Demodulation device

Details Demodulation device Demodulation device

1999-07-22

2002-06-25

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Angle modulation

C375S325000, C375S327000, C375S373000, C375S376000, C329S307000, C329S346000

Reexamination Certificate

active

06411658

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a demodulation device, and more particularly to, a demodulation device of quasi-coherent detection system equipped with a carrier recovery circuit.
BACKGROUND OF THE INVENTION
In demodulation devices used in communication equipment for satellite communications etc., in order to make the device free from adjustment to reduce the maintenance cost and to stabilize the characteristic, the digitization of device starts coming into practical use. As the system of digitizing the carrier recovery circuit of a demodulation device, there is the quasi-coherent detection system. The quasi-coherent detection is conducted by that the local oscillation signal of LO (local oscillator) which has a frequency near to the carrier frequency but asynchronous and IF (intermediate frequency) signal to be input are multiplied to give BB (base band) signal, further converted into digital signal, where a frequency difference remained a little is compensated by using the carrier recovery circuit formed as a digital circuit. By such a composition, the composition of LO can be changed from VCO (voltage controlled oscillator) to a composition (quasi-coherent detection system) with fixed oscillation frequency by quartz oscillator. Therefore, the adjustment of analogue circuit becomes unnecessary and the variation of characteristic can be prevented.
For the carrier recovery circuit, pull-in range that can compensate the frequency difference of transmission/reception LO of IF and RF (radio frequency) is required. In case of analogue carrier recovery circuit, with a loop bandwidth of carrier recovery PLL (phase-locked loop) in stationary state, the range of pulling into synchronization for carrier cannot be sufficiently secured. Therefore, an oscillator called a sweeper is provided.
FIG. 1
shows a conventional digitized demodulation device of quasi coherent detection system, which is disclosed in Japanese patent application laid-open No. 7-177194 (1995). The demodulation device comprises a demodulator (DEM)
51
, a local oscillator (LO)
52
, an endless phase shifter (EPS)
53
as a complex multiplier, a synchronous detector (SYNCDET)
54
, a sweeper
55
, an adder
56
, a numerical controlled oscillator (NCO)
57
, a phase detector (PD)
58
and a loop filter (LPF)
59
.
The sweeper
55
comprises a delay circuit
55
a
and an adder
55
b.
Data k and one-bit output of the delay circuit
55
a
are added by the adder
55
b.
The delay output of the added value becomes sweep data, which will be control data of NCO
57
through the adder
56
.
In the composition of device in
FIG. 1
, input signal Si is demodulated by DEM
51
and a local oscillator
52
, where the demodulated signal is signal that is phase-rotated according to the difference between the carrier frequency of received modulated signal and the oscillation frequency of the local oscillator
52
. So, a given correction is made by EPS
53
. In PD
58
, the phase error of demodulated signal is detected, and further high region is cut by LPF
59
. The output signal of LPF
59
is applied through the adder
56
to NCO
57
as a control data. In the synchronous detector
54
, the synchronous detection to output of EPS
53
is conducted and the result is applied to the sweeper
55
. The sweeper
55
outputs a waveform (saw-tooth wave) with an amplitude corresponding to a desirable range of pulling into synchronization, and this is added to output of the LPF
59
at the adder
56
. By this addition result, the oscillation of NCO
57
is controlled so that the oscillation range of NCO
57
is enlarged. By the output frequency of NCO
57
, the amount of phase shift at EPS
53
is controlled.
In the carrier pull-in process (in pulling out of synchronization), at first, the sweeper
55
operates so that the oscillation frequency of NCO
57
comes near to the oscillation frequency of input signal Si. When the difference between the carrier frequency of input signal Si and the oscillation frequency of NCO
57
falls within the range of pulling into synchronization of LPF
59
, the value of LPF
59
varies to establish the synchronization, when the establishment of synchronization is detected, the operation of the sweeper
55
stops and thereby the operation of pulling into synchronization is completed. A variation of carrier frequency occurred thereafter is followed by using output of LPF
59
to keep the synchronization with carrier.
In recent years, for the purpose of reducing the cost and making the frequency variable, LO of RF section employs a synthesizer system increasingly. In this system, since the phase noise of LO is large, it is necessary to enhance the durability against phase noise, i.e., it is necessary to keep such a condition that pulling out of carrier synchronization is hard to cause by cluttering from outside. The most durable against cluttering from outside is a state that LPF operates around the center value (=0).
FIG. 2
shows a conventional demodulation device of quasi-coherent detection system composed as a modification of the device in FIG.
1
. This demodulation device is disclosed in Japanese patent application laid open No. 7-177194 (1995) (ibid. FIG.
1
), and the difference from the device in
FIG. 1
is a sweeper
60
. The sweeper
60
comprises a controller (CONT)
61
as a detection control block, a selector
62
to select either of set values k
1
and k
2
, a register (REG)
63
, a selector
64
to select either of clocks CLK
1
and CLK
2
, and an accumulator
65
to conduct a given operation based on outputs of the register (REG)
63
and the selector (SEL)
64
. The other components are the same as those in
FIG. 1
, therefore its explanation is omitted herein.
In SEL
62
either of the set value k
1
for addition and the set value k
2
for subtraction can be selected, and is SEL
64
either of CLK
1
with high speed and CLK
2
with low speed can be selected. The sweeper
60
is actuated when the synchronous detector
54
detects the pulling into synchronization or pulling out of synchronization.
When the pulling out of synchronization is detected, CONT
61
starts controlling so that the set value k
1
for addition is selected by SEL
62
and CLK
1
with high speed is selected by SEL
64
. According to CLK
1
, ACC
65
conducts the accumulation, and output of ACC
65
is applied through the adder
56
to NCO
57
as control data. Also, when the pulling into synchronization is detected by the synchronous detector
54
. ACC
65
stops accumulating and the output value is held. Hereupon, SEL
62
is switched into the set value k
2
for subtraction and SEL
64
is switched into CLK
2
with low speed.
In the pulling out of synchronization, based on pull out detection signal from the synchronous detector
54
, SEL
62
selects the set value k
1
for addition and SEL
64
selects CLK
1
with high speed. Thereby, REG
63
is set to the set value k
1
for addition, and ACC
65
accumulates the set value k
1
for addition based on CLK
1
, this accumulation result is applied through the adder
56
to NCO
57
as control data. Also, when pull-in detection signal from the synchronous detector
54
is input, REG
63
is cleared by CONT
61
and the sweep data is held. Then, CONT
61
controls SEL
62
to select the set value k
2
for subtraction and controls SEL
64
to select CLK
2
with low speed. According to CLK
2
with low speed, ACC
65
accumulates the set value k
2
for subtraction set at REG
63
. Hereupon, the sweep data reduces gradually so that PLL can follow sufficiently, when coming to zero or near to zero, CONT
61
clears REG
63
.
In the pulling into synchronization, based on pull in detection signal from the synchronous detector
54
, CONT
61
clears REG
63
. Thereby, input of ACC
65
becomes zero, the sweep data at that time is held. Then, as described above, REG
63
is set to the set value k
2
for subtraction and the sweep data reduces gradually. When CONT
61
detects the sweep data comes to zero or near to zero in the period that pull-in detection signal is

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