U4479B Datasheet, PDF (9/11 Pages) TEMIC Semiconductors – DVB-Cable IF Converter and Full Multi-Standard Video-/Sound IF Processing

U4479B
IM3 Measurements and Application Recommendations for DVB Operation
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 each
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:
50
48
46
44
42
40
38
36
34
32
30
40
14778
V_out = 1.5 V_pp
V_out = 1.8 V_pp
50 60 70 80 90 100
Input voltage (each carrier) ( dBmV )
Figure 4. IM3 vs. IF input voltage and IF2out voltage
Test conditions: input: f₁ = 38 MHz,
f₂ = 39 MHz, LO = 43 MHz
Circuitry for External Gain Control
from DSP
+5 V
S852T
to Pin 7
10 kΩ
100 nF
14581
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 dBµV. The variation of the
LO input amplitude should not exceed ± 5 dB (better:
±3 dB).
100 pF
10 kΩ
S852T
+5 V
490 nH
4.7 kΩ
560 Ω
68 pF
47 pF
1 nF
to Pin 22
560 Ω
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 µA. Therefore, the
minimum current capacity of the external voltage source
should be ± 30 µA or more.
The circuitry shown in figure 5 takes some more current.
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.
TELEFUNKEN Semiconductors
9 (11)
Rev. A2, 14-Jan-98
Preliminary Information