SNAS601G August   2012  – September 2014 LMX2581

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Simplified Schematic
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements, MICROWIRE Timing
    7. 7.7 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Typical Performance Characteristics
        1. 8.3.1.1 Phase Noise Typical Performance Plot Explanations
        2. 8.3.1.2 Other Typical Performance Plot Characteristics Explanations
      2. 8.3.2  Impact of Temperature on VCO Phase Noise
      3. 8.3.3  OSCin INPUT and OSCin Doubler
      4. 8.3.4  R Divider
      5. 8.3.5  PLL N Divider And Fractional Circuitry
        1. 8.3.5.1 Programmable Dithering Levels
        2. 8.3.5.2 Programmable Delta Sigma Modulator Order
      6. 8.3.6  PLL Phase Detector and Charge Pump
      7. 8.3.7  External Loop Filter
      8. 8.3.8  Low Noise, Fully Integrated VCO
        1. 8.3.8.1 VCO Digital Calibration
      9. 8.3.9  Programmable VCO Divider
      10. 8.3.10 0-Delay Mode
      11. 8.3.11 Programmable RF Output Buffers
        1. 8.3.11.1 Choosing the Proper Pull-Up Component
        2. 8.3.11.2 Choosing the Best Setting for the RFoutA_PWR and RFoutB_PWR Words
      12. 8.3.12 Fastlock
      13. 8.3.13 Lock Detect
        1. 8.3.13.1 Vtune Lock Detect
        2. 8.3.13.2 Digital Lock Detect (DLD)
      14. 8.3.14 Part ID and Register Readback
        1. 8.3.14.1 Uses of Readback
        2. 8.3.14.2 Serial Timing for Readback
      15. 8.3.15 Optimization of Spurs
        1. 8.3.15.1 Phase Detector Spur
        2. 8.3.15.2 Fractional Spur - Integer Boundary Spur
        3. 8.3.15.3 Fractional Spur - Primary Fractional Spurs
        4. 8.3.15.4 Fractional Spur - Sub-Fractional Spurs
        5. 8.3.15.5 Summary of Spurs and Mitigation Techniques
    4. 8.4 Device Functional Modes
      1. 8.4.1 Full Synthesizer Mode
      2. 8.4.2 External VCO Mode
      3. 8.4.3 Powerdown Modes
    5. 8.5 Programming
      1. 8.5.1 Serial Data Input Timing
      2. 8.5.2 Recommended Initial Power on Programming Sequence
      3. 8.5.3 Recommended Sequence for Changing Frequencies
      4. 8.5.4 Triggering Registers
    6. 8.6 Register Maps
      1. 8.6.1 Programming Word Descriptions
        1. 8.6.1.1  Register R15
          1. 8.6.1.1.1 VCO_CAP_MAN — Manual VCO Band Select
          2. 8.6.1.1.2 VCO_CAPCODE[7:0] — Capacitor Value for VCO Band Selection
        2. 8.6.1.2  Register R13
          1. 8.6.1.2.1 DLD_ERR_CNT[3:0] - Digital Lock Detect Error Count
          2. 8.6.1.2.2 DLD_PASS_CNT[9:0] - Digital Lock Detect Success Count
          3. 8.6.1.2.3 DLD_TOL[2:0] — Digital Lock Detect
        3. 8.6.1.3  Registers R10, R9, and R8
        4. 8.6.1.4  Register R7
          1. 8.6.1.4.1 FL_PINMODE[2:0], MUXOUT_PINMODE[2:0], and LD_PINMODE[2:0] — Output Format for Status Pins
          2. 8.6.1.4.2 FL_INV, MUX_INV, LD_INV - Inversion for Status Pins
          3. 8.6.1.4.3 FL_SELECT[4:0], MUXOUT_SELECT[4:0], LD_SELECT[4:0] — State for Status Pins
        5. 8.6.1.5  Register R6
          1. 8.6.1.5.1 RD_DIAGNOSTICS[19:0] — Readback Diagnostics
          2. 8.6.1.5.2 RDADDR[3:0] — Readback Address
          3. 8.6.1.5.3 uWIRE_LOCK - Microwire lock
        6. 8.6.1.6  Register R5
          1. 8.6.1.6.1  OUT_LDEN — Mute Outputs Based on Lock Detect
          2. 8.6.1.6.2  OSC_FREQ[2:0] — OSCin Frequency for VCO Calibration
          3. 8.6.1.6.3  BUFEN_DIS - Disable for the BUFEN Pin
          4. 8.6.1.6.4  VCO_SEL_MODE — Method of Selecting Internal VCO Core
          5. 8.6.1.6.5  OUTB_MUX — Mux for RFoutB
          6. 8.6.1.6.6  OUTA_MUX — Mux for RFoutA
          7. 8.6.1.6.7  0_DLY - Zero Delay Mode
          8. 8.6.1.6.8  MODE[1:0] — Operating Mode
          9. 8.6.1.6.9  PWDN_MODE - Powerdown Mode
          10. 8.6.1.6.10 RESET - Register Reset
        7. 8.6.1.7  Register R4
          1. 8.6.1.7.1 PFD_DLY[2:0] — Phase Detector Delay
          2. 8.6.1.7.2 FL_FRCE — Force Fastlock Conditions
          3. 8.6.1.7.3 FL_TOC[11:0] — Fastlock Timeout Counter
          4. 8.6.1.7.4 FL_CPG[4:0] — Fastlock Charge Pump Gain
          5. 8.6.1.7.5 CPG_BLEED[5:0]
        8. 8.6.1.8  Register R3
          1. 8.6.1.8.1 VCO_DIV[4:0] — VCO Divider Value
          2. 8.6.1.8.2 OUTB_PWR[5:0] — RFoutB Output Power
          3. 8.6.1.8.3 OUTA_PWR[5:0] — RFoutA Output Power
          4. 8.6.1.8.4 OUTB_PD — RFoutB Powerdown
          5. 8.6.1.8.5 OUTA_PD — RFoutA Powerdown
        9. 8.6.1.9  Register R2
          1. 8.6.1.9.1 OSC_2X — OSCin Doubler
          2. 8.6.1.9.2 CPP - Charge Pump Polarity
          3. 8.6.1.9.3 PLL_DEN[21:0] — PLL Fractional Denominator
        10. 8.6.1.10 Register R1
          1. 8.6.1.10.1 CPG[4:0] — PLL Charge Pump Gain
          2. 8.6.1.10.2 VCO_SEL[1:0] - VCO Selection
          3. 8.6.1.10.3 FRAC_ORDER[2:0] — PLL Delta Sigma Modulator Order
          4. 8.6.1.10.4 PLL_R[7:0] — PLL R divider
        11. 8.6.1.11 Register R0
          1. 8.6.1.11.1 ID - Part ID Readback
          2. 8.6.1.11.2 FRAC_DITHER[1:0] — PLL Fractional Dithering
          3. 8.6.1.11.3 NO_FCAL — Disable Frequency Calibration
          4. 8.6.1.11.4 PLL_N - PLL Feedback Divider Value
          5. 8.6.1.11.5 PLL_NUM[21:12] and PLL_NUM[11:0] — PLL Fractional Numerator
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Clocking Application
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Fractional PLL Application
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
    3. 9.3 Do's and Don'ts
  10. 10Power Supply Recommendations
    1. 10.1 Supply Recommendations
    2. 10.2 Regulator Output Pins
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LMX2581 can be used in a broad class of applications. In general, they tend to fall in the categories where the output frequency is a nicely related input frequency and those that require fractional mode. The following schematic generally applies to most applications.

9.2 Typical Applications

sch_main.pngFigure 21. Typical Schematic

9.2.1 Clocking Application

When the output and input frequencies are nicely related, the LMX2581 can often achieve this in integer mode. In integer mode, fractional spurs and noise are much less of a concern, so higher phase detector frequency and wider loop bandwidth are typically used for optimal phase noise performance.

9.2.1.1 Design Requirements

For this example, consider a design for a fixed 1500 MHz output clock to be generated from a 100 MHz input clock. Good close in phase noise and maximizing the output power are desired in this particular example

9.2.1.2 Detailed Design Procedure

For this kind of application, the design goal is typically to minimize the jitter.

PARAMETER VALUE REASON for CHOOSING
Fout 1500 MHz This parameter was given.
Fosc 100 MHz This parameter was given.
Fpd 200 MHz Choose a highest possible phase detector frequency. There are no fractional spurs and this increases the value of C1
Fvco 3000 MHz The VCO needs to be a multiple of 1500 MHz, which restricts it to be 3000 MHz.
Kpd 31x This maximizes the C1 capacitor and also the phase noise
Loop Bandwidth 256 kHz Theoretically, optimal jitter is obtained by choosing the loop bandwidth to the frequency where the open loop PLL and closed loop VCO noise are equal, which would be about 250 kHz. The phase margin is typically chosen around 70 degrees, but is chosen to be 50 degrees to increase the value of the C1 capacitor to be at least 1 nF to reduce VCO phase noise degradation.
Phase Margin 50 deg
OUT_A_PWR 45 This yields the maximum output power.
C1 1 nF Calculated with TI clock design software
C2 6.8 nF
R2 270 Ω
Pull-Up Component 18 nH Inductor This gives maximum output power.

9.2.1.3 Application Curves

Figure 22 is an example of the performance that one might see for an application like this. The achieved results show an output power of about 14 dBm (single-ended) and a jitter from 100 Hz to 10 MHz of 100 fs. Note that the output power is higher than +12 dBm as claimed in the electrical specifications because this is at a lower frequency than 2.7 GHz.

ta03_ClockingExample.gif
Figure 22. Measured Data and Loop Bandwidth Choice
ta_plot_inverted_nopanel.png
Figure 23. Measured Plot

9.2.2 Fractional PLL Application

For applications where the output frequency is not always related nicely to the input frequency, lowering the loop bandwidth and reducing the phase detector frequency can often improve spurs at the cost of in-band phase noise.

9.2.2.1 Design Requirements

Consider generating 1880 to 3760 MHz from a 100 MHz input frequency with a channel spacing of 200 kHz. This is the situation similar that was used for the EVM board.

9.2.2.2 Detailed Design Procedure

PARAMETER VALUE REASON for CHOOSING
Fout 1880 - 3760 MHz This parameter was given.
Fosc 100 MHz This parameter was given.
Fpd 25 MHz By trial and error and experimenting with the clock design tool, we see that this gives a good trade-off between the integer boundary spur and phase noise.
Loop Bandwidth 28.7 KHz This is around where the PLL and VCO noise meet. The VCO is at 2700 MHz
Kpd 31x Choose the highest charge pump gain to maximize the capacitor next to the VCO.
C1_LF 1.8 nF The loop filter can be calculated with the clock design tool. Note that we need to keep the loop bandwidth not too wide so that the capacitor next to the VCO is larger. Also, it is put in C4_LF spot, not C3_LF spot. Both are electrically equivalent, but layoutwise, C4_LF makes more sense. See the board layout in sections to come.
C2_LF 56 nF
C3_LF Open
C4_LF 3.3 nF
R2_LF 390 Ω
R3_LF 270 Ω
R4_LF 0 Ω
OUT_A_PWR 30 This combination of pull-up component and output power settings yields optimal noise floor.
Pull-Up Component 18 nH Inductor

9.2.2.3 Application Curves

ta02_EVMplot_integer_channel_inverted_nopanel.png
Figure 24. Integer Channel
ta02_spurplot_fractional_channel_inverted_nopanel.png
Figure 25. Fractional Channel 2703 MHz

9.3 Do's and Don'ts

CATEGORY DO DON'T WHY
Output Pull-Up Components Place pull-up components close to RFoutA and RFoutB Go through a Via before getting to the pull-up component. The output impedance is determined by this component and if it is far away, there will be loss in output power.
Fractional Spurs
  • Take advantage of TI tools that can simulate these.
  • Read the section on spurs to better understand them.
  • Use a systematic process to optimize them
  • Assume that raising the phase detector frequency always improves the integer boundary spur.
  • Assume that changing the loop bandwidth will always impact integer boundary spurs.
Fractional spurs can have more than one mechanism, especially the integer boundary spur.
Dithering
  • Understand the trade-offs and when it is appropriate to use.
  • Combine with larger equivalent fractions.
Use on simple fractions . Dithering is very effective in eliminating some spurs, but useless for eliminating others. Dithering adds PLL phase noise, so it should be only used for appropriate situations.
VbiasCOMP and VbiasVCO Put as much capacitance as possible, up to 32 µF
  • Use less than 10 µF of capacitance
  • Ignore capacitor de-rating factors.
This capacitance impacts the VCO phase noise.