Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-08-07
2001-02-20
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Receivers
Automatic frequency control
C375S316000
Reexamination Certificate
active
06192089
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to digital receivers, and more specifically to digital receivers that utilize automatic frequency control.
BACKGROUND OF THE INVENTION
The reception and processing of broadcast RF (radio frequency) signals are known in the art. Historically, the reception and processing of RF signals was performed using analog circuitry in which a number of analog components received the signals, and passed the information contained in the amplitude and phase portions of the signals along for further processing and output. More recently, the advent of digital circuitry has allowed designers to create communication devices which employ digital signal processing in the processing of the RF signals.
A conventional digital receiver
10
utilized in radio communication systems is illustrated in FIG.
1
. The conventional digital receiver
10
receives a broadcast input signal
14
sent from a radio communication system
12
in an analog format using an antenna
16
. A (radio frequency) RF interface
18
then amplifies the broadcast input signal
14
, passes the frequency component of the broadcast input signal
14
, and remodulates the desired frequency component to an (intermediate frequency) IF signal
20
. An analog mixer
22
then mixes down to baseband the IF signal
20
using a reference frequency
26
, such as the output of a voltage controlled oscillator, to produce a baseband signal
24
. An analog to digital converter
28
receives the baseband signal
24
and converts it from an analog signal to a digital signal
30
which includes an in-phase signal and a quadrature signal. The digital signal
30
is then digitally filtered by a filter
32
to improve the selectivity of the receiver and reduce the adjacent channel interference. The filtered signal
34
is then decimated to a lower sampling frequency, the decimation step consisting of down-sampling with a down-sampler
36
; and low pass filtering with a low pass filter
40
to a new sampling rate. Downsampling comprises selectively discarding input data and adjusting the output data such that the transmission rate of the output data is at a different clock rate than the input data clock rate. Once the signal
42
is sampled at a lower rate, it can be further processed digitally to remove induced noise and to retrieve the signal that was encoded by the radio communication system
12
.
Due to frequency fluctuations between the carrier frequency and the local oscillator frequency, continual adjustment of a frequency difference between the two frequencies is required to avoid errors in the retrieval of the signal. This procedure of adaptively adjusting the local oscillator frequency to track the carrier frequency is known as automatic frequency control (AFC). The frequency shifts in the signal due to drifts in the local oscillator frequency in the digital receiver's front end (such as RF interface
18
) and other noise factors make AFC a challenge to implement.
One conventional method of AFC is shown in
FIG. 1
, comprising a demodulator
44
, a (digital signal processor) DSP
48
, a digital to analog converter
52
and a (voltage controlled oscillator) VCO
56
. The demodulator
44
demodulates the signal to produce a conventional FM demodulated signal
46
. FM information is then extracted from the conventional FM demodulated signal
46
. The DSP
48
determines a frequency offset and compensation value, outputs the digital frequency compensation value
50
, which is converted to an analog frequency compensation value
54
by the digital to analog converter
52
. This analog frequency compensation value
54
is used to vary the frequency of the VCO
56
and the output of the VCO
56
is the reference frequency
26
. This technique requires the use of a high precision digital to analog converter
52
. In most cases the high precision digital to analog converter
52
, which is essential to accurate frequency compensation, can be difficult and expensive to realize.
A second conventional method of AFC eliminates the digital to analog converter
52
and performs the AFC using software within the DSP
48
. In this method the DSP
48
mixes the signal to the appropriate frequency by using a software complex multiplier; and additionally performs the a numerically controlled oscillator function to generate a complex exponential.
FIG. 2
is a block diagram illustrating a second conventional digital receiver
60
utilizing this method.
In
FIG. 2
, the second conventional digital receiver
60
receives a broadcast input signal
14
sent from a radio communication system
12
using an antenna
16
. The broadcast input signal
14
is processed through the RF interface
18
, the analog mixer
22
, the analog to digital converter
28
, the filter
32
, and the down-sampler
36
similarly to previously described for the conventional digital receiver
10
. In some scenarios, the analog mixer coarsely adjusts the signal using the AFC computations as the reference
21
.
In the second conventional AFC method shown in
FIG. 2
, the output of the down-sampler
36
, the downsampled signal
64
, is received by a digital mixer
62
, which then quadrature mixes down the downsampled signal
64
using an AFC reference frequency
82
. The signal continues to be processed through a filter
40
, and a demodulator
44
as previously described for the conventional digital receiver
10
of FIG.
1
.
In this second conventional method, the output of the demodulator
44
, the demodulated signal
70
, is fed into a complex multiplier consisting of an integrator
72
, and a (numerically controlled oscillator) NCO
80
. The integrator
72
receives a plurality of demodulated signals
70
, and performs a successive accumulation function including all past values of the demodulated signals
70
. The NCO
80
receives the output of the integrator
72
, the integrated signal
76
, and generates a complex value representation of the phase of this integrated signal
76
. The output of the NCO
80
is the AFC reference signal
82
which is mixed with the downsampled signal
64
in the digital mixer
62
.
The implementation of this second conventional method of automatic frequency control using DSP software is computationally complex and may require a significant amount of processing power.
The two conventional apparatus and techniques for automatic frequency control described above are representative of the design approaches currently utilized. Analysis of these two apparatus indicates that there exists a need in the art for an apparatus that efficiently, with minimal circuit components and minimal power consumption, performs automatic frequency control.
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patent: 5668749 (1997-09-01), Corleto et al.
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Corleto Jose G.
Kelley Brian Todd
Rahman Shareq
Corrielus Jean B.
Dulaney Randi L.
Motorola Inc.
Pham Chi H.
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