Dual Fractional-N Synthesizers/PLLs This Application Note provide

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Dual Fractional-N Synthesizers/PLLs This Application Note provide

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FREQUENTLY ASKED QUESTIONS
Dual Fractional-N Synthesizers/PLLs
This Application Note provides answers following frequently asked questions (FAQs) related fractional-N synthesizers general, Skyworks SKY72300, SKY72301, SKY72302, SKY74038 fractional-N synthesizers their respective Evaluation Boards, specifically. What Direct Digital Modulation feature SKY7230x synthesizers? does SKY7230x "frequency power steering" feature operate? When SKY7230x power-down mode, crystal oscillator still operable? What expected lock-up time? calculate loop filter component values? While using SKY74038, have noticed pulsating phase noise effect when viewing output spectrum span setting). appears like wave washing towards carrier from both sides then washes out, away from carrier repeats slow rate about once every second. this normal behavior? eliminated? there dithering disable function SKY74038? What design trade-offs associated with fractional-N synthesizers?
What design trade-offs associated with fractional-N synthesizers? What primary benefit fractional-N synthesizers? lower reference frequency with fractional-N synthesizers? What spurs expected from fractional-N synthesizers versus integer-N synthesizers? What meant term "spur-free"? Does fractional-N offer improved phase noise over integer-N? Does fractional-N offer improved switching time over integer-N?
SKY7230x SKY74038 Evaluation Boards
Where purchase Evaluation Boards? SKY7230x Evaluation Boards baluns divider inputs. baluns necessary?
This subject fully discussed Skyworks White Paper, Basics Dual Fractional-N Synthesizers/PLLs, document number 101463 (available http://www.skyworksinc.com/). What primary benefit fractional-N synthesizers?
Fractional-N synthesizers afford greater design flexibility over integer-N synthesizers that allows: Very fine step size Very high comparison frequency Wide loop bandwidth implementations Rapid lock-up times Good phase noise performance lower reference frequency with fractional-N synthesizers?
Skyworks Synthesizer Package Types
SKY7230x package 28-pin EP-TSSOP. What does "EP" mean? SKY7230x synthesizers available other package types? what package type SKY74038 available?
Synthesizer Functioning
What differences between SKY7230x group synthesizers SKY74038? only interested using SKY7230x main synthesizer. need apply power auxiliary supply pins? Does charge pump current setting affect phase noise? there charge pump polarity reversal function? What SKY7230x divider input impedance?
When fractional-N synthesizer used, best highest possible reference source. This yields lowest value (i.e., best phase noise). Also, modulator quantization noise subject maximum attenuation. quantization noise peaks frequency equal comparison frequency from carrier with null comparison frequency. Quantization noise rises dB/decade. Depending loop bandwidth reference frequency, quantization noise appear Voltage Controlled Oscillator (VCO) output spectrum.
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FRACTIONAL-N SYNTHESIZER/PLL
Generally, comparison frequency above, with loop bandwidth kHz, quantization noise does present problem. What spurs expected from fractional-N synthesizers versus integer-N synthesizers?
Whether these spurs pose problem depends following: Required frequency band plan (actual channel frequency assignments) Comparison frequency Loop bandwidth System spur specification What meant term "spur-free"?
primary spurs associated with integer-N Phase Locked Loops (PLLs) reference spurs. relationship between comparison frequency (step size) loop bandwidth ultimately determines level attenuation afforded. Fractional-N synthesizers exhibit fractional, reference, integer-N boundary spurs. Fractional Spurs. most serious spurs found fractional-N synthesizers (but present Skyworks synthesizer designs) fractional spurs. They appear around carrier regardless frequency which programmed. spacing these spurs usually equal channel step size fractional-N synthesizer half channel step size). example, 18-bit fractional-N implementation using comparison frequency would have fractional spurs spaced MHz/218 around carrier. Fractional spurs buried phase noise Skyworks fractional-N synthesizers. Reference Spurs. With fractional-N synthesizers, comparison frequency very high, which allows large loop filter attenuation reference spur (even with wide loop bandwidth implementation). Integer-N Boundary Spurs. Integer-N boundary spurs appear when VCO/PLL programmed frequencies that near harmonic multiples comparison frequency. They exhibit amplitude between when measured inside loop bandwidth SKY7230x when measured SKY74038). exact mechanism that precipitates these spurs entirely understood. spurs tend lessen when drive level into divider port lowered. spurs spaced distance equal comparison frequency. example, comparison frequency yields spur-free VCO/PLL tuning range nearly MHz. fractional-N synthesizers exhibit these spurs. However, Skyworks family SKY7230x synthesizers offer best integerN boundary spur performance currently available. There also spur that occurs near divide values 1/2, 1/4, 1/8.1/2n (e.g., 41.5, 41.25, 41.125). present, these spurs have lower amplitude than primary integer-N spur located harmonic comparison frequency. example, comparison frequency divider value 41.5005 yields output frequency 415.005 MHz. present, spur would appear 415.000 MHz.
term "spur free" refers fact that there fractional spurs. Fractional spurs buried phase noise Skyworks fractional-N synthesizers. Integer-N boundary spurs occur harmonic multiples internal reference (comparison) frequency typically measured inside loop bandwidth (SKY7230x, measured inside loop bandwidth offset from spur). spur also occur near divide values (e.g., 41.50005). present, these spurs have lower amplitude than primary integer-N boundary spur located harmonic comparison frequency. comparison frequency would offer nearly spur-free tuning spectrum. fractional-N synthesizers subject integer-N boundary spurs. Spur performance specifications fractional-N synthesizers usually refer fractional spurs that occur around carrier regardless frequency. There fractional-N spurs present with Skyworks synthesizers (both SKY7230x family SKY74038). Does fractional-N offer improved phase noise over integer-N?
Yes. Depending channel step size (and system architecture), fractional-N design offer improved phase noise high comparison frequency. channel step size needed 1800 MHz, integer-N yields value 9000. fractional-N offers value only (assuming comparison fractional-N implementation). This translates into 20log(N) gain (20log[9000/180]). this figure applied phase detector noise floor good integer-N phase detector (e.g., -165 dBc/Hz), typical noise floor value would -165 -131 dBc/Hz This typical phase noise floor Skyworks fractional-N synthesizers when operating high comparison frequency MHz). this particular case resulting phase noise identical (comparing integer-N versus fractional-N implementation).
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FRACTIONAL-N SYNTHESIZER/PLL
However, when Skyworks fractional-N synthesizers used comparison frequencies greater than MHz, resulting value smaller (<180). This further improves system phase noise beyond that achievable with integer-N PLL. Does fractional-N offer improved switching time over integer-N?
SKY7230x synthesizers available other package types? SKY72300, SKY72301, SKY72302 only available 28-pin Exposed Thin Shrink Small Outline Packages (EPTSSOPs). what package type SKY74038 available? SKY74038 only available 20-pin TSSOP. What differences between SKY7230x group synthesizers SKY74038? SKY7230x SKY74038 designs result completely different efforts from different design facilities. such, system architectures completely different. Their only similarity that they both apply fractional-N techniques. SKY74038 evolved Global System Mobile Communications (GSM) chipset group. Therefore, offers power consumption package size conducive battery operated, portable, handheld applications. currently being used Code Division Multiple Access (CDMA) Wideband CDMA (W-CDMA) handset designs. SKY7230x synthesizer designs evolved Skyworks Bluetooth® design effort. These components offer broader, general purpose applications specifically address handset market. SKY7230x devices have additional features (such on-board crystal oscillator circuit Direct Digital Modulation) that allow generation rate FSK, MSK, GMSK, modulation schemes writing device register control instantaneous frequency deviation. designs similar terms phase noise performance. main difference that SKY74038 offers integer-N synthesizer auxiliary/IF side, SKY7230x devices offer both integer-N 10-bit fractional-N modes auxiliary/IF side. typical integer-N boundary spurs SKY7230x devices also better than those SKY74038. only interested using SKY7230x main synthesizer. need apply power auxiliary supply pins? auxiliary synthesizer going used specific SKY7230x device, power down using registers. Alternatively, leave VCCcp_aux VCCcml_aux pins disconnected. Note that charge pump left disconnected while synthesizer powered, poor spectral performance occurs with other synthesizer (all Skyworks synthesizers dual devices).
Yes. small step size fast lock-up time design goals, integer-N solutions provide needed performance. loop bandwidth broadened achieve switching time integer-N PLL, reference spurs rise excessively. divider value also large large comparison frequency. Therefore, phase noise also poor this case. Fractional-N synthesizers have been applied next generation handsets because they offer rapid switching times with small channel step size (and good phase noise). Compare integer-N with fractional-N that same comparison frequency MHz, example) same loop bandwidth. Both exhibit similar switching times reference spur attenuation (all other system parameters being equal). However, integer-N much larger step size, equal comparison frequency MHz. fractional-N offers step size (assuming 21-bit modulator). design goal very large channel step size more), integer-N implementation yield suitable switching time performance. Where purchase Evaluation Boards?
Evaluation Boards Skyworks line fractional-N synthesizers available from authorized distributor. Refer http://www.skyworksinc.com/ nearest distributor. SKY7230x Evaluation Boards baluns divider inputs. baluns necessary?
However, operation divider single-ended, effective operating range limits divider input increase noticed SKY7230x package 28-pin EP-TSSOP. What does "EP" mean? Exposed Pad. This pad, located bottom side device, provides ground return path auxiliary synthesizer divider power supply. There several other substrate pads connected ground connection. good design practice attach this pad, even though necessary when only main synthesizer going used.
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FRACTIONAL-N SYNTHESIZER/PLL
Does charge pump current setting affect phase noise? However, Evaluation Board loop filters designed used with maximum charge pump settings. Operating Evaluation Board with lower charge pump current give appearance degraded near-in phase noise performance modified loop dynamics. there charge pump polarity reversal function? SKY7230x family synthesizers does have charge pump polarity reversal function. However, SKY74038 synthesizer does have this ability. external, noise op-amp must used implement polarity reversal SKY7230x devices. What SKY7230x divider input impedance? Divider input impedances SKY72300 (balanced measurement through balun) are:
2500 2000 1500 j141 j205
synthesizers. When programmed this mode (bits address LD/PSmain LD/PS pins configured power steering operation. external hardware circuit connection needs made between LD/PSmain LD/PSaux pins loop filter (simplest configuration install low-value resistor between each pins largest capacitor loop filter). These pins have three possible states. Under locked condition, state high impedance. This allows charge pump circuitry control VCO. During lock condition, pins drive high ground, effectively steering VCO. These pins actually toggle between high impedance either ground comparison frequency rate during lock condition. LD/PSmain LD/PSaux pins current limited with charge pump circuitry. Therefore, they slew faster rate. When frequency close final destination frequency, synthesizer switches back normal charge pump control mode. resistor value hundred Ohms good starting point external hardware circuit connection made. impedance value low, system becomes unstable able lock. When programmed power steering, there Lock Detect function. Systems that require Lock Detect signal cannot power steering feature. When SKY7230x power-down mode, crystal oscillator still operable? When entire device power-down mode (bit address crystal circuitry also powered down draws minimal current (see below). However, main auxiliary synthesizers power-down (bits address crystal oscillator left running. Current consumption shown below except VCCxtal which approximately typical current consumption each SKY7230x power signals power-down mode noted Table below:
Power Signals VCCcml_main VCCcp_main VCCxtal VCCcp_aux VCCcml_aux (Min) Total: (Max)
Divider impedances SKY72301 (unbalanced measurement, input ground through capacitor) are:
1200 1000 j141 j156 j168 j176 j164
divider input impedance SKY72302 been precisely measured. What Direct Digital Modulation feature SKY7230x synthesizers? Direct Digital Modulation eliminates many previously required components from radio transmitter, which creates simple flexible, cost-effective alternative. This feature Skyworks fractional-N synthesizers allows quick stepping carrier (VCO) through range frequencies effectively create FSK, MSK, GMSK, other constant envelope) continuous phase signal. details this feature, refer Skyworks Application Note Direct Digital Modulation Using SKY72300, SKY72301, SKY72302 Dual Synthesizers/PLLs, document number 101349. does SKY7230x "frequency power steering" feature operate? intent power steering feature provide percent reduction acquisition time from other fractional-N
VCCdigital
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FRACTIONAL-N SYNTHESIZER/PLL
What expected lock-up time? Lock-up time large frequency steps (approximately more) approximately equal divided loop bandwidth. calculate loop filter component values? Calculations loop filter components described respective User Guide SKY72300, SKY72301, SKY72302 Evaluation Board (document numbers 101395, 101407, 101408, respectively) available http://www.skyworksinc.com/. While using SKY74038, have noticed pulsating phase noise effect when viewing output spectrum span setting). appears like wave washing towards carrier from both sides then washes out, away from carrier repeats slow rate about once every second. this normal behavior? eliminated? Yes, this normal behavior when SKY74038 synthesizer programmed integer divide value
260.300, etc.). Internal dithering causes phase noise pulsate slow rate. This eliminated disabling internal dithering when integer value. disable dithering, bits word must programmed 112. fractional (non-integer) values these bits should programmed 102. there dithering disable function SKY74038? previous question.
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Dual Fractional-N Synthesizers/PLLs This Application Note provide

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