SNAS624B March   2014  – May 2015 LMX2492 , LMX2492-Q1


  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 Storage Conditions
    3. 7.3 ESD Ratings
    4. 7.4 Recommended Operating Conditions
    5. 7.5 Thermal Information
    6. 7.6 Electrical Characteristics
    7. 7.7 Timing Requirements, Programming Interface (CLK, DATA, LE)
    8. 7.8 Typical Characteristics
  8. Detailed Description
    1. 8.1  Overview
    2. 8.2  Functional Block Diagram
    3. 8.3  Feature Description
      1. 8.3.1  OSCin Input
      2. 8.3.2  OSCin Doubler
      3. 8.3.3  R Divider
      4. 8.3.4  PLL N Divider
      5. 8.3.5  Fractional Circuitry
      6. 8.3.6  PLL Phase Detector and Charge Pump
      7. 8.3.7  External Loop Filter
      8. 8.3.8  Fastlock and Cycle Slip Reduction
      9. 8.3.9  Lock Detect and Charge Pump Voltage Monitor
        1. Charge Pump Voltage Monitor
        2. Digital Lock Detect
      10. 8.3.10 FSK/PSK Modulation
      11. 8.3.11 Ramping Functions
        1. Ramp Count
        2. Ramp Comparators and Ramp Limits
      12. 8.3.12 Power on Reset (POR)
    4. 8.4  Device Functional Modes
      1. 8.4.1 Continuous Frequency Generator
        1. Integer Mode Operation
        2. Fractional Mode Operation
      2. 8.4.2 Modulated Waveform Generator
    5. 8.5  Programming
      1. 8.5.1 Loading Registers
    6. 8.6  Register Map
    7. 8.7  Register Field Descriptions
      1. 8.7.1 POWERDOWN and Reset Fields
      2. 8.7.2 Dividers and Fractional Controls
        1. Speed Up Controls (Cycle Slip Reduction and Fastlock)
    8. 8.8  Lock Detect and Charge Pump Monitoring
    9. 8.9  TRIG1,TRIG2,MOD, and MUXout Pins
    10. 8.10 Ramping Functions
    11. 8.11 Individual Ramp Controls
  9. Applications and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
      3. 9.2.3 Application Performance Plot - Sawtooth Waveform Example
      4. 9.2.4 Application Performance Plot - Flat Top Triangle Waveform
      5. 9.2.5 Applications Performance Plot -- Complex Triggered Ramp
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Development Support
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Related Links
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Detailed Description

8.1 Overview

The LMX2492/92-Q1 is a microwave PLL, consisting of a reference input and divider, high frequency input and divider, charge pump, ramp generator, and other digital logic. The Vcc power supply pins run at a nominal 3.3 volts, while the charge pump supply pin, Vcp, operates anywhere from Vcc to 5 volts. The device is designed to operate with an external loop filter and VCO. Modulation is achieved by manipulating the MASH engine.

8.2 Functional Block Diagram

LMX2492 LMX2492-Q1 fbd_snas624.gif

8.3 Feature Description

8.3.1 OSCin Input

The reference can be applied in several ways. If using a differential input, this should be terminated differentially with a 100 ohm resistance and AC coupled to the OSCin and GND/OSCin* terminals. If driving this single-ended, then the GND/OSCin* terminal may be grounded, although better performance is attained by connecting the GND/OSCin* terminal through a series resistance and capacitance to ground to match the OSCin terminal impedance.

8.3.2 OSCin Doubler

The OSCin doubler allows the input signal to the OSCin to be doubled in order to have higher phase detector frequencies. This works by clocking on both the rising and falling edges of the input signal, so it therefore requires a 50% input duty cycle.

8.3.3 R Divider

The R counter is 16 bits divides the OSCin signal from 1 to 65535. If DIFF_R = 0, then any value can be chosen in this range. If DIFF_R=1, then the divide is restricted to 2,4,8, and 16, but allows for higher OSCin frequencies.

8.3.4 PLL N Divider

The 16 bit N divider divides the signal at the Fin terminal down to the phase detector frequency. It contains a 4/5 prescaler that creates minimum divide restrictions, but allows the N value to increment in values of one.

Modulator Order Minimum N Divide
Integer Mode, 1st Order Modulator 16
2nd Order Modulator 17
3rd Order Modulator 19
4th Order Modulator 25

8.3.5 Fractional Circuitry

The fractional circuitry controls the N divider with delta sigma modulation that supports a programmable first, second, third, and fourth order modulator. The fractional denominator is a fully programmable 24-bit denominator that can support any value from 1,2,..., 224, with the exception when the device is running one of the ramps, and in this case it is a fixed size of 224.

8.3.6 PLL Phase Detector and Charge Pump

The phase detector compares the outputs of the R and N dividers and generates a correction voltage corresponding to the phase error. This voltage is converted to a correction current by the charge pump. The phase detector frequency, fPD, can be calculated as follows: fPD = fOSCin × OSC_2X / R.

The charge pump supply voltage on this device, Vcp, can be either run at the Vcc voltage, or up to 5.25 volts in order to get higher tuning voltages to present to the VCO.

8.3.7 External Loop Filter

The loop filter is external to the device and is application specific. Texas Instruments website has details on this at

8.3.8 Fastlock and Cycle Slip Reduction

The Fastlock™ and Cycle Slip Reduction features can be used to improved lock time. When the frequency is changed, a timeout counter can be used to engage these features for a prescribed number of phase detector cycles. During this time that the timeout counter is counting down, the device can be used to pull a terminal from high impedance to ground switch in an extra resistor (R2pLF), change the charge pump current (FL_CPG), or change the phase detector frequency. TRIG2 is recommended for switching the resistor with a setting of TRIG2_MUX = Fastlock (2) and TRIG2_PIN = Inverted/Open Drain (5).

LMX2492 LMX2492-Q1 sch_fastlock_snas624.gif
Parameter Normal Operation Fastlock Operation
Charge Pump Gain CPG FL_CPG
Device Pin
(TRIG1, TRIG2, MOD, or MUXout)
High Impedance Grounded

The resistor and the charge pump current are changed simultaneously so that the phase margin remains the same while the loop bandwidth is by a factor of K as shown in the following table:

Parameter Symbol Calculation
Charge Pump Gain in Fastlock FL_CPG Typically use the highest value.
Loop Bandwidth Multiplier K K=sqrt(FL_CPG/CPG)
External Resistor R2pLF R2 / (K-1)

Cycle slip reduction is another method that can also be used to speed up lock time by reducing cycle slipping. Cycle slipping typically occurs when the phase detector frequency exceeds about 100x the loop bandwidth of the PLL. Cycle slip reduction works in a different way than fastlock. To use this, the phase detector frequency is decreased while the charge pump current is simultaneously increased by the same factor. Although the loop bandwidth is unchanged, the ratio of the phase detector frequency to the loop bandwidth is, and this is helpful for cases when the phase detector frequency is high. Because cycle slip reduction changes the phase detector rate, it also impacts other things that are based on the phase detector rate, such as the fastlock timeout-counter and ramping controls.

8.3.9 Lock Detect and Charge Pump Voltage Monitor

The LMX2492/92-Q1 offers two methods to determine if the PLL is in lock, charge pump voltage monitoring and digital lock detect. These features can be used individually or in conjunction to give a reliable indication of when the PLL is in lock. The output of this detection can be routed to the TRIG1, TRIG2, MOD, or MUXout terminals. Charge Pump Voltage Monitor

The charge pump voltage monitor allows the user to set low (CMP_THR_LOW) and high (CMP_THR_HIGH) thresholds for a comparator that monitors the charge pump output voltage.

Vcp Threshold Suggested Level
= (Vthresh + 0.08) / 0.085
6 for 0.5V limit
= (Vthresh - 0.96) / 0.044
42 for 2.8V limit
= (Vthresh + 0.056) / 0.137
4 for 0.5V limit
= (Vthresh -1.23) / 0.071
46 for 4.5V limit Digital Lock Detect

Digital lock detect works by comparing the phase error as presented to the phase detector. If the phase error plus the delay as specified by the PFD_DLY bit is outside the tolerance as specified by DLD_TOL, then this comparison would be considered to be an error, otherwise passing. The DLD_ERR_CNT specifies how may errors are necessary to cause the circuit to consider the PLL to be unlocked. The DLD_PASS_CNT specifies how many passing comparisons are necessary to cause the PLL to be considered to be locked and also resets the count for the errors. The DLD_TOL value should be set to no more than half of a phase detector period plus the PFD_DLY value. The DLD_ERR_CNT and DLD_PASS_CNT values can be decreased to make the circuit more sensitive. If the circuit is too sensitive, then chattering can occur and the DLD_ERR_CNT, DLD_PASS_CNT, or DLD_TOL values should be increased.

Note that if the OSCin signal goes away and there is no noise or self-oscillation at the OSCin pin, then it is possible for the digital lock detect to indicate a locked state when the PLL really is not in lock. If this is a concern, then digital lock detect can be combined with charge pump voltage monitor to detect this situation..

8.3.10 FSK/PSK Modulation

Two level FSK or PSK modulation can be created whenever a trigger event, as defined by the FSK_TRIG field is detected. This trigger can be defined as a transition on a terminal (TRIG1, TRIG2, MOD, or MUXout) or done purely in software. The RAMP_PM_EN bit defines the modulation to be either FSK or PSK and the FSK_DEV register determines the amount of the deviation. Remember that the FSK_DEV[32:0] field is programmed as the 2's complement of the actual desired FSK_DEV value. This modulation can be added to the modulation created from the ramping functions as well.

RAMP_PM_EN Modulation Type Deviation
0 2 Level FSK Fpd × FSK_DEV / 224
1 2 Level PSK 360° × FSK_DEV / 224

8.3.11 Ramping Functions

The LMX2492/92-Q1 supports a broad and flexible class of FMCW modulation formed by up to 8 linear ramps. When the ramping function is running, the denominator is fixed to a forced value of 224 = 16777216. The waveform always starts at RAMP0 when the LSB of the PLL_N (R16) is written to. After it is set up, it will start at the initial frequency and have piecewise linear frequency modulation that deviates from this initial frequency as specified by the modulation. Each of the eight ramps can be individually programmed. Various settings are as follows

Ramp Characteristic Programming Field Name Description
Ramp Length RAMPx_LEN
The user programs the length of the ramp in phase detector cycles. If RAMPx_DLY=1, then each count of RAMPx_LEN is actually two phase detector cycles.
Ramp Slope RAMPx_LEN
The user does not directly program slope of the line, but rather this is done by defining how long the ramp is and how much the fractional numerator is increased per phase detector cycle. The value for RAMPx_INC is calculated by taking the total expected increase in the frequency, expressed in terms of how much the fractional numerator increases, and dividing it by RAMPx_LEN. The value programmed into RAMPx_INC is actually the two's complement of the desired mathematical value.
Trigger for Next Ramp RAMPx_NEXT_TRIG The event that triggers the next ramp can be defined to be the ramp finishing or can wait for a trigger as defined by TRIG A, TRIG B, or TRIG C.
Next Ramp RAMPx_NEXT This sets the ramp that follows. Waveforms are constructed by defining a chain ramp segments. To make the waveform repeat, make RAMPx_NEXT point to the first ramp in the pattern.
Ramp Fastlock RAMPx_FL This allows the ramp to use a different charge pump current or use Fastlock
Ramp Flags RAMPx_FLAG This allows the ramp to set a flag that can be routed to external terminals to trigger other devices. Ramp Count

If it is desired that the ramping waveform keep repeating, then all that is needed is to make the RAMPx_NEXT of the final ramp equal to the first ramp. This will run until the RAMP_EN bit is set to zero. If this is not desired, then one can use the RAMP_COUNT to specify how may times the specified pattern is to repeat. Ramp Comparators and Ramp Limits

The ramp comparators and ramp limits use programable thresholds to allow the device to detect whenever the modulated waveform frequency crosses a limit as set by the user. The difference between these is that comparators set a flag to alert the user while a ramp limits prevent the frequency from going beyond the prescribed threshold. In either case, these thresholds are expressed by programming the Extended_Fractional_Numerator.
Extended_Fractional_Numerator = Fractional_Numerator + (N-N*) × 224
In the above, N is the PLL feedback value without ramping and N* is the instantaneous value during ramping. The actual value programmed is the 2's complement of Extended_Fractional_Numerator.

Type Programming Bit Threshold
Ramp Limits RAMP_LIMIT_LOW Lower Limit
Ramp Comparators RAMP_CMP0
For the ramp comparators, if the ramp is increasing and exceeds the value as specified by RAMP_CMPx, then the flag will go high, otherwise it is low. If the ramp is decreasing and goes below the value as specified by RAMP_CMPx, then the flag will go high, otherwise it will be low.

8.3.12 Power on Reset (POR)

The power on reset circuitry sets all the registers to a default state when the device is powered up. This same reset can be done by programming SWRST=1. In the programming section, the power on reset state is given for all the programmable fields.

8.4 Device Functional Modes

The two primary ways to use the LMX2492/92-Q1 are to run it to generate a set of frequencies

8.4.1 Continuous Frequency Generator

In this mode, the LMX2492/92-Q1 generates a single frequency that only changes when the N divider is programmed to a new value. In this mode, the RAMP_EN bit is set to 0 and the ramping controls are not used. The fractional denominator can be programmed to any value from 1 to 16777216. In this kind of application, the PLL is tuned to different channels, but at each channel, the goal is to generate a stable fixed frequency. Integer Mode Operation

In integer mode operation, the VCO frequency needs to be an integer multiple of the phase detector frequency. This can be the case when the output frequency or frequencies are nicely related to the input frequency. As a rule of thumb, if this an be done with a phase detector of as high as the lesser of 10 MHz or the OSCin frequency, then this makes sense. To operate the device in integer mode, disable the fractional circuitry by programming the fractional order (FRAC_ORDER) , dithering (FRAC_DITH), and numerator (FRAC_NUM) to zero. Fractional Mode Operation

In fractional mode, the output frequency does not need to be an integer multiple of the phase detector frequency. This makes sense when the channel spacing is more narrow or the input and output frequencies are not nicely related. There are several programmable controls for this such as the modulator order, fractional dithering, fractional numerator, and fractional denominator. There are many trade-offs with choosing these, but here are some guidelines

Parameter Field Name How to Choose
Fractional Numerator and Denominator FRAC_NUM
The first step is to find the fractional denominator. To do this, find the frequency that divides the phase detector frequency by the channel spacing. For instance, if the output ranges from 5000 to 5050 in 5 MHz steps and the phase detector is 100 MHz, then the fractional denominator is 100 MHz/5 = 20. So for a an output of 5015 MHz, the N divider would be 50 + 3/20. In this case, the fractional numerator is 3 and the fractional denominator is 20. Sometimes when dithering is used, it makes sense to express this as a larger equivalent fraction.
Note that if ramping is active, the fractional denominator is forced to 224.
Fractional Order FRAC_ORDER There are many trade-offs, but in general try either the 2nd or 3rd order modulator as starting points. The 3rd order modulator may give lower main spurs, but may generate others. Also if dithering is involved, it can generate phase noise.
Dithering FRAC_DITH Dithering can reduce some fractional spurs, but add noise. Consult application note AN-1879 for more details on this.

8.4.2 Modulated Waveform Generator

In this mode, the device can generate a broad class of frequency sweeping waveforms. The user can specify up to 8 linear segments in order to generate these waveforms. When the ramping function is running, the denominator is fixed to a forced value of 224 = 16777216

In addition to the ramping functions, there is also the capability to use a terminal to add phase or frequency modulation that can be done by itself or added on top of the waveforms created by the ramp generation functions.

8.5 Programming

8.5.1 Loading Registers

The device is programmed using several 24 bit registers. The first 16 bits of the register are the address, followed by the next 8 bits of data. The user has the option to pull the LE terminal high after this data, or keep sending data and it will apply this data to the next lower register. So instead of sending three registers of 24 bits each, one could send a single 40 bit register with the 16 bits of address and 24 bits of data. For that matter, the entire device could be programmed as a single register if desired.

8.6 Register Map

Registers are programmed in REVERSE order from highest to lowest. Registers NOT shown in this table or marked as reserved can be written as all 0's unless otherwise stated. The POR value is the power on reset value that is assigned when the device is powered up or the SWRST bit is asserted.

Table 1. Register Map

Register D7 D6 D5 D4 D3 D2 D1 D0 POR
0 0 0 0 0 1 1 0 0 0 0x18
1 0x1 Reserved 0x00
2 0x2 0 0 0 0 0 SWRST POWERDOWN[1:0] 0x00
3-15 0x3 - 0xF Reserved -
16 0x10 PLL_N[7:0] 0x64
17 0x11 PLL_N[15:8] 0x00
18 0x12 0 FRAC_ORDER[2:0] FRAC_DITHER[1:0] PLL_N[17:16] 0x00
19 0x13 FRAC_NUM[7:0] 0x00
20 0x14 FRAC_NUM[15:8] 0x00
21 0x15 FRAC_NUM[23:16] 0x00
22 0x16 FRAC_DEN[7:0] 0x00
23 0x17 FRAC_DEN[15:8] 0x00
24 0x18 FRAC_DEN[23:16] 0x00
25 0x19 PLL_R[7:0] 0x04
26 0x1A PLL_R[15:8] 0x00
27 0x1B 0 FL_CSR[1:0] PFD_DLY[1:0] PLL_R_
0 OSC_2X 0x08
28 0x1C 0 0 CPPOL CPG[4:0] 0x00
29 0x1D FL_TOC[10:8] FL_CPG[4:0] 0x00
30 0x1E 0 CPM_
CPM_THR_LOW[5:0] 0x0a
31 0x1F 0 CPM_
CPM_THR_HIGH[5:0] 0x32
32 0x20 FL_TOC[7:0] 0x00
33 0x21 DLD_PASS_CNT[7:0] 0x0f
34 0x22 DLD_TOL[2:0] DLD_ERR_CNTR[4:0] 0x00
35 0x23 MOD_
1 MUXout
0 0 1 0x41
36 0x24 TRIG1_MUX[4:0] TRIG1_PIN[2:0] 0x08
37 0x25 TRIG2_MUX[4:0] TRIG2_PIN[2:0] 0x10
38 0x26 MOD_MUX[4:0] MOD_PIN[2:0] 0x18
39 0x27 MUXout_MUX[4:0] MUXout_PIN[2:0] 0x38
40-57 0x28-0x39 Reserved -
58 0x3A RAMP_TRIG_A[3:0] 0 RAMP_
RAMP_EN 0x00
59 0x3B RAMP_TRIG_C[3:0] RAMP_TRIG_B[3:0] 0x00
60 0x3C RAMP_CMP0[7:0] 0x00
61 0x3D RAMP_CMP0[15:8] 0x00
62 0x3E RAMP_CMP0[23:16] 0x00
63 0x3F RAMP_CMP0[31:24] 0x00
64 0x40 RAMP_CMP0_EN[7:0] 0x00
65 0x41 RAMP_CMP1[7:0] 0x00
66 0x42 RAMP_CMP1[15:8] 0x00
67 0x43 RAMP_CMP1[23:16] 0x00
68 0x44 RAMP_CMP1[31:24] 0x00
69 0x45 RAMP_CMP1_EN[7:0] 0x00
70 0x46 0 FSK_TRIG[1:0] RAMP_
71 0x47 FSK_DEV[7:0] 0x00
72 0x48 FSK_DEV[15:8] 0x00
73 0x49 FSK_DEV[23:16] 0x00
74 0x4A FSK_DEV[31:24] 0x00
75 0x4B RAMP_LIMIT_LOW[7:0] 0x00
76 0x4C RAMP_LIMIT_LOW[15:8] 0x00
77 0x4D RAMP_LIMIT_LOW[23:16] 0x00
78 0x4E RAMP_LIMIT_LOW[31:24] 0x00
79 0x4F RAMP_LIMIT_HIGH[7:0] 0xff
80 0x50 RAMP_LIMIT_HIGH[15:8] 0xff
81 0x51 RAMP_LIMIT_HIGH[23:16] 0xff
82 0x52 RAMP_LIMIT_HIGH[31:24] 0xff
83 0x53 RAMP_COUNT[7:0] 0x00
84 0x54 RAMP_TRIG_INC[1:0] RAMP_
RAMP_COUNT[12:8] 0x00
85 0x55 Reserved 0x00
86 0x56 RAMP0_INC[7:0] 0x00
87 0x57 RAMP0_INC[15:8] 0x00
88 0x58 RAMP0_INC[23:16] 0x00
89 0x59 RAMP0_
RAMP0_INC[29:24] 0x00
90 0x5A RAMP0_LEN[7:0] 0x00
91 0x5B RAMP0_LEN[15:8] 0x00
92 0x5C RAMP0_NEXT[2:0] RAMP0_
RAMP0_FLAG[1:0] 0x00
93 0x5D RAMP1_INC[7:0] 0x00
94 0x5E RAMP1_INC[15:8] 0x00
95 0x5F RAMP1_INC[23:16] 0x00
96 0x60 RAMP1_
RAMP1_INC[29:24] 0x00
97 0x61 RAMP1_LEN[7:0] 0x00
98 0x62 RAMP1_LEN[15:8] 0x00
99 0x63 RAMP1_NEXT[2:0] RAMP1_
RAMP1_FLAG[1:0] 0x00
100 0x64 RAMP2_INC[7:0] 0x00
101 0x65 RAMP2_INC[15:8] 0x00
102 0x66 RAMP2_INC[23:16] 0x00
103 0x67 RAMP2
RAMP2_INC[29:24] 0x00
104 0x68 RAMP2_LEN[7:0] 0x00
105 0x69 RAMP2_LEN[15:8] 0x00
106 0x6A RAMP2_NEXT[2:0] RAMP2_
RAMP2_FLAG[1:0] 0x00
107 0x6B RAMP3_INC[7:0] 0x00
108 0x6C RAMP3_INC[15:8] 0x00
109 0x6D RAMP3_INC[23:16] 0x00
110 0x6E RAMP3_
RAMP3_INC[29:24] 0x00
111 0x6F RAMP3_LEN[7:0] 0x00
112 0x70 RAMP3_LEN[15:8] 0x00
113 0x71 RAMP3_NEXT[2:0] RAMP3_
RAMP3_FLAG[1:0] 0x00
114 0x72 RAMP4_INC[7:0] 0x00
115 0x73 RAMP4_INC[15:8] 0x00
116 0x74 RAMP4_INC[23:16] 0x00
117 0x75 RAMP4_
RAMP4_INC[29:24] 0x00
118 0x76 RAMP4_LEN[7:0] 0x00
119 0x77 RAMP4_LEN[15:8] 0x00
120 0x78 RAMP4_NEXT[2:0] RAMP4_
RAMP4_FLAG[1:0] 0x00
121 0x79 RAMP5_INC[7:0] 0x00
122 0x7A RAMP5_INC[15:8] 0x00
123 0x7B RAMP5_INC[23:16] 0x00
124 0x7C RAMP5_
RAMP5_INC[29:24] 0x00
125 0x7D RAMP5_LEN[7:0] 0x00
126 0x7E RAMP5_LEN[15:8] 0x00
127 0x7F RAMP5_NEXT[2:0] RAMP5_
RAMP5_FLAG[1:0] 0x00
128 0x80 RAMP6_INC[7:0] 0x00
129 0x81 RAMP6_INC[15:8] 0x00
130 0x82 RAMP6_INC[23:16] 0x00
131 0x83 RAMP6_
RAMP6_INC[29:24] 0x00
132 0x84 RAMP6_LEN[7:0] 0x00
133 0x85 RAMP6_LEN[15:8] 0x00
134 0x86 RAMP6_NEXT[2:0] RAMP6_
RAMP6_FLAG[1:0] 0x00
135 0x87 RAMP7_INC[7:0] 0x00
136 0x88 RAMP7_INC[15:8] 0x00
137 0x89 RAMP7_INC[23:16] 0x00
138 0x8A RAMP7_
RAMP7_INC[29:24] 0x00
139 0x8B RAMP7_LEN[7:0] 0x00
140 0x8C RAMP7_LEN[15:8] 0x00
141 0x8D RAMP7_NEXT[2:0] RAMP7_
RAMP7_FLAG[1:0] 0x00
142-32767 0x8E-0x7fff Reserved 0x00

8.7 Register Field Descriptions

The following sections go through all the programmable fields and their states. Additional information is also available in the applications and feature descriptions sections as well. The POR column is the power on reset state that this field assumes if not programmed.

8.7.1 POWERDOWN and Reset Fields

Table 2. POWERDOWN and Reset Fields

Field Location POR Description and States
R2[1:0] 0 POWERDOWN Control Value POWERDOWN State
0 POWERDOWN, ignore CE
1 Power Up, ignore CE
2 Power State Defined by CE terminal state
3 Reserved
SWRST R2[2] 0 Software Reset. Setting this bit sets all registers to their POR default values. Value Reset State
0 Normal Operation
1 Register Reset

8.7.2 Dividers and Fractional Controls

Table 3. Dividers and Fractional Controls

Field Location POR Description and States
R18[1] to R16[0] 16 Feedback N counter Divide value. Minimum count is 16. Maximum is 262132. Writing of the register R16 begins any ramp execution when RAMP_EN=1.
R18[3:2] 0 Dither used by the fractional modulator Value Dither
0 Weak
1 Medium
2 Strong
3 Disabled
R18[6:4] 0 Fractional Modulator order Value Modulator Order
0 Integer Mode
1 1st Order Modulator
2 2nd Order Modulator
3 3rd Order Modulator
4 4th Order Modulator
5-7 Reserved
R21[7] to R19[0] 0 Fractional Numerator. This value should be less than or equal to the fractional denominator.
R24[7] to R22[0] 0 Fractional Denominator. If the RAMP_EN=1, this field is ignored and the denominator is fixed to 224.
R26[7] to R25[0] 1 Reference Divider value. Selecting 1 will bypass counter.
OSC_2X R27[0] 0 Enables the Doubler before the Reference divider Value Doubler
0 Disabled
1 Enabled
PLL_R _DIFF R27[2] 0 Enables the Differential R counter.
This allows for higher OSCin frequencies, but restricts PLL_R to divides of 2,4,8 or 16.
Value R Divider
0 Single-Ended
1 Differential
R27[4:3] 1 Sets the charge pump minimum pulse width. This could potentially be a trade-off between fractional spurs and phase noise. Setting 1 is recommended for general use. Value Pulse Width
0 Reserved
1 860 ps
2 1200 ps
3 1500 ps
R28[4:0] 0 Charge pump gain Value Charge Pump State
0 Tri-State
1 100 uA
2 200 uA
31 3100 uA
CPPOL R28[5] 0 Charge Pump Polarity
Positive is for a positive slope VCO characteristic, negative otherwise.
Value Charge Pump Polarity
0 Negative
1 Positive Speed Up Controls (Cycle Slip Reduction and Fastlock)

Table 4. FastLock and Cycle Slip Reduction

Field Location POR Description and States
R27[6:5] 0 Cycle Slip Reduction (CSR) reduces the phase detector frequency by multiplying both the R and N counters by the CSR value while either the FastLock Timer is counting or the RAMPx_FL=1 and the part is ramping. Care must be taken that the R and N divides remain inside the range of the counters. Cycle slip reduction is generally not recommended during ramping. Value CSR Value
0 Disabled
1 x 2
2 x 4.
3 Reserved
R29[4:0] 0 Charge pump gain only when Fast Lock Timer is counting down or a ramp is running with RAMPx_FL=1 Value Fastlock Charge Pump Gain
0 Tri-State
1 100 uA
2 200 uA
31 3100 uA
R29[7:5] and R32[7:0] 0 Fast Lock Timer. This counter starts counting when the user writes the PLL_N(Register R16). During this time the FL_CPG gain is sent to the charge pump, and the FL_CSR shifts the R and N counters if enabled. When the counter terminates, the normal CPG is presented and the CSR undo’s the shifts to give a normal PFD frequency. Value Fastlock Timer Value
0 Disabled
1 1 x 32 = 32
2047 2047 x 32 = 65504

8.8 Lock Detect and Charge Pump Monitoring

Table 5. Lock Detect and Charge Pump Monitor

Field Location POR Description and States
R30[5:0] 0x0A Charge pump voltage low threshold value. When the charge pump voltage is below this threshold, the LD goes low. Value Threshold
0 Lowest
63 Highest
CPM_FLAGL R30[6] - This is a read only bit.
Low indicates the charge pump voltage is below the minimum threshold.
Value Flag Indication
0 Charge pump is below CPM_THR_LOW threshold
1 Charge pump is above CPM_THR_LOW threshold
R31[5:0] 0x32 Charge pump voltage high threshold value. When the charge pump voltage is above this threshold, the LD goes low. Value Threshold
0 Lowest
63 Highest
CPM_FLAGH R31[6] - This is a read only bit.
Charge pump voltage high comparator reading. High indicates the charge pump voltage is above the maximum threshold.
Value Threshold
0 Charge pump is below CPM_THR_HIGH threshold
1 Charge pump is above CPM_THR_HIGH threshold
R33[7:0] 0xff Digital Lock Detect Filter amount. There must be at least DLD_PASS_CNT good edges and less than DLD_ERR edges before the DLD is considered in lock. Making this number smaller will speed the detection of lock, but also will allow a higher chance of DLD chatter.
R34[4:0] 0 Digital Lock Detect error count. This is the maximum number of errors greater than DLD_TOL that are allowed before DLD is de-asserted. Although the default is 0, the recommended value is 4.
R34[7:5] 0 Digital Lock detect edge window. If both N and R edges are within this window, it is considered a “good” edge. Edges that are farther apart in time are considered “error” edges. Window choice depends on phase detector frequency, charge pump minimum pulse width, fractional modulator order and the users desired margin. Value Window and Fpd Frequency
0 1 ns (Fpd > 130 MHz)
1 1.7 ns (80 MHz , Fpd ≤ 130 MHz)
2 3 ns (60 MHz , Fpd ≤ 80 MHz)
3 6 ns (45 MHz , Fpd ≤ 60 MHz)
4 10 ns (30 MHz < Fpd ≤ 45 MHz)
5 18 ns ( Fpd ≤ 30 MHz)
6 and 7 Reserved

8.9 TRIG1,TRIG2,MOD, and MUXout Pins

Table 6. TRIG1, TRIG2, MOD, and MUXout Terminal States

Field Location POR Description and States
R36[2:0] 0 This is the terminal drive state for the TRIG1, TRIG2, MOD, and MUXout Pins Value Pin Drive State
0 TRISTATE (default)
1 Open Drain Output
2 Pullup / Pulldown Output
R37[2:0] 0 3 Reserved
R38[2:0] 0 4 GND
MUXout_ _PIN
R39[2:0] 0 5 Inverted Open Drain Output
6 Inverted Pullup / Pulldown Output
7 Input

Table 7. TRIG1, TRIG2, MOD, and MUXout Selections

Field Location POR Description and States



R36[7:3], R37.3

R36[7:3], R35.3

R37[7:3], R35.4

R38[7:3], R35.7



These fields control what signal is muxed to or from the TRIG1,TRIG2, MOD, and MUXout pins.
Some of the abbreviations used are:
COMP0, COMP1: Comparators 0 and 1
LD, DLD: Lock Detect, Digital Lock Detect
CPM: Charge Pump Monitor
CPG: Charge Pump Gain
CPUP: Charge Pump Up Pulse
CPDN: Charge Pump Down Pulse
Value MUX State
1 Input TRIG1
2 Input TRIG2
3 Input MOD
4 Output TRIG1 after synchronizer
5 Output TRIG2 after synchronizer
6 Output MOD after synchronizer
7 Output Read back
8 Output CMP0
9 Output CMP1
10 Output LD (DLD good AND CPM good)
11 Output DLD
12 Output CPMON good
13 Output CPMON too High
14 Output CPMON too low
16 Output R Divide/2
17 Output R Divide/4
18 Output N Divide/2
19 Output N Divide/4
20 Reserved
21 Reserved
22 Output CMP0RAMP
23 Output CMP1RAMP
24 Reserved
25 Reserved
26 Reserved
27 Reserved
28 Output Faslock
29 Output CPG from RAMP
30 Output Flag0 from RAMP
31 Output Flag1 from RAMP
32 Output TRIGA
33 Output TRIGB
34 Output TRIGC
35 Output R Divide
36 Output CPUP
37 Output CPDN
38 Output RAMP_CNT Finished
39 to 63 Reserved

8.10 Ramping Functions

Table 8. Ramping Functions

Field Location POR Description and States
RAMP_EN R58[0] 0 Enables the RAMP functions. When this bit is set, the Fractional Denominator is fixed to 224. RAMP execution begins at RAMP0 upon the PLL_N[7:0] write. The Ramp should be set up before RAMP_EN is set. Value Ramp
0 Disabled
1 Enabled
RAMP_CLK R58[1] 0 RAMP clock input source. The ramp can be clocked by either the phase detector clock or the MOD terminal based on this selection. Value Source
0 Phase Detector
1 MOD Terminal
RAMP_PM_EN R58[2] 0 Phase modulation enable. Value Modulation Type
0 Frequency Modulation
1 Phase Modulation




0 Trigger A,B, and C Sources Value Source
0 Never Triggers (default)
1 TRIG1 terminal rising edge
2 TRIG2 terminal rising edge
3 MOD terminal rising edge
4 DLD Rising Edge
5 CMP0 detected (level)
6 RAMPx_CPG Rising edge
7 RAMPx_FLAG0 Rising edge
8 Always Triggered (level)
9 TRIG1 terminal falling edge
10 TRIG2 terminal falling edge
11 MOD terminal falling edge
12 DLD Falling Edge
13 CMP1 detected (level)
14 RAMPx_CPG Falling edge
15 RAMPx_FLAG0 Falling edge
R63[7] to R60[0]
0 Twos compliment of Ramp Comparator 0 value. Be aware of that the MSB is in Register R70.
R64[7:0] 0 Comparator 0 is active during each RAMP corresponding to the bit. Place a 1 for ramps it is active in and 0 for ramps it should be ignored. RAMP0 corresponds to R64[0], RAMP7 corresponds to R64[7]
R70[1], R68[7] to R65[0] 0 Twos compliment of Ramp Comparator 1 value. Be aware of that the MSB is in Register R70.
R69[7:0] 0 Comparator 1 is active during each RAMP corresponding to the bit. Place a 1 for ramps it is active in and 0 for ramps it should be ignored. RAMP0 corresponds to R64[0], RAMP7 corresponds to R64[7].
R76[4] to R75[3] 0 Deviation trigger source. When this trigger source specified is active, the FSK_DEV value is applied. Value Trigger
0 Always Triggered
1 Trigger A
2 Trigger B
3 Trigger C
R74[7] to R71[0]
0 Twos compliment of the deviation value for frequency modulation and phase modulation. This value should be written with 0 when not used. Be aware that the MSB is in Register R70.
R78[7] to 75[0]
0x00000000 Twos compliment of the ramp lower limit that the ramp can not go below . The ramp limit occurs before any deviation values are included. Care must be taken if the deviation is used and the ramp limit must be set appropriately. Be aware that the MSB is in Register R70.
R82[7] to 79.0[0]
0xffffffff Twos compliment of the ramp higher limit that the ramp can not go above. The ramp limit occurs before any deviation values are included. Care must be taken if the deviation is used and the ramp limit must be set appropriately. Be aware that the MSB is in Register R70.
R84[4] to R83[0] 0 Number of RAMPs that will be executed before a trigger or ramp enable is brought down. Load zero if this feature is not used. Counter is automatically reset when RAMP_EN goes from 0 to 1.
RAMP_AUTO R84[5] 0 Automatically clear RAMP_EN when RAMP Count hits terminal count. Value Ramp
0 RAMP_EN unaffected by ramp counter (default)
1 RAMP_EN automatically brought low when ramp counter terminal counts
R84[7:6] 0 Increment Trigger source for RAMP Counter. To disable ramp counter, load a count value of 0. Value Source
0 Increments occur on each ramp transition
1 Increment occurs on trigA
2 Increment occurs on trigB
3 Increment occurs on trigC

8.11 Individual Ramp Controls

These bits apply for all eight ramps. For the field names, x can be 0,1,2,3,4,5,6, or 7.

Table 9. Individual Ramp Controls

Field Location POR Description and States
RAMPx _INC[29:0] Varies 0 Signed ramp increment.
RAMPx _FL Varies 0 This enables fastlock and cycle slip reduction for ramp x. Value CPG
0 Disabled
1 Enabled
RAMPx _DLY Varies 0 During this ramp, each increment takes 2 PFD cycles per LEN clock instead of the normal 1 PFD cycle. Slows the ramp by a factor of 2. Value Clocks
0 1 PFD clock per RAMP tick.(default)
1 2 PFD clocks per RAMP tick.
RAMPx _LEN Varies 0 Number of PFD clocks (if DLY is 0) to continue to increment RAMP. 1=>1 cycle, 2=>2 etc. Maximum of 65536 cycles.
RAMPx _FLAG[1:0] Varies 0 General purpose FLAGS sent out of RAMP. Value Flag
0 Both FLAG1 and FLAG0 are zero. (default)
1 FLAG0 is set, FLAG1 is clear
2 FLAG0 is clear, FLAG1 is set
3 Both FLAG0 and FLAG1 are set.
RAMP0 _RST Varies 0 Forces a clear of the ramp accumulator. This is used to erase any accumulator creep that can occur depending on how the ramps are defined. Should be done at the start of a ramp pattern. Value Reset
0 Disabled
1 Enabled
Varies 0 Determines what event is necessary to cause the state machine to go to the next ramp. It can be set to when the RAMPx_LEN counter reaches zero or one of the events for Triggers A,B, or C. Value Operation
RAMP0 _NEXT[2:0] Varies 0 The next RAMP to execute when the length counter times out