Transmitter method, apparatus, and frequency plan for minimizing spurious energy
View Patent ↗A translational-loop transmitter includes a local oscillator (LO) generator for generating first and second LO signals, a modulator for generating a modulated reference signal using the second LO signal, and an offset phase-locked-loop (PLL) for phase-locking an output signal to the reference signal, and for tuning the output signal in accordance with the first LO signal. The PLL includes an offset mixer in a feedback path of the PLL, and operates in accordance with a frequency plan that minimizes the effects of on- and off-channel spurs at the output of the offset mixer.
1. A transmitter apparatus, comprising:
a local oscillator (LO) generator that generates an LO signal having a tunable frequency f LO1 ;
a phase-locked-loop (PLL) that generates an output signal that is phase-locked to a reference signal, the output signal having a frequency f TX responsive to the LO signal and tunable within at least a first and second alternative frequency bands, the PLL including a feedback path for generating a feedback signal having a desired frequency f FB responsive to the frequencies f TX and f LO1 ; and
means for establishing the frequency f FB and the frequency f LO1 based on whether the frequency f TX is in the first and/or second alternative frequency bands.
2. The apparatus of claim 1 , wherein:
the frequency f FB is approximately equal to 1/6 the frequency f TX when the frequency f TX is in each of the first and second alternative frequency bands; and
the frequency f LO1 is approximately equal to 7/6 the frequency f TX when the frequency f TX is in each of the first and second alternative frequency bands.
3. The apparatus of claim 1 , wherein the frequency f FB represents a difference between frequency f LO1 and frequency f TX .
4. The apparatus of claim 1 , wherein the LO generator includes:
a source oscillator for generating a source signal having a frequency f SLO ; and
a frequency divider circuit to produce the LO signal from the source signal, wherein a ratio of the frequency f SLO when the frequency f TX is in the first frequency band to the frequency f SLO when the frequency f TX is in the second frequency band is approximately 2:1.
5. The apparatus of claim 4 , wherein the LO generator includes:
a source oscillator for generating a source signal having a frequency f SLO ; and
a frequency divider circuit to produce the LO signal from the source signal, wherein
the frequency f SLO is approximately equal to 14/3 the frequency f TX when the frequency f TX is in the first frequency band, and
the frequency f SLO is approximately equal to 7/3 the frequency f TX when the frequency f TX is in the second frequency band.
6. The apparatus of claim 5 , wherein the frequency f TX is tunable within a third frequency band, and when the frequency f TX is in the third frequency band
the frequency f SLO is approximately equal to 7/3 the frequency f TX ,
the frequency f LO1 is approximately equal to 7/6 the frequency f TX , and
the frequency f FB is approximately equal to 1/6 the frequency f TX .
7. The apparatus of claim 4 , wherein said means for establishing includes a frequency plan relating f SLO , f LO1 , and f FB to f TX .
8. The apparatus of claim 1 , the PLL feedback path comprising:
an offset mixer for generating the feedback signal having the desired frequency f FB responsive to the frequencies f TX and f LO1 , wherein the offset mixer generates
on-channel spurs at a spur power level significantly lower than the feedback signal power level; and
off-channel spurs at a spur frequency significantly separated from the desired frequency f FB .
9. The apparatus of claim 1 , wherein the frequency f FB is a first intermediate frequency when the frequency f TX is in the first frequency band and a second intermediate frequency separated from the first intermediate frequency when the frequency f TX is in the second frequency band, the PLL feedback path comprising:
an offset mixer for generating the feedback signal having the desired frequency F FB responsive to the frequencies f TX and f LO1 ;
a first filter, following the offset mixer, that passes the first intermediate frequency and rejects first spurious energy offset in frequency from the first intermediate frequency when the frequency f TX is in the first frequency band; and
a second filter, following the offset mixer, that passes the second intermediate frequency and rejects second spurious energy offset in frequency from the second intermediate frequency when the frequency f TX is in the second frequency band.
10. The apparatus of claim 9 , wherein the combination of frequencies f LO1 and f TX causes
the first spurious energy to be offset in frequency from the first intermediate frequency by at least an integer multiple of the first intermediate frequency, and
the second spurious energy to be offset in frequency from the second intermediate frequency by at least an integer multiple of the second intermediate frequency.
11. The apparatus of claim 9 , wherein the first and second bandpass filters are each adapted to produce a respective filtered feedback signal, the PLL further including:
a phase or frequency detector that derives an error signal responsive to the reference signal and a respective one of the filtered feedback signals;
a control signal deriving circuit following the phase or frequency detector that derives a control signal from the error signal; and
a Voltage Controlled Oscillator (VCO), following the control signal deriving circuit, that generates the phase-locked output signal responsive to the control signal.
12. The apparatus of claim 1 , wherein:
the first frequency band extends approximately between 880 MHz and 915 MHz; and
the second frequency band extends between one of the following,
approximately between 1710 MHz and 1785 MHz, or
approximately between 1850 MHz and 1910 MHz.
13. The apparatus of claim 1 , wherein the frequency f TX is tunable within a third frequency band, and
the first frequency band extends approximately between 880 MHz and 915 MHz,
the second frequency band extends approximately between 1710 MHz and 1785 MHz, and
the third frequency band extends approximately between 1850 MHz and 1910 MHz.
14. The apparatus of claim 1 , wherein the reference signal and the output signal each include at least one of a phase modulated signal component and a frequency modulated signal component.
15. The apparatus of claim 14 , wherein the LO generator is adapted to generate a second LO signal, the apparatus further comprising: a modulator that modulates the second LO signal with a baseband signal to produce the reference signal such that the reference signal has a frequency f MOD substantially equal to the feedback frequency f FB .
16. A transmitter apparatus, comprising:
(a) means for generating a local oscillator (LO) signal having a tunable frequency f LO1 ;
(b) means for phase-locking an output signal to a reference signal, the output signal having a frequency f TX responsive to the LO signal and tunable within at least a first and second alternative frequency bands, said means for phase-locking including generating a feedback signal having a desired frequency f FB responsive to the frequencies f TX and f LO1 ; and
(c) means for establishing the frequency f FB and the frequency f LO1 based on whether the frequency f TX is in the first and/or second alternative frequency bands.
17. The apparatus of claim 16 , wherein:
the frequency f FB is approximately equal to 1/6 the frequency f TX when the frequency f TX is in each of the first and second alternative frequency bands; and
the frequency f LO1 is approximately equal to 7/6 the frequency f TX when the frequency f TX is in each of the first and second alternative frequency bands.
18. The apparatus of claim 16 , wherein step (a) includes:
means for generating a source signal having a frequency f SLO ; and
means for frequency dividing the source signal to produce the LO signal, wherein a ratio of the frequency f SLO when the frequency f TX is in the first frequency band to the frequency f SLO when the frequency f TX is in the second frequency band is approximately 2:1.
19. The apparatus of claim 18 , wherein
the frequency f SLO is approximately equal to 14/3 the frequency f TX when the frequency f TX is in the first frequency band, and
the frequency f SLO is approximately equal to 7/3 the frequency f TX when the frequency f TX is in the second frequency band.
20. The apparatus of claim 19 , wherein the frequency f TX is tunable within a third alternative frequency band, and when the frequency f TX is in the third frequency band
the frequency f SLO is approximately equal to 7/3 the frequency f TX ,
the frequency f LO1 is approximately equal to 7/6 the frequency f TX , and
the frequency f FB is approximately equal to 1/6 the frequency f TX .
21. The apparatus of claim 16 , wherein the frequency f FB is a first intermediate frequency when the frequency f TX is in the first frequency band and a second intermediate frequency separated from the first intermediate frequency when the frequency f TX is in the second frequency band, and wherein step (b) further comprises:
means for producing the feedback signal by frequency-mixing the output signal with the LO signal;
means for filtering the feedback signal to pass the first intermediate frequency and reject first spurious energy offset in frequency from the first intermediate frequency when the frequency f TX is in the first frequency band; and
means for filtering the feedback signal to pass the second intermediate frequency and reject second spurious energy offset in frequency from the second intermediate frequency when the frequency f TX is in the second frequency band.
22. The apparatus of claim 21 , wherein said frequency-mixing comprises generating the frequency f FB as a difference between the frequency f LO1 and the frequency f TX .
23. The apparatus of claim 21 , wherein each said means for filtering produces a respective filtered feedback signal, and wherein step (b) further comprises:
means for deriving an error signal responsive to the reference signal and a respective one of the filtered feedback signals;
means for deriving a control signal from the error signal; and
means for generating the phase-locked output signal responsive to the control signal.
24. The apparatus of claim 16 , wherein:
the first frequency band extends approximately between 880 MHz and 915 MHz; and
the second frequency band extends between one of the following,
approximately between 1710 MHz and 1785 MHz, or
approximately between 1850 MHz and 1910 MHz.
25. The apparatus of claim 16 , wherein the frequency f TX is tunable within a third alternative frequency band, and
the first frequency band extends approximately between 880 MHz and 915 MHz,
the second frequency band extends approximately between 1710 MHz and 1785 MHz, and
the third frequency band extends approximately between 1850 MHz and 1910 MHz.
26. The apparatus of claim 16 , wherein the reference signal and the output signal each include at least one of a phase modulated component and a frequency modulated component.
27. The apparatus of claim 26 , further comprising:
means for generating a second LO signal; and
means for producing the reference signal from a baseband signal using the second LO signal.