Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2001-01-29
2002-12-17
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C348S678000, C342S092000
Reexamination Certificate
active
06496017
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an arrangement for measuring antenna signal strength, the arrangement comprising:
a first amplifier section for receiving an antenna signal for providing a first output signal, and having a first automatic gain control stage with a first attenuation factor controlled by a first automatic gain control signal;
a second amplifier section for receiving a first input signal, derived from said first output signal, for providing a second output signal, and having a second automatic gain control stage with a second attenuation factor controlled by a second automatic gain control signal;
automatic gain control means arranged to receive a second input signal derived from the second output signal and to generate said first and second automatic gain control signals in dependence on the second output signal;
the arrangement being arranged such that:
for values of said antenna signal below a predetermined threshold level, only said second automatic gain control stage may be active with increasing second attenuation factor with increasing values of the antenna signal; and
for values of said antenna signal above said predetermined threshold level, said first automatic gain control stage is active with increasing first attenuation factor with increasing values of the antenna signal.
DESCRIPTION OF THE PRIOR ART
Such an arrangement is known from U.S. Pat. No. 4,403,346. The arrangement known from this prior art document will be illustrated with reference to
FIG. 1
which is also presented as
FIG. 7
in that document.
FIG. 1
shows an automatic gain control (AGC) circuit for a wide band tuner as used in television systems. An antenna (not shown) receives a signal and generates an output signal ANT INPUT applied to a first AGC stage
2
. The output of the first AGC stage
2
is sent to an UHF/VHF RF circuit
40
. Typically, the UHF/VHF RF circuit will comprise three parallel circuits, two for two different VHF frequency bands and one for one UHF frequency band. Instead of three different bands, one band may also be applied depending on the application concerned.
The output of the RF circuit is connected to the input of an UHF/VHF selection switch
7
. The selection switch comprises an U/V switch
7
a
and a second AGC stage
7
B in series. The output of the selection switch
7
is fed to two superheterodyne sections which are not shown in FIG.
1
. Superheterodyne sections and the way they operate are known to persons skilled in the art and need no further explanation here. The output of the second superheterodyne section is a demodulated picture signal. Moreover, the second superheterodyne section generates an AGC control voltage V
AGC
which is an input to an AGC signal source
50
.
The AGC signal source
50
comprises a differential amplifier A
50
and an inverting amplifier A
52
. The input of the inverting amplifier A
52
is connected to the inverting input of the differential amplifier A
50
. The non-inverting input of the differential amplifier A
50
is connected to a node E
50
which is connected to the AGC control voltage V
AGC
through a resistor R
50
. Node E
50
is also connected to the input of inverting amplifier A
52
and to the anode of a diode D
50
. The cathode of diode D
50
is connected to a slider of a trimmer resistor VR
50
, one side of which is connected to a power supply voltage and the other side of which is connected to ground.
The connection point between resistor R
52
and the inverting amplifier A
52
is called E
52
and is also connected to the anode of a diode D
52
. The cathode of the diode D
52
is connected to a slider of a trimmer resistor VR
52
, one side of which is connected to the power supply voltage and the other side of which is connected to ground.
When the voltage on node E
50
is above a threshold voltage V
TH1
, diode D
50
will conduct. Below this value, diode D
50
will be blocked. When the voltage on node E
52
is above a threshold voltage V
TH2
, diode D
52
will conduct. Below this value, diode D
52
will be blocked. The trimmer resistors are set such that V
TH1
>V
TH2
.
The output voltage of differential amplifier A
50
is used as AGC control signal V
AGC1
for the first AGC stage
2
. The output voltage of inverting amplifier A
52
is used as AGC control signal V
28
for the second AGC stage
7
B.
Now, the operation of the arrangement of
FIG. 1
will be explained with reference to
FIGS. 2 and 3
which are identical to
FIGS. 8 and 9
, respectively, of U.S. Pat. No. 4,403,346. As shown in
FIG. 2
, as long as V
AGC
is below V
TH2
, neither diode D
50
nor diode D
52
is conducting. Then, V
AGC2
(input voltage of inverting amplifier A
52
) shows a linear relationship with V
AGC
. Since almost no voltage difference will be present between the differential inputs of differential amplifier A
50
, V
AGC1
will be substantially equal to 0 V.
When V
AGC
exceeds threshold voltage V
TH2
, diode D
52
starts conducting and the input voltage V
AGC2
of inverting amplifier A
52
remains fixed at a predetermined level P
2
. However, from that moment onwards, a current flows through resistor R
52
and a voltage difference will be built up between the inputs of differential amplifier A
50
. Then, V
AGC1
shows a linear relationship with V
AGC
.
However, upon V
AGC
exceeding threshold level V
TH1
, diode D
50
starts conducting, resulting in node E
50
remaining at a fixed voltage. Therefore, for V
AGC
values above V
TH1
, the value of V
AGC1
remains fixed at a predetermined level P
1
.
FIG. 3
shows the total gain of the tuner including components
2
,
40
,
7
in dependence on the value of control voltage V
AGC
. The first AGC stage
2
and the second AGC stage
7
B are both implemented in such a way that their respective gains are at maximum when their AGC control voltages V
AGC1
and V
AGC2
, respectively, equal 0 V and their gains can only decrease with increasing values of AGC control voltages V
AGC1
and V
AGC2
, respectively.
As long as V
AGC
is below a predetermined threshold value V
TH0
(V
TH0
<V
TH2
), both AGC stages
2
,
7
B show maximum gains and the total tuner gain is G
0
. When V
AGC
exceeds threshold voltage V
TH0
, the second AGC stage
7
B starts to show a linearly decreasing gain relation with increasing values of V
AGC
until V
AGC
reaches threshold V
TH2
. From that moment onwards, V
AGC2
remains fixed and the attenuation factor of the second AGC stage
7
B remains fixed. Moreover, from that moment onwards, V
AGC1
starts to show an increasing linear relation with V
AGC
(
FIG. 2
) and the first AGC stage
2
starts to show a linearly decreasing gain relation with increasing V
AGC
. When V
AGC
exceeds threshold V
TH1
also control voltage V
AGC1
remains fixed (see FIG.
2
). So, for values of V
AGC
above V
TH1
both AGC stages
2
,
7
B remain at a fixed attenuation level. The total tuner gain will then remain fixed at a predetermined value of G
1
.
Since the attenuation of the first AGC stage
2
only starts after the attenuation of the second AGC stage
7
B has reached its maximum, the concept of AGC shown in
FIGS. 1
,
2
and
3
is also known as “delayed AGC”.
As described above, the signal V
AGC
is generated by the superheterodyne section (not shown) that outputs the demodulated picture signal. Thus, essentially, the first and second AGC stages
2
,
7
B in the prior art according to
FIG. 1
are controlled by the output of the arrangement itself. In practice, the value of threshold V
TH0
(
FIG. 3
) is determined by a reference voltage with which the demodulated picture signal is to be compared. The thresholds V
TH1
and V
TH2
are determined by the settings of the sliders of variable resistors VR
50
and VR
52
. These settings are made in the tuner factory and will, normally, not be accessible to buyers of the tuners.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a tuner arrangement arranged to measure and possibly display the strength of the antenna signal supplied to the tuner arrangement.
This object is obtained by
Kerveros James
Oda Christine
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