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Troubleshooting Test Setup
Some synthesizer section problems require placing the YTO PLL in an unlocked condition.
This is done by moving jumper A14J23 to the TEST position. This grounds the YTO
ERROR signal, disabling the ability of the CPU to detect an unlocked YTO. The FM coil
driver output is set to its mid-range level causing the YTO to be controlled only by the main
coil tune DAC.
Synthesizer section troubleshooting is best done with the spectrum analyzer SPAN set to 0 Hz
(even though it is still possible to sweep the Main and FM coils of the YTO).
With the YTO in its unlocked conditions and the SPAN set to 0 Hz, the nominal YTO
frequency is not necessarily the value listed as LO FREQ in the Frequency Diagnose menu.
The YTO has an initial pretune accuracy of
f20
MHz. To display the nominal YTO
frequency, press (CAL),
MORE
1 OF 2,
PREQ
DIAGNOSE, LO
FREQ
.
The fractional N oscillator frequency is the same as the desired sampler IF. To display the
fractional N oscillator frequency, press
a,
MORE
1
OF
2 ,
FREQ
DIAGEQSE
,
FRAC
Bl
FREQ .
If the sampler IF is negative (YTO frequency is lower than the desired sampling oscillator
harmonic), the fractional N frequency will be displayed as a negative number.
Confirming a Faulty Synthesizer Section
The All YTO (the spectrum analyzer first LO) is a YIG-tuned oscillator which tunes from
2.95 to 6.8107 GHz. The A7 switched LO distribution amplifier (SLODA) levels the output
of All and distributes the signal to the A8 low band mixer, A10 YIG-tuned mixer/filter
(RYTHM),
A15UlOO
sampler, and the front panel 1ST LO OUTPUT. The synthesizer section
includes the following
PLLs
(Phase Locked Loops):
YTO PLL
A7, All, Al4 and Al5 assemblies
Offset PLL (sampling oscillator PLL) Al5 RF assembly
Fractional N PLL
Al4 frequency control assembly
Reference PLL
Al5 RF assembly
The fractional N PLL is sometimes swept backwards (higher frequency to lower frequency).
This is necessary because of the way in which the sampler IF signal is produced.
Note
n The frequency control board is digitally controlled. If multiple failures
appear in unrelated areas of the circuitry, the control may be at fault.
Refer to the troubleshooting procedures in this chapter for further help on
isolating those failures.
w
The TAM tests the signal path circuitry by digitally controlling the
hardware and monitoring the control lines to make sure they are responding
properly. Use the TAM automatic fault isolation routine or verify the RF
levels manually to ensure proper operation.
1 l-8 Synthesizer Section