528 Chapter11
Synthesizer Section
Confirming a Faulty Synthesizer Section
Confirming a Faulty Synthesizer Section
The A11 YTO (the 1st LO of the spectrum analyzer) is a YIG-tuned
oscillator which tunes from 2.95 to 6.8107 GHz. The A7 LO
multiplier/amplifier (LOMA) levels the A11 output and distributes the
signal to the following:
A8 low band mixer
A10 YIG-tuned mixer/filter (RYTHM)
A12 switched barium-tuned mixer (SBTX)
A15U100 sampler
1ST LO OUTPUT to the front panel
The synthesizer section includes the following PLLs (Phase Locked
Loops):
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 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.
Check Sampling
Oscillator Tune
Voltage (steps
1-4)
1. Connect the positive lead of a DVM to A15J200 pin 13 and the
negative lead to A15J200 pin 6. This measures the sampling
oscillator tune voltage which is an input to the ADC MUX of the A3
interface assembly.
2. Set the spectrum analyzer to the following settings:
Span .................................................................................0Hz
Center frequency .................................................. 389.5MHz
Trigger ......................................................................SINGLE
3. Use the data entry keys to tune the
CENTER FREQ to the values
listed in Table 11-2 on page 529.
YTO PLL A7, A11, A14 and A15
assemblies
Offset PLL (sampling
oscillator PLL)
A15 RF assembly
Fractional N PLL A14 frequency control
assembly
Reference PLL A15 RF assembly
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