Mhz Phase Locked Loop - Motorola R-2001A Service Manual

11-9. 500-1000 MHz Phase Locked Loop. The 500-1000 MHz output is locked to either the sum or the
difference of the 310-440 MHz and 60.5 MHz loop output frequencies. In the locked condition, mixing the
divide by two output of the 500-1000 MHz VCO's with the 310-440 MHz signal gives a difference frequency
equal to the 60.5 MHz output. There are two frequencies at the divide by two output, the 310-440 MHz
frequency plus or minus the 60.5 frequency, which will mix down to the correct frequency. However, the sense
of the loop is inverted for one compared to the other. Thus the phase switch following the Phase/Frequency
Detector determines at which frequency the loop will lock.
11-10. Modulation Control. Modulation of the tuning voltage for the 60.5 MHz VCO provides the frequency
modulation of the RF output. Since the modulation sensitivity changes by a factor of two when the 250-500
MHz source is selected, the modulation control provides programmable gain control to maintain constant
sensitivity at the FM MOD and SWEEP inputs. Additionally, the wideband modulation mode requires a gain of
four beyond that for the narrowband mode. Thus under the control of the processor the Modulation Control
selects between the SWEEP and FM MOD inputs, provides gains of 1, 2, 4, and 8 for the FM MOD input and
gains of 1 and 2 for the SWEEP input. Input modulation sensitivites are 5 kHz/volt and 20 kHz/volt for narrow
and wideband FM input and 2 MHz/volt for the sweep input.
11-11. Module Control. Control information is latched in four bit control latches which are loaded by the
processor through the RF control bus. The four bit RF ADD BUS 0-3 is decoded by the Address Decoder to
determine which Control Latch the four bit RF DATA BUS 0-3 is to be stored. Synchronization of the data
transfer is the function of the RF BUS EN line. Two decoded address outputs, RF LATCH 13 and 14, select
latches on the receiver module for receiver control. One control latch output, LO/HI BAND SEL, goes to the RF
Input module to control the frequency range of the output amplifer.
11-2