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2015 IEEE International Symposium on Radio-Frequency Integration Technology FR2B-1 An 80 GHz Programmable Frequency Divider for Wideband mm-Wave Frequency Ramp Synthesis M. van Delden∗, G. Hasenaecker∗, N. Pohl† , K. Aufinger‡ and T. Musch∗ † Fraunhofer ∗ Institute of Electronic Circuits, Ruhr-Universität Bochum, Germany Institute for High Frequency Physics and Radar Techniques, Wachtberg, Germany ‡ Infineon Technologies AG, Neubiberg, Germany Abstract—A programmable frequency divider operating at input frequencies from DC to 80 GHz for the use in fractionalN synthesizers is presented. The division factor can be set to all integer values between 12 and 259 and is applied by an 8 bit parallel interface for fast modulation. The remarkably high input frequency in combination with the programmability is achieved by a dual-modulus concept and differential emittercoupled logic with a consequent merging of logic gates into flipflops. Among this, a reset function has been implemented to synchronize multiple synthesizers. The frequency divider has been realized in a SiGe BiCMOS technology (fT /fmax =250/360 GHz). The divider works at a supply voltage of 3.3 V with a power consumption of less than 390 mW. fref PFD fS LF fV fLO f0 fref ÷N PFD LF fS fractional logic ÷N ÷N fractional logic const. (a) (b) Fig. 1. Block diagram of the divider’s proposed applications: (a) in the feedback loop of an offset-PLL and (b) as a reference frequency ramp generator. Prescaler Prog. Dividers A&B Sync. Output FF Buffer Delay fin fout I. I NTRODUCTION Q S The progress in SiGe technologies enables the realization 1 fA A D 4/5 of monolithic microwave integrated circuits with constantly fps DA R L increasing frequencies and bandwidths at low cost. Thus MC B improved frequency synthesizers offer access to high-end miDB L crowave measurement systems, such as high precision FMCW 6 2 radar systems [1] or vector network analyzers. Control Logic R N 8 The key to high performance systems are highly linear frequency ramps with ultra-low phase noise and high bandwidth. Fig. 2. Block diagram of the dual-modulus divider with synchronizing flipA common concept for microwave frequency synthesizers is flop. to stabilize a voltage-controlled oscillator (VCO) in a phaselocked loop (PLL). Among others, two principles are appliII. A RCHITECTURE cable for wideband frequency ramp generation in this case. The presented divider is based on the dual-modulus concept Firstly, as shown in Fig. 1(a) and described in detail in [1], a as shown in Fig. 2. This concept enables high input frequencies constant reference frequency is used and the the division factor for programmable dividers. On the one hand only the prescaler of the frequency divider in the feedback loop of an offset(Fig. 3) has to handle the very high input frequencies with PLL is switched. The second principle is applying a frequency just a simple logic, switching the prescaling factor between ramp to the reference input of a conventional PLL and using 4 and 5. On the other hand the complex logic is shifted into a constant division factor in the feedback loop, as depicted in the following fully programmable dividers A and B, which Fig. 1(b). The reference frequency ramp is generated by a high therefore operate maximum at a quarter of the input frequency. frequency signal source and a second frequency divider, whose In [4] the division factor N of the complete circuit is derived: division factor is alternating, as described in [2]. In both cases a frequency divider with a wide division N = 4 · NA + NB (1) factor range starting at a low value, a high input frequency as well as low residual phase noise is crucial, due to the required Due to the wideband application, division factors N selectable ultra-low phase noise at the VCO’s output. Furthermore, the from the entire integer range from 12 to 259 are beneficial. This high demand for ramp linearity can be achieved by a fractionalis realized as we use a 6 bit divider A and 2 bit divider B. The N frequency division, which requires a change of the division division factors 256 to 259 are caused by a clock cycle delay factor with every output cycle of the divider. Compared to in divider A. An 8 bit parallel interface, applying the division our earlier work presented in [3], we improved this frequency factor, enables fast modulation in fractional-N synthesizers. divider especially by the integration of a reset function, a The core of divider A, shown in Fig. 4, consists of a larger output-pulse-width and the implementation in a new synchronous up counter built with T flip-flops. Reaching a technology for increasing the maxmimum input frequency value of 62 is detected by the 6-times AND. Synchronized from 57 GHz to 80 GHz. by the flip-flop S3 this creates the load signal L with a pulse978-1-4673-7794-2/15/$31.00 © 2015 IEEE 181 MC & D Q & D Q D fps Q fin S2 S3 & Q1 Q0 D Q T Q L L SR SR & Q2 T Q L SR & Q3 T Q L SR & Q4 A0 A1 VCC Q Q L T Q L SR & A3 A2 6 bit data register A4 VCS Q5 T Q L SR L fin R Emitter Follower RS C C output pulse generation QD& D-Latch 2 RC RC L up-counter Q D ≥1 Emitter Follower RT T S Fig. 3. Block diagram of the 4/5-prescaler based on a Johnson ring counter with logic integrated into the D flip-flops for setting the prescaling factor. fA D-Latch 1 XOR Set/Reset on Load RC RC Rbias REF1 REF1 Rbias REF2 REF2 Fig. 5. Schematic of the T flip-flop consisting of a D flip-flop with merged XOR and set/reset function on load [3] as an example for merged emittercoupled logic. A5 III. D ESIGN During the design of each gate several aspects have to be considered to maximize the dividers operating frequency [6]. All gates are realized in differential emitter-coupled logic using differential bipolar transistor pairs as current switches, which are applied to emitter followers for fast loading and unloading the input capacitance of the following stages. To increase the operating speed we consequently merged logic gates into flipflops, by series conjunction of current switches. For a supply voltage of 3.3 V four stacked logic levels, provided by resistive as well as emitter follower based level shifting, can be realized, as shown in Fig. 5. The divider is designed and realized in Infineon’s new SiGe BiCMOS technology B11HFC. This technology includes high speed hetero bipolar transistors with a transit frequency f T = 250 GHz and a maximum oscillation frequency f max = 360 GHz, which were consequently used. As the transistor capacitances, especially C CB , are the dominating limiting factors for maximum operating speed without increasing the power consumption dramatically, it is crucial to choose a small transistor size of 0.22 µm × 1 µm. We designed the transistors maximum peak current densities with 14.3 mA/µm2 , but allways made sure that the effective current densities are still below the recommended design current density for maximum f T of 11.5 mA/µm2 . Simulations during designing of the prescaler have been solved with the delivered HICUM model [7] due to the very high current densities. Where this model is attesting a maximum input frequency of 88 GHz, the Spice Gummel-Poon model (SGPM) ends at 69 GHz. Designing the programmable dividers was mostly supported by SGPM simulations and validated by a few HICUM simulations, because the current densities here are lower and the large amount of transistors results in a dramatically high simulation time using the HICUM model. All simulations have been done without parasitic extraction of the wire capacitances. Fig. 4. Block diagram of 6 bit divider A with up-counter and output-pulse generation. width of one clock cycle of divider A’s input frequency f ps . This signal is buffered and loads the value DA in the next cycle into the counter, resulting in a division factor NA = 64 − DA , due to the 2 clock cycle delay. This enables different division factors every cycle of the whole divider, which is essential for fractional-N synthesizers. In the usual concept, as presented e.g. in [4], the load signal would be the output signal of divider A as well. But we extended the output pulse generation for two reasons. First, the pulse-width of the output signal will usually be one clock cycle of f ps (50 ps at f in = 80 GHz), if neither a third programmable divider, which dramatically increases power consumption, nor an additional divide-by-two stage at the output, which disables the beneficial low division factors from 12 to 23 and all odd ones, is used. This leads to high requirements for the following phase-frequency detector in a PLL and low energy in the output signal at larger division factors. To overcome this disadvantage we doubled the pulsewidth by a disjunction of the 6-times AND output and the load signal. The second reason for extending the output signal generation is the implementation of a global reset function by the signal R. This feature is beneficial to synchronizing multiple PLLs, e.g. in a MIMO FMCW radar system. Mainly divider B is a 2 bit version of divider A and responsible for controlling the prescaling factor by the modulus control signal MC (for a block diagram see [3]). The modulus control logic is realized by multiple conjunctions and is synchronized to f ps by a flip-flop. The load signal is delivered by divider A, because only the first cycle of divider B is of interest. Because in frequency synthesizers a low residual phase noise of the divider is essential, we use the flip-flop S1 to synchronize the output of divider A to the input frequency f in . Therefore the prescaler and programmable dividers are no longer in the direct signal path from input to output of the dual-modulus divider, which results in a reduction of the phase noise as proved in [5]. To always ensure a synchronization within one clock cycle of f in we added a delay gate consisting of two differential amplifiers. Otherwise the synchronization would take one cycle for f in ≤ 51 GHz and two cycles for f in ≥ 51 GHz. IV. E XPERIMENTAL R ESULTS A photograph of the fabricated chip is shown in Fig. 6(a). The divider itself occupies an area of 180 µm × 380 µm and has a power consumption of less than 390 mW, so it is easy to be integrated in complex MMICs such as single-chip radar transceivers. For measurements the chip has been mounted on a Rogers RT/duroid 5880 substrate to apply the division factor and the power supply as well as to derive the output signal. The single-ended input signal was fed by probes (GGB Model 40A 182 334 0 -0.2 V (fout) (V) 99.8 Prescaler (b) 1 in 0 0 0.2 0.4 0.6 t (ns) 0.8 1 3 2 1 0 -1 0 50 0 50 (a) t (ns) t (ns) 100 150 100 150 (b) Fig. 8. (a) Measured output signal with a pulse-width of 8/f in . (b) Output signal after using the reset function is immediately tout = N/f in . N = 12 N = 13 N = 14 N = 81 N = 255 Sim. 20 0.8 tout = 13.5ns -0.5 f = 40 GHz, N = 32 Residual phase noise L (dBc/Hz) Input power P (dBm) Fig. 6. (a) Photograph of the realized frequency divider. (b) Distribution of the power consumption and number of transistors in the different blocks. 0.4 0.6 t (ns) 0.2 -0.2 10 5 0 -5 -10 -15 -20 -25 0 0.2 fin = 6 GHz, N = 81 0 -1 0 0.4 71 (a) fin = 40 GHz, N = 16 0.2 V (fout ) (V) 0.4 R (V) 169.6 Counter A 4 23 113 V (fout) (V) 59.3 Counter B Number of transistors Divider B Prescaler 100 μm Divider A Output Buffer Delay + Sync. Power consumption (mW) 20.0 Buffer 29.0 Del.+Sync. Logic Interface 40 60 80 Input frequency fin (GHz) Fig. 7. Measured and simulated input sensitivity of the divider for different division N factors at a temperature of 25◦ C. Fig. 9. and 110H), while the complementary input was left open. Input frequencies from DC to 40 GHz were directly delivered by a signal source (R&S SMR40) and for frequencies above 43 GHz a V-band frequency quadrupler (Milltech AMC-15-RFH00) with an adjustable attenuator (Parzich WR15) was used. As there was no proper signal source available, the gap from 40 GHz to 43 GHz has only been validated by simulations. -125 f = 1 GHz, N = 12 f = 1 GHz, N = 24 out -130 out -135 -140 -145 -150 2 10 3 4 5 10 10 10 Offset frequency from carrier Df (Hz) 10 6 Measured phase noise of the divider. the extremely high maximum input frequency this divider is well suited for fractional-N synthesizers in high performance microwave measurement systems. The reset function of the divider gives the opportunity to easily synchronize multiple synthesizers, e.g. in MIMO systems. ACKNOWLEDGMENT The authors would like to thank Infineon Technologies AG for fabricating the chips and the Federal Ministry of Education and Research (BMBF) for financial support in frame of the project RAWIS (Funding number: 13N13232). The sensitivity curve in Fig. 7 shows the required minimum power of the sinusoidal input signal for an appropriate output signal, measured with a spectrum analyzer (HP 8565). For all measured division factors a maximum input frequency of 80 GHz is achieved. The difference of 8 GHz compared to HICUM simulations is probably caused by wire parasitics. To reduce the minimum input power at lower frequencies the 1 pF on chip coupling capacitor can be shorted and a larger capacitor can be applied externally. The single-ended output signal is measured with a subsampling oscilloscope (HP 54120B) and shown in Fig 8(a). As mentioned before, the pulse-width is 8/f in and independent from the division factor. The reset function is illustrated in Fig. 8(b). After the falling edge of the reset signal, the divider immediately starts dividing with the proper division factor. To characterize the residual phase noise we set up two identical dividers with their output signals in quadrature. The residual phase noise, measured with a signal source analyzer (R&S FSUP26), is shown in Fig. 9. A low noise floor of −149 dBc/Hz and a flicker noise corner frequency of 13.3 kHz is reached. [1] [2] [3] [4] [5] V. C ONCLUSION We presented an 8 bit programmable frequency divider operating at input frequencies up to 80 GHz. A parallel interface enables fast programming of the divison factor in the entire integer range between 12 and 259. The realized divider consumes a power of less than 390 mW at a 3.3 V power supply. Because of the wide division factor range, low residual phase noise and [6] [7] 183 R EFERENCES N. Pohl, T. Jaeschke, and K. Aufinger, “An Ultra-Wideband 80 GHz FMCW Radar System Using a SiGe Bipolar Transceiver Chip Stabilized by a Fractional-N PLL Synthesizer,” IEEE Transactions on Microwave Theory and Techniques, vol. 60, no. 3, pp. 757–765, Mar. 2012. R. Storch, G. Hasenaecker, and T. Musch, “Reference Signal Synthesis for FMCW MIMO Radar Systems Using a Fractional-N Frequency Divider,” in German Microwave Conference (GeMIC), Aachen, GER, Mar. 2014. G. 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