The KD2BD ATV AM Video Modulator


"[This modulator] has none of the problems of a typical design. The series output transistor is a VHF power transistor. It has flat response out past 20 MHz, less than 1% total harmonic distortion, and does not generate significant levels of IMD." "This circuit is probably the best ATV video modulator I have run across over the years and is well-worth the modest effort needed to build it."

-- Dr. Ralph Taggart, WB8DQT, The ARRL Image Communications Handbook, 2002



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.


modulator


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.


simulation


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.



telegraphy key John A. Magliacane
Amateur Radio Operator: KD2BD