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參數(shù)資料
| 型號： | U4476B-MFLG3 |
| 英文描述： | TV Video Receiver Circuit |
| 中文描述： | 電視視頻接收電路 |
| 文件頁數(shù)： | 5/7頁 |
| 文件大?。?/td> | 81K |
| 代理商： | U4476B-MFLG3 |
U4476B
Preliminary Information
Rev. A3, 08-Mar-01
5 (7)
IM3 Measurements and Application Recommendations
IM3 Measurement
For correct measurement of IM3 with the 2-carrier
method, it is necessary to control the internal IF amplifier
via an external voltage at Pin 7. The criteria for a correct
value of the external control voltage is the output
amplitude of 1.8 V
pp
(or lower). Please note that
change of the input level requests a correction of the
external gain control voltage at Pin 7 in order to achieve
the 1.8-V
pp
output level again. In this case, IM3 is nearly
43 dB, independent from the input level. With an output
voltage of 1.5 V
pp
only, the IM3 is about –50 dB.
Anyway, if the output amplitude is more than 1.8 V
pp
(specified), the IM3 will decrease rapidly. In the graphic
below, IM3 is shown for two different conditions:
each
30
32
34
36
38
40
42
44
46
48
50
40
50
Input voltage (each carrier) ( dB V )
60
70
80
90
100
I
14778
V
out
= 1.5 V
pp
V
out
= 1.8 V
pp
Figure 4. IM3 vs. IF input voltage and IF2out voltage
Test conditions: input: f
1
= 38 MHz,
f
2
= 39 MHz, LO = 43 MHz
Circuitry for External Gain Control
If the gain control of the IC is carried out by an external
DSP, the internal charge pumps of the AGC (Pin 7) have
to be overlapped by an external voltage source. The
internal AGC currents are
±
20
μ
A. Therefore, the
minimum current capacity of the external voltage source
should be
±
30
μ
A or more.
The circuitry shown in figure 5 takes some more current.
14581
S852T
10 k
100 nF
to Pin 7
+5 V
from DSP
Figure 5.
External LO Application
If the internal VCO is not used, the LO can be fed
unbalanced or balanced to the VCO Pins 22 and 23. The
application shown below is generating an unbalanced LO
signal by using a 36-MHz third-overtone quartz
reference. The application circuit is taken from IQD
Limited Crystal Product Data Book. The LO signal is fed
unbalanced with 1 nF coupling capacity to Pin 22. Pin 23
has to be blocked to ground with an 1-nF capacitor. The
typical LO amplitude is 100 dB
μ
V. The variation of the
LO input amplitude should not exceed
±
5 dB (better:
±
3 dB).
560
100 pF
490 nH
4.7 k
47 pF
68 pF
560
1 nF
S852T
to Pin 22
+5 V
10 k
14582
Figure 6.
Note:
With external LO operation, Pin 18 (VCO tuning voltage)
should be connected to the supply voltage, and Pin 20
(LO output) should not be connected to the PCB.
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