TM8100/TM8200 Service Manual Frequency Synthesizer Fault Finding 179© Tait Electronics Limited June 20069 Frequency Synthesizer Fault FindingIntroduction This section covers the diagnosis of faults in the frequency synthesizer.The sections are divided into the following:■ Initial checks■ Fault diagnosis of RF PLL circuitry■ Fault diagnosis of FCL circuitry.The initial checks will indicate whether it is the RF PLL or the FCL that issuspect. Note that the synthesizer is a closed-loop control system. A fault inone area can cause symptoms to appear elsewhere. Locating the fault cantherefore be difficult.MeasurementTechniques The radio must be in CCTM for all the fault-diagnosis procedures of thissection. The CCTM commands required are listed in Table 9.1. Full detailsof the commands are given in “Computer-Controlled Test Mode(CCTM)” on page 118. Use an oscilloscope with a x10 probe for all voltagemeasurements required. The signals should appear stable and clean.Consider any noise or unidentified oscillations as evidence of a faultrequiring investigation. Use a frequency counter for all measurements ofhigh frequencies. The RF power output from the frequency synthesizer willnot exceed 10mW. If a probe is used for frequency measurements, use thex1 setting.Table 9.1 CCTM commands required for the diagnosis of faults in the frequency synthesizerCommand Description72 Read lock status of RF PLL, FCL and LO2 — displays xyz (0=not in lock, 1=in lock)101 x y 0 Set transmit frequency (x in hertz) and receive frequency (y in hertz) to specified values205 Reset calibration parameters to their default values301 0 10 Calibrate VCXO of FCL302 0 10 Calibrate VCO(s) of RF PLL334 x Set synthesizer on (x=1) or off (x=0) via DIG SYN EN line335 x Set transmit-receive switch on (x=1) or off (x=0) via DIG SYN TR SW line389 x Set synthesizer mode to slow (x=0) or fast (x=1)393 1 x Write data x to FPGA
