KD2BD ATV AM Video Modulator


Introduction

PC Electronics ATV transmitters seem to suffer from poor linearity, and generate
excessive intermodulation distortion products that extend well
beyond the 9 MHz bandwidth expected of a double sideband NTSC AM
television signal with subcarrier audio. A test was once
conducted where the RF output of a PC Electronics model PC-70 ATV
transceiver was examined on a spectrum analyzer, and results
published in Mike Stone’s The Complete Amateur Television
Workbook
, Second Edition. The results show an emission
bandwidth greater than 18 MHz at -38dbc levels! The use of
a Mirage D24 solid state power amplifier after the transceiver
served to increase the bandwidth of the transmitted signal even
further.

Building an in-band,
duplexed ATV repeater system is not a trivial task. Just ask
anyone who’s tried. The output spectrum of the repeater’s
transmitter must be extremely clean, and both the transmitter’s
output as well as the receiver’s input must be tightly filtered
to achieve adequate isolation and reject interference from strong
signals on neighboring frequencies.

When the Brookdale ATV
Repeater System was first constructed, a PC Electronics model
RTX-70 exciter was used in the transmitter portion of the
repeater. However, the intermodulation distortion products
generated by the exciter when amplified by a Mirage D100ATV-R
amplifier were so severe, they completely overwhelmed the
repeater’s receiver, despite the extensive combline and
interdigital filtering used in the system.

After much study and
research, the cause of the exciter’s strong IMD products was
found to be the result of distortion and non-linearity in the
exciter’s video modulator, and not the modulated RF stages
themselves. Once the original video modulator was replaced with
the one described on this page, the harmful effects of IMD were
reduced to imperceptible levels. An added bonus was a noticeable
increase in the exciter’s RF power output.


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Circuit
Description

The first section of the video modulator performs a video level
clamping function. The high input impedance Darlington amplifier
and the small series coupling capacitor combine to achieve an
excellent low-frequency response and a clamping function with
very rapid response time. A three terminal voltage regulator sets
the clamping level to 8 volts.

The clamped video is then level shifted through a zener diode and applied to a single
video amplifier stage. A PNP transistor in the collector circuit
functions as a constant current source for the video amplifier.
The use of a constant current source rather than a collector load
resistor in the video amplifier results in very high voltage gain
with nearly rail-to-rail output voltage capability. Negative
feedback around the video amplifier reduces the gain to the
amount required, while it increases the bandwidth, flattens the
frequency response, increases the linearity, and reduces any
distortion products to extremely low levels.

A 2N3904 in the base
circuit sets the `Q’ point of the amplifier. FM subcarrier audio
at 4.5 MHz is also injected at this point through a 3300 ohm
resistor and a DC blocking capacitor. A high frequency 5-watt RF
power transistor (2SC1909) serves as the last stage of the
modulator. An NTE-235, NTE-236, or equivalent is suitable in this
application.

Clamped video is also
fed into an LM311 voltage comparator that serves as a video sync
detector. Some builders have used an LM1881 sync separator chip
for this stage. Either way, sync pulses detected by this circuit
trigger a CD4016B silicon bilateral switch that lowers the
negative feedback around the video amplifier and voltage follower
at sync time to compensate for high power gain compression in any
solid-state linear power amplifiers that may follow the video
modulated stages. This method of sync expansion is unique in that
it also compensates for audio subcarrier compression at sync
time, resulting in a clean output signal with no sync buzz in the
transmitted audio.

R1 is a video gain
control. It is used to set the white modulation level. R2 is a
video bias control. It is used to set the black modulation level.
R3 is adjusted so that the LM311 delivers clean sync pulses to
the CD4016B with an input signal present. R4 adjusts the sync
modulation level. Up to 9 dB of sync expansion is possible with
this circuit.

David Sudduth, KB0ZNP,
was kind enough to develop a PC board design for this modulator
using Circad 98
software
, and offer the layout and foil patterns to anyone who wishes to
duplicate this modulator. The original Circad 98 PCB data file is also available.

Video Modulator
Performance

The following graphs
show the results of a computer simulation of the video modulator
when adjusted for a voltage gain of 20 dB. This simulation was
performed using MicroCAP IV software, an electronic circuit
analysis program by Spectrum Software.


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The top graph plots modulator voltage
gain as function of frequency. The graph shows a wide and flat
frequency response that extends to 20 MHz.

The lower graph plots modulator phase
shift as a function of frequency. the graph shows a wide and flat
phase response that is essential for accurate color reproduction.
The phase response around the chrominance subcarrier (3.58 MHz)
falls well within the +/- 10 degree specification used by
commercial broadcasters.

Conclusion

Feedback received from others who have duplicated
this modulator and used it in their own ATV transmitters and
repeaters confirm its excellent performance. This modulator has
also been used in a completely new
exciter that has recently replaced the PC Electronics exciter
originally used in the Brookdale ATV Repeater System.


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