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Fig. 6. Effects of angular field; crystal thickness, 0.045 in.
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However, this technique has proven unnecessary, since the 75% level is sufficient for most needs.
Angular Field
The standard modulator slit has a relatively large dimension in one direction, i.e., 0.1 in. Since this slit is the virtual light source for the modulator, all the light passing through the crystal cannot be normally incident. This angular field problem may be overcome by adjusting the length of the optical system and in that manner control the angular field. Figure 6 summarizes how the angular field affects distortion. For zero angular field and 74% modu
lation, the second harmonic distortion is zero; the third harmonic distortion is approximately 2.2% of the fundamental as noted earlier. As the angular field is increased, the third harmonic distortion remains substantially constant while the second harmonic distortion increases parabolically so that at 2.28° for the half angle, the total distortion from all sources is approximately 3.3%. To keep the modulator slit to the above-mentioned angular field, the overall modulator length is approximately 5 in.
Frequency Response
Before investigating the frequency response of a crystal modulator, it is well to study the laminated crystal structure as it is used. This is necessary since the laminar structure affects the frequency response. The term laminar structure is used since the polarizers, mica quarterwave plate and crystal, with its electrodes, are all cemented together. This entire assembly is then mounted in a bakelite block. A schematic diagram of this laminar structure is shown in Fig. 7. Figure 8 shows a picture of this structure with a handling arm for fitting
210
March 1953 Journal of the SMPTE Vol. 60