Analog Devices EE168 Application Notes

Engineer To Engineer Note EE-168

Technical Notes on using Analog Devices' DSP components and development tools

Using Third Overtone Crystals with the ADSP-218x DSP

Contributed by Larry Hurst August 8, 2002

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OT crystal has a lower activity, (i.e.

For these reasons, extra care should be

OT crystal oscillators

Note that there is often no indication,

OT operation verses

OT operation.

Introduction

DSPs frequently require an input clock frequency (CLKIN) that is over 35MHz. Unfortunately fundamental mode crystals over 35MHz are not popular and tend to be expensive and fragile. Packaged clock oscillators cost considerably more than a crystal so, for some applications, using a 3 may be a sensible choice.

While the current trend is to incorporate PLL frequency multiplication into the DSP, using a low frequency input clock to generate internal core clocks of several hundred MHz, there are still occasions when it might be useful to consider using a 3

This note discusses using readily available 3

overtone crystals, at frequencies

over 35MHz, with the ADSP-218x family of

overtone (3

OT crystal.

OT) crystal

Second, a 3 requires a higher minimum drive level to start reliably).

taken when designing 3 and careful testing should be performed over temperature, voltage and with a representative batch of crystals to ensure that all parts operate reliably.

marked on the crystal package, to show that a crystal is intended for 3 fundamental mode operation. Care should be taken to determine this information. If a crystal is used in a traditional (two capacitor fundamental mode circuit) appears to be oscillating at approximately one third of the frequency marked on it’s package, it is very likely that it is intended for 3

DSPs. A design procedure is developed for calculating the optimum values for the support components. This procedure can be extended to CODECs and other applications requiring input clocks over 35MHz.

Design Method

When a 3rd OT crystal is chosen, two additional circuit components must the traditional parallel, or fundamental mode

be added to

circuit, to force oscillation at the overtone

Cautionary Note

There are a number of cautions that should be noted when deciding to use a 3 crystal oscillator.

First, a 3

OT crystal normally has a

higher ESR, typically more than twice that of a

frequency marked on the crystal. The added components consist of a series inductor and capacitor as shown in Figure 1. If L

and C3 are

not added to the circuit, the crystal will oscillate at its fundamental frequency, which is approximately

one third of the desired overtone

frequency.

fundamental mode crystal at the same frequency.

Copyright 2002, Analog Devices, Inc. All rights reserved. Analog Devices assumes no responsibility for customer product design or the use or application of customers’ products or for any infringements of patents or rights of others which may result from Analog Devices assistance. All trademarks and logos are property of their respective holders. Information furnished by Analog Devices Applications and Development Tools Engineers is believed to be accurate and reliable, however no responsibility is assumed by Analog Devices regarding the technical accuracy and topicality of the content provided in all Analog Devices’ Engineer-to-Engineer Notes.

-=5V

ADSP-218x-/L/M/N DSP

DDINT

GND

L=3.3V

•

L=3.3V M=2.5V

M=2.5V

N=1.8V

••

GND

CLKIN XTAL

•• C

••

13 14

••

FBS

•• C

OUT

CLKOUT

••

CIS, C

ARE STRAY

CAPACITANCES

FBS

, C

••

•

DDEXT

GND

•

COMPONENTS

ADDED

FOR 3

OSCILLATOR

-=5V

-=5V L=3.3V

L=3.3V M=2.5-3.3V

M=2.5-3.3V N=1.8-3.3V

N=1.8-3.3V

Figure 1: Schematic of 3rd Overtone Crystal Oscillator

Note that the three capacitors, C

, must be ‘RF’ types with low loss dielectrics

, C2 and

at the frequencies being used. Examples of capacitors with suitable dielectrics include silver mica, polystyrene and ceramic NP0.

The inductor, L

, must also be chosen for

low RF losses (i.e. high ‘Q’). At these frequencies and inductance values this usually means an air core type, although there are some inductors that use special formulations of iron dust and/or ferrites that result in high Q. As a guide, look for an inductor with a Q greater than 30, DC resistance less than 1.0Ω and a selfresonant frequency (SRF) greater than 120MHz.

The crystal’s load capacitance (C

) is

required to ensure the crystal operates at the labeled frequency and will be specified by the crystal manufacturer. This is usually a ‘standard’ value and 18pF is very common. It is up to the engineer to choose the correct values for C C

and L1 in conjunction with the amplifier and

, C2,

stray PCB capacitance, to provide the correct load capacitance, C

. C1 and C2 will usually be

between 20pF and 70pF.

is only required for blocking DC

current that would otherwise load the output of the oscillator. Its value is not critical and a value of 1nF NP0 should be satisfactory.

with C

The inductor, L

and the stray output capacitance at a

, is chosen to resonate

frequency fR ≈ ⅔ of the 3rd OT frequency, fOT. This provides the correct loading reactance for the crystal and closed loop phase relationship to start and maintain oscillation. In addition, the parallel combination of L an effective capacitance, C frequency, f

, to correctly load the crystal.

and C2 must provide

at the 3rdOT

2EFF

We have the following two equations

with two unknown values, L

and C2 …

Equation 1

)( ++

LCCC

OSOUT

EFF

EFFOT

Equation 2

EE-168: Using Third Overtone Crystals with the ADSP-218x DSP Page 2 of 11

where fR is the actual resonant frequency of L1 combined with the total output capacitance, C C

and COS. Note that C2 is the actual

OUT

capacitor value used while C capacitance at f of C

and L1.

The reactance of C

due to the parallel combination

is the effective

2EFF

is small enough to be

ignored. Similarly the contributions of the feedback capacitances, C

and C

, are very

FBS

small and can be ignored in determining the required values of C

and L1.

With some simple arithmetic manipulation we have the resulting design equations for C

and L1 …

C++=

L++=

where: ω

Equation 3

()

Equation 4

= 2πfOT

CCC

OSOUTEFF

CCC

OSOUTEFFOT

()

Summarizing, the crystal manufacturer will specify a total load capacitance for the crystal. This is the TOTAL value of capacitance that must appear across the two terminals of the crystal for the operating frequency to be within the specified tolerance of the value stamped on the package. The total capacitance is usually called the load capacitance, C of the amplifier input capacitance, C capacitance, C

and output capacitance, C

, and will consist

, feedback

OUT

Added to these is the PCB stray capacitances, C

, C

and COS. Finally we have

FBS

to add the external capacitors C1 and the parallel combination of C

and L1.

Example: Determining External Load Capacitors, C Inductor L

Assume a manufacturer specifies a

37.5MHz 3 C

=18pF. For the ADSP-218xM/N oscillator

amplifier, typical values are C 7pF and C capacitances, assume C C

=1pF. These are all reasonable

FBS

approximations and, in practice, a couple of pF either way will not make much difference.

To calculate the equivalent capacitance across the crystal output capacitances are effectively in series.

Therefore, the amplifier total capacitance, C

For the PCB total capacitance, C

Therefore, total Amplifier and PCB stray capacitance, C

The total load capacitance is specified by the crystal manufacturer. In this case, C 18pF. We have 6pF provided by the amplifier in the DSP and stray PCB capacitance, as noted above. Hence we have to add another 12pF in parallel to make a total of 18pF. This capacitance is provided by C combination of C2 in parallel with L1.

OT crystal with a load capacitance,

= 1pF. For the PCB stray

=2pF, COS=3pF and

, first note that the input and

= C

+ CINC

= [1+ 5×7/(5 + 7)]

≈ 4pF

PCBT

= C

+ CISCOS/(C

FBS

= [1 + 2×3/(2 + 3)]

≈ 2pF

= CAT + C

≈ 6pF

, C2 and

= 5pF, C

/(C

OUT

+ C

PCBT

+ COS)

PCBT

and the

OUT

)

EE-168: Using Third Overtone Crystals with the ADSP-218x DSP Page 3 of 11

NOTE: It is most common to make C1 and C the crystal, the resulting values for C

equal, and, since they are in series across

and C

2EFF

will each be 24pF, the series combination making the 12pf required to make-up the specified total load capacitance.

NOTE that this ‘sleight of hand’ introduction of capacitance C

in place of C2

2EFF

which is the effective capacitance of the parallel combination of C2 and L1 required to make 24pF at the 3rd OT frequency.

At this point we have determined the value of C

- in this example,

= 24pF

From the design equations, 3 & 4, we can determine the values of C

& L1,

= 662.2×10

-9

∴L1 = 662.2nH

Checking Calculated Values

To check the effective capacitance of the

//L1 combination at fOT, we can use the

expression;





 

which simplifies to;

EFF







−=

()

C++=

= [9*24 + 4(7+3)]/5 = 51.2pF

∴C2 = 51.2pF

Also, knowing the required crystal overtone frequency, ω

2π×37.5MHz;

L++=

()

= 5/[4(2π37.5×10E6)2(24+7+5)10E-12]

CCC

OSOUTEFF

= 2πfOT =

CCC

OSOUTEFFOT

Substituting values;

= 51.2pF – 1/(2π37.5E6)2×662.2E-9

2EFF

∴C

= 24pF 3

2EFF

Also, to confirm the frequency of

resonance, from equation 1;

= 1/[2π√{(51.2pF + 7pF + 5pF)662.2nH}]

∴ fr = 25.0MHz = 2fOT/3 3

So all the calculations look good. Using preferred values, we can complete our design as shown in Figure 2. (See Appendix A for a detailed component list)

EE-168: Using Third Overtone Crystals with the ADSP-218x DSP Page 4 of 11

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Analog Devices EE168 Application Notes

Specifications and Main Features

Frequently Asked Questions

User Manual

Introduction

Design Method

Cautionary Note

Checking Calculated Values