A SiGe Monocycle Impulse Generator for Impulse Radio Ultra-Wideband Applications Jochen Dederer, Andreas Trasser and Hermann Schumacher University of Ulm, Dept. of Electron Devices and Circuits, Albert-Einstein-Allee 45, 89069 Ulm, Germany, phone: +49 731 5031590, e-mail:
[email protected]
I. I NTRODUCTION There are several possibilities for generation of UWB impulses. Step recovery diodes (SRD’s) are used to generate step functions with extremely small transition times. Various impulse generators using SRD’s together with a pulse shaping network have been published [1], [2]. The major drawback here is that these circuits can not be fully integrated. Other approaches use an oscillator that is time-gated with an external switch. Most previous work based on a gated-oscillator method includes an integrated monocycle impulse generator and transmitter [3]. However, the spectrum of the generated pulse sequences does not meet the FCC emission restrictions. An example for an impulse generator with active pulse shaping circuits is a CMOS impulse generator that generates output impulses which are similar to the fifth derivative of a Gaussian pulse [4]. Based on simulation results, the authors conclude that this approach will meet the FCC mask without additional filtering. A common approach for an impulse generator with passive pulse shaping circuits is to use an analog differentiator in the form of a L-C network together with a resistive load. The impulse duration and the achievable amplitude is determined by the L-C pulse shaping network and the risetime of the signal that controls the pulse shaping stage. Similar to approaches with hybrid pulse generators using SRD’s, the main difficulty here is to provide a control signal with very short transition times. Previously published work on impulse generators with passive pulse shaping circuits shows simulated pulse amplitudes in the range of 30 mV pp [5]. In this work we present a novel, fully integrated SiGe HBT impulse generator that exhibits pulse amplitudes of 260 mV pp . The pulse shape of the generated pulses is an excellent approximation of the first derivative of the Gaussian pulse. The broadband output spectrum of the ultra-short pulses is well centered in the allocated spectrum mask and meets the emission restrictions for UWB transmission without the need of complex filter structures. The presented design is optimized with regard to a simple and compact circuit topology together with a minimum
power consumption of the actual pulse shaper circuit. II. T ECHNOLOGY The presented circuit has been designed and fabricated using the commercially available ATMEL SiGe2 HBT technology [6] which offers both, SIC (selectively implanted collector) transistors with an fT of 80 GHz and non-SIC transistors with a lower fT of 50 GHz but a larger collector-emitter breakdown voltage BVCE0 . Both types of transistors are used in this approach. The passive and active devices are realized on low-resistivity 20 Ωcm substrate. The process offers three metallization layers. III. M ONOCYCLE P ULSE S HAPE Several possible monocycle pulse shapes for UWB are described in [7]. Most commonly used is a monocycle pulse shape that can be obtained by taking the first derivative of the Gaussian pulse. This monocycle can be represented by f (t) = − √
At t2 exp − 2 2σ 2π σ 3
(1)
where A defines the amplitude and σ determines the pulse width. The normalized pulse shape and the corresponding power spectral density of such a Gaussian monocycle with σ = 25 ps are shown in Fig. 1 and Fig. 2, respectively. The center frequency of the pulse spectrum is about 6.8 GHz and corresponds to the center of the allocated UWB frequency band from 3.1 GHz to 10.6 GHz. 1
Normalized Amplitude
Abstract— A compact 0.56 mm x 0.56 mm SiGe ultra-wideband (UWB) impulse generator is presented. The realized circuit generates ultra-short, symmetrical pulses with a peak-to-peak amplitude of 260 mV. The spectrum of the generated pulses has a -10 dB bandwidth of 7.5 GHz centered around 6 GHz. The pulse shape is similar to the first derivative of the Gaussian pulse and the corresponding power spectral density fits very well into the FCC regulatory spectrum mask.
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Fig. 1.
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Normalized pulse shape of Gaussian monocycle with σ = 25 ps
signal into an output signal with a pulse shape similar to the Gaussian pulse and accordingly the falling edge of the squarewave into a pulse with negative amplitude. Only the positive pulse activates the pulse shaper, see below. The third and last function block is the actual monocycle pulse shaper. As stated earlier, the targeted pulse shape is obtained by taking the first derivative of the Gaussian pulse. The pulse shaper circuit consists of a single transistor in common emitter configuration with a resistive feedback RE and the analog differentiator which is formed by inductor L and capacitor C together with the off-chip resistor RL as shown in Fig. 4.
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Fig. 2. Normalized power spectral density (PSD) of Gaussian monocycle with σ = 25 ps
IV. C IRCUIT D ESIGN
AND
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L AYOUT
In Out
Fig. 3.
Out
In
The pulse generator circuit can be divided into three function blocks. The objective of the first function block is the generation of a squarewave signal with sufficiently short transition times for the actual pulse shaping in the succeeding function blocks. The first function block is realized as a limiting amplifier with two differential amplifier stages, each followed by a pair of emitter followers for decoupling of the stages as shown in Fig. 3.
VCC
C
RE
Fig. 4.
RL
Monocycle pulse shaper circuit
The sizes of the transistor, the inductor and the capacitor are chosen with regard to a maximum amplitude of the generated pulses while maintaining the targeted pulse duration together with a moderate pulse ringing due to the resulting L-C parallel resonance circuit. The transistor serves as a switch which is controlled by the output signal of the preceding differentiating network. Therefore the monocycle pulse shaper circuit only draws current in the presence of the positive, ultra short pulses at the base of the transistor. The complete chip, pads included, has a size of 0.56 mm x 0.56 mm. A microphotograph of the chip is shown in Fig. 5.
Single stage of limiting amplifier
This first function block provides a control signal with very short transition times and makes the following pulse shaping almost independent of the risetime of the input signal - for the evaluation of the chip the pulse generator can even be operated with sinusoidal inputs. The two amplifier stages draw 5 mA at 3 V and 9 mA at 6.5 V, respectively. For low repetition rate input signals, this function block could be powered down in between the generated pulses. The limiting amplifier is followed by a C-R differentiating highpass network. This function block transforms the rising edge of the squarewave
Fig. 5.
Microphotograph of the monocycle impulse generator
V. M EASUREMENT R ESULTS
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All presented measurements were performed on-wafer in a 50 Ω test environment. Measurements in the time- and frequency domain were carried out using a digital sampling scope together with a 50 GHz sampling head and a spectrum analyzer, respectively. A conventional sinusoidal RF source generated the input signal to the circuit. Two on-wafer groundsignal-ground microwave probes were used to contact the input and output ports of the circuit.
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A. Time-Domain Measurements
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Fig. 7. Comparison of measured power spectral density with FCC regulatory mask for indoor UWB devices
circuit. Increasing the data rate leads to a concentration of the signal energy on less spectral lines while increasing the level of each spectral line. A random pulse-to-pulse interval smoothes the output spectrum as the RF energy is distributed more uniformely accross the band. In the past, several papers have been published, presenting time-hopping modulation schemes with the focus on the spectral shaping of Impulse Radio UWB systems [9].
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Fig. 6 shows the measured output pulses with a peak-to-peak amplitude of 260 mV and a pulse duration of 150 ps. The latter is defined as the time interval between the 10 % points of the negative and positive peak of the monocycle.
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VI. C ONCLUSION
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Measurement FCC Frequency Mask for Indoor UWB
Measurement Gaussian Monocycle 0
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Fig. 6. Comparison of measured monocycle waveform with Gaussian monocycle (σ=25 ps, scaled to the amplitude of the measured pulse)
As shown in Fig. 6, excellent agreement between the mathematical derived, targeted monocycle pulse shape with σ = 25 ps (as described in Equation (1)) and the measured pulse shape has been achieved. The measured pulses show a good symmetry and a moderate ringing level. B. Frequency-Domain Measurements The spectrum of pulse sequences with regular pulse-to-pulse intervals shows discrete energy spikes corresponding to the repetitive period of consecutive pulses. The measured output spectrum of such a 50 MHz pulse train and its compliance with the FCC regulatory spectrum mask for indoor UWB devices [8] is depicted in Fig. 7. For this pulse sequence, the monocycle pulse shaper circuit as shown in Fig. 4 draws 84 µA at 2.5 V which equals a power consumption of only 210 µW. The signal energy of the pulse sequence is spread across the full allocated bandwidth and the corresponding output spectrum is well centered in the FCC spectrum mask. The spectral regrowth for low frequencies up to approximately 1 GHz is due to feed-through of spectral components of the squarewave signal to the output of the
We have demonstrated a novel UWB monocycle impulse generator in a commercially available SiGe HBT technology. The impulse generator exhibits pulses with a peak-to-peak amplitude of 260 mV. The corresponding spectral power density has a -10 dB bandwidth of 7.5 GHz which fits very well into the allocated spectrum mask. Good agreement between the targeted, mathematical derived pulse shape and the measured pulse shape has been achieved. The overall power consumption of the presented chip is 73.7 mW for a 50 MHz pulse repetition frequency. However, it can be reduced to a few milliwatts using a suitable power-down circuit for the limiting amplifier. The presented results qualify this circuit for operation in Impulse Radio UWB applications. ACKNOWLEDGMENT The authors wish to thank ATMEL GERMANY GmbH Heilbronn for the excellent support. R EFERENCES [1] J. Han and C. Nguyen, ”Ultra-wideband electronically tunable pulse generators”, IEEE Microwave and Wireless Components Letters, vol.14, issue 3, pp. 112-114, March 2004. [2] J. Han and C. Nguyen, ”A new ultra-wideband, ultra-short monocycle pulse generator with reduced ringing”, IEEE Microwave and Wireless Components Letters, vol.12, no. 6, pp. 206-208, 2002. [3] A. Azakkour, M. Regis, F. Pourchet, G. Alquie, ”A new integrated monocycle generator and transmitter for ultra-wideband (UWB) communications”, IEEE Radio Frequency Integrated Circuits (RFIC) Symp., 12-14 June 2005, Long Beach, CA, USA. [4] H. Kim and Y. Joo, ”Fifth-derivative Gaussian pulse generator for UWB system”, IEEE Radio Frequency Integrated Circuits (RFIC) Symp., 12-14 June 2005, Long Beach, CA, USA.
[5] Y. Zheng, H. Dong and Y. P. Xu, ”A novel CMOS/BiCMOS UWB pulse generator and modulator”, IEEE MTT-S International Microwave Symposium Digest, vol. 2, pp. 1269-1272, 2004. [6] A. Sch¨uppen, J. Berntgen, P. Maier, M. Tortschanoff, W. Kraus and M. Averweg, ”An 80 GHz SiGe production technology”, III-V Review, vol. 14, pp. 42-46, August 2001. [7] X. Chen and S. Kiaei, ”Monocycle shapes for ultra wideband system”, IEEE Proc. of Int. Symp. Circuits and Systems (ISCAS), vol.1, pp. 597600, 2002. [8] Federal Communications Commission: ”First report and order: Revision of Part 15 of the commission’s rule regarding ultra-wideband transmission systems”, ET Docket 98-153, April 2002. [9] J. Romme and L. Piazzo, ”On the power spectral density of timehopping Impulse Radio”, IEEE Conference on Ultra Wideband Systems and Technologies (UWBST), Baltimore, pp. 241-244, May 2002.
