Dual Fractional-N Synthesizers/PLLs Frequently
Asked Questions
This document provides answers to the following frequently
asked questions (FAQs) related to fractional-N synthesizers in
general, and to Skyworks鈥?CX72300, CX72301, CX72302, and
CX74038 fractional-N synthesizers and their respective
Evaluation Boards, specifically:
I am only interested in using the CX7230x main synthesizer. Do
I need to apply power to the auxiliary supply pins?
Does charge pump current setting affect phase noise?
Is there a charge pump polarity reversal function?
What is the CX7230x VCO divider input impedance?
When the CX7230x is in power-down mode, is the crystal
oscillator still operable?
What is the expected lock-up time?
How do I calculate loop filter component values?
What is the Direct Digital Modulation feature of the CX7230x
synthesizers?
How does the CX7230x 鈥渇requency power steering鈥?feature
operate?
While using the CX74038, I have noticed a pulsating phase
noise effect when viewing the VCO output spectrum (on a
1 MHz span setting). It appears like a wave washing in towards
the carrier from both sides and then it washes out, away from
the carrier and repeats at a slow rate of about once every
second. Is this normal behavior? How can it be eliminated?
Why is there a dithering disable function on the CX74038?
What are the design trade-offs associated with fractional-N
synthesizers?
This subject is fully discussed in the
Basics of Dual Fractional-N
Synthesizers/PLLs
White Paper (document number 101463)
available at
http://www.skyworksinc.com/.
General Questions
What are the design trade-offs associated with fractional-N
synthesizers?
What is the primary benefit of fractional-N synthesizers?
Can I use a lower reference frequency with the fractional-N
synthesizers?
What spurs can be expected from fractional-N synthesizers
versus integer-N synthesizers?
What is meant by the term 鈥渟pur-free鈥?
Does fractional-N offer improved phase noise over integer-N?
Does fractional-N offer improved switching time over integer-N?
CX7230x and CX74038 Evaluation Boards
Where do I purchase Evaluation Boards?
The CX7230x Evaluation Boards use baluns on the VCO divider
inputs. Are the baluns necessary?
Skyworks鈥?Synthesizer Package Types
I noticed the CX7230x package is a 28-pin EP-TSSOP. What
does 鈥淓P鈥?mean?
Are the CX7230x synthesizers available in other package
types?
In what package type is the CX74038 available?
Synthesizer Functioning
What are the differences between the CX7230x group of
synthesizers and the CX74038?
FAQ
Proprietary Information and Specifications Are Subject to Change
Skyworks
103037A
November 4, 2002
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