U.S. patent application number 12/978394 was filed with the patent office on 2012-06-28 for digital phase lock loop.
Invention is credited to Paolo Madoglio, Georgios Palaskas, Ashoke Ravi, Pin-En Su.
Application Number | 20120161831 12/978394 |
Document ID | / |
Family ID | 46272899 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120161831 |
Kind Code |
A1 |
Ravi; Ashoke ; et
al. |
June 28, 2012 |
DIGITAL PHASE LOCK LOOP
Abstract
An apparatus may comprise a time-to-digital circuit
architecture. Other embodiments are described and claimed.
Inventors: |
Ravi; Ashoke; (Hillsboro,
OR) ; Su; Pin-En; (Fountain Valley, CA) ;
Madoglio; Paolo; (Beaverton, OR) ; Palaskas;
Georgios; (Portland, OR) |
Family ID: |
46272899 |
Appl. No.: |
12/978394 |
Filed: |
December 23, 2010 |
Current U.S.
Class: |
327/156 |
Current CPC
Class: |
H03L 7/1976 20130101;
H03L 2207/50 20130101; H03L 7/085 20130101 |
Class at
Publication: |
327/156 |
International
Class: |
H03L 7/08 20060101
H03L007/08 |
Claims
1. An apparatus, comprising: a digital phase locked loop circuit
(DPLL) having a time-to-digital converter (TDC), the TDC
comprising: a first ring comprising a first delay cell and an
inverter arranged to receive a reference signal, the reference
signal circulating in the reference ring over one or more stages
that each comprise a reference delay period 2t.sub.df; a second
ring comprising a second delay cell and an inverter arranged to
receive from a voltage controlled oscillator (VCO) a VCO signal
having a phase delay with respect to the reference signal, the VCO
signal circulating in the second ring over one or more stages that
each comprise a VCO delay period 2t.sub.ds that is different from
the reference delay period; and a flip-flop circuit coupled to
outputs of the first and second rings, the flip-flop circuit
arranged to periodically sample first and second outputs from the
first and the second ring, respectively, wherein the phase delay is
determined by an amount N of reference delay periods elapsed
between when a first reference signal is detected and when an
output of the flip-flop corresponds to a zero to one or one to zero
transition.
2. The apparatus of claim 1, wherein the DPLL circuit is arranged
to provide a later-arriving signal of the VCO and reference signals
to a ring having a shorter delay period.
3. The apparatus of claim 2, wherein the DPLL is arranged to delay
the VCO signal, wherein the reference signal is received at the
first ring before the VCO signal is received at the second ring,
and wherein 2t.sub.ds<2t.sub.df.
4. The apparatus of claim 1, the DPLL circuit further comprising:
an adder coupled to an input of a loop filter of the DPLL circuit
and arranged to add a phase offset to the output of the TDC; and a
multi-modulus divider whose input is coupled to an output of the
VCO and whose output is coupled to the input of the second
ring.
5. The apparatus of claim 1, wherein the DPLL circuit is arranged
to adjust a frequency of the VCO according to the phase delay, the
phase delay equal to a product of N.times.2|t.sub.ds-t.sub.df|,
wherein 2|t.sub.ds-t.sub.df| is TDC resolution.
6. The apparatus of claim 1, further comprising: a VCO delay
circuit arranged to add a delay equal to one VCO period into the
VCO signal received by the TDC; and a period estimation loop
arranged to unwrap an output from the TDC, wherein the period
estimation loop converges on a number M, wherein M is equal to a
ratio of the VCO period to TDC resolution.
7. A method of measuring a phase delay of a voltage controlled
oscillator (VCO) signal of a digital phase locked loop (DPLL)
circuit, comprising: circulating a reference signal in a first ring
oscillator circuit of a time-to-digital converter (TDC), the first
ring oscillator circuit arranged with a reference delay period
2t.sub.df; circulating the VCO signal in a second ring oscillator
circuit of the TDC, the second ring oscillator circuit arranged
with a VCO delay period 2t.sub.ds that is different from the
reference delay period; counting a number N of reference delay
periods from when a first reference signal is detected and when an
output of a flip-flop that samples the VCO and reference signals
corresponds to a zero to one transition; and determining the phase
delay according to N.times.2|t.sub.ds-t.sub.df|.
8. The method of claim 7, further comprising providing a later
arriving signal of the VCO and reference signals to a ring having a
shorter delay period.
9. The method of claim 8, wherein 2t.sub.ds<2t.sub.df, the
method further comprising: adding a phase offset to an output of
the TDC before the output enters a loop filter of the DPLL;
receiving an output signal of the VCO containing the phase offset
at a multimodulus divider; and sending a rising edge of the output
signal having the phase offset to the second ring.
10. The method of claim 7, further comprising adjusting a frequency
of the VCO according to the determined phase delay.
11. The method of claim 7, wherein the counting the number N of
reference delay periods comprises sampling a rising edge of the
reference signal in a D-flip-flop to determine the zero to one
transition.
12. The method of claim 7, further comprising: adding a delay equal
to one VCO period (T.sub.VCO) into the VCO signal before the VCO
signal is received by the TDC; and unwrapping the output of the TDC
in a loop circuit that converges on a value M equal to
T.sub.VCO/(2|t.sub.ds-t.sub.df|.
13. A communications system, comprising: a wireless interface that
comprises a digital phase locked loop (DPLL) having a
time-to-digital converter (TDC) for comparing phase difference
between a voltage controlled oscillator (VCO) signal and a
reference signal, the TDC comprising: a first ring comprising a
first delay cell and an inverter arranged to receive the reference
signal, the reference signal circulating in the reference ring over
one or more stages that each comprise a reference delay period
2t.sub.df; a second ring comprising a second delay cell and an
inverter arranged to receive the VCO signal, the VCO signal
circulating in the second ring over one or more stages that each
comprise a VCO delay period 2t.sub.ds that is different from the
reference delay period; and a flip-flop that uses output from the
first ring to sample output from the second ring, wherein the phase
delay is determined by an amount N of reference delay periods
elapsed between when a first reference signal is detected and when
an output of the flip-flop corresponds to a zero to one
transition.
14. The communications system of claim 13, wherein the wireless
interface comprises one of: a transmitter, a receiver, and a
transceiver.
15. The communications system of claim 13, wherein the system
comprises one or more of: a wireless access point, a wireless
client device, a wireless station, a laptop computer, ultra-laptop
computer, portable computer (PC), personal computer, notebook PC,
handheld computer, personal digital assistant (PDA), cellular
telephone, combination cellular telephone/PDA, smartphone, pager,
messaging device, media player, digital music player, set-top box,
appliance, workstation, user terminal, mobile unit, consumer
electronics, television, digital television, high-definition
television, television receiver, and a high-definition television
receiver.
16. The communications system of claim 13, wherein the wireless
interface comprises a wireless communications integrated circuit
that includes a frequency synthesizer controlled by the DPLL.
17. The communications system of claim 13, wherein the DPLL is
arranged to delay the VCO signal, wherein the reference signal is
received at the first ring before the VCO signal is received at the
second ring, and wherein 2t.sub.ds<2t.sub.df.
18. The communications system of claim 13, the DPLL further
comprising: an adder coupled to an input of a loop filter of the
DPLL circuit and arranged to add a phase offset to the output of
the TDC; and a multi-modulus divider whose input is coupled to an
output of the VCO and whose output is coupled to the input of the
second ring.
19. The communications system of claim 13, wherein the DPLL circuit
is arranged to adjust a frequency of the VCO according to the phase
difference, the phase difference equal to a product of
N.times.2|t.sub.ds-t.sub.df|, wherein 2|t.sub.ds-t.sub.df| is TDC
resolution.
20. The communications system of claim 13, the DPLL further
comprising: a VCO delay circuit arranged to add a delay equal to
one VCO period into the VCO signal received by the TDC; and a
period estimation loop arranged to unwrap an output from the TDC,
wherein the period estimation loop converges on a number M, wherein
M is equal to a ratio of the VCO period to TDC resolution.
21. A method of measuring a phase delay of a voltage controlled
oscillator (VCO) signal of a digital phase locked loop (DPLL)
circuit, comprising: circulating a reference signal in a ring
oscillator circuit of a time-to-digital converter (TDC), the ring
oscillator circuit arranged with a reference delay period
.tau..sub.REF that is different than a period T.sub.vco of the VCO;
sending an output of the the ring oscillator circuit and the VCO
signal to a flip-flop for sampling; counting a number N of
reference delay periods from when a first reference signal is
detected and when an output of the flip-flop corresponds to a
transition between zero and one; and determining the phase delay
according to N.times.|T.sub.VCO-.tau..sub.REF|.
Description
BACKGROUND
[0001] In a digital phase locked loop (DPLL) architecture for local
oscillator (LO) generation, the voltage controlled oscillator (VCO)
phase is typically measured and compared to a high purity
low-frequency reference signal using a time-to-digital converter
(TDC). The error phase between the VCO and reference signals is
filtered and used to set the VCO frequency. In typical
implementations of a TDC, a mismatch occurs between buffer cells of
the TDC. This mismatch may produce a non-uniform quantization
characteristic. As a result, the VCO phase, as measured by the TDC,
shows an error that inherits the same periodicity as that of the
VCO phase, which in turn is set by the desired ratio between output
frequency and reference signal frequency. This periodic error
appears in the DPLL output signal spectrum as spurious sidebands
around a carrier. Such spurs limit the ability to use a DPLL
employing a conventional TDC architecture for frequency generation
where high spectral purity is desired.
[0002] In addition, the resolution of a TDC is typically not known
precisely because the resolution may be process-, voltage-, and
temperature dependent. Without knowledge of TDC resolution, the TDC
cannot output a number that exactly represents the phase difference
between its two inputs, but rather a scaled phase difference
between the inputs. The phase wrapping effect in a TDC device
serves to periodically reset this incorrect phase difference such
that the periodicity appears at the DPLL output signal spectrum as
spurious sidebands around the carrier. These spurs further limit
the application of a DPLL as a frequency generator where high
spectral purity is required, because filtering of such spurs would
require a low bandwidth DPLL. It is with respect to these and other
considerations that the present improvements have been needed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] FIG. 1 illustrates one embodiment of a time-to-digital
converter.
[0004] FIG. 2 illustrates one exemplary output of the
time-to-digital converter of FIG. 1.
[0005] FIG. 3 illustrates one embodiment of a digital phase locked
loop circuit.
[0006] FIGS. 4A and 4B illustrate operation of the digital phase
locked loop circuit of FIG. 3.
[0007] FIG. 5 illustrates one embodiment of a period estimation
loop circuit.
[0008] FIG. 6 illustrates one embodiment of a time-to-digital
converter of the circuit of FIG. 5.
[0009] FIG. 7 illustrates one embodiment of a communications
system.
[0010] FIG. 8 illustrates one embodiment of a computing system.
[0011] FIG. 9 illustrates an embodiment of a time-to-digital
converter which directly compares a VCO signal against output of a
single ring oscillator
[0012] FIG. 10 illustrates exemplary timing diagrams of the
embodiment in FIG. 9.
DETAILED DESCRIPTION
[0013] Various embodiments may be generally directed to systems
that employ digital frequency synthesizers. Some embodiments may be
particularly directed to architecture for digital phase locked
loops.
[0014] As digital phase locked loops are more widely deployed,
techniques and architecture to enhance their functioning as
frequency synthesizers may be desirable. Various embodiments
provide improved DPLL performance that reduces the spurs in
spectral output that may result from the use of TDCs in a DPLL
circuit arrangement.
[0015] Some embodiments include DPLLs having a recirculating TDC
architecture that eliminates buffer mismatches produced in
conventional TDC architecture. Other embodiments may employ a
period estimation loop architecture that may be integrated into a
DPLL circuit arrangement. Other DPLL embodiments may employ a
recirculating TDC architecture in conjunction with a period
estimation loop architecture.
[0016] Various embodiments may comprise one or more elements. An
element may comprise any structure arranged to perform certain
operations. Although an embodiment may be described with a limited
number of elements in a certain arrangement by way of example, the
embodiment may include more or less elements in alternate
arrangement as desired for a given implementation. It is worthy to
note that any reference to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment is included in at least
one embodiment. The appearances of the phrase "in one embodiment"
in various places in the specification are not necessarily all
referring to the same embodiment.
[0017] FIG. 1 illustrates a block diagram of one embodiment of a
recirculating TDC 100. In various embodiments, the recirculating
TDC 100 may be arranged in a circuit that comprises digital and
analog components, or a circuit that comprises only digital
components. In particular embodiments, TDC 100 may be arranged in a
DPLL. In some embodiments, TDC 100 may be arranged in a DPLL
frequency synthesizer. In some embodiments a DPLL frequency
synthesizer may be included in a radio receiver, mobile
communication device, wireless communication device including WiFi,
WiMax, Bluetooth device, and the like.
[0018] The recirculating TDC 100 includes a pair of rings 102, 104
that are arranged to receive a pair of signals, such as a voltage
controlled oscillator signal (VCO) and a reference signal,
respectively, as illustrated in FIG. 1. In some embodiments, the
TDC 100 may be arranged to receive a digital controlled oscillator
(DCO) signal and a reference signal. Each ring comprises a
multiplexer (MUX) 112, 118 that feeds a signal to a respective
inverter 110, 114, which, in turn, feed respective inverted signals
to inputs of delay cells 108, 116. Each delay cell 108, 116 may be
characterized by a respective delay time t.sub.ds, t.sub.df, which
may be any convenient delay period. In accordance with embodiments
of the TDC 100 the delay time t.sub.ds of the VCO delay cell 108
differs from the delay time t.sub.df of the reference delay cell
108.
[0019] The delay cells 108 and 116 may have known components that
may comprise, for example, inverters (not shown). In some
embodiments, the delay cells 108, 116 are arranged such that the
number of inverters in each delay cell are each an even number.
Accordingly, each ring 102, 104 may include an odd number of
inverters in series including inverters 110, 114. In various
embodiments, the delay cells 108, 116 and inverters 110, 114 may
comprise CMOS devices having known designs.
[0020] The output of delay cells 102, 104 are fed to respective MUX
devices 112, 118. In this manner, a VCO signal may be recirculated
in ring 102 with a delay corresponding to 2t.sub.ds in each period
of circulation and a reference signal may be recirculated in ring
102 with a delay corresponding to 2t.sub.df in each period of
circulation. In particular, each ring may act as a ring oscillator
in which the respective signals that are output by inverters 110,
114 are inverted after each period of circulation.
[0021] Signals output by VCO ring 102 may be fed to flip-flop 106.
Signals output by reference ring 104 may also be fed to flip-flop
106. Is some embodiments flip-flop 106 may comprise a D-flip-flop.
In one embodiment depicted in FIG. 1, the data input to flip-flop
106 is from the VCO ring 102 while the clock pin receives input
from the reference ring 104. In some embodiments, the flip-flop 106
may be edge triggered, as depicted in FIG. 1. In another
embodiment, the VCO ring signal may be used to trigger the
reference ring signal.
[0022] In one embodiment D-flip-flop 106 may sample the outputs
from rings 102, 104 and detect a zero-to-one transition. By
determining the number of reference ring stages from the time the
rising edge of the reference ring is detected until the zero-to-one
transition is detected, TDC 100 may provide a quantized calculation
of the phase delay between reference and VCO signals.
[0023] As detailed further below, the TDC 100 circuit architecture
calculates the phase delay of a VCO signal while avoiding the
mismatch problem in conventional TDCs or in Vernier TDCs, which
both exhibit buffer mismatches between stages. Instead, TDC 100 is
arranged to recirculate a reference and VCO signal in respective
reference and VCO ring oscillators, which each provide the same
inverter and delay cell for their respective signals for each
stage, thereby eliminating the buffer mismatch problem.
[0024] FIG. 2 depicts exemplary signals that may be generated in
some TDC embodiments, such as the TDC of FIG. 1. With reference
also to FIG. 1, the reference signal ref(t) 202 may represent a
signal fed to reference ring 104 and the VCO signal vco(t) 206 may
represent a signal fed to VCO ring 102. In operation, TDC 100 may
provide an output that represents the phase delay between the
reference signal 202 and VCO signal 206. This TDC output may be
used to adjust an oscillator in a DPLL in a manner that facilitates
producing an output spectrum without spurs that are characteristic
of known TDCs used in DPLL circuits. In particular, the
recirculated reference signals 208 and VCO signals 212 generate
sampled output 216, which results in a detected transition in
flip-flop 106 when the signals converge.
[0025] In some embodiments, the rising edge 204 of the reference
signal arrives before that of the VCO signal 224, as depicted in
FIG. 2. In an embodiment depicted in FIG. 2, the reference ring
delay period 2t.sub.df 210 is longer than the VCO ring delay period
ring delay period 2t.sub.ds 214. Because the delay in ring 102 is
less, the rising edge 228 of the recirculated VCO signal eventually
catches up with the rising edge 226 of the recirculated reference
signal after a fixed amount of periods.
[0026] Before the VCO rising edge 228 catches up to the reference
ring rising edge 226, sampled outputs 218 and 220 do not detect a
zero-to-one transition. Once the VCO ring rising edge catches up to
the reference ring rising edge, a sampled output 222 produces a
zero-to-one detection. TDC 100 may accordingly output a number
corresponding to the number N of reference ring oscillating periods
that have elapsed (elapsed reference ring periods) between
detection of the first rising edge 204 in the reference signal and
the time of sampled output 222 when the zero-to-one output at
flip-flop 106 occurs.
[0027] The quantized time duration between arrival of ref(t) 204
and VCO(t) 224 may be calculated as the product of the number N of
elapsed reference ring periods output by the TDC and the TDC
resolution, the latter of which is given by a difference in the
absolute values of the VCO and reference periods 2|tds-tdf| for the
architecture of TDC 100. In various embodiments, this value,
2N|tds-tdf|, may be used to adjust the VCO frequency in a DPLL
circuit.
[0028] In order for the TDC circuit 100 of FIG. 1 to output
respective reference and VCO signals whose rising edges converge,
the embodiment of FIG. 2 provides a later arriving signal (VCO)
with a shorter delay period. In accordance with some embodiments, a
circuit architecture may be provided that ensures that a later
arriving signal has a shorter delay period. In this manner, the
recirculated VCO and reference signals are ensured of converging
after N stages, wherein the TDC flip-flop detects a zero-to-one
transition.
[0029] FIG. 3 depicts an exemplary DPLL. In the illustrated
embodiment shown in FIG. 3, a DPLL circuit 300 is arranged to
ensure that a faster ring 102 of the recirculating TDC is provided
with a later-arriving signal. FIG. 3 depicts the fast ring as VCO
ring 102, which may operate as described hereinabove. DPLL circuit
300 includes an adder 302 that may receive an output from TDC 100
and add a phase offset .phi. to the output signal before it is
received in loop filter 304, which may be a known loop filter. DPLL
circuit 300 may also include a digital-to-analog converter (DAC)
and tunable oscillator 308, which may be arranged in series with a
multimodulus divider (MMD) 310. The MMD may be operable to output a
signal to an input of VCO ring 102.
[0030] MMD 310 may be arranged as a .DELTA.-.SIGMA. modulated
multi-modulus divider, as depicted in FIG. 3. In operation MMD 310
may select one of many output VCO signal rising edges and send the
selected rising edge to TDC 100 for conversion, as described above
with respect to FIGS. 1 and 2. Circuit 300 may be arranged to add
the phase offset .phi. to a VCO signal in such a manner that the
rising edge output from MMD 310 has a desired phase relationship
with a reference signal ref(t) that is input to ring 104. For DPLL
circuit 300 to be in the locked state, the input to loop filter 304
may be driven to zero, thereby ensuring that the phase offset .phi.
causes the rising edge of the VCO signal div(t) to be delayed with
respect to ref(t).
[0031] FIGS. 4A and 4B depict the result of a phase offset
operation that may be provided by an embodiment using DPLL 300.
Rising edge 402 of ref(t) signal 400 is depicted as fixed in phase
space. In the range of scenarios illustrated in FIG. 4a, before a
phase offset .phi. is added, the rising edge 408 of VCO signal
div(t) 406 may occur over a range of phase space 410, which may be
equal to the VCO signal 206 period T.sub.VCO In this manner, the
rising edge 408 may occur before 408a or after 408b rising edge
402. In the scenario depicted by rising edge 408a, if the div(t)
edge arrives earlier than the ref(t) edge, the rising edges may not
properly converge. On the other hand, when phase offset .phi. is
added, the rising edge 412 of offset VCO signal div(t) 410 may
occur over a range of phase space 414 that may be equal to the
T.sub.VCO However, the offset .phi. is sufficient that both the
latest possible rising edge 412b and earliest possible rising edge
412a both occur after reference rising edge 402, thereby ensuring
convergence of the rising edges.
[0032] In other embodiments, convergence of a VCO and reference
signal in a recirculating TDC may be arranged by providing a slower
VCO ring in which a feedback signal rising edge arrives earlier
than the reference rising edge. This may be accomplished in one
embodiment by arranging VCO ring 102 to be slower than reference
ring 104 and by providing a phase offset to the ref(t) signal.
Accordingly, the faster reference rising edge is forced to arrive
after the div(t) rising edge, ensuring that the reference and
div(t) rising edges converge.
[0033] In one embodiment, in order to pull a DPLL such as DPLL 300
within dynamic range of a TDC, such as TDC 100, a simple counter
may be used to handle large initial phase errors.
[0034] FIG. 9 depicts a further embodiment of a recirculating TDC
150. In this embodiment, a reference signal may be recirculated
with a reference period .tau..sub.REF through reference ring
oscillator circuit 104 similar to that described above with respect
to FIG. 1. A VCO signal having a period T.sub.VCO may be fed to
flip-flop 106. As illustrated, control logic 152 may send signals
to MUX 118 to control the input to inverter 114, such that the
input is switched between the recirculated reference signal in ring
104 and the reference input (REF input).
[0035] FIG.10 depicts exemplary signals that may be generated in
some TDC embodiments, such as the TDC of FIG. 9. With reference
also to FIG. 9, the flip-flop output signal 160 samples the rising
edge 226 of the reference signal and may initially output a "1"
when the VCO signal is high, as depicted in FIG. 10. Eventually,
the reference ring signal may overtake the rising edge of the VCO
signal 224 and output a "0" as shown. Shift register 154 may then
latch the output of flip-flop 106 for further processing. The
quantized time duration between arrival of ref(t) 204 and VCO(t)
206 may be calculated as the product of the number N of elapsed
reference ring periods output by the TDC and the TDC resolution,
|T.sub.VCO-.tau..sub.REF|. In one example, .tau..sub.REF may be
designed to be (63/64) T.sub.VCO As with the aforementioned
embodiments, the duration of the reference and VCO periods may be
arranged so that the later arriving signal corresponds to the
signal having the shorter period.
[0036] In other embodiments, the resolution of a TDC may be
estimated using a circuit arrangement and method that estimates a
period of the VCO. In some embodiments, the VCO period estimation
is operable to return a number M that represents the ratio of the
VCO period to the TDC resolution. In some embodiments, the
resolution of a conventional TDC, such as a linear TDC or a Vernier
TDC, may be estimated. In this manner, the TDC resolution can be
calculated based upon the returned value of M. This may ensure that
a TDC can output a number that represents the phase difference
between its two inputs.
[0037] In one embodiment, a divider-assisted DPLL circuit
architecture may be used to implement a VCO period estimation
process. As depicted in FIG. 5, a TDC resolution estimating circuit
500 includes a delay circuit 510 and period estimation loop 520. In
some embodiments, delay circuit 510 may be arranged to delay a
received feedback signal by one VCO period, as described below.
[0038] Delay circuit 510 includes a D-flip-flop 508, which may
receive a VCO signal VCO(t) and a DPLL feedback divider signal
div(t). The divider signal may be fed to MUX 504, which outputs a
signal to a TDC 502. The MUX 504 may be also arranged to receive an
output signal from D-flip-flop 508. The select input of MUX 504 may
receive a pseudo random bit sequence (PRBS), b[n] 506. In this
manner, the feedback divider signal div(t) may be randomly delayed
by one VCO period according to the pseudo-random bit sequence
received from a generator (not shown). In one embodiment, as
illustrated in FIG. 5, the delay of one VCO period may be
accomplished by sampling the divider output with the VCO
signal.
[0039] In another embodiment, the delay of one VCO period may be
accomplished by selectively incrementing the division modulus in a
multi-modulus divider.
[0040] In one embodiment, the TDC output may be sent to VCO period
estimation loop 520 for unwrapping. This may be accomplished by
running an estimation loop in the frequency domain, in which the
unwrapped phase is differentiated (1-z.sup.-1) to extract
frequency. If the TDC input is delayed by one VCO period, the
unwrapped output may be subtracted by an estimated VCO period
number +/-M. If the estimated VCO period number M is incorrect, the
subtraction output u[n] will contain a residual error. In the
embodiment of FIG. 5, VCO period estimation loop 520 may scale and
multiply the error with its corresponding random sequence b[n], and
then accumulate the result. The PRBS b[n] may also be
differentiated accordingly when used in the VCO period estimation
loop. This effectively calculates the correlation between u[n] and
b[n]. The accumulated result c[n] may be smoothed by a digital
low-pass filter whose output may be used to update the VCO period
number M. Once the VCO period number M converges, u[n] contains no
residue of b[n]. As a result, the correlation output c[n] may
remain relatively constant so that M averages to the target VCO
period number.
[0041] In some embodiments, TDC 502, which interacts with delay
circuit 510 and VCO period estimation loop 520, may be a
conventional TDC that exhibits a known quantization error, as
discussed above. Because of this quantization error, the estimated
output M may fluctuate about a correct mean value over time.
[0042] In other embodiments, instead of a divider-assisted
architecture, the VCO period estimation loop 520 may be used in a
counter-assisted DPLL architecture. In such architecture, the
feedback signal may be a VCO signal that contains a series of
rising edges in a reference period. In this case, a TDC, such as a
Vernier or a conventional TDC may be designed to have a large
enough conversion range.
[0043] FIG. 6 depicts a TDC 600 that may be employed in some
embodiments with period estimation loop 520 to estimate TDC
resolution. This may be accomplished by adding a delay of one VCO
period using TDC 600. Instead of detecting a first VCO rising edge,
an embodiment of VCO 600 may be arranged to detect a second VCO
rising edge corresponding to a delay of one VCO period. In some
embodiments, as depicted in FIG. 6, TDC 600 may be designed with a
sufficient amount of stages such that a second edge detection
occurs within the TDC. Accordingly, the second edge detection may
be used as an output from the TDC when a delay in the feedback
signal of one VCO period is required.
[0044] In some embodiments, the recirculating TDC architecture may
be used in microprocessors and in wireless communication circuits,
as well as any circuits capable of generating multi-phase clock
signals. Embodiments of the recirculating TDC and of the period
estimation loop may be used in frequency synthesizers for wireless
data standards, such as as WiFi, WiMax and 3G-LTE.
[0045] FIG. 7 illustrates a block diagram of one embodiment of a
communications system 700 that may incorporate the TDC 300 and/or
TDC resolution estimating circuit 500. As shown in FIG. 7, the
communications system 700 may comprise a network 702 that
communicates over links 708-m with a plurality of nodes 704-n,
where m and n may represent any positive integer value. In various
embodiments, the nodes 704-n may be implemented as various types of
wireless devices. Examples of wireless devices may include, without
limitation, a station, a subscriber station, a base station, a
wireless access point (AP), a wireless client device, a wireless
station (STA), a laptop computer, ultra-laptop computer, portable
computer, personal computer (PC), notebook PC, handheld computer,
personal digital assistant (PDA), cellular telephone, combination
cellular telephone/PDA, smartphone, pager, messaging device, media
player, digital music player, set-top box (STB), appliance,
workstation, user terminal, mobile unit, consumer electronics,
television, digital television, high-definition television,
television receiver, high-definition television receiver, and so
forth.
[0046] In some embodiments, the nodes 704-n may comprise one more
wireless interfaces and/or components for wireless communication
such as one or more transmitters, receivers, transceivers, radios,
chipsets, amplifiers, filters, control logic, network interface
cards (NICs), antennas, antenna arrays, modules and so forth.
[0047] FIG. 8 is a diagram of an exemplary computing system
embodiment. In particular, FIG. 8 is a diagram showing a system
800, which may include various elements. As shown in FIG. 8, I/O
device 806, RAM 808, and ROM 810 are coupled to processor 802 by
way of chipset 804. Chipset 804 may be coupled to processor 802 by
a bus 812. Accordingly, bus 812 may include multiple lines. In
various embodiments, components of system 800 may be clocked by
DPLL signals generated in accordance with the TDC 100 and/or TDC
resolution estimating circuit 500. The embodiments, however, are
not limited to these elements.
[0048] Numerous specific details have been set forth herein to
provide a thorough understanding of the embodiments. It will be
understood by those skilled in the art, however, that the
embodiments may be practiced without these specific details. In
other instances, well-known operations, components and circuits
have not been described in detail so as not to obscure the
embodiments. It can be appreciated that the specific structural and
functional details disclosed herein may be representative and do
not necessarily limit the scope of the embodiments.
[0049] Various embodiments may be implemented using hardware
elements, software elements, or a combination of both. Examples of
hardware elements may include processors, microprocessors,
circuits, circuit elements (e.g., transistors, resistors,
capacitors, inductors, and so forth), integrated circuits,
application specific integrated circuits (ASIC), programmable logic
devices (PLD), digital signal processors (DSP), field programmable
gate array (FPGA), logic gates, registers, semiconductor device,
chips, microchips, chip sets, and so forth. Examples of software
may include software components, programs, applications, computer
programs, application programs, system programs, machine programs,
operating system software, middleware, firmware, software modules,
routines, subroutines, functions, methods, procedures, software
interfaces, application program interfaces (API), instruction sets,
computing code, computer code, code segments, computer code
segments, words, values, symbols, or any combination thereof.
Determining whether an embodiment is implemented using hardware
elements and/or software elements may vary in accordance with any
number of factors, such as desired computational rate, power
levels, heat tolerances, processing cycle budget, input data rates,
output data rates, memory resources, data bus speeds and other
design or performance constraints.
[0050] Some embodiments may be described using the expression
"coupled" and "connected" along with their derivatives. These terms
are not intended as synonyms for each other. For example, some
embodiments may be described using the terms "connected" and/or
"coupled" to indicate that two or more elements are in direct
physical or electrical contact with each other. The term "coupled,"
however, may also mean that two or more elements are not in direct
contact with each other, but yet still co-operate or interact with
each other.
[0051] Some embodiments may be implemented, for example, using a
computer-readable medium or article which may store an instruction
or a set of instructions that, if executed by a computer, may cause
the computer to perform a method and/or operations in accordance
with the embodiments. Such a computer may include, for example, any
suitable processing platform, computing platform, computing device,
processing device, computing system, processing system, computer,
processor, or the like, and may be implemented using any suitable
combination of hardware and/or software. The computer-readable
medium or article may include, for example, any suitable type of
memory unit, memory device, memory article, memory medium, storage
device, storage article, storage medium and/or storage unit, for
example, memory, removable or non-removable media, erasable or
non-erasable media, writeable or re-writeable media, digital or
analog media, hard disk, floppy disk, Compact Disk Read Only Memory
(CD-ROM), Compact Disk Recordable (CD-R), Compact Disk Rewriteable
(CD-RW), optical disk, magnetic media, magneto-optical media,
removable memory cards or disks, various types of Digital Versatile
Disk (DVD), a tape, a cassette, or the like. The instructions may
include any suitable type of code, such as source code, compiled
code, interpreted code, executable code, static code, dynamic code,
encrypted code, and the like, implemented using any suitable
high-level, low-level, object-oriented, visual, compiled and/or
interpreted programming language.
[0052] Unless specifically stated otherwise, it may be appreciated
that terms such as "processing," "computing," "calculating,"
"determining," or the like, refer to the action and/or processes of
a computer or computing system, or similar electronic computing
device, that manipulates and/or transforms data represented as
physical quantities (e.g., electronic) within the computing
system's registers and/or memories into other data similarly
represented as physical quantities within the computing system's
memories, registers or other such information storage, transmission
or display devices. The embodiments are not limited in this
context.
[0053] Although the subject matter has been described in language
specific to structural features and/or methodological acts, it is
to be understood that the subject matter defined in the appended
claims is not necessarily limited to the specific features or acts
described above. Rather, the specific features and acts described
above are disclosed as example forms of implementing the
claims.
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