8.10 pF.
Taking into account the effect of parasitic capacitance, after the first simulation the capacitor values were recalculated:
C1 = C5 =1.89 pF, C2 = C4 = 4.96 pF, C3 = 6.13 pF.
The values of the TAs needed for interconnection gyrators are the following:
Gn = G55 = 173 uA/V, G22 = G44 = 453 uA/V, G33 = 560 uA/V.
These TAs were designed by the same method as the main TA. The characteristics do not change linearly with a proportional increase (or decrease) of transistor sizes. Using a parametric analysis, amplifiers with nearly the same linearity and Gm/I ratio were realized.
The complex filter amplitude response |K(p)| is shown in Fig. 2 b. The dotted curve refers to a filter response before capacitors recalculation. The solid line refers to a final response of the designed filter. Table 2 compares complex filter characteristics. There are no available works where the bandwidth would be the same, but it is worth noting that even for lower frequencies and narrower bandwidth the linearity of the designed filter is significantly better.
Tuning
It is known that in Gm-C filters the tuning system requires the compensation of the technological variation of parameters. There are different approaches to the problem of building a tuning circuit, e. g. in accordance with [3], it could be a phase-locked loop with the G -based VCO and reference oscillator
m
ff as shown in Fig. 3 a. The idea is always in tuning by changing the control voltage Vc. Taking into account a significant difference in values of interconnection TAs, which could lead to a different change for the same control voltage change, it is imperative to check if the two tuning systems are required.
Fig. 3 b shows tuning characteristics of the filter. The control voltage Vc was changed simultaneously for all TAs of the filter structure from 0.9 V (dotted curve) to 1.1 V (dotted-dashed curve). The solid line refers to an initial value of 1 V. The frequency shift is not the
4
b)
Fig. 2. G -C complex filter structure (a); frequency response of complex filter (b)
Comparison of complex filter characteristics
Table 2
Work [6], 2006 [7], 2013 This work
Technology G -C, 180 nm m ARC, 65 nm G -C, 180 nm m '
Prototype Butterworth, 6th order 6th order Butterworth, 5 th order
Center frequency, MHz 2 4-28 27.5
Bandwidth, MHz 3 4.2-18 25
Image rejection ratio, dB 55 > 31 > 47
Linearity THD 45 dB (0.2 Vpp 2 MHz) IIP3 25-28 dBm THD >80 dB (1 Vpp 25 MHz) IIP3 33 dBm (100 Ohm)
Power consumption, mW 0.72 8.5-26 «36
а)
b)
fref
f. MHz
I5ÍL12 16 18 20 22 24 26 28 30 32 34 36 38 40
Fig. 3. Block diagram of tuning circuit [3] (a); tuning characteristic of proposed filter (b)
same for V = 1.1 V and V = 0.9 V, because
c c '
of non-linear dependence of Gm versus control voltage, as could be seen from Fig. 1 b. A more important thing is that the relative change of the center frequency and bandwidth is not the same (23.63 and 28 %, respectively for 0.9 V and 12.73 and 12 % for 1.1 V). Depending on the target range of the frequency shift and bandwidth change, both the second frequency tuning system and automatic gain control circuit could be required.
A complex 180-nm CMOS Gm-C filter with high linearity was designed. The obtained
linearity is more than 80 dB THD @1V
J pp
25 MHz, for 25 MHz bandwidth filter centered at 27.5 MHz. The power consumption is about 36 mW for 5+5 order Butterworth complex filter. The achieved linearity in terms of IIP3 is at least 5 dBm higher. The problem of filter tuning was discussed. It was confirmed that for a relatively high frequency shift more than one tuning system would be required.
REFERENCES / СПИСОК ЛИТЕРАТУРЫ
1. Phanumas Khumsat, Apisak Worapishet Design of 1.8-V CMOS Polyphase Filter for Dual-Mode Bluetooth/Zig-Bee Transceiver. ECTI Transactions on Electrical Eng., Electronics and Communications, 2006, Vol. 4, No.1, pp. 29-34.
2. Ha Le-Thai, Huy-Hieu Nguyen, Hoai-Nam Nguyen, Hong-Soon Cho, Jeong-Seon Lee, Sang-Gug Lee An IF Bandpass Filter Based on a Low Distortion Transconductor. IEEE Journal Solid-State Circuits, 2010, Vol. 45, No. 11, pp. 2250-2261.
3. Korotkov A.S., Morozov D.V., Hauer H., Unbehauen R. A 2.5-V, 0.35-um CMOS trans-conductance-capacitor filter with enhanced linearity. Proc. MWSCAS'02, Tulsa, 2002, Vol. 3, pp. 141-144.
4. Vijaya Bhadauria, Krishna Kant, Swapna
Banerjee A Tunable Transconductor With High Linearity. Proc. IEEE Asia Pacific Conference on Circuits and Systems, 2010, pp. 5—8.
5. Shin-Jye Hsu, Che-Yu Lu, Chung-Chih Hung. 40MHz Gm-C Filter with High Linearity OTA for Wireless Applications. Proc. Internat. Symp. on VLSI Design, Automation, and Test, Apr. 2012, pp. 1-4.
6. Ramin Zanbaghi, Mojtaba Atarodi, Armin
Tajalli An Ultra-Low Power Gm-C Complex Filter for Low-IF Wireless PAN Applications. Proc. TENCON 2006, pp. 1-4.
7. Nan Qi, Zheng Song, Baoyong Chi Albert Wang, Tianling Ren Zhihua Wang. A Multimode Complex Bandpass Filter With Gm-assisted Power Optimization and I/Q Calibration. Proc. ISCAS 2013', pp. 1845-1848.
IVANOV, Nikita V. St. Petersburg State Polytechnical University. 195251, Politekhnicheskaya Str. 29, St. Petersburg, Russia E-mail: ivanovnick@mail.ru
ИВАНОВ Никита Валерьевич — инженер первой категории кафедры интегральной электроники Санкт-Петербургского государственного политехнического университета, студент шестого курса. 195251, Россия, Санкт-Петербург, ул. Политехническая, д. 29 E-mail: ivanovnick@mail.ru
KOROTKOV, Alexander S. St. Petersburg State Polytechnical University. 195251, Politekhnicheskaya Str. 29, St. Petersburg, Russia. E-mail: korotkov@rphf.spbstu.ru
КОРОТКОВ Александр Станиславович — заведующий кафедрой интегральной электроники Института физики, нанотехнологий и телекоммуникаций Санкт-Петербургского государственного политехнического университета, доктор технических наук, профессор. 195251, Санкт-Петербург, ул. Политехническая, д. 29. E-mail: korotkov@rphf.spbstu.ru
© St. Petersburg State Polytechnical University, 2014
