HP 85662A, Troubleshooting And Repair Manual

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complementary pair The sense of this current is such that the feed back is positive.
The more load, the more positive feedback is developed. The more positive feedback, the
more gain the amplifier has. This implies negative output impedance. The magnitude of the
negative output impedance is the resistance to which R41 is set.
and the SYM cap are added to cancel the effect of the case (parallel) capacitance of the
crystal. (See Figure 4.) R94, R96 and other resistors are switched in to change bandwidths.
C31 and C62 (not shown) are in series with the crystal to tune the center frequency to
precisely 3 MHz. The negative output impedances of the amplifier is set to compensate for
the series resistance of the crystal so that as different bandwidths are selected, the amplitude
(overall gain) does not change. If the relative gain of the filter assembly is too low in the
10 Hz BW, and cannot be corrected by R41 and R30, three common problems are:
n
All five crystals are not tuned close enough to the same center frequency.
n
The series resistance of one or more of the crystals is too high.
n
The output impedance of one or more of the buffer amplifiers is not negative enough.
C R 2 3 C R 2 4
Figure 4. Fourth Pole, Equivalent Circuit
3