PLL circuit and method for generating a modulated carrier signal
A PLL circuit for generating a modulated carrier signal includes a digitally controlled oscillator (DCO) to generate the modulated signal. The PLL circuit receives a desired phase change as a modulation signal at each cycle of a non-uniform clock, derived from the a DCO output and a uniform reference clock. This phase change adjusts the DCO's frequency. The circuit also receives a frequency control word, representing the ratio of the desired carrier frequency to the reference clock frequency. The phase change and frequency control word are accumulated to predict the DCO's output phase. A non-uniform clock compensation circuit calculates a compensation value for the phase change. A phase detector estimates the error between the predicted phase and the time offset between the reference clock and DCO output, generating a control signal for the DCO based on this error.
1 . A phased locked loop, PLL, circuit for generating a modulated carrier signal, the PLL circuit comprising
a controlled oscillator configured to generate a controlled oscillator output signal as the modulated carrier signal;
a first input configured to receive a desired phase change as a modulation signal at an n-th cycle of a non-uniform clock derived from the controlled oscillator output signal and a uniform reference clock;
a first path for the desired phase change to the controlled oscillator to change its oscillation frequency based on the desired phase change;
a second input configured to receive a frequency control word at the n-th cycle of the non-uniform clock, the frequency control word being a ratio of a desired carrier frequency of the carrier signal and a frequency of the uniform reference clock;
a second path for the desired phase change to a frequency control word accumulator configured to accumulate the desired phase change and the FCW to generate a first phase prediction of the controlled oscillator's output phase at the (n+1)-th cycle of the non-uniform clock;
a non-uniform clock compensation circuit configured to recursively calculate, using the desired phase change, the frequency control word, and the first phase prediction, a non-uniform clock compensation value for the desired phase change on the first path and/or a second phase prediction of the controlled oscillator's output phase at the (n+1)-th cycle of the uniform reference clock;
a phase detector configured to estimate an error between the second phase prediction and an instantaneous time offset between the uniform reference clock and the controlled oscillator output signal and to generate a control signal for the controlled oscillator based on the estimated error.
2 . The PLL circuit of claim 1 , wherein the controlled oscillator output signal and the uniform reference clock have different respective frequencies and are asynchronous signals.
3 . The PLL circuit of claim 1 , wherein the non-uniform clock and the uniform reference clock have equal respective average frequencies, and wherein instantaneous periods and start points of the non-uniform clock relative to the uniform reference clock vary from cycle to cycle.
4 . The PLL circuit of claim 1 , wherein the non-uniform clock is phase aligned with the controlled oscillator output signal.
5 . The PLL circuit of claim 1 , wherein the first input is configured to receive a desired modulation phase for the modulated carrier signal at the n-th cycle of the non-uniform clock, and wherein the PLL circuit further comprises
a differentiator circuit configured to differentiate the desired modulation phase to generate the desired phase change at the n-th cycle of the non-uniform clock.
6 . The PLL circuit of claim 1 , wherein the phase detector is configured to estimate the error between a predicted time difference between two nearest falling/rising edges of the uniform reference clock and the controlled oscillator output signal and a measured time difference between two nearest falling/rising edges of the uniform reference clock and the controlled oscillator output signal.
7 . The PLL circuit of claim 1 , wherein the non-uniform clock compensation circuit is configured to calculate the non-uniform clock compensation value for the n-th non-uniform clock cycle based on the desired phase change of the (n−1)-th non-uniform clock cycle, an instantaneous period of the n-th non-uniform clock cycle, and a period of the uniform reference clock.
8 . The PLL circuit of claim 1 , wherein the non-uniform clock compensation circuit is configured to calculate the non-uniform clock compensation value for the n-th non-uniform clock cycle based on the second phase prediction of the controlled oscillator's output phase for the (n−1)-th uniform reference clock cycle, the second phase prediction of the controlled oscillator's output phase for the n-th uniform reference clock cycle, the frequency control word; and the desired phase change of the (n−1)-th non-uniform clock cycle.
9 . The PLL circuit of claim 1 , wherein the non-uniform clock compensation circuit is configured to calculate the non-uniform clock compensation value (ϕ DMC [n]) for the n-th non-uniform clock (CKR 2 ) cycle based on
ϕ
DMC
[
n
]
=
ϕ
S
,
frac
[
n
-
1
]
-
ϕ
S
,
frac
[
n
-
2
]
FCW
·
Δϕ
M
[
n
-
1
]
,
wherein ϕ S,frac [·] denotes a fractional part of the second phase prediction ϕ S [·] used to track the phase of the modulated carrier signal when the PLL circuit operates in a locked state.
10 . The PLL circuit of claim 1 , wherein the non-uniform clock compensation circuit is configured to calculate the second phase prediction of the controlled oscillator's output phase for the (n+1)-th uniform reference clock cycle based on the first phase prediction (ϕ R [n]), the desired phase change (Δϕ M [n]), the non-uniform clock compensation value (ϕ DMC [n]), a period (T REF ) of the uniform reference clock, and a non-uniform delay (T update [n]) between an edge of the n-th uniform reference clock cycle and an edge of the (n+1)-th non-uniform clock cycle plus a constant analog circuit delay (D ana ).
11 . The PLL circuit of claim 10 , wherein the non-uniform clock compensation circuit is configured to calculate the second phase prediction (ϕ S [n]) of the controlled oscillator's output phase for the (n+1)-th uniform reference clock (FREF) cycle based on
ϕ
S
[
n
]
=
ϕ
R
[
n
]
-
T
update
[
n
]
T
REF
·
(
Δϕ
M
[
n
]
+
ϕ
DMC
[
n
]
)
.
12 . The PLL circuit of claim 10 , wherein the non-uniform clock compensation circuit is configured to calculate the second phase prediction (ϕ S [n]) of the controlled oscillator's output phase for the (n+1)-th uniform reference clock (FREF) cycle based on
ϕ
S
[
n
]
≈
ϕ
R
[
n
]
-
[
(
2
FCW
+
D
ana
T
REF
)
-
ϕ
S
,
frac
[
n
-
1
]
FCW
]
·
(
Δϕ
M
[
n
]
)
,
wherein ϕ S,frac [·] denotes a fractional part of ϕ S [·] used to track the phase of the modulated carrier signal when the PLL circuit operates in a locked state.
13 . A polar transmitter comprising the PLL circuit of claim 1 .
14 . A method for generating a modulated carrier signal, the method comprising
providing a controlled oscillator configured to generate a controlled oscillator output signal (CKV) as the modulated carrier signal;
receiving a desired phase change (Δϕ M [n]) as a modulation signal at an n-th cycle of a non-uniform clock (CKR 2 ) derived from the controlled oscillator output signal (CKV) and a uniform reference clock (FREF) of a PLL circuit;
providing the desired phase change (Δϕ M [n]) to the controlled oscillator to change its oscillation frequency based on the desired phase change (Δϕ M [n]);
receiving a frequency control word (FCW) at the n-th cycle of the non-uniform clock (CKR 2 ), the frequency control word (FCW) being a ratio of a desired carrier frequency (f c ) of the carrier signal and a frequency (f REF ) of the uniform reference clock;
providing the desired phase change (Δϕ M ) to a frequency control word accumulator configured to accumulate the desired phase change (Δϕ M [n]) and the FCW to generate a first phase prediction (ϕ R [n]) of the controlled oscillator's output phase at the (n+1)-th cycle of the non-uniform clock (CKR 2 );
recursively calculating, using the desired phase change (Δϕ M [n]), the frequency control word FCW, and the first phase prediction (ϕ R [n]), a non-uniform clock compensation value (ϕ DMC [n]) for the desired phase change Δϕ M [n] on the first path and/or a second phase prediction (ϕ S [n]) of the controlled oscillator's output phase at the (n+1)-th cycle of the uniform reference clock (FREF); and
estimating an error between the second phase prediction (ϕ S [n]) and an instantaneous time offset (Δt S ) between the uniform reference clock (FREF) and the controlled oscillator output signal (CKV) and generating a control signal for the controlled oscillator based on the estimated error.