Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1999-12-06
2003-05-20
Vo, Don N. (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S316000, C375S344000
Reexamination Certificate
active
06567488
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a tuner and to a method for correcting the alignment of a phase-locked-loop demodulating stage in a tuner.
2. Description of the Prior Art
A known form of tuner for use within an analogue TV satellite receiver is illustrated in
FIG. 1
of the accompanying drawings as reference designator
10
and comprises an AGC (automatic gain control) filter stage
12
, a downconverter and SAW-filter stage
13
fed from the AGC filter stage, a frequency synthesiser stage
14
connected so as to control the frequency of the local oscillator
15
in the downconverter stage, and a PLL (phase-locked loop) FM demodulator stage
16
fed from the output of the downconverter stage
13
. A microcontroller
17
controls the various functions of the tuner, including setting up the synthesiser to tune the local oscillator
15
to the requisite frequency for the selection of the RF channel of interest, and receiving an AFC (automatic frequency control) flag signal
18
from the PLL stage for purposes to be described later. Preceding the tuner
10
is a head unit
20
connected to a satellite dish
21
and containing a down-converter and low-noise amplifier stage
22
, which feeds the RF input of the tuner
10
.
The PLL block
16
contains a reference-signal generator in the form of a voltage-controlled oscillator (VCO)
23
which generates a signal whose frequency is compared with the incoming signal from the downconverter stage
13
in a phase comparator
24
. Control of the VCO frequency is achieved by, for example, the use of a varactor diode
25
, the capacitance of which is varied from the output of the phase comparator
24
by way of an amplifier
26
. The varactor diode
25
is shunted by a reactance, in this case the inductor
27
, for reasons to be explained shortly.
There are three centre-frequencies associated with the tuner
10
. These are: the centre-frequency of the IF (intermediate frequency) following down-conversion in the down-converter stage
13
, this centre-frequency being a function of the frequency accuracy of the head-unit down-conversion stage
22
and the frequency-accuracy of the local oscillator
15
; the centre-frequency of the SAW filter
19
in the down-converter stage
13
, and the centre-frequency of the VCO in the PLL stage
16
. In the interest of providing optimum signal-to-noise ratio, it is desirable that all three of these coincide as far as possible. This is shown diagrammatically in
FIG. 2
of the accompanying drawings, in which curve
30
represents the bandwidth of the SAW filter
19
, curve
31
the frequency characteristic of the IF entering the SAW filter
19
and curve
32
is the controllable frequency range of the VCO in the PLL stage
16
. In the ideal state, the centre of the VCO's frequency range, the centre of the IF and the centre of the SAW's response should coincide at
33
.
In the known alignment procedure two alignment processes are carried out: one manual and mechanical and performed during alignment of the tuner in production, the other automatic and electronic and performed during normal operation of the tuner. The manual/mechanical alignment procedure generally consists in injecting a signal of a known carrier frequency into the tuner, allowing this signal to be downconverted to IF by the downconverter stage
13
, then adjusting the physical dimensions of an air coil of the inductor
27
to centre the VCO output frequency onto the IF frequency. The alignment is monitored either by measuring the video output signal, or by sampling the AFC data generated by the PLL demodulator on line
18
. In a typical set-up involving the first of these monitoring alternatives, a signal of, say, 1.5 GHz frequency is input into the AGC section
12
modulated with, say, a ±20 MHz ramp signal. Two signals will then be fed into the X- and Y-terminals of an oscilloscope: the ramp modulating signal into the X-terminals and the PLL-demodulated output signal (“Video Output”) into the Y-terminals. The air core of the inductor
27
is then adjusted until the appropriate waveform is obtained on the screen, at which point the VCO is correctly aligned.
The automatic alignment involves the AFC function already mentioned. This function compares the centre-frequency of the VCO to the IF centre-frequency at the output of the SAW filter and, if the latter is higher than the former, the output on line
18
goes, say, logic low (“0”), whereas if the IF centre-frequency is lower than the VCO centre-frequency the AFC output on line
18
goes high (“1”). This is illustrated in
FIG. 3
of the accompanying drawings, in which &Dgr;F is the difference in the centre-frequencies. The logic signal on line
18
is used by the micro-controller
17
to adjust the frequency of the downconverter local oscillator
15
(via the synthesiser
14
) in a direction such as to bring the two centre-frequencies into alignment.
Use of an AFC function in this way corrects for errors in the centre-frequency of the head unit
20
due mainly to the susceptibility of the unit to temperature fluctuations. These can be quite severe, since the unit is usually situated in the open air and may therefore be subject to extreme weather conditions.
The afore-mentioned inductor adjustment is a procedure which conventionally takes place on the bench in a production-line setting and is, by its very nature, quite cumbersome and expensive, since it requires either costly manual labour to perform it, as described above, or some kind of test machine. By way of example, if the alignment costs $40 per hour and three units are aligned per minute, then the alignment cost per unit is $0.22. This known alignment procedure is not just expensive, however, it is also far from perfect. This is because it does not compensate for drift in the frequencies of the local oscillators
15
and
23
which can arise due to ageing, temperature fluctuations (not so serious as in the head unit) or the use of a different tuner supply voltage form the voltage applied during adjustment of the inductor
27
. Also, if the centre-frequency of the VCO
23
, specifically, is in error, the AFC function will assume this is a head-unit error and so wilt “correct” for it by incorrectly aligning the tuner. The result, in an analogue TV system, is a compromised performance with visual impairments to the picture being apparent to the viewer.
There is therefore a need for an alignment arrangement which takes into account the effects of factors such as ageing and temperature and supply-voltage variations in the tuner and does so, ideally, in a more cost-effective manner than the known alignment arrangement. It would also be advantageous if such a new arrangement were to involve minimal redesign of the known arrangement, especially as far as hardware is concerned.
Basic Concept of the Invention
In accordance with the present invention, there is provided a tuner comprising an RF input stage, a downconverter stage fed from said RF input stage for providing an if signal, a phase-locked-loop demodulator stage fed from said downconverter stage and comprising a reference-signal generator, characterised in that said tuner comprises an alignment arrangement for use during an alignment phase of said tuner, said alignment arrangement comprising: means for injecting into said demodulator stage during said alignment phase an alignment signal having a frequency corresponding to a nominal value of a centre-frequency of said IF signal; means for adjusting the frequency of said reference signal until it is substantially aligned with the frequency of said alignment signal; and means for thereupon removing said alignment signal.
The reference-signal generator may be a voltage-controlled generator having a control input, and the means for adjusting the reference-signal frequency may comprise a variable voltage means connected to the control input. The variable voltage means may be a digital-to-analogue converter.
The tuner may comprise an automatic frequency-control (AFC) arrangement c
Kirschstein, etal.
Vo Don N.
Zarlink Semiconductor Limited
LandOfFree
Relating to demodulator alignment correction in tuners does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Relating to demodulator alignment correction in tuners, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Relating to demodulator alignment correction in tuners will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3016731