19-2166; Rev 0; 9/01 Low-Jitter 155MHz/622MHz Clock Generator Features ♦ Single +3.3V or +5.0V Supply ♦ Power Dissipation: 150mW at +3.3V Supply ♦ External VCO Center Frequencies (fVCO): 155MHz to 670MHz ♦ Reference Clock Frequencies: fVCO, fVCO/2, fVCO/8 ♦ Main Clock Output Frequency: fVCO The MAX3670 operates from a single +3.3V or +5.0V supply, and dissipates 150mW (typ) at 3.3V. The operating temperature range is from -40°C to +85°C. The chip is available in a 5mm ✕ 5mm, 32-pin QFN package. ♦ Optional Output Clock Frequencies: fVCO, fVCO/2, fVCO/4, fVCO/8 ♦ Low Intrinsic Jitter: <0.4psRMS ♦ Loss-of-Lock Indicator Applications ♦ PECL Clock Output Interface OC-12 to OC-192 SONET/WDM Transport Systems Ordering Information Clock Jitter Clean-Up and Frequency Synchronization PART TEMP. RANGE PIN-PACKAGE MAX3670EGJ Frequency Conversion -40°C to +85°C 32 QFN-EP* (5mm x 5mm) *Exposed pad System Clock Distribution Pin Configuration appears at end of data sheet. Typical Application Circuit 3.3V 142Ω 155MHz REFCLK+ VCCD 16:1 SERIALIZER MOUT- REFCLK3.3V MAX3892 MOUT+ 142Ω 142Ω VCOIN+ VCO KVCO = 25kHz/V 155MHz 100Ω VCOIN- RSEL N.C. VSEL N.C. GSEL1 N.C. MAX3670 142Ω 332Ω GSEL2 VC 0.01µF 4700pF GSEL3 500kΩ OPAMPOPAMP+ 4700pF POLAR 3.3V GND 500kΩ SETUP FOR 10kHz LOOP BANDWIDTH REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX3670 General Description The MAX3670 is a low-jitter 155MHz/622MHz reference clock generator IC designed for system clock distribution and frequency synchronization in OC-48 and OC-192 SONET/SDH and WDM transmission systems. The MAX3670 integrates a phase/frequency detector, an operational amplifier (op amp), prescaler dividers and input/output buffers. Using an external VCO, the MAX3670 can be configured easily as a phase-lock loop with bandwidth programmable from 15Hz to 20kHz. MAX3670 Low-Jitter 155MHz/622MHz Clock Generator ABSOLUTE MAXIMUM RATINGS Supply Voltage .........................................................-0.5V to +7V Voltage at C2+, C2-, THADJ, CTH, GSEL1, GSEL2, GSEL3, LOL, RSEL, REFCLK-, REFCLK+, VSEL, VCOIN+, VCOIN-, VC, POLAR, PSEL1, PSEL2, COMP, OPAMP+, OPAMP- ..................................-0.5V to (VCC + 0.5V) PECL Output Current (MOUT+, MOUT-, POUT+, POUT-).................................................56mA Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range. ............................-65°C to +160°C Lead Temperature (soldering, 10s) .................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. DC ELECTRICAL CHARACTERISTICS (VCC = +3.3V ±10% or VCC = +5.0V ±10%, TA = -40°C to +85°C. Typical values are at VCC = +3.3V and TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER Supply Current SYMBOL ICC CONDITIONS MIN (Note 2) TYP MAX UNITS 48 72 mA INPUT SPECIFICATIONS (REFCLK±, VCOIN±) Input High Voltage VIH VCC 1.16 VCC 0.88 V Input Low Voltage VIL VCC 1.81 VCC 1.48 V VCC 1.3 Input Bias Voltage V Common-Mode Input Resistance 7.5 11.5 17.5 kΩ Differential Input Resistance 12.8 21.0 32.5 kΩ mVp-p Differential Input Voltage Swing AC-coupled 300 1900 0°C to +85°C VCC 1.025 VCC 0.88 -40°C to 0°C VCC 1.085 VCC 0.88 0°C to +85°C VCC 1.81 VCC 1.62 -40°C to 0°C VCC 1.83 VCC 1.556 2.4 VCC V 0.4 V PECL OUTPUT SPECIFICATIONS Output High Voltage Output Low Voltage VOH V VOL V TTL SPECIFICATIONS 2 Output High Voltage VOH Sourcing 20µA Output Low Voltage VOL Sinking 2mA _______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator (VCC = +3.3V ±10% or VCC = +5.0V ±10%, TA = -40°C to +85°C. Typical values are at VCC = +3.3V and TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS OPERATIONAL AMPLIFIER SPECIFICATIONS (Note 3) Op Amp Output Voltage Range VCC = +3.3V ±10% 0.3 VCC 0.3 VCC = +5.0V ±10% 0.5 VCC 0.5 VO Op Amp Input Offset Voltage | VOS | Op Amp Open-Loop Gain 3 AOL 90 V mV dB PHASE FREQUENCY DETECTOR (PFD)/CHARGE-PUMP (CP) SPECIFICATIONS (Note 4) Full-Scale PFD/CP Output Current | IPD | PFD/CP Offset Current High gain 16 20 24.4 Low gain 4 5 6.2 High gain 0.80 Low gain 1.08 µA % | IPD | AC ELECTRICAL CHARACTERISTICS (VCC = +3.3V ±10% or VCC = +5.0V ±10%, TA = -40°C to +85°C. Typical values are at VCC = +3.3V and TA = +25°C, unless otherwise noted.) (Note 5) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 670 MHz CLOCK OUTPUT SPECIFICATIONS Clock Output Frequency Optional Clock Output Frequency fVCO = 622MHz 622/311/ 155/78 fVCO = 155MHz 155/78/ 38/19 Clock Output Rise/Fall Time Measured from 20% to 80% Clock Output Duty Cycle (Note 6) 45 MHz 280 ps 55 % 1.14 µVRMS /√Hz NOISE SPECIFICATIONS Random Noise Voltage at LoopFilter Output VNOISE Spurious Noise Voltage at LoopFilter Output Power-Supply Rejection at LoopFilter Output Freq > 1kHz (Note 7) (Note 8) PSR (Note 9) 50 µVRMS 30 dB REFERENCE CLOCK INPUT SPECIFICATIONS 622/ 155/78 Reference Clock Frequency Reference Clock Duty Cycle 30 670 MHz 70 % _______________________________________________________________________________________ 3 MAX3670 DC ELECTRICAL CHARACTERISTICS (continued) MAX3670 Low-Jitter 155MHz/622MHz Clock Generator AC ELECTRICAL CHARACTERISTICS (continued) (VCC = +3.3V ±10% or VCC = +5.0V ±10%, TA = -40°C to +85°C. Typical values are at VCC = +3.3V and TA = +25°C, unless otherwise noted.) (Note 5) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 20,000 Hz 0.1 dB PLL SPECIFICATIONS PLL Jitter Transfer Bandwidth BW (Note 10) 15 FJITTER ≤ BW (Note 11) Jitter Transfer Function OP AMP SPECIFICATION Unity-Gain Bandwidth 7 MHz VCO INPUT SPECIFICATION VCO Input Frequency VCO Input Slew Rate fVCO 622/155 0.5 670 MHz V/ns Specifications at -40°C are guaranteed by design and characterization. Measured with PECL outputs unterminated. OPAMP specifications met with 10kΩ load to ground or 5kΩ load to VCC (POLAR = 0 and POLAR = VCC). PFD/CP currents are measured from pins OPAMP+ to OPAMP-. See Table 3 for gain settings. AC characteristics are guaranteed by design and characterization. Measured with 50% VCO input duty cycle. Random noise voltage at op amp output with 800kΩ resistor connected between VC and OPAMP-, PFD/CP gain (KPD) = 5µA/UI, and POLAR = 0. Measured with the PLL open loop and no REFCLK or VCO input. Note 8: Spurious noise voltage due to PFD/CP output pulses measured at op amp output with R1 = 800kΩ, KPD = 5µA/UI, and compare frequency 400 times greater than the higher-order pole frequency (see Design Procedure). Note 9: PSR measured with a 100mVp-p sine wave on VCC in a frequency range from 100Hz to 2MHz. External resistors R1 matched to within 1%, external capacitors C1 matched to within 10%. Measured closed loop with PLL bandwidth set to 200Hz. Note 10: The PLL 3dB bandwidth is adjusted from 15Hz to 20kHz by changing external components R1 and C1, by selecting the internal programmable divider ratio and phase-detector gain. Measured with VCO gain of 220ppm/V and C1 limited to 2.2µF. Note 11: Measured at BW = 20kHz. When input jitter frequency is above PLL transfer bandwidth (BW), the jitter transfer function rolls off at -20dB/decade. Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: 4 _______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator EDGE SPEED vs. TEMPERATURE 40 30 20 -20 0 20 40 60 80 -30 LOOP FILTER OUTPUT -40 -50 155.52MHz -60 -40 -20 TEMPERATURE (°C) 0 20 40 60 80 1k TEMPERATURE (°C) 10k 100k 1M 10M FREQUENCY (Hz) 622MHz CLOCK OUTPUT (DIFFERENTIAL OUTPUT) 200mV/ div MAX3670 toc03 -20 155MHz CLOCK OUTPUT (DIFFERENTIAL OUTPUT) MAX3670 toc05 -40 622.08MHz BW = 1kHz HOP = 5kHz -10 SUPPLY REJECTION (dB) 3.3V 0 MAX3670 toc04 SUPPLY CURRENT (mA) 5.0V 50 280 270 260 250 240 230 220 210 200 190 180 170 160 150 POWER-SUPPLY REJECTION vs. FREQUENCY MAX3670 toc02 MAX3670 toc01 60 EDGE SPEED 20%-80% (ps) SUPPLY CURRENT vs. TEMPERATURE 200mV/ div 500ps/div 2.0ns/div _______________________________________________________________________________________ 5 MAX3670 Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) Low-Jitter 155MHz/622MHz Clock Generator MAX3670 Pin Description 6 PIN NAME FUNCTION 1 C2+ Positive Filter Input. External capacitor connected between C2+ and C2- used for setting the higher-order pole frequency (see Setting the Higher-Order Poles). 2 C2- Negative Filter Input. External capacitor connected between C2+ and C2- used for setting the higherorder pole frequency (see Setting the Higher-Order Poles). 3, 9, 15 VCCD 4 THADJ Positive Digital Supply Voltage Threshold Adjust Input. Used to adjust the Loss-of-Lock threshold (see LOL Setup). 5 CTH Threshold Capacitor Input. A capacitor connected between CTH and ground used to control the Loss-ofLock conditions (see LOL Setup). 6 GSEL1 Gain Select 1 Input. Three-level pin used to set the phase-detector gain (KPD) and the frequency-divider ratio (N2) (see Table 3). 7 GSEL2 Gain Select 2 Input. Three-level pin used to set the phase-detector gain (KPD) and the frequency-divider ratio (N2) (see Table 3). 8 GSEL3 Gain Select 3 Input. Three-level pin used to set the phase-detector gain (KPD) and the frequency-divider ratio (N2) (see Table 3). 10 LOL Loss-of-Lock. LOL signals a TTL low when the reference frequency differs from the VCO frequency. LOL signals a TTL high when the reference frequency equals the VCO frequency. 11 GND Supply Ground 12 RSEL Reference Clock Select Input. Three-level pin used to set the predivider ratio (N3) for the input reference clock (see Table 1). 13 REFCLK+ 14 REFCLK- Positive Reference Clock Input Negative Reference Clock Input VCO Clock Select Input. Three-level pin used to set the predivider ratio (N1) for the input VCO clock (see Table 2). 16 VSEL 17 POUT- Negative Optional Clock Output, PECL Positive Optional Clock Output, PECL 18 POUT+ 19, 22 VCCO Positive Supply Voltage for PECL Outputs 20 MOUT- Negative Main Clock Output, PECL 21 MOUT+ Positive Main Clock Output, PECL 23 VCOIN- Negative VCO Clock Input 24 VCOIN+ Positive VCO Clock Input 25 VC 26 POLAR Polarity Control Input. Polarity control of op amp input. POLAR = GND for VCOs with positive gain transfer. POLAR = VCC for VCOs with negative gain transfer. 27 PSEL1 Optional Clock Select 1 Input. Used to set the divider ratio for the optional clock output (see Table 4). 28 PSEL2 Optional Clock Select 2 Input. Used to set the divider ratio for the optional clock output (see Table 4). 29 VCCA Positive Analog Supply Voltage for the Charge Pump and Op Amp 30 COMP Compensation Control Input. Op amp compensation reference control input. COMP = GND for VCOs whose control pin is VCC referenced. COMP = VCC for VCOs whose control pin is GND referenced. 31 OPAMP- 32 OPAMP+ Positive Op Amp Input (POLAR = 0), Negative Op Amp Input (POLAR = 1) EP Exposed Pad Ground. The exposed pad must be soldered to the circuit board ground plane for proper thermal and electrical performance. Control Voltage Output. The voltage output from the op amp that controls the VCO. Negative Op Amp Input (POLAR = 0), Positive Op Amp Input (POLAR = 1) _______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator R3 VCO KVCO C3 LOL THADJ CTH VC C1 R1 C1 R1 COMP POLAR OPAMP- OPAMP+ OPAMP LOL REFCLK+ REFCLK- DIV (N3) DIV (N2) PFD/CP KPD RSEL C2- VSEL VCOIN+ DIV (N1) C2+ DIV (N2) MOUT+ PECL VCOIN- MOUTPOUT+ GAIN-CONTROL LOGIC MAX3670 GSEL1 GSEL2 GSEL3 Detailed Description The MAX3670 contains all the blocks needed to form a PLL except for the VCO, which must be supplied separately. The MAX3670 consists of input buffers for the reference clock and VCO, input and output clock-divider circuitry, LOL detection circuitry, gain-control logic, a phase-frequency detector and charge pump, an op amp, and PECL output buffers. This device is designed to clean up the noise on the reference clock input and provide a low-jitter system clock output. DIV 1/2/4/8 PSEL1 PECL POUT- PSEL2 Input Buffer for Reference Clock and VCO The MAX3670 contains differential inputs for the reference clock and the VCO. These inputs can be DC-coupled and are internally biased with high impedance so that they can be AC-coupled (Figure 1 in the Interface Schematic section). A single-ended VCO or reference clock can also be applied. Input and Output Clock-Divider Circuitry The reference clock and VCO input buffers are followed by a pair of clock dividers that prescale the input frequency of the reference clock and VCO to 77.76MHz. _______________________________________________________________________________________ 7 MAX3670 Functional Diagram MAX3670 Low-Jitter 155MHz/622MHz Clock Generator Depending on the input clock frequency of 77.76MHz, 155.52MHz, or 622.08MHz, the clock divider ratio must be set to 1, 2, or 8, respectively. The POUT output buffer is preceded by a clock divider that scales the main clock output by 1, 2, 4, or 8 to provide an optional clock. Table 1. Reference Clock Divider INPUT PIN RSEL REFERENCE CLOCK INPUT FREQ. (MHz) DIVIDER RATIO N3 PREDIVIDER OUTPUT FREQ. (MHz) 77.76 VCC 77.76 1 LOL Detection Circuitry OPEN 155.52 2 77.76 The MAX3670 incorporates a loss-of-lock (LOL) monitor that consists of an XOR gate, filter, and comparator with adjustable threshold (see “LOL Setup” in the Applications section). A loss-of-lock condition is signaled with a TTL low when the reference clock frequency differs from the VCO frequency. GND 622.08 8 77.76 Gain-Control Logic The gain-control circuitry facilitates the tuning of the loop bandwidth by setting phase-detector gain and frequency-divider ratio. The gain-control logic can be programmed to divide from 1 to 1024, in binary multiples, and to adjust the phase detector gain to 5µA/UI or 20µA/UI (see Table 3 in Setting the Loop Bandwidth section). Phase-Frequency Detector and Charge Pump The phase-frequency detector incorporated into the MAX3670 produces pulses proportional to the phase difference between the reference clock and the VCO input. The charge pump converts this pulse train to a current signal that is fed to the op amp. Op Amp The op amp is used to form an active PLL loop filter capable of driving the VCO control voltage input. Using the POLAR input, the op amp input polarity can be selected to work with VCOs having positive or negative gaintransfer functions. The COMP pin selects the op amp internal compensation. Connect COMP to ground if the VCO control voltage is VCC referenced. Connect COMP to VCC if the VCO control voltage is ground referenced. Design Procedure Setting Up the VCO and Reference Clock The MAX3670 accepts 77.76MHz, 155.52MHz, or 622.08MHz (including FEC rates) reference clock frequencies. The RSEL input must be set so that the reference clock is prescaled to 77.76MHz (or FEC rate), to provide the proper range for the PFD and LOL detection circuitry. Table 1 shows the divider ratio for the different reference frequencies. 8 The MAX3670 is designed to accept 77.76MHz, 155.52MHz, or 622.08MHz (including FEC rates) voltage-controlled oscillator (VCO) frequencies. The VSEL input must be set so that the VCO input is prescaled to 77.76MHz (or FEC rate), to provide the proper range for the PFD and LOL detection circuitry. Table 2 shows the divider ratio for the different VCO frequencies. Table 2. VCO Clock Divider INPUT PIN VSEL VCO CLOCK INPUT FREQ. (MHz) DIVIDER RATIO N1 PREDIVIDER OUTPUT FREQ. (MHz) VCC 77.76 1 77.76 OPEN 155.52 2 77.76 GND 622.08 8 77.76 Setting the Loop Bandwidth To eliminate jitter present on the reference clock, the proper selection of loop bandwidth is critical. If the total output jitter is dominated by the noise at the reference clock input, then lowering the loop bandwidth will reduce system jitter. The loop bandwidth (K) is a function of the VCO gain (KVCO), the gain of the phase detector (KPD), the loop filter resistor (R1), and the total feedback-divider ratio (N = N1 ✕ N2). The loop bandwidth of the MAX3670 can be approximated by K RK K = PD 1 VCO 2πN For stability, a zero must be added to the loop in the form of resistor R1 in series with capacitor C1 (see Functional Diagram). The location of the zero can be approximated as fZ = 1 2πR1C1 Due to the second-order nature of the PLL jitter transfer, peaking will occur and is proportional to fZ/K. For certain applications, it may be desirable to limit jitter _______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator INPUT PIN GSEL1 VCC INPUT PIN GSEL2 VCC INPUT PIN GSEL3 VCC 20 DIVIDER RATIO N2 1 OPEN VCC VCC 20 2 GND VCC VCC 20 4 VCC OPEN VCC 20 8 OPEN OPEN VCC 20 16 GND OPEN VCC 20 32 VCC GND VCC 20 64 OPEN GND VCC 20 128 GND GND VCC 20 256 VCC VCC GND 20 512 OPEN VCC GND 20 1024 KPD (µA/UI) quency to be (K ✕ 4) < fHOP ≤ fCOMPARE, where K is the loop bandwidth. The HOP can be implemented either by providing a compensation capacitor C2, which produces a pole at f HOP = 1 2π(20kΩ)(C2 ) or by adding a lowpass filter, consisting of R3 and C3, directly on the VCO tuning port, which produces a pole at f HOP = 1 2πR3C3 Using R3 and C3 may be preferable for filtering more noise in the PLL, but it may still be necessary to provide filtering via C2 when using large values of R1 and N1 ✕ N2 to prevent clipping in the op amp. VCC VCC OPEN 5 1 OPEN VCC OPEN 5 2 Setting the Optional Output GND VCC OPEN 5 4 The MAX3670 optional clock output can be set to binary subdivisions of the main clock frequency. The PSEL1 and PSEL2 pins control the binary divisions. Table 4 shows the pin configuration along with the possible divider ratios. VCC OPEN OPEN 5 8 OPEN OPEN OPEN 5 16 GND OPEN OPEN 5 32 VCC GND OPEN 5 64 OPEN GND OPEN 5 128 GND GND OPEN 5 256 VCC OPEN GND 5 512 OPEN OPEN GND 5 1024 peaking in the PLL passband region to less than 0.1dB. This can be achieved by setting fZ ≤ K/100. The three-level GSEL pins (see Functional Diagram) select the phase-detector gain (KPD) and the frequencydivider ratio (N2). Table 3 summarizes the settings for the GSEL pins. A more detailed analysis of the loop filter is located in application note HFDN-13.0 on www.maxim-ic.com. Setting the Higher-Order Poles Spurious noise is generated by the phase detector switching at the compare frequency, where fCOMPARE = fVCO/(N1 ✕ N2). Reduce the spurious noise from the digital phase detector by placing a higher-order pole (HOP) at a frequency much less than the compare frequency. The HOP should, however, be placed high enough in frequency that it does not decrease the overall loop-phase margin and impact jitter peaking. These two conditions can be met by selecting the HOP fre- Table 4. Setting the Optional Clock Output Driver INPUT PIN PSEL1 INPUT PIN PSEL2 VCO TO POUT DIVIDER RATIO 1 VCC VCC GND VCC 2 VCC GND 4 GND GND 8 Applications Information PECL Interfacing The MAX3670 outputs (MOUT+, MOUT-, POUT+, POUT-) are designed to interface with PECL signal levels. It is important to bias these ports appropriately. A circuit that provides a Thévenin equivalent of 50Ω to VCC - 2V can be used with fixed-impedance transmission lines with proper termination. To ensure best performance, the differential outputs must have balanced loads. It is important to note that if optional clock output is not used, it should be left floating to save power (see Figure 2). _______________________________________________________________________________________ 9 MAX3670 Table 3. Gain Logic Pin Setup MAX3670 Low-Jitter 155MHz/622MHz Clock Generator Layout The MAX3670 performance can be significantly affected by circuit board layout and design. Use good highfrequency design techniques, including minimizing ground inductance and using fixed-impedance transmission lines on the reference and VCO clock signals. Power-supply decoupling should be placed as close to VCC pins as possible. Take care to isolate the input from the output signals to reduce feedthrough. quencies above the loop bandwidth may degrade LOL functionality. The user can set the amount of frequency or phase difference between VCO and reference clock at which LOL indicates an out-of-lock condition. The frequency difference is called the beat frequency. The CTH pin can be connected to an external capacitor, which sets the lowpass filter frequency to approximately f L= VCO Selection The MAX3670 is designed to accommodate a wide range of VCO gains, positive or negative transfer slopes, and VCC-referenced or ground-referenced control voltages. These features allow the user a wide range of options in VCO selection; however, the proper VCO must be selected to allow the clock generator circuitry to operate at the optimum levels. When selecting a VCO, the user needs to take into account the phase noise and modulation bandwidth. Phase noise is important because the phase noise above the PLL bandwidth will be dominated by the VCO noise performance. The modulation bandwidth of the VCO contributes an additional higher-order pole (HOP) to the system and should be greater than the HOP set with the external filter components. 1 2πCTH 60kΩ This lowpass filter frequency should be set about 10 times lower than the beat frequency to make sure the filtered signal at CTH does not drop below the THADJ threshold voltage. The internal compare frequency of the part is 77.78MHz. For a 1ppm sensitivity (beat frequency of 77Hz), the filter needs to be at 7.7Hz, and CTH should be at 0.33µF. The voltage at THADJ will determine the level at which the LOL output flags. THADJ is set to a default value of 0.6V which corresponds in a 45° phase difference. This value can be overridden by applying the desired threshold voltage to the pin. The range of THADJ is from 0V (0°) to 2.4V (180°). Noise Performance Optimization Depending on the application, there are many different ways to optimize the PLL performance. The following are general guidelines to improve the noise on the system output clock. Interface Schematics 1) If the reference clock noise dominates the total system-clock output jitter, then decreasing the loop bandwidth (K) reduces the output jitter. 2) If the VCO noise dominates the total system clock output jitter, then increasing the loop bandwidth (K) reduces the output jitter. 3) Smaller total divider ratio (N1 ✕ N2), lower HOP, and smaller R1 reduce the spurious output jitter. 4) Smaller R1 reduces the random noise due to the op amp. VCC VCC - 1.3V 10.5kΩ 10.5kΩ REFLCK+ LOL Setup The LOL output indicates if the PLL has locked onto the reference clock using an XOR gate and comparator. The comparator threshold can be adjusted with THADJ, and the XOR gate output can be filtered with a capacitor between CTH and ground (Figure 3 in the Interface Schematic section). When the voltage at pin CTH exceeds the voltage at pin THADJ, then the LOL output goes low and indicates that the PLL is not locked. Note that excessive jitter on the reference clock input at fre- 10 REFLCK- MAX3670 Figure 1. Input Interface ______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator VCC LOL 60kΩ THADJ OUT+ OUT- 0.6V CTH 60kΩ REFCLK MAX3670 VCO Figure 2. Output Interface Figure 3. Loss-of-Lock Indicator Pin Configuration OPAMP+ OPAMP- COMP VCCA PSEL2 PSEL1 POLAR VC 32 31 30 29 28 27 26 25 1 24 VCOIN+ C2- 2 23 VCOIN- VCCD 3 22 VCCO THADJ 4 21 MOUT+ 20 MOUT- MAX3670* 18 POUT+ GSEL3 8 17 POUT- 11 12 13 14 15 16 VSEL 7 VCCD GSEL2 REFCLK- VCCO REFCLK+ 19 RSEL 6 GND GSEL1 LOL 5 VCCD CTH 10 Chip Information TRANSISTOR COUNT: 2478 C2+ 9 MAX3670 *THE EXPOSED PAD MUST BE SOLDERED TO SUPPLY GROUND. ______________________________________________________________________________________ 11 MAX3670 Interface Schematics (continued) Low-Jitter 155MHz/622MHz Clock Generator MAX3670 Package Information 12 ______________________________________________________________________________________ Low-Jitter 155MHz/622MHz Clock Generator Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX3670 Package Information (continued)