Method and system for controlling frequency

Details Method and system for controlling frequency Method and system for controlling frequency

1999-11-19

2003-06-10

Chin, Vivian (Department: 2681)

Telecommunications

Receiver or analog modulated signal frequency converter

Local control of receiver operation

C455S192200, C455S208000

Reexamination Certificate

active

06577853

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and system for controlling frequency in general and to a method and system for providing an appropriate frequency, within predetermined error limits.
BACKGROUND OF THE INVENTION
In the art, there are known methods and systems for controlling and moderating frequency. A conventional transceiver includes therein a local oscillator which, for example in radio frequency (RF) systems, is usually a frequency generating crystal. When the transceiver receives a signal in a given frequency f
S
, it has to shift the frequency f
S
of the signal to a desired frequency f
d
by producing an internal frequency f
LO
, using the local oscillator. Conventional frequency crystals are known to include a frequency shift which needs to be corrected, according to the received signal frequency f
S
.
Reference is now made to
FIGS. 1 and 2
.
FIG. 1
is a schematic illustration of a prior art system, generally referenced
1
, for controlling the internal frequency of a transceiver.
FIG. 2
is a graphic illustration of the frequency versus voltage curve, referenced of a typical voltage-controlled-oscillator (VCO).
System
1
includes a voltage-controlled-oscillator
2
(VCO), an input frequency shift unit
5
and an output frequency shift unit
7
, both connected to VCO
2
, and a frequency estimator
4
, connected to VCO
2
and to input frequency shift unit
5
, for determining the required frequency correction.
Conventionally, a demodulator
6
is connected after the input frequency shift unit
5
and a modulator
8
is connected before the output frequency shift unit
7
.
A received signal provided to the input frequency unit
5
, incorporates a frequency f
S
. The VCO
2
generates an internal frequency f
IN
and provides it to the input frequency shift unit
5
. The input frequency shift unit
5
produces a new signal having an intermediate frequency of f
I
, wherein f
I
=f
S
−f
IN
, and provides the new signal to the frequency estimating unit
4
as well as to the demodulator
6
. It will be appreciated that conventional demodulators define a maximal allowed frequency difference &Dgr;f
ALLOWED
between f
I
and f
d
, so that |f
I
−f
d
|≦&Dgr;f
ALLOWED
. A conventional oscillator oscillates at a frequency which changes due to poor manufacturing quality, temperature changes and the like so that a situation where |f
I
−f
d
|>&Dgr;f
ALLOWED
often occurs.
The frequency estimator
4
detects the signal provided by the frequency shift unit
5
. If |f
I
−f
d
|>&Dgr;f
ALLOWED
then f
I
has to be adjusted to equal f
d
. The frequency estimator
4
determines the frequency correction &Dgr;f=f
d
−f
I
, determines a respective voltage V(&Dgr;f) and provides the voltage V(&Dgr;f) to the VCO
2
. Accordingly, the VCO
2
generates a corrected internal frequency f
IN
.
It will be noted that, as long as |f
I
−f
d
|>&Dgr;f
ALLOWED
, the demodulator cannot properly process the incoming signal.
The modulator
8
produces a signal for transmitting at a frequency of f
T
and provides the signal to the output frequency shift unit.
7
. The VCO
2
provides an internal transmitting frequency f
OUT
to the output frequency shift unit
7
. In turn, the output frequency shift unit
7
produces a transmittable signal bearing a frequency of f
OUT
+f
T
which is the transmitted signal b broadcasting frequency.
It is noted that f
IN
and F
OUT
are generated around a main frequency f
LO
, generated by the local oscillator of the VCO unit
2
.
Referring now to
FIG. 2
, when provided with a voltage at a value of v
1
, the VCO
2
is expected to provide the respective frequency f. A conventional VCO, having an internal inaccuracy, may provide, for a given voltage value of v
1
, a frequency in the range between f
MIN
and f
max
. When maximum(f
MAX
−f,f−f
MIN
)>&Dgr;f
ALLOWED
, the frequency correction cannot be deemed final.
Conventionally, after several cycles of detection and correction, the intermediate frequency f
I
should differ from the desired frequency f
d
by no more than &Dgr;f
ALLOWED
. Until then, the demodulator
6
cannot perform properly.
It will be appreciated that the modulator
8
is also dependent on the accuracy of the frequency provided thereto by the VCO
2
.
SUMMARY OF THE PRESENT INVENTION
It is an object of the present invention to provide a novel method and a novel system for providing an appropriate frequency, within predetermined error limits.
According to the present invention there is thus provided a system for receiving a signal in a received frequency and shifting the received frequency to become a desired frequency, the system including a controllable oscillator for generating a first internal frequency, a frequency estimating unit connected to the controllable oscillator, a first frequency shift unit, connected to the controllable oscillator and to the frequency estimating unit, for shifting the received frequency according to the first internal frequency, thereby obtaining an initially shifted frequency and a second frequency shift unit connected to the first frequency shift unit and the frequency estimating unit for shifting the initially shifted frequency.
The frequency estimating unit determines a total frequency shift value from the desired frequency, the received frequency and the first internal frequency and it also determines a first frequency shift value and a second frequency shift value from the total frequency shift value.
Furthermore, the frequency estimating unit provides a first frequency change command to the controllable oscillator, so as to correct the first internal frequency according to the first frequency shift value, a second frequency change command to the second frequency shift unit, so as to shift the initially shifted frequency according to the second frequency shift value.
The frequency estimating unit may provides the first frequency change command to the controllable oscillator as an electrical voltage, with respect to the controllable oscillator characteristics.
The second frequency shift unit can be a hardware unit or a software unit. The second frequency shift unit may include means for performing a frequency shift by multiplying the initially shifted frequency by e
j2&pgr;&Dgr;f
, wherein &Dgr;f is the second frequency shift value.
The controllable oscillator may generates a second internal frequency, for example, for use in a combined transmitter receiver implementations a. In such a case, the system further includes a first output frequency shift unit, connected to the controllable oscillator for shifting a transmittable signal according to the second internal frequency.
For providing enhanced frequency correction according to the invention, the system further includes a second output frequency shift unit connected before the first output frequency shift unit and to the frequency estimating unit. The frequency estimating unit provides a command to the second output frequency shift unit to shift the transmittable signal according to the second frequency shift value thereby producing an initially shifted transmittable signal. Then, the first output frequency shift unit shifts the initially shifted transmittable signal according to the second internal frequency.
According to another aspect of the present invention there is provided a method for operating a system generating a controllable internal frequency f
IN
, a method for shifting a signal from an initial frequency f
S
towards a desired frequency f
D
to a final frequency f
F
, wherein |f
D
−f
F
| is less than a predetermined allowed frequency error f
ALLOWED
.
The method includes the steps of:
determining a total frequency shift value F
TOTAL
according to the internal frequency f
IN
and the initial frequency value f
S
;
determining a first frequency shift value &Dgr;f
1
from the total frequency shift value f
TOTAL
and the predetermined allowed frequency error f
ALLOWED
;
deter

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