60 Ghz Transceiver Circuits In Sige Circuits I Sige

Proceedings of the 30th European Solid-State Circuits Conference September 20 - September 24, 2004, Leuven, Belgium, pp. 83-86

60 GHz Transceiver Circuits in SiGe:C BiCMOS Technology Wolfgang Winkler, Johannes Borngräber, Hans Gustat, Falk Korndörfer IHP, Im Technologiepark 25, D-15236 Frankfurt (Oder), Germany Phone: +49-(0)-335-5625-150 E-mail: [email protected]

Abstract This paper presents the design and measurement of key circuit building blocks for a high-data-rate transceiver in the 60 GHz band. The adopted modulation scheme is ASK for simple configuration with high data rate. The circuits presented are: LNA, oscillator, mixer, modulator and demodulator. The circuits are fabricated in a 0.25 µm SiGe:C BiCMOS technology.

1. Introduction Traditionally, multimedia content is transferred with wired transmission systems. Because of the limited data rates, conventional WLAN systems cannot take over this task. That’s why new wireless transmission systems with data rates > 150 Mb/s are under investigation [1]. In [2] a wireless transceiver module at 60 GHz is presented showing a data-rate as high as 1.25 Gb/s. The chip set consists of different MMICs based on 0.15 µm AlGaAs/InGaAs heterojunction FET technology. For future low-cost systems it is required to realize the full transceiver in a silicon-based technology on a single chip or at least with a low chip-count. With the modern CMOS, bipolar and BiCMOS technologies developed in the last few years it seems certain to make true this goal in short term [3]. This paper presents the design and measurement of key circuits for the implementation in a high data-rate 60 GHz transceiver. At the present state, the circuits are completed for on-wafer measurement.

analog part of the system without having to deal with complexity, throughput and power consumption of high-data rate digital processing. - The bandwidth available in the 61 GHz ISM band is 500 MHz. Thus, bandwidth efficiency is less important compared to lower frequency bands. - In a bandwidth-efficient modulation scheme like OFDM, the A/D converter (ADC) is a major problem. With the required low power consumption and throughput in mind, it is difficult to design an ADC that would suit the system needs. In summary, ASK seems well suited for a firstgeneration 60 GHz transceiver, due to simplicity and power efficiency. Later implementations will certainly make use of more sophisticated modulation schemes. The transmit path of the transceiver consists of a 61.25 GHz fundamental mode oscillator, a switch and a power amplifier. Between the modulator and the power amplifier a filter is inserted to reduce spurs. The receiver path consists of a low noise amplifier (LNA), a mixer, a 56 GHz oscillator, a variable gain amplifier and an ASK demodulator. The circuit blocks in dashed boxes in Figure 1 are described in this paper. .

2. Circuit Design A. Transceiver Architecture Figure 1 shows the block diagram of the proposed transceiver. It is based on amplitude shift keying modulation principle (ASK). The reasons for the selection of this modulation technique are the following. - ASK is a very simple modulation scheme allowing designing and demonstrating a 60 GHz transceiver in a relatively short time-scale. It allows focusing on the

Figure 1 : Transceiver block diagram

B. Low Noise Amplifier The three-stage LNA circuit is shown in Figure 2 and Figure 3. One task of the LNA design was to get unconditional stability for both on-wafer measurements and for the use of the amplifier in test board modules together with the other receiver components. Especially the use of bond wires for the connection of the ground potential (and the associated inductance) causes serious stability problems if a single-ended configuration is used. That’s why all the stages are designed in differential configuration. The input and output are trafo-coupled. The transformer coupling is useful in two ways. First, it acts as a balun for connection to a single-ended antenna. Second, the primary and secondary windings of the transformer are tuned to gat a bandpass characteristic. With this, an additional input filter in the RX path can be omitted and this function is integrated in the LNA. The circuit of one single stage of the LNA is shown in Figure 3. It is a differential stage with inductive load and with matching network at the output.

interference via the silicon substrate is reduced in comparison to a single-ended version. The diodes and resistors in the circuit are used to define the operating point of the transistors. The tank is a symmetric circuit of the Inductors L1 and the MIM capacitor C1. The base-emitter capacitors CBE of the bipolar transistor acts in parallel to the MIM capacitors C1. For explanation, the tank can be divided into two half circuits separated by the symmetry line shown in Figure 4. In operation, the oscillator-halves are working in the odd mode, such that the outputs are 180º out of phase. The nodes of the tank indicated by the symmetry line are fixed at virtual ground for the fundamental tone. The inductors L1 are designed as arms of a single loop of metal layer 4 (Aluminium with 2 µm thickness and 10 µm width). The simulated inductance is 85 pH per arm. The capacitor C2 is a varicap formed by an nchannel MOS device. In order to get a wide range of capacitance variation the device is working in the whole range from depletion to accumulation. The voltage for frequency-control is applied to the n-well of the structure. With VCtrl becoming more positive, the MOS structure is driven into depletion and the capacitance will be reduced. The output of the oscillator core is connected to an amplifier in the case of the receive path. The oscillator of the transmit path is directly connected to the amplifier with switch (Figure 1).

Figure 2 : 60 GHz LNA.

Figure 3 : Circuit of one stage of differential LNA.

C. Oscillators The transceiver architecture of Figure 1 requires two oscillators working at different frequencies. One is for the transmit path at the centre of the ISM band at 61.25 GHz. The other frequency in the receive path is the transmit frequency reduced by the intermediate frequency (IF). The circuit principle of both oscillators is based on a modified Colpitts principle in a symmetric configuration of negative-resistance type as shown in Figure 4 [4]. With the symmetric circuit the signal

Figure 4 : Circuit of the LC oscillators used in the receive

and the transmit path. D. Mixer The mixer circuit is a balanced Gilbert cell with symmetric LO and RF inputs. The differential output is connected to the VGA giving the differential signal to the demodulator. E. ASK Modulator The modulator circuit consists of bipolar transistors and a MOS transistor (Figure 5). It is a symmetric common-

base amplifier with the base connected to the drain of an n-channel MOS transistor. The gate of the MOS transistor is connected to the data input. With low voltage level at the input the MOS transistor is switched off and the common-base circuit acts as an amplifier transferring the RF to the power amplifier. With highlevel at the input the MOS transistor is switched on and the base-emitter voltage approaches zero. In this manner the amplifier is switched off and the power amplifier input is isolated from the oscillator. The advantage of the circuit is the shielding function of the bipolar base region so that a good isolation can be expected.

Figure 5 : RF switch for modulation of the RF signal.

3. Measurement Results The circuits were fabricated in the IHP 0.25 µm SiGe:C BiCMOS technology. The bipolar transistors of this technology have a maximum transit frequency fT of 200 GHz and also a maximum frequency of oscillation fmax of 200 GHz. In the circuits mainly bipolar transistors and passives were used. The only MOS transistor so far is the switch in the modulator circuit. Figure 7 shows photos of the presented circuits. The oscillators were characterized using an on-wafer test-system with GSprobes. The supply voltage of the oscillators and mixer is 3V and the voltage of the switch, demodulator and the LNA is 2.5 V.

b)

a)

F. Demodulator The amplitude-shift keying demodulation uses a fullwave rectifier together with a lowpass filter (LPF) corresponding to the maximum data rate of 1Gb/s. The block diagram is shown in Figure 6. Full-wave rectification is easily achieved using both differential inputs. The diode function is implemented using highperformance transistors, resulting in wide input frequency range (3-30 GHz) giving large IF flexibility. However, the summation is a differential-to-single-ended conversion. To regenerate the differential mode, the output of an additional common-mode block (CMB) acts as the corresponding inverted signal. A subsequent differential amplifier provides sufficient gain for bit slicing.

c)

d)

e) Figure 6 : ASK demodulator block diagram.

Figure 7 : Chip Photo of a) LNA, b) mixer, c) oscillator of

the receive-path, d) oscillator and switch in the transmit-path, e) receiver test structure.

The LNA was measured on wafer with a 110 GHz vector network analyser. The maximum gain is 9.6 dB. The gain-maximum is reached at 61 GHz, which is exactly the target frequency of the requested ISM band (Figure 8). The LNA has a bandpass characteristic resulting from the tuned transformers at input and output. With this characteristic, no additional bandpass filter is needed.

Figure 10 : Tuning curve of the LC oscillator in the

receiver.

4. Summary and Conclusions

Figure 8 : Gain curves of the 60 GHz LNA.

The transmit path was measured by applying a rectangular waveform to the data input of the modulator while the integrated oscillator generates the RF power. Figure 9 shows the output of the switch measured with an oscillograph. The signal at the data input has a frequency of 200 MHz. The output amplitude is 210 mVpp. The output frequency of the oscillator is 65 GHz. This frequency is too high in comparison to the target of 61.25 GHz. A redesign of this oscillator is required.

Key circuit blocks for 60 GHz high data-rate transceiver system were successfully designed and fabricated in a SiGe:C BiCMOS technology. The adopted modulation scheme was ASK for simple configuration with high data rate. The measurement results show 9.6 dB gain of the LNA at 61 GHz, 4 GHz tuning range of the oscillator in the receiver with a centre frequency of 58 GHz and an ASK modulator with good isolation properties and 210 mVpp output voltage. Further measurements and a redesign is in progress.

Acknowledgements The authors acknowledge the IHP technology team for chip fabrication and the modelling team for supplying accurate models of the devices.

References

Figure 9 : Signal of the oscillator with switching the

output on and off. The LC oscillator in the receive path has a tuning range from 56 GHz to 60 GHz. The chosen IF frequency for the VGA and the demodulator is 4 GHz. Figure 10 shows the tuning curve of the VCO.

[1] P. Smulders, “Exploiting the 60GHz band for local wireless multimedia access: prospects and future directions,” IEEE Communications Magazine, pp.140-147, Jan. 2002, pp. 118-121. [2] K. Ohta et al., “Wireless 1.25 Gb/s transceiver module at 60GHz band,” ISSCC Dig. Tech. Papers, pp. 298-299, Feb. 2002. [3] S. Reynolds et al., “60 GHz transceiver circuits in SiGe bipolar technology,” ISSCC Dig. Tech. Papers, pp. 442-443, Feb. 2004. [4] W.Winkler et al., “60 GHz and 76 GHz oscillators in 0.25 µm SiGe:C BiCMOS“, IEEE Int. Solid-State Circuits Conf., February 2003, pp. 454-455.