Krizhanovskii V., Sang-Gug Lee Information and Communications University 119 Munjiro, Yuseong-gu, Daejeon, 305-714, Korea Tel: +83(042)-8666133, email: sglee@icu.ac.kr

Channel-gate DC bias

Fig. 1. Mixer schematic and simulation setup

Abstract – This paper describes sub-harmonic mixer designed in 0.18 |jm CMOS process and intended for use in low- power IEEE 802.15.4-compliant 2.4 GHz transceiver. Device addresses the following issues of the direct conversion architecture: flicker-noise, LO leakage and oscillator pulling.

I.  Introduction

In the recent years, low-power integrated system design was established as one of the major trends in integrated circuit design industry and research. Requirements of low cost and close integration with digital circuitry dictate the usage of CMOS IC processes for RF and analog parts of transceiver. At the same time, while pursuing toward low-cost low-power receiver, direct conversion architecture is the natural selection.

Mixer operation principle under assumption of ideal switching is illustrated in Fig. 2 and Fig. 3.

Fig. 2. Gate-channel LO differential waveforms minus their DC levels for MOSFET keys driven by l-LO (M1-M2 and M7-M8 keys) and Q-LO (M3-M4 and M5-M6 keys) signals

At a first glance, it looks like low-frequency noise in a down-conversion mixer should be up-converted by LO and should not appear at the output frequency. In reality, not only it does appear at the output, but represents a severe impediment in usage of direct down-conversion architecture in wireless communication systems, especially in ones with narrow channel bandwidth. Two mechanisms responsible for non-complete up-conver- sion of the flicker noise were identified in literature [1]. One of them, which is the random modulation of the mixer switching instances by the low-frequency noise, is the dominant one in most practical situations. It was also pointed out that this dominant mechanism could be efficiently suppressed by using the passive mixer without DC-current.

Fig. 3. Two cases of I- and Q-driven keys opening and closing timing resulting from different switching thresholds shown in Fig. 2

Among other issues of Direct Conversion receiver design are self-mixing DC-offset due to LO signal coupling, and VCO pulling due to transmitter output signal and interferer signals coupling to the VCO circuitry [2]. The only feasible approach to deal with this issue is to operate VCO at the frequency not equal to the incoming RF carrier. This can be done by either the VCO frequency transformation [3] (division or multiplication, rational or fractional ones) or by using mixer in which the effective frequency mixing appears between input RF signal and harmonic of applied LO signal. In case of direct-conversion architecture it means that the mixer achieves desired down-conversion by accepting LO signal at sub-harmonic of RF carrier (sub-harmonic mixer) [4, 5].

Sub-harmonic passive mixer in Direct Conversion receiver can provide the advantages described above, as well as flicker noise-free operation.

II.  Main part

The schematic of simulation setup is shown in Fig. 1. The differential-driven key is used instead of single- MOSFET key in the conventional switching bridge. Accepting multiple-phase quadrature LO signal at half the frequency of RF carrier, the sub-harmonic mixer exhibit time-domain switching function similar to conventional mixer driven by LO signal with double frequency, thus implementing desired direct down-conversion.

Even though the period of switching function is half of the period of each differential signal for any gate- channel DC voltage bias (as long as the switch operates), opening and closing timing of I- and Q-driven pairs of keys depends on gate-channel bias and in turn affects the linearity and noise performance of the mixer, as it was found through simulation.

One of the design goals was the flicker-noise-free operation, therefore, mixer’s Noise Figure for different gate-channel DC biases was simulated, Fig.4. Gate- channel DC bias is swept from 0.2 to 0.4 V. Under assumption of ideal switching, the DC bias of 0.32 V corresponds to switching when only one pair of keys (l-driven or Q-driven) is open at a time.

Fig. 4. Single side-band spot NF of the mixer versus output frequency for different gate-channel DC biases

As can be seen from Fig.4, flicker-noise corner frequency does not exceed few kilohertz, meaning that simulation shows flicker-noise-free operation.

The simulated mixer performance after the optimization of transistor sizes and voltage bias is summarized in Table 1.

As a way to provide quadrature LO signal required for the mixer operation, quadrature Voltage Controlled Oscillator (QVCO) was designed and submitted to fabrication at the same chip with mixer.

Table 1

Mixer simulated performance_____________

Voltage gain

-6 dB20

MP3

+ 5.5 dBm

SSB NF flat-band

10.6 dB

SSB NF @ 100 Hz

11 dB

LO differential amplitude

0.63 V

Current consumption

No current consumption in the mixer core

Intended transceiver supply voltage

1.25 V

III.  Conclusion

Simulation results of passive sub-harmonic mixer are presented. Flicker-noise behavior that needs further theoretical explanation is observed. Sub-harmonic mixer with QVCO at sub-harmonic of incoming frequency constitute architecture that addresses the following issues of direct conversion receiver: flicker-noise, LO leakage and oscillator pulling. To the best of authors’ knowledge, given mixer topology is first time utilized as a passive mixer. Simulation results of the mixer show flicker-noise-free operation with NF 10.6 dB, voltage gain -6 dB20 and IIP3 +5.5 dBm without DC power consumption. Mixer interconnected with QVCO is submitted for fabrication using 0.18 |jm CMOS process and is intended for use in IEEE 802.15.4- compiant2.4 GHz low-power transceiver.

IV.  References

[1]Darabi H.. Abidi A. A. Noise in RF-CMOS mixers: a simple physical model // IEEE Journal of Solid-State Circuits, V.

35,- № . 1.- Jan. 2000,- P. 15 – 25

[2]Razavi B. Design considerations for direct-conversion receivers // IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, Volume: 44, Issue: 6, June 1997, P. 428-435

[3] J. Strange. S. Atkinson. «А Direct Conversion Transceiver for Multi-Band GSM Application» / IEEE Radio Frequency Integrated Circuit Symposium Digest of Papers, P. 25-28,

2000.

[4] Z. Zhang. Z. Chen. J. Lau. «А CMOS Self-Mixing-Free Front-End for Direct Conversion Applications» / IEEE International Symposium on Circuits and Systems, Vol.4, P. 386 – 389, 2001.

[5]K. G. Lee. J. B. Park. J. W. Lee. S. W. Lee. H. K. Huh.

D.       K. Jeong. W. C. Kim. «А Single-Chip 2.4 GHz Direct-Conversion CMOS Receiver for Wireless Local Loop Using Multiphase Reduced Frequency Conversion Technique» / / IEEE Journal of Solid-State Circuits, Vol.36, № 5, P. 800- 809, May 2001.

0. 18 мкм КМОП субгармонійних ЗМІШУВАЧ ДЛЯ Трансивер IEEE802.15.4 НА 2,4 ГГц

Крижанівський В. В., Лі Сан-Гук Університет інформатики і комунікацій 119 Munjiro, Yuseong-gu, Daejeon, 305-714, Korea Tel: +83 (042) -8666133, email: sgiee@icu.ac.kr

Анотація – Описано субгармонійних змішувач, спроектований для 0.18 мкм КМОП процесу і призначений для малопотужного приемопередатчика на частоту 2.4 ГГц, що задовольняють стандарту IEEE 802.15.4. Пристрій орієнтований на вирішення наступних проблем архітектури з прямим перетворенням: фліккер-шум, зсув вихідного сигналу по постійному струму і затягування частоти гетеродина.

I.  Введення

фліккер-шум змішувача є однією з проблем в архітектурі приймачів прямого перетворення. В літературі [1] підкреслюється, що він може бути зменшений використанням пасивних змішувачів. Серед інших проблем приймачів прямого перетворення – постійна напруга на виході змішувача внаслідок наведень і самосмешенія сигналу гетеродина, і затягування частоти ГУН внаслідок проникнення вихідного сигналу передавача на ГУН [2]. Як метод усунення зазначених явищ пропонується схемне рішення, описане в доповіді.

II. Основна частина

На рис. 2 показані змодельованих частотні залежності коефіцієнта шуму пропонованого змішувача (Мал. 1) при різному зсуві на затворі. Продемонстровано роботу змішувача без фліккер-шуму, але хід залежностей потребує додаткового теоретичному дослідженні.

Для створення квадратурного сигналу гетеродина, необхідного для роботи змішувача, останній був виготовлений на одному кристалі з квадратурних ГУН.

III. Висновок

Топологія змішувача вперше використовується в пасивному варіанті. Моделювання показує режим роботи без фліккер-шуму. Очікувані параметри змішувача: коефіцієнт шуму 10.6 дБ, коефіцієнт посилення по напрузі -6 дБ і умовна межа динамічного діапазону по інтермодуляційних спотворень третього порядку +5.5 дБм. Схема була розроблена для використання в малопотужних прийомопередавача діапазону 2.4 ГГц за стандартом IEEE 802.15.4.

Джерело: Матеріали Міжнародної Кримської конференції «СВЧ-техніка і телекомунікаційні технології»