Pulse or digital communications – Synchronizers – Synchronizing the sampling time of digital data
Reexamination Certificate
2000-09-18
2004-03-23
Tran, Khai (Department: 2631)
Pulse or digital communications
Synchronizers
Synchronizing the sampling time of digital data
C375S376000
Reexamination Certificate
active
06711222
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communication systems and more particularly those systems where digital frequency synthesis is used for remote communication applications.
DESCRIPTION OF THE PRIOR ART
Frequency synthesis is used to track the frequency of an incoming data stream. Such synthesis typically uses some means to lock on to the frequency of the incoming data stream. One such means is the well known phase lock loop (PLL) a block diagram for which is shown in FIG.
1
.
As is shown in
FIG. 1
, frequency synthesizer
10
includes a divide by n circuit
12
in the form of a digital counter which produces an output cycle for each n input cycles. Thus the signal at the output of circuit
12
has a frequency of f
i
, where f
i
is the frequency of the signal at the input to synthesizer
10
.
Phase detector
14
receives the output of circuit
12
and also receives an input from loop
16
comprising loop filter
16
a
, voltage controlled oscillator (VCO)
16
b
and divide by m circuit
16
c
which is also in the form of a digital counter that produces an output cycle for each m input cycles. The output of the PLL is at the output of the VCO
16
b
and if that output has a frequency of f
o
then the signal produced at the output of divide by m circuit
16
c
has a frequency of f
o
/m. Thus the input frequencies to phase detector
14
are f
i
and f
0
/m.
Under locked steady state conditions the PLL causes the input frequencies to phase detector
14
to be exactly equal, that is, f
i
=f
o
/m. The output frequency can then be expressed as:
f
o
=(
m
)*
f
i
.
One of the drawbacks to this method of frequency synthesis is that the PLL needs time to lock on to the frequency. During this time the data can not be reliably retrieved from the data stream necessitating a sacrificial preamble in front of every transmission.
An alternative technique to frequency synthesis in overcoming communication issues is commonly referred to as Store-Forward. One such prior art implementation for the Store-Forward technique is circuit
18
shown in FIG.
2
. The Store-Forward method samples the incoming data stream
20
at sampler
28
with either a clock
30
supplied with that stream or with a clock recovered from the data stream by a clock recovery circuit
22
which is shown in dashed lines in FIG.
2
. After a completed data frame has been received it is stored in a buffer
24
until it is retransmitted. Circuit
18
also includes a serial to parallel converter
32
, a parallel to serial converter
34
, an oscillator
26
and control logic
36
.
The Store-Forward method requires that enough data be stored to compensate for any differences between the oscillator
26
and the oscillator of the device that originated the message. The major deficiency of this approach is that a significant time delay is introduced into the communication link and therefore can reduce the overall system bandwidth.
SUMMARY OF THE INVENTION
The invention is a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also uses the synchronized received message data stream to generate a first time reference to sample the synchronized received message data stream into a sampler. The method further uses the voltage controlled oscillator frequency to generate a second time reference to read the synchronized received message data stream out of the sampler. The method also further uses the first and second time references to determine how much time has accumulated between the voltage controlled oscillator frequency and the remote oscillator frequency. The method further samples the accumulating time difference at a predetermined rate; and adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
The invention is also a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also determines an accumulating difference in time between the voltage controlled oscillator Frequency and the remote oscillator frequency using both a first time reference that is generated from the synchronized received message data stream and a second time reference that is generated from the voltage controlled oscillator frequency. The method further samples the accumulating time difference at a predetermined rate. The method also further adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
The invention is further a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also generates a first time reference from the synchronized received message data scream. The method further generates a second time reference from the voltage controlled oscillator frequency. The method also further determines an accumulating difference in tame between the voltage controlled oscillator frequency and the remote oscillator frequency using the first and second time references. The method further samples the accumulating time difference at a predetermined rate; and adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
An apparatus for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The apparatus has a voltage controlled oscillator having an adjustable frequency and a sampler. The apparatus also has a first circuit for synchronizing the received message data stream to the adjustable frequency of the voltage controlled oscillator and for generating from the synchronized received message data stream a first time reference to sample the synchronized received message data stream into the sampler and for generating from the voltage controlled oscillator frequency a second time reference to read the synchronized received message data stream out of the sampler. The apparatus further has a frequency discriminator responsive to the first and second time references to determine an accumulating difference in time between the voltage controlled oscillator adjustable frequency and the remote oscillator frequency, the frequency discriminator sampling the accumulating time difference at a predetermined rate; and a control loop to adjust the voltage controlled oscillator adjustable frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
REFERENCES:
patent: 5361044 (1994-11-01), Norimatu et al.
patent: 6549587 (2003-04-01), Li
Heilman Thomas Richard
Piunno, Jr. John Albert
ABB Automation Inc.
Rickin Michael M.
Tran Khai
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