Low noise microwave frequency synthesizer having loop accumulation
View Patent ↗A frequency synthesizer and a method for synthesizing a microwave output signal frequency. The synthesizer uses a reference signal having a variable frequency and accumulation in feedback of a phase locked loop for synthesizing a microwave output frequency. The feedback accumulation rate is derived from a first seed word and the variable reference frequency is derived from a second seed word. A spur suppressor having arithmetic frequency conversions reduces the levels of in-band spurious signals of the variable reference frequency signal in order to reduce the spurious signal levels in the output signal.
1. A frequency synthesizer for synthesizing an output frequency, comprising:
a feedback accumulator for accumulating a first seed word at said output frequency for updating a feedback word having a most significant bit (MSB) at an output of the feedback accumulator, wherein said first seed word is based on a desired said output frequency;
a variable reference generator for synthesizing a variable reference frequency for a reference signal;
an oscillator for providing said output frequency controlled by a phase comparison between said reference signal and an MSB signal corresponding to changes in said MSB; and wherein:
said variable reference frequency is synthesized according to a second seed word based on said desired output frequency.
2. The frequency synthesizer of claim 1 , wherein:
the variable reference generator includes a spur suppressor using arithmetic frequency conversions for suppressing spurious signal levels for said reference signal for frequencies other than said variable reference frequency.
3. The frequency synthesizer of claim 2 , wherein:
said spur suppressor includes an intermediate frequency translator for frequency translating a first signal at a synthesized first frequency to an intermediate signal at a variable intermediate frequency, and a frequency divider for dividing said intermediate frequency for providing a second signal at a variable second frequency, said variable reference frequency based on said variable second frequency.
4. The frequency synthesizer of claim 3 , wherein:
the variable reference generator further includes a reference frequency translator for frequency translating said second signal from said variable second frequency to said reference signal at said variable reference signal.
5. The frequency synthesizer of claim 1 , wherein:
the variable reference generator includes a low frequency synthesizer using said second seed word for generating a first signal having a synthesized first frequency; and a frequency converter for converting said synthesized first frequency to said variable reference frequency.
6. The frequency synthesizer of claim 5 , wherein:
the variable reference generator further includes at least one frequency multiplier for frequency multiplying a base frequency to at least one conversion signal having a conversion frequency;
said low frequency synthesizer uses a first said conversion signal with said second seed word for synthesizing said synthesized first frequency; and
said frequency converter uses said at least one conversion signal for frequency conversion of said synthesized first frequency to said variable reference frequency.
7. The frequency synthesizer of claim 6 , further comprising:
a seed calculator for using said desired output frequency, at least one said conversion frequency, and an accumulator modulus of the feedback accumulator for generating said first seed word.
8. The frequency synthesizer of claim 7 , wherein:
the seed calculator generates said first seed word according to
K=Fx ref*2 M /F out
where K is said first seed word; Fout is said desired output frequency; Fxref is a center frequency of said variable reference frequency; and 2 M is said accumulator modulus.
9. The frequency synthesizer of claim 8 , wherein:
the seed calculator further generates said first seed word according to:
K =integer{[( Fx ref*2 M /F out)+Δ K/ 2 ]/ΔK}*ΔK
where K is said first seed word as an integer and ΔK is an incremental resolution of said first seed word K.
10. The frequency synthesizer of claim 7 , wherein:
the seed calculator is a dual seed calculator using said desired output frequency, said first seed word, an accumulator modulus of the feedback accumulator, said at least one conversion frequency, and a low frequency modulus of said low frequency synthesizer for synthesizing said synthesized first frequency.
11. The frequency synthesizer of claim 10 , wherein:
said dual seed calculator generates said second seed word according to:
L=[F out*( K/ 2 M )− Ffxd ]*(2 ML /Fsclk )
where L is said second seed word; K is said first seed word K; Fout is said desired output frequency; Fsclk is said first conversion frequency; Ffxd is one said conversion frequency; 2 M is said accumulator modulus; and 2 ML is said low frequency modulus.
12. The frequency synthesizer of claim 10 , wherein:
the frequency converter includes a frequency divider for frequency dividing an intermediate signal having a variable intermediate frequency by a divide factor for providing a variable second frequency where said intermediate frequency is based on said synthesized first frequency and where said variable reference frequency is derived from said variable second frequency; and
the seed calculator uses said divide factor for generating said second seed word.
13. The frequency synthesizer of claim 12 , wherein:
said dual seed calculator generates said second seed word according to:
L={N*[F out*( K/ 2 M )− Ffxd 2]− Ffxd 1}*(2 ML /Fsclk )
where L is said second seed word; K is said first seed word K; Fout is said desired output frequency; Fsclk is said first conversion frequency; Ffxd 1 is one said conversion frequency; Ffxd 2 is one said conversion frequency; 2 M is said accumulator modulus; 2 ML is said low frequency modulus; and N is said divide factor.
14. A method for synthesizing an output frequency, comprising:
synthesizing a variable reference frequency for a reference signal;
accumulating a first seed word at said output frequency for updating a feedback word having a most significant bit (MSB) at an output of a feedback accumulator, said first seed word based on a desired said output frequency;
controlling said output frequency by a phase comparison between said reference signal and an MSB signal corresponding to changes in said MSB; and wherein:
synthesizing said variable reference frequency includes deriving said reference frequency from a second seed word based on said desired output frequency.
15. The method of claim 14 , wherein:
synthesizing said variable reference frequency includes using arithmetic frequency conversions for suppressing spurious signal levels for said reference signal for frequencies other than said variable reference frequency.
16. The method of claim 15 , wherein:
using said arithmetic frequency conversions for suppressing said spurious signal levels includes:
frequency translating a first signal at a synthesized first frequency to an intermediate signal at a variable intermediate frequency; and
frequency dividing said intermediate frequency by a divide factor for providing a second signal at a variable second frequency, said variable reference frequency based on said variable second frequency.
17. The method of claim 16 , wherein:
synthesizing said variable reference frequency further includes frequency translating said second signal from said variable second frequency to said reference signal at said variable reference signal.
18. The method of claim 14 , wherein:
synthesizing said variable reference frequency includes:
using said second seed word for generating a first signal having a synthesized first frequency; and
frequency converting said synthesized first frequency to said variable reference frequency.
19. The method of claim 18 , wherein:
synthesizing said variable reference frequency further includes frequency multiplying a base frequency to at least one conversion signal having a conversion frequency; and synthesizing said synthesized first frequency using a first said conversion signal with said second seed word; and
frequency converting includes using said at least one conversion signal for frequency conversion of said synthesized first frequency to said variable reference frequency.
20. The method of claim 19 , further comprising:
generating said first seed word based on said desired output frequency, at least one said conversion frequency, and an accumulator modulus of the feedback accumulator.
21. The method of claim 20 , wherein:
generating said first seed word includes generating said first seed word according to
K=Fx ref*2 M /F out
where K is said first seed word; Fout is said desired output frequency; Fxref is a center frequency of said variable reference frequency; and 2 M is said accumulator modulus.
22. The method of claim 21 , wherein:
generating said first seed word further includes generating said first seed word according to:
K =integer{[( Fx ref*2 M /F out)+Δ K/ 2 ]/ΔK}*ΔK
where K is said first seed word as an integer and ΔK is an incremental resolution of said first seed word K.
23. The method of claim 20 , further comprising:
generating said second seed word using said desired output frequency, said first seed word, said accumulator modulus, said at least one conversion frequency, and a low frequency modulus used with said first conversion signal for synthesizing said synthesized first frequency.
24. The method of claim 23 , wherein:
generating said second seed word includes generating said second seed word according to:
L=[F out*( K/ 2 M )− Ffxd]* (2 ML /Fsclk )
where L is said second seed word; K is said first seed word K; Fout is said desired output frequency; Fsclk is said first conversion frequency; Ffxd is one said conversion frequency; 2 M is said accumulator modulus; and 2 ML is said low frequency modulus.
25. The method of claim 23 , wherein:
frequency converting includes frequency dividing an intermediate signal having a variable intermediate frequency by a divide factor for providing a variable second frequency where said variable intermediate frequency is based on said synthesized first frequency and where said variable reference frequency is derived from said variable second frequency; and
generating said second seed word includes using said divide factor for generating said second seed word.
26. The method of claim 25 , wherein:
generating said second seed word further includes generating said second seed word according to:
L={N*[F out*( K/ 2 M )− Ffxd 2]− Ffxd 1}*(2 ML /Fsclk )
where L is said second seed word; K is said first seed word K; Fout is said output frequency; Fsclk is said first conversion frequency; Ffxd 1 is one said conversion frequency; Ffxd 2 is one said conversion frequency; 2 M is said accumulator modulus; 2 ML is said low frequency modulus; and N is said divide factor.