Video Modulator for
FM/AM-Audio
MGM 3000X
Bipolar IC
Features
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FM- and AM-audio modulator
Audio carrier output for suppression of harmonics
Sync level clamping of video input signal
Controlling of peak white value
Continuous adjustment of modulation depth for
positive or negative values
Symmetrical mixer output with separate ground area
Symmetrical oscillator with separate RF-ground
Low spurious radiation
High stability of the RF-oscillator frequency
High stability of the audio oscillator
Internal reference voltage
12 V supply voltage
Ordering Code
Q67000-A5179
Q67006-A5179
P-DSO-20-1
Type
MGM 3000X
MGM 3000X
Package
P-DSO-20-1 (SMD)
P-DSO-20-1 Tape & Reel (SMD)
Functional Description and Application
The monolithic integrated circuit MGM 3000X is especially suitable as a modulator for
the 48 to 860 MHz frequency range.
Video recorders, cable converters, TV-converter networks, demodulators, video
generators, video security systems, amateur TV-applications and personal computers.
Circuit Description
Oscillator
The RF-oscillator is available at pins 3 - 7. The oscillator operates as a symmetrical
Colpitts circuit. The oscillator chip ground, pin 5, should be connected to ground at the
resonance circuit shielding point. An external oscillator can be injected inductively or
capacitively via pins 3 and 7. The layout of the PCB should be such as to provide a
minimum shielding attenuation between the oscillator pins 3 - 7 and modulator output
pins 15 - 17 of approximately 80 dB.
Semiconductor Group
1
12.94
MGM 3000X
For optimal residual carrier suppression, the symmetrical mixer outputs at pins 15, 17
should be connected to a matched balanced-to-unbalanced broadband transformer, e.g.
a Guanella transformer with good phase precision at 0
°
and 180
°.
The transmission
loss should be less than 3 dB. In addition, an LC-low pass filter combination is required
at the output. The cut-off frequency of the low pass filter combination must exceed the
maximum operating frequency.
Video
The video signal with the negative synchronous level is capacitively connected to pin 12.
The internal clamping circuit is referenced to the synchronizing level. Should the video
signal change by 6 dB, this change will be compensated by the resonance circuit which
is set by the peak white value. At pin 13, the current pulses of the peak white detector
are filtered through the capacitor which also determines the control time constant. The
RF-carrier switches from negative to positive video modulation, when pin 14 is
connected to ground. By varying the value of resistance
R
at pin 14 between
∞
… 0
Ω
the modulation depth can be increased from 70 % to 100 % when the modulation is
negative and decreased from 100 % to 70 % when the modulation is positive.
Audio
Via pin 1, the audio signal is capacitively coupled to the AF-input for the FM-modulation
of the oscillator. A parallel resonance circuit is connected to the audio carrier oscillator
at pins 19, 20. The unloaded
Q
of the resonant circuit must be
Q
= 25 and the parallel
resistor
R
T
= 6.8 kΩ to ensure a video to audio carrier ratio of 12.5 dB. At the same time,
capacitative and/or inductive reactance for the resonance frequency should have a value
of
X
C
≈
X
L
≈
800
Ω.
Via pin 18, the audio signal is capacitively coupled to the AF-input for the AM-modulation
of the oscillator. This signal is forwarded to a mixer which is influenced by the AM-
modulation input of pin 18. The video to audio carrier ratio can be changed by connecting
an external voltage to pin 18, which deviates from the internal reference voltage.
Through an additional external dc voltage at pin 18, the set AM-modulation index can be
changed by overriding the internally adjusted control voltage for a fixed AM-modulation
index.
At the output of the above described mixer the FM and/or AM modulated audio signal is
added to the video signal and mixed with the oscillator signal in the RF-mixer.
For optimal suppression of harmonics of the audio carrier an audio filter should be
connected between pin 10 and 11.
Source
The internal reference voltage is available at pin 2 and has to be capactively blocked
there.
Semiconductor Group
2
MGM 3000X
Pin Configuration
(top view)
Semiconductor Group
3
MGM 3000X
Pin Definitions and Functions
Pin No. Symbol
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
FM-Audio
Function
AF-input for FM-modulation
Internal reference voltage (7.5 V)
Symmetrical oscillator input
Symmetrical oscillator output
Oscillator ground
Symmetrical oscillator output
Symmetrical oscillator input
Supply voltage (12 V)
Not connected
Audio output; symmetrical inputs for audio filter
Audio output; symmetrical inputs for audio filter
Video input with clamping
Connection for smoothing capacitor for video control loop
Switch-over for positive and negative modulation
Symmetrical RF-output
Ground
Symmetrical RF-output
Video to audio carrier ratio adjustment and AF-input for
AM-modulation
FM-audio oscillator, symmetrical inputs for tank circuit
FM-audio oscillator, symmetrical inputs for tank circuit
V
REF
OSC-Input 1
OSC-Out 1
OSC-Ground
OSC-Out 2
OSC-Input 2
V
S
N.C.
Audio Out 1
Audio Out 2
Video
Video-Cap.
Modulation
Output 2
Ground
Output 1
AM-Audio
FM-Audio OSC
FM-Audio OSC
Semiconductor Group
4
MGM 3000X
Block Diagram
Semiconductor Group
5
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