TROUBLESHOOTING AND REPAIR
FREQUENCY AND PHASE MODULATION
Modulation Section 6E-7.
The modulation section consists of a high rate modulation path and a low rate
modulation path. The modulation signal comes from the Modulation Control PCA at
J6. The signal frequency at this point can range from DC to 200 kHz. Full scale
amplitude for each range is 4V AC-peak for full deviation at the modulation
frequency. The type of modulation is determined following this point. The logic
control signals for range switching and type of modulation are generated in the two
"PAL" ICs, U29 and U33 and selector U30. This was pointed out previously under the
heading "Phase Detector, Loop Circuits, and Logic Section". See Tables 6E-1 and
6E-2.
The high rate path consists of U37, U39, U40, U41, U42, U43, U45, U46, U47, K2, and
Z6. U37, U45 and U46 are level translators from TTL (CMOS) level to the drive level
for the analog FET (or DMOS) switches which require levels for "off" of nominal -12V
DC and for "on" of +12V DC. The Mode switch for ACFM, DCFM (as well as
Low-rate FM), PHMOD (normal), and High-rate PHMOD is U39. This switch
functions as a one-of-four selector on the input of an amplifier, U40. The adjustments,
R104 and C75, are used to balance the different modes of operation. The feedback
resistors, R107 and R108, around this amplifier determine the gain of this path. The
amplifier output drives the range resistor network Z6 and range switches. The range
switches are relay switch K2 and analog switches U47 and part of U43. These are
controlled by level translators U45 and U46. The modulation signal is also amplified
by U41, which drives a analog multiplier, U42, to generate a second harmonic. The
second harmonic is added to the fundamental modulation signal for predistorting the
signal to the modulation port of the 80-MHz VCO. This predistortion cancels the
distortion of the VCO. The analog switches in U43, along with the associated resistors,
control and adjust the correct amount of predistortion for each range. The output of
the range network and switches and the output of the predistortion network are added
in the summing resistors R126 and R127. The relay K1 shorts out the large resistor,
R127, for the low deviation ranges (high "Q" mode). The 49.9 ohm resistor R126 is for
low noise performance. The ranges are labeled for FM modulation; however, there are
corresponding phase modulation ranges, i.e., 4 MHz, is 400 (40) radians, etc. The
range and predistortion paths are interactive and require interactive adjustment for
each range; range match: R139, R140, or R141, distortion match: R115, R117, or R119
respectively.
The high rate modulation signal and some of the range control logic signals are sent to
the Sum Loop PCA to maintain correct operation there. Since this causes an
interaction between the Sum Loop and the FM PCA, a lead-lag compensation is made
with R120 and C99 controlled by analog switch Q15 and translator U45. The lead-lag
compensation is controlled by the range bit FMRN2H.
The low rate modulation path consists of Z7, U32, U38, U36, Z5, and U35 and
associated components. This path operates in all modes of modulation except in the
DCFM mode and the CW mode. The modulation range is determined by a range,
network and switch, Z7 and U32, in conjunction with a range network and switch, Z5
and U35, relative to the reference frequency. The modulation signal is applied to the
range resistor network Z7, selected by analog switch U32, and applied to the virtual
ground input of a first section of dual op-amp U38. The selected feedback network
determines the gain and function. The output of the first op-amp U38 is processed by
the range network Z5 and a range switch U35. The resistors R102 and R145 determine
the gain of the low rate path for ACFM and PHASE-MOD respectively. The selected
feedback network consists of capacitors and resistors: C70 and R95 for ACFM, R98
and C71 for PHASE-MOD, and R146, R147, and C76 for high rate PHASE-MOD.
6E-7
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