Table of Contents

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Application Report

Dramatically Improve Your Lock Time with VCO
Instant Calibration

Dean Banerjee, Jacob Mieso
ABSTRACT
Lock time is the time it takes a PLL to settle to a final frequency (and phase) within a given tolerance whenswitching from one frequency to another and is critical for fast frequency hopping applications. The lock time
consists of the time it takes to write to the device, the VCO calibration time, and the analog settling time of the
PLL loop. For very fast switching applications, the VCO calibration time can dominate and this brings the need toensure that this is fast as possible. This article discusses integrated PLL and VCOs on Si technology, why there
is a need for calibration, and some methods to improve for ultra-fast lock time. The LMX2820 synthesizer from
Texas instruments is breaking barriers with the ability to do this VCO calibration in under 5 μs.

Table of Contents
1 Overview of Frequency Hopping ..........................................................................................................................................2
2 Integrated VCO Overview ......................................................................................................................................................3
2.1 Integrated Silicon VCO Structure....................................................................................................................................... 3
2.2 Robustness and Consistency of VCO Calibration..............................................................................................................3
3 Components of Synthesizer Lock Time................................................................................................................................4
3.1 Write Time.......................................................................................................................................................................... 4
3.2 VCO Calibration Time........................................................................................................................................................ 4
3.3 Analog Settling Time.......................................................................................................................................................... 6
4 Improving VCO Calibration Time With the LMX2820...........................................................................................................6
4.1 Improving LMX2820 VCO Calibration With Partial Assist.................................................................................................. 7
4.2 Full Assist Method of Improving VCO Calibration Time..................................................................................................... 7
4.3 Instant Calibration – The Ultimate to Blazing-Fast VCO Calibration..................................................................................8
5 Conclusion.............................................................................................................................................................................11
6 References.............................................................................................................................................................................11

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Overview of Frequency Hopping

1 Overview of Frequency Hopping
The ability to quickly switch between multiple frequencies, also known as frequency hopping, is a critical element
in many systems, such as communications, defense radio, and electronic warfare. For these applications, it isimperative that switching between frequencies happens as quickly as possible.
For example, the Link-16 network used by NATO for transferring real-time tactical information requires frequency
hopping under 13 μs.[1] [2]Figure 1-1 illustrates the typical receiver architecture that requires one or more local oscillators (LOs) to generate
the different frequency signals and mix them with an incoming receive signal.

Mixer

Amp

BPF

LO
Figure 1-1. Typical Receiver Architecture
In such an architecture, the LMX2820 device can be used as an LO and simplifies the design with two key
features: (1) integrating multiple VCO cores into one chip and (2) having ultra-fast VCO calibration. The eight
VCO cores in the LMX2820 allow for wideband frequency coverage from 43.75 MHz to 22.4 GHz from a single
IC, removing the need for multiple LOs. To switch quickly between these VCO cores, the Instant Calibration
function of the LMX2820 device greatly reduces VCO calibration time to less than 5 µs, thus reducing the overall
lock time.
This report gives an example of achieving a lock time of under 11 μs with the LMX2820. When compared withother calibration methods available today, it is clear that Instant Calibration provides the fastest solution for
designing frequency hopping systems.

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Integrated VCO Overview

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2 Integrated VCO Overview
The Voltage Controlled Oscillator (VCO) converts a tuning voltage to an output frequency. It has become
increasingly popular to integrate these on integrated circuits along with other functionality. To achieve goodphase noise performance and wide tuning range, the VCO frequency range is typically divided into several
overlapping bands. This allows the individual bands to have lower VCO gain, which corresponds to better VCO
phase noise. By adding all the bands together, they can jointly create a wide tuning range. For this technique towork, a calibration routine has to be used to tell the VCO which is the proper band to use for a given frequency.

2.1 Integrated Silicon VCO Structure
Nowadays, PLLs with integrated VCOs on silicon technologies are very common for the reasons cost, size,
wideband frequency coverage, and performance. To achieve good wideband performance while keeping goodphase noise performance, it is common practice to use a series of switched capacitors and inductors.

CVaractor

2ÂC

4Â&

VCO2

C

VCO1

CFixedSwitched Capacitor Bank

Selectable VCO Cores

Figure 2-1. Simplified Tank Circuit for Silicon VCO
For this particular example, there are eight capacitor combinations and two inductor combinations, which allow
16 possible frequency bands to be achieved as Figure 2-2 shows.

Tuning Voltage

VCO1

VCO2

VMax

VMin

0

C 2C 3C 4C 5C 6C 7C

0C

2C 3C 4C 5C 6C 7C

fMax

fMin
Frequency

Figure 2-2. Frequency Bands
Modern day synthesizers typically have more capacitors and inductors. For instance, the LMX2820 device has
eight VCO cores and 192 different values in the capacitor bank to create 1536 total bands. Between thesebands, there is overlap to allow for temperature drift and process variation. In addition to this, there is also aninternal amplitude setting that varies as a function of frequency and temperature. So in summary, the three key
parameters for the VCO Calibration to determine are:•••

VCO_ CORE: This is the inductor choice that greatly impacts the frequency band
VCO_CAPCODE: This is the capacitor bank setting that changes the frequency band
VCO_DACISET: This is the amplitude setting that optimizes phase noise

The purpose of the VCO calibration is to find the proper values for these three parameters. This calibration is
activated by programmable serial commands when a change in frequency is desired.

2.2 Robustness and Consistency of VCO Calibration
The VCO calibration should be designed to be robust overtemperature. Even if the VCO is calibrated at one
extreme temperature and it drifts to the other extreme temperature, the VCO should retain lock and not require

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Components of Synthesizer Lock Timereprogramming. For integrated silicon VCOs from Texas Instruments, one can choose to re-calibrate the VCO forsome improvement, but this improvement in doing this is less than 1 dB in phase noise. As VCO is designed totolerate wide temperature drifts, this implies that there is considerable overlap between the frequency bands.
This leads to the next aspect of consistency of VCO calibration. As there is considerable overlap of thefrequency bands, there are multiple frequency bands that can hit the same frequency. When the VCO does notchoose the same calibration settings (VCO_CORE, VCO_CAPCTRL, VCO_DACISET), then there can bedifferences in spurs, VCO gain, VCO phase noise, and propagation delay. This is especially true if a different
VCO core is chosen.

3 Components of Synthesizer Lock TimeThe time that it takes a synthesizer to change from one frequency to another is composed of three major
components. The first component is the time that it actually takes to tell the synthesizer to change the frequency.
This is typically done through a serial programming bus. The second component is the VCO calibration time,
which is the time it takes for the VCO to find the correct settings (VCO_CORE, VCO_CAPCODE,VCO_DACISET). After the VCO calibration is finished, there will be some frequency error (Δf). The final
component is the analog settle time, which is the time it takes for this final frequency error after the VCO
calibration to settle within a given tolerance.

3.1 Write TimeThe first thing that needs to be done to get a synthesizer to change frequencies is that this command needs tobe sent. Typically several register values need to be changed and the write time is the amount of time it takes towrite these registers. For instance, if the SPI programming bus can be run at 75 MHz and one needs to program
three 24-bit registers, then this can be done in slightly under 3 μs. The most intuitive way to reduce the write timeis to increase the clock speed, which is definitely an effective method.
Another technique is to use shadow registers, also known as double buffering. For this approach, multiple
registers can be written before they are needed and then the end of one write can trigger all the register to takeplace at the same time. This is assuming that one knows the next frequency that is required. In addition toeliminating the write time, shadow registers also can help prevent glitches. For instance, suppose it is required towrite the feedback divider (N) in one register and then trigger the VCO calibration by writing to a different
register. If the N divider is written first, the VCO will slam into the tuning rail because the frequency is invalid forthat band. Then the VCO is programmed to calibrate and it will go to the correct frequency. However, having the
VCO tuning voltage at the rail can increase the analog settling time after the VCO calibration is finished. So insummary, shadow registers can eliminate programming time and prevent unnecessarily long analog settling
times.

3.2 VCO Calibration Time
Another concern is the time that it takes the VCO to calibrate, which is dependent on the device and setup
conditions. The VCO will start at some initial starting point and then the VCO calibration searches for the correct

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calibration values. The simplest approach is “no assist” calibration in which no effort is used to choose the initialstarting point. This is demonstrated with the LMX2594 PLL calibration as Figure 3-1 shows.

Figure 3-1. LMX2594 VCO Calibration With no Assist
The first step is for the VCO to choose the correct core, that we can see as several step responses that finish at
marker 2 at 2.9 μs. After this, it slews through the CAPCODE values to find to the correct one, which iscompleted at marker 4. The amplitude calibration is where the correct DACISET value is chosen and after thishappens, the final analog settling time is finished at marker 5.
A natural question when looking at Figure 3-1 is to wonder if there is a way to assist the VCO calibration with an
initial guess to have the VCO start at a closer frequency. Indeed there is a way and this is known as partial
assist. By giving an initial guess for VCO_CORE, VCO_CAPCODE, and VCO_DACISET, this improves the VCO
calibration, even if the guess is slightly off. Figure 3-2 illustrates this.

Figure 3-2. LMX2594 VCO Calibration With Partial Assist

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Components of Synthesizer Lock Time

Beyond simply giving the VCO an educated starting point, there is also Full Assist Calibration, which involves
forcing the exact value. For this method, it requires the device to be locked to the frequency at a previous time
and the calibration settings stored in an external memory. Then these values can be read in and forced.
Although this does require this to be done for every device, the benefit of eliminating the calibration time is worth
it for many. For devices such as the LMX2594 and LMX2820, these same settings can be used over the entire
temperature range.

3.3 Analog Settling Time
The VCO calibration will find the correct core (VCO_CORE), band (VCO_CAPCODE), and amplitude setting(VCO_DACISET). During this time, the VCO tuning voltage is forced to a centered voltage which is close to theactual value, but after this, the final frequency error must settle out.

4 Improving VCO Calibration Time With the LMX2820
In Figure 4-1, we can see the raw unassisted calibration time of 153 μs before any enhancements are used. The
LMX2820 synthesizer from Texas instruments has several enhancements for fast switching time.

Figure 4-1. LMX2820 Unassisted Lock Time

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4.1 Improving LMX2820 VCO Calibration With Partial Assist
The partial assist method can be used to improve the lock time for this example with the LMX2820. To do this,some values were chosen close to, but not equal to, the starting values Table 4-1 shows.
Table 4-1. VCO Calibration Start Values
Setting

Actual ValueNo Assist Start Value

Partial Assist Start ValueVCO_SEL

1

7

1

VCO_CAPCTRL

186

0

180

VCO_DACISET

336

300

330

Using this partial assist method, the VCO calibration time was reduced from 174 μs to 80 μs. This is a worthy
reduction, but instant calibration can do better. For more information and examples on partial assist VCO
calibration, see the Streamline RF Synthesizer VCO Calibration and Optimize PLL Lock Time Application
Report.

4.2 Full Assist Method of Improving VCO Calibration TimeThe benefits of partial assist depend on the situation, but typically might be on the order of a 50% reduction in
VCO calibration time. This is helpful, but many applications demand more. Full assist addresses this bybypassing the calibration altogether and forcing the correct value. The challenge with this is that these values
vary from part to part so one has to lock the device beforehand with the standard calibration and read back thesettings. Then one can force them in and bypass the calibration time. Full assist provides the benefit of thefastest calibration time and also consistent values, although it requires a lot of programming overhead. Also, if
the write speed is slow, then this will limit how fast the VCO can change frequencies. Full assist VCO calibration
can dramatically improve the VCO calibration time, but requires significant setup, so this is why no results areshown for this approach. Another drawback with the full assist mode is that it takes several register writes tochange the values of VCO_SEL, VCO_CAPCTRL, VCO_DACISET, PLL N value, and PLL fraction. Between
these writes, the PLL will not be at the correct state and will slam the VCO tuning voltage to the rails. To mitigate
this, the charge pump can be tri-stated, but this is more programming effort. In summary, full assist calibration isfast and it also has the benefit of always giving the same calibration settings for the same frequency. The
drawback is increased programming overhead and also it requires the setup effort of locking the device to all therequired frequencies and reading back and storing the calibration values so that later on they can be forced forfull assist calibration. For more details and examples with full assist calibration, see the Streamline RF
Synthesizer VCO Calibration and Optimize PLL Lock Time Application Report.

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Improving VCO Calibration Time With the LMX2820

4.3 Instant Calibration – The Ultimate to Blazing-Fast VCO Calibration
The LMX2820 introduces a new upgrade to full assist calibration called instant calibration. With this instantcalibration, the same fast VCO calibration results can be obtained without so much programming and setup
overhead. This has several advantages.
1. The calibration values for CAPCODE, CORE, and DACISET are always the same for the same frequency –giving consistent performance. This does assume power is not removed from the device, and there is an
initial setup routine for the calibration that is required.
2. The VCO calibration time can be reduced to below 5 μs without compromising performance or reliability.
3. The LMX2820 features double buffering (shadow registers), which not only simplifies switching frequencies,
but applies all the register settings for the new frequency at once, so that the VCO tuning voltage does not
get slammed to the wrong value
Many different situations were measured using the initial setup condition as shown in Figure 4-2. The VCO
frequency, dividers, and INSTCAL_FNUM values were changed, but the other conditions were the same.
FCAL_INSTCAL_DLY was set to 250, which makes the VCO calibration time last for 2.5 μs.

Figure 4-2. TICSPro Setup

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The rest of the lock time is due to the analog settling time of the loop. The frequency error at the start of theanalog calibration was measured (Δf) and the lock time was measured using the LD pin output and the results
are recorded in Table 4-2.
Table 4-2. Lock Time Measurements
Output Frequency (MHz)VCO Frequency (MHz)Output Divider

FstartFstop

FstartFstop

DivStart

DivEnd

5600

6000

5600

6000

1

1

6000

5063

6000

10126

1

5600

6053

5600

6053

1

6053

5600

6053

5600

1

5595

6028

11190

6028

2

6028

5595

6028

11190

2800

5600

11200

5600

2800

11200

dF (MHz)

Lock Time (μs)

6.7

11.8

2

0.5

9.8

1

-3.2

9.7

1

1.4

10.9

1

14.5

12.2

1

2

2.2

9.5

11200

4

2

0.9

11.9

11200

2

4

2.2

10.8

The loop filter impacts the analog settling time of the loop and has some influence.

Figure 4-3. Loop Filter Setup

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Figure 4-4. Lock Time Simulation

Figure 4-5. Wide Span Instant Calibration With the LMX2820

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Figure 4-6. Narrow Span Instant Calibration With the LMX2820
Figure 4-6 shows that at 10.8 μs, which is where the lock detect pin indicated lock, the frequency is slightly off on
the order of 30 kHz. There is a long tail showing it gradually settling to this point, which sometimes can be
improved by changing the capacitor type from X7R to C0G, although this was not attempted. In Figure 4-4 thefrequency error after calibration was measured and the settling tolerance of 30 kHz was used and the simulationsuggested 8.2 μs, compared to the actual measurement of 10.8 μs. In any case, regardless of measurement
technique or capacitor types used, this lock time is blazing fast.

5 Conclusion
For applications requiring fast frequency hopping, the lock time for PLL synthesizers is a critical specification.
This time includes the programming time, VCO calibration time, and analog settling time. Silicon based
synthesizers with integrated VCOs offer a wide tuning range and some offer advantages to reduce lock time. In
particular, the LMX2820 features shadow registers and Instant Calibration allowing for lightning fast calibration
without compromising performance.

6 References
1. Understanding Voice and Data Link Networking. Northrum Grumman Corporation. December 2014.
2. LINK 16 ELECTROMAGNETIC COMPATIBILITY (EMC) FEATURES CERTIFICATION PROCESS AND
REQUIREMENTS. US Department of Defense. April 2005.
3. Texas Instruments, LMX2820 22.4-GHz Wideband PLLatinum™ RF Synthesizer With Phase Synchronization
and JESD204B Support Data Sheet
4. Texas Instruments, Streamline RF Synthesizer VCO Calibration and Optimize PLL Lock Time Application
Report

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