to 8.3 MHz, the two waves will be less than 180 degrees out of phase. The output will be an almost
square pulse whose width from center positive edge to the next center positive edge would be equivalent
to the period of the luminance frequency being differentiated. The placement of this center on reference
would give inaccurate measurement of the period and an inaccurate reading when the RF is
demodulated. Resistor R2 on the DM-7 board sets this reference level. The luminance FM signal,
which leaves the cosine equalizer through buffer Q5, is a series of almost square waves whose period
represents the particular luminance frequency. The cosine equalizer emphasizes the higher frequencies
to give a sharper rise time to the square waves going to IC-1 pin 1 on the DM-7 board.
Learning Event 4:
FUNDAMENTALS OF Y DEMODULATION (DM-7 BOARD) CIRCUITS
1.
Refer to Figure 4-2 (fo) at end of lessons. The Y-RF signal from the HF-3 board (fig 4-3) is
supplied to pin 1 of IC-I on the DM-7 board. Pin 1 is the input to a limiter stage that is necessary to
eliminate amplitude variations in the Y-RF signal prior to demodulation. The output of the limiter is
split into two paths in the Y demodulator. One path goes directly to the phase comparator. The other
path delays the pulse-coded luminance before it reaches the phase comparator. This delay is constant
for all frequencies. These two signals are then mixed to produce a series of pulses whose period is one-
half the input luminance frequency. These pulses are then integrated by the low-pass filters, FL1 and
FL2, to the luminance DC levels found in the video signal.
Figure 4-3. Y-FM demodulator
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