Video Transmitter Tuning, Part 1

Lesson 1 - Video Transmitter Basics

When tuning a transmitter of any kind, you want to insert a signal into the transmitter's normal input, that represents the entire band that the transmitter is capable of transmitting on. For instance, Let's say your transmitter normally transmits at a carrier frequency of 50 MHz, and has a maximum bandwidth of 1 MHz.

You would want to inject a signal that is 2 MHz in bandwidth on the carrier frequency ( 50 MHZ ) of the transmitter. In other words, you would want a signal that sweeps from 49 MHz to 51 MHz at a constant level. It is suggested that you use a bandwidth that is at least TWICE the bandwidth capabilities of the transmitter, so that you can check for out of band frequencies that shouldn't pass through the output filters.

A signal generator that is specifically designed for this purpose is called a

" SWEEP GENERATOR ".

One of the best, and my favorite tool for this purpose is the old military SG-677/U. In addition to having an overall frequency range of 500 KHz to 1.2 GHz CW or variable 10 KHz-400MHz sweep, it has provisions for markers. Included with it are 3 markers at 1MHz, 10MHz, and 50MHz with options for 5 more! It isn't too difficult to add 3.58MHz and 4.5 MHz markers for Aural and Chroma sub carriers for checking TV Transmitters. Not to mention that it's "Milspec" and rugged as a tank!

Proper equipment setup for the SG-677 and similar sweep generators is above. .

Note* that the sweep generator's output goes into the tuned cavity, or transmitter, and the output of the transmitter is viewed on the scope, via a detector, in this case using special circuitry within the sweep generator.

Note that this setup can be used on ANY tuned circuit, whether it is a tuned cavity, an RCL filter, or a 9 Cavity Klystron with turbocharger.

If all you are checking out is Television Transmitters, though, I recommend the Tektronix 1405, which sweeps the entire video band, injecting sync pulse at the proper point, and has the added advantage of having a built in marker generator.

The proper setup for a Tek 1405 is as shown. Instead of going directly into the cavity or RF portion of the transmitter, the Tek 1405 is designed to inject a signal into the video modulator portion of the exciter.

The resultant signal is then viewed on a frequency - vs. amplitude display, to see it's characteristics across the entire band. This can be accomplished on a standard oscilloscope, with special adaptation. However, this is most commonly done using a spectrum analyzer or network analyzer. Note* that the marker generator is only truly useful if it is used along with it's sister spectrum analyzer, the Tektronx 271x series. Tektronix does not guarantee the accuracy of this device with any other scope - which kind of limits it

Depending on the transmitter, it may be desirable to have a very sharp peak, with no harmonics. In the case of a Television Transmitter, the signal we want to see looks like this:

A TV transmitter is a very wide bandwidth transmitter. In the united states, the Television Signal is 6 MHz wide, and is actually 2 transmitters combined into one. It is an Amplitude Modulated ( AM ) Video transmitter, and a Frequency Modulated ( FM ) Audio Transmitter.

The video transmitter has a video ( visual ) carrier, which carries all the black & white ( luminance ) video details on it, and a Color ( chromanance ) sub-carrier that carries all the color details on it.

The sound ( aural ) carrier may or may have mono, stereo, second audio, "pro-channel", or other data riding on it.

It is because there is so much data and information riding on this complicated signal, that it takes up so much bandwidth. Normally, a generator that can sweep from 0 to 20 MHz is used to test a TV Transmitter's output signal.

If we inject a 20 MHz wide swept signal on the visual carrier frequency of a television transmitter, the following signal will be ( hopefully ) the outcome.

The single peak you see at the top, is the visual carrier. Notice the long flat horizontal line along the top. This indicates that the transmitter shown has very good bandwidth characteristics. Ther is no "slant" to the signal, and it is relatively flat along the top. This is what we are looking for; A flat signal. Note, there is no audio carrier shown here.

If the signal we have is not flat, then the characteristics of the final output will not be "linear", and the output of the transmitter will be distorted. It will not truly amplify the input signal, but will cause imperfections in the output, which might cause interference to other signals.

If the signal is low on one end or the other, we say it has a slope to it. This will cause the transmitter to amplify part of the signal more than another part. For instance, if the left side were lower, then when we apply a normal video signal, the video carrier will be lower than normal, possibly even lower than the aural carrier. There will also be distortions in the video signal.

If, on the other hand, the right side of the signal were lower, then the audio, chroma, and right most portions of the video carrier modulated signals will be lower than normal.

The object then, is to achieve a flat signal across the entire 6 MHz spectrum, with little or no distortions, and as high in overall amplitude as possible within physical and legal limits. This can be a magic trick in itself!

Not taking into consideration the idiosyncrasies of each individual transmitter type, ( Klystron, IOT, Tetrode, etc ), there are many paramaters which must be taken into consideration.

Using the visual carrier as a reference, the chroma subcarrier should be exactly 3.579545 MHz above the visual ( +/- 10 Hz ).

Also, the sound carrier should be 4.5 MHz above the visual. Both the visual and sound carriers have a 1 KHz center frequency tolerance, however, they should be kept as close as possible, and the sound carrier should be locked to the visual carrier ( if the visual drifts upward slightly, the sound should drift along with it).

The lower knee of the band should be located .075 MHz below the visual carrier, with the center of the left slope ( -1.25 MHz ) not being greater than 20 dB down from the peak.

The upper knee should be located at 4.18 MHz above the visual carrier, with the center of the right slope ( +4.75 MHz ) also not being greater than 20 dB down from the peak.

The top line should be flat and linear within +/- 2 dB, and the second null to the left ( located at
-3.579545 ) should be 42 dB down or more. Note* This is the negative of the chroma subcarrier.

Here is a picture of a normal analog video signal superimposed on top of a swept signal. Notice the placement of the visual and aural carriers, as well as the chroma sub-carrier. If you say that the visual carrier is at the "0" line, and count major divisions to the right, the chroma is at 3.5 major divisions, and the aural is at 4.5. That is because each major division ( as the scope is presently set up ) represents 1 MHz.

(On The Following Indicator... PURPLE will indicate your current location)

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