Oscillators – Automatic frequency stabilization using a phase or frequency... – Tuning compensation
Reexamination Certificate
1999-01-28
2001-01-30
Grimm, Siegfried H. (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Tuning compensation
C331S025000, C455S118000, C455S314000, C455S316000
Reexamination Certificate
active
06181212
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to phase locked loops. More particularly, the present invention pertains to phase locked loops capable of providing multiple output frequencies.
BACKGROUND OF THE INVENTION
While phase locked loop design in accordance with the present invention is applicable to any device utilizing a phase locked loop, it is particularly adapted for radio frequency communication devices in which frequency conversion of a base band signal to the RF (radio frequency) transmission frequency or vice versa is performed in two frequency conversion steps. Using the transmission path of a cellular telephone as an example, a cellular telephone will frequency up-convert a base band signal to the RF transmission frequency, in two frequency mixing (heterodyning) steps. Depending on the particular country and the particular band, the RF transmission frequency for cellular communications may be in the range of about 800 MHz-900 MHz or about 1800 MHz-1900 MHz.
Commonly, a base band signal will first be frequency up-converted to an intermediate frequency (IF) of, for example, about 270 MHz by mixing the baseband signal with an IF reference signal generated at 270 MHz. The IF information signal generated thusly is then frequency up-converted to the RF transmission frequency by mixing it with an RF reference signal generated by a second PLL.
FIG. 1
is a block diagram of an exemplary cellular telephone frequency conversion circuit
10
of the prior art. Both the transmit
11
and receive
13
paths are shown. This circuit is found, for instance, in the W3020 and W3000 chips manufactured by Lucent Technologies, Inc. of Murray Hill, N.J., U.S.A. An incoming differential base band signal, represented in the figure as differential signals TX
I
, and TX
Q
, are fed to mixers
12
and
14
, respectively. TX
Q
is the real part of the signal and TX
I
is the imaginary part (phase shifted 90° from the real part) of the signal. Mixers
12
and
14
mix the incoming signals TX
I
and TX
Q
, respectively, with a signal generated by a first local oscillator 16 to perform the first frequency up-conversion on the data signal from the baseband to the intermediate frequency. Local oscillator
16
is, for example, a PLL generating a signal at 540 MHz. That signal is forwarded to a divider circuit
18
which frequency divides the oscillator signal by two or by three depending on the frequency band of operation. Particularly, this circuit is adapted for a dual-band cellular telephone and therefore selectively provides two separate intermediate frequencies, namely, 180 MHz and 270 MHz, depending on the final RF frequency to be achieved. The IF reference signal from the divider
18
is forwarded to phase quadrature circuitry
20
. The phase quadrature circuitry creates two signals at the IF reference frequency 90° out of phase with each other and forwards them to the mixers
12
and
14
, respectively. The mixers
12
and
14
mix the real and imaginary parts of the base band signal with the real and imaginary portions of the IF reference frequency, respectively, to create real and imaginary signals
22
and
24
at the intermediate frequency. An adder
26
sums the signals to create a complete signal on line
28
at the intermediate frequency containing the information in the original base band signal. That signal is filtered by a band pass filter
30
to eliminate noise and harmonics created by the mixers
12
and
14
and is forwarded to a second mixer
32
. Mixer
32
is the RF frequency mixer which will frequency up-convert the IF frequency information signal to the RF transmission frequency. RF mixer
32
mixes the intermediate frequency signal on line
33
with a signal generated by a second local oscillator
34
at an RF frequency displaced 270 MHZ from the desired transmission frequency to generate a side band signal at the desired RF transmission frequency. This local oscillator circuit
34
comprises two alternately selectable PLLs
34
a
and
34
b
since many countries, including the U.S. and European countries, have two broad frequency bands within which cellular communications are permitted. Accordingly, a local oscillator is provided for each broad frequency band. Within each of the broad bands is a series of narrower frequency channels from which each cellular telephone will use one channel for a given call. Switch
36
will select the signal from one of the local oscillators depending upon the selected cellular broad band and forward it to amplifier
38
. The output of amplifier
38
is fed to the second input of the RF mixer
32
to mix the intermediate signal on line
30
with the RF local oscillator reference signal to create an RF information signal on line
33
. That signal is filtered further by band pass filter
43
to eliminate harmonics and background noise, amplified by amplifiers
48
and
50
and forwarded to the antenna
52
.
The receive path circuitry
13
is basically the same circuitry in reverse. It comprises a pair of filters
60
and
62
, respectively, tuned to the two broad bands permitted for cellular telephone communications by the particular country. Those signals are amplified by low noise amplifiers
64
and
66
, respectively, and forwarded to additional filters
68
and
70
, respectively. Those signals are mixed by mixers
72
and
74
, respectively, with the signals generated by the RF frequency PLL
34
a
or
34
b
at a frequency that is displaced 270 MHz from the received RF signal, respectively, to create a side band signal at an intermediate frequency of 270 MHz. Up to this point, both paths (i.e., through elements
60
,
64
,
68
and
72
and through elements
62
,
66
,
70
and
74
) process the received signal regardless of which broad band it is within even though the output of only one of the paths (the one that is tuned to the frequency of the particular incoming signal) will be used. The outputs of both mixers
72
and
74
are filtered by filter
76
and forwarded to an amplifier
78
. Filter
76
not only cleans up the signal, but also acts as a selector of the signal from the appropriate path. Specifically, it will pass only the signal in the path that was tuned for the RF frequency of the particular received signal. The signal sent to the filter that was generated in the other path will not be at 270 MHz because it was mixed with an RF reference frequency that was not offset therefrom by 270 MHz.
The intermediate frequency signal on line
79
is mixed by mixers
80
and
82
with the 270 MHz reference signal generated by the IF PLL
16
as divided by divide by two divider
84
and broken into two components phase shifted 90° from each other by phase differentiator
86
. This separates the signal into its real and imaginary parts. The outputs are filtered by filters
88
and
90
, respectively, amplified by amplifiers
92
and
94
, respectively, filtered further by filters
96
and
98
, respectively, and amplified again by amplifiers
100
and
102
, respectively. The signals are then passed on to base band processing circuitry.
Each local oscillator
16
,
34
a
, and
34
b
is comprised of a phase locked loop (PLL) for generating a well regulated frequency signal. A typical PLL includes at least a phase comparator, a voltage controlled oscillator, a charge pump, a feedback loop, an oscillator, filters, amplifiers, and multiple frequency dividers.
The circuit of
FIG. 1
requires an IF PLL
16
and a separate RF PLL (in this case a pair of RF PLLs
34
a
and
34
b
).
FIG. 2
is a block diagram of a typical phase locked loop circuit that can be used for any of the local oscillator circuits
16
,
34
a
or
34
b
in FIG.
1
. For exemplary purposes, we will consider it to be used for the local oscillator
16
for frequency up-converting the base band signal to an IF signal centered at 270 MHz. The circuit comprises a crystal oscillator
202
generating an oscillating signal at a frequency of 13 MHz. The output of the crystal oscillator is provided to a divide by 13 circuit
204
to generate a 1 MHz signal on line
2
Khoini-Poorfard Ramin
Mecklai Hussein K.
Grimm Siegfried H.
Lucent Technologies - Inc.
Synnestvedt & Lechner
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