520 - 44 - 00
The GT4122 and GT4124 are available in both 20 pin PDIP and
20 pin SOIC packaging. They each represent a dedicated
video mixer function in one package and offer professional
video mixing with very few external parts. These devices are
specifically designed for the professional broadcast market
and are used in Production Switchers (Vision Mixers) and
Multilayer Keyers.
Full data is available from the device data sheets in the
GENNUM Data Book. Applications engineers at GENNUM
will be pleased to answer technical questions about any of the
wide range of Video & Broadcast products made by GENNUM
Corporation.
-6k -3k 1M 3k 6k
0
-20
-40
-60
-80
-100
Fig. 11 Spectrum of Double Sideband Signal
FREQUENCY (Hz)
GT4122
RF CHOKE or
1-5k RESISTOR
0.5V REF
14 -INA
OUT 20
13 +INA
18 -INB
17 +INB
9 V
C
7 REF
1.0μF
10μF
-
+
-
+
-
+
DSB OUTPUT
GND
CARRIER
AUDIO
2200
2200
Fig. 10 Double Sideband Modulator Circuit
CONCLUSION
Photograph 10. Envelope Waveform
of Double Sideband Signal
The spectrum is shown in Figure 11 and indicates a carrier null
of at least 50dB.
The carrier is a 1V peak to peak, 1MHz signal superimposed
on a +0.5V bias. The audio level is varied to control the amount
of modulation.
Photograph 10 shows the resulting envelope waveform. Again,
this signal may be prescaled to place it on any desired
channel. One sideband may also be filtered in order to
produce a Single Sideband Suppressed Carrier signal.
In order to produce a suppressed carrier signal, mixing must
occur between in-phase and anti-phase signals. To achieve
this, the A-INPUT is configured as an inverting amplifier with
unity gain by using two 2.2k
resistors in the feedback loop.
This is illustrated in Figure 10.
The audio signal is now applied to both the A-INPUT and the
B-INPUT. The signals reach the summing circuits within the
device 180
°
out of phase. The carrier applied to the CONTROL
input modulates these signals and produces a suppressed
carrier output.
DOUBLE SIDEBAND SUPPRESSED CARRIER MODULATOR
G