U4476B-MFLG3 TEMIC [TEMIC Semiconductors], U4476B-MFLG3 Datasheet - Page 5

• Manufacturer Part Number: U4476B-MFLG3
• Description: IF Amplifier / Converter for DVB (Cable)
• Manufacturer: TEMIC [TEMIC Semiconductors]

TELEFUNKEN Semiconductors
Rev. A2, 15-Jan-98
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
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
43 dB, independent from the input level. With an output
voltage of 1.5 V
Anyway, if the output amplitude is more than 1.8 V
(specified), the IM3 will decrease rapidly. In the graphic
below, IM3 is shown for two different conditions:
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
minimum current capacity of the external voltage source
should be 30 A or more.
The circuitry shown in figure 5 takes some more current.
14778
Figure 4. IM3 vs. IF input voltage and IF2out voltage
50
48
46
44
42
40
38
36
34
32
30
pp
40
output level again. In this case, IM3 is nearly
Test conditions: input: f
f
Input voltage (each carrier) ( dB
2
50
= 39 MHz, LO = 43 MHz
pp
pp
(or lower). Please note that
only, the IM3 is about –50 dB.
60
V
V
out
out
70
= 1.5 V
= 1.8 V
20 A. Therefore, the
1
80
Preliminary Information
pp
= 38 MHz,
pp
90
m
V )
100
*
each
pp
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 V. The variation of the
LO input amplitude should not exceed
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.
3 dB).
490 nH
100 pF
from DSP
560
4.7 k
10 k
10 k
Figure 5.
Figure 6.
S852T
+5 V
68 pF
47 pF
100 nF
U4476B
S852T
to Pin 7
560
1 nF
14581
5 dB (better:
to Pin 22
+5 V
14582
5 (7)