Synthesizer method utilizing variable frequency comb lines and frequency toggling
A variable frequency synthesizer and method of outputting the variable frequency is disclosed. The synthesizer comprises a first reference frequency, a direct digital synthesizer (DDS) receiving the first reference frequency and outputting a tuned frequency, a variable frequency comb generator receiving the tuned frequency and outputting a variable frequency comb comprised of a plurality of comb lines, a mixer receiving the variable frequency comb and a signal from an oscillator and outputting an intermediate frequency, a phase lock loop (PLL) receiving a second reference frequency and the intermediate frequency and outputting a phase lock signal, and the oscillator receiving the phase lock signal and outputting a variable synthesized frequency.
1. A variable frequency synthesizer, comprising:
a reference frequency;
a direct digital synthesizer (DDS) receiving the reference frequency and outputting a tuned frequency;
a variable frequency comb generator receiving the tuned frequency and outputting a variable frequency comb comprised of a plurality of comb lines; and
a tunable filter adapted to tune the variable frequency synthesizer to a specific comb line from the plurality of comb lines;
wherein the variable frequency synthesizer outputs a range of synthesized frequencies based on the specific comb line the variable frequency synthesizer is tuned to.
2. The synthesizer of claim 1 , wherein the reference frequency is fixed.
3. The synthesizer of claim 1 , wherein the reference frequency is variable.
4. The synthesizer of claim 1 , further comprising a second fixed reference frequency, wherein the DDS receives the second fixed reference frequency.
5. The synthesizer of claim 1 , wherein there are no tuning holes in the outputted synthesized frequency range.
6. The synthesizer of claim 1 , wherein the DDS is tunable over a range of about ⅙ of the reference frequency to about ⅖ of the reference frequency.
7. The synthesizer of claim 6 , wherein the DDS output frequency is within a first Nyquist zone.
8. The synthesizer of claim 6 , wherein the DDS output frequency is in a Nyquist zone higher than the first Nyquist zone.
9. The synthesizer of claim 1 , wherein the relationship between the lowest outputted synthesized frequency (F MIN ), the maximum DDS tuning range (ΔDDS MAX ), and the nominal comb line spacing is governed by:
F MIN ×ΔDDS MAX ≧(comb spacing) 2 .
10. The synthesizer of claim 1 , wherein the frequency comb lines cover a range of 2 to 20 GHz.
11. The synthesizer of claim 1 , wherein the DDS is toggled to tune the synthesizer to a desired frequency not otherwise attainable by the DDS absent toggling.
12. The synthesizer of claim 11 , wherein the specific frequency is attained by switching between a first frequency attainable by the DDS without toggling and a second frequency attainable by the DDS without toggling, wherein the first frequency and the second frequency surround the desired frequency.
13. The synthesizer of claim 12 , wherein the DDS is held at a first tuning word for a first sequence length and subsequently held at a second tuning word for a second sequence length.
14. The synthesizer of claim 13 , wherein the first tuning word and the second tuning word differ by one and the total time of the first sequence length and the second sequence length is minimized.
15. The synthesizer of claim 14 , wherein the first tuning word is determined by
a 1 =└2 b v/f┘;
the second tuning word is determined by
a
2
=
⌈
2
b
v
f
⌉
;
the first sequence length is determined by
k
1
=
mf
g
a
2
-
2
b
n
Δ
g
;
and
the second sequence length is determined by
k
2
=
2
b
n
Δ
g
-
mf
g
a
1
;
wherein, f is the clock frequency, b is the phase register length, v is a frequency related to f by a ratio of integers, m and n are integers, Δ is the synthesizer tuning step size, and g=gcd(mf,2 b nΔ), wherein gcd is the greatest common divisor.
16. The synthesizer of claim 12 , wherein the DDS is varies between a first tuning word and a second tuning word for a first sequence length and subsequently the DDS is held at a second tuning word for a second sequence length.
17. A method of outputting a variable frequency, comprising:
outputting a reference frequency at a clock;
receiving the reference frequency at a direct digital synthesizer (DDS);
outputting a tuned frequency from the DDS;
receiving the tuned frequency at a variable frequency comb generator;
outputting a variable frequency comb comprised of a plurality of comb lines from the variable frequency comb generator;
receiving the variable frequency comb at a tunable filter;
tuning to a specific comb line of the plurality of comb lines; and
outputting a range of synthesized frequencies based on the specific comb line.
18. The method of claim 17 , wherein the reference frequency is fixed.
19. The method of claim 17 , wherein the reference frequency is variable.
20. The method of claim 17 , wherein the DDS receives a second reference frequency.
21. The method of claim 17 , wherein there are no tuning holes in the outputted synthesized frequency range.
22. The method of claim 17 , wherein the DDS is tunable over a range of about ⅙ of the fixed reference frequency to about ⅖ of the reference frequency.
23. The method of claim 22 , wherein the DDS output frequency is within the first Nyquist zone.
24. The method of claim 22 , wherein the DDS output frequency is in a Nyquist zone higher than the first Nyquist zone.
25. The method of claim 1 , wherein the relationship between a lowest outputted synthesized frequency (F MIN ), a maximum DDS tuning range (ΔDDS MAX ), and comb line spacing is governed by:
F MIN ×ΔDDS MAX ≧(comb spacing) 2 .
26. The method of claim 17 , wherein the frequency comb lines cover a range of 2 to 18 GHz.
27. The method of claim 17 , wherein the DDS is toggled to tune the synthesizer to a desired frequency not otherwise attainable by the DDS absent toggling.
28. The method of claim 27 , wherein the specific frequency is attained by tuning between a first frequency attainable by the DDS without toggling and a second frequency attainable by the DDS without toggling, wherein the first frequency and the second frequency surround the desired frequency.
29. The method of claim 28 , wherein the DDS is held at a first tuning word for a first sequence length and subsequently held at a second tuning word for a second sequence length.
30. The method of claim 29 , wherein the first tuning word and the second tuning word differ by one and the total time of the first sequence length and the second sequence length is minimized.
31. The method of claim 30 , wherein the first tuning word is determined by
a
1
=
⌊
2
b
v
f
⌋
;
the second tuning word is determined by
a
2
=
⌈
2
b
v
f
⌉
;
the first sequence length is determined by
k
1
=
mf
g
a
2
-
2
b
n
Δ
g
;
and
the second sequence length is determined by
k
2
=
2
b
n
Δ
g
-
mf
g
a
1
;
wherein, f is the clock frequency, b is the phase register length, v is a quantized frequency, m and n are integers, Δ is the synthesizer tuning step size, and g=gcd(mf, 2 b nΔ), wherein gcd is the greatest common divisor.
32. The method of claim 27 , wherein the DDS is varies between a first tuning word and a second tuning word for a first sequence length and subsequently the DDS is held at a second tuning word for a second sequence length.