Chapter 7 357
General Troubleshooting
Block Diagram Description
Synthesizer Section
The 1st LO is phase-locked to the 10 MHz standard internal to the
instrument by four PLLs. See Figure 7-7 on page 358.
The reference PLL supplies reference frequencies for the instrument.
The three remaining PLLs tune and phase-lock the LO through its
frequency range. To tune the LO to a particular frequency, the
instrument microprocessor must set the programmable feedback
dividers (N) and reference dividers (R) contained in each PLL.
Sweeping the 1st LO
The spectrum analyzer uses a method called lock and roll to sweep the
1st LO (A11 YTO) for LO spans >2 MHz. This involves phase-locking
the spectrum analyzer at the start frequency during the retrace of the
sweep, then sweeping through the desired frequency range in an
unlocked condition. The sweep ramp, which sweeps the LO during the
roll part of the lock and roll process, is generated on the A14 frequency
control assembly. It is applied to either the main coil or the FM coil of
the A11 YTO. For LO spans≤2.0 MHz, the YTO PLL remains locked and
the fractional N PLL sweeps while remaining phase locked. The
frequency/span relationships are as follows:
When the sweep ramp is applied to the YTO, the spectrum analyzer
must prevent this loop from trying to compensate for changes in the
output frequency. To accomplish this, the spectrum analyzer opens the
PLL by disconnecting the YTO PLL phase detector output.
Reference PLL (part of A15)
The 100 MHz PLL output is routed through multipliers to provide
600 MHz for the second LO, and 300 MHz for the third LO and the
sampling oscillator reference.
ThereferencePLLisalsodivideddownto10MHz
for the fractional N PLL. The reference PLL is locked to a 10 MHz
OCXO (oven-compensated crystal oscillator) or a TCXO (Option 103).
The PLL can also be locked to an external frequency reference.
The 10 MHz reference also supplies the reference for the frequency
counter on the A2 controller assembly, and the cal oscillator on the A4
assembly.
A11 YTO Spanwidth Sweep Applied To
20.1 MHz to 3.8107 GHz A11 YTO main coil
2.01 MHz to 20.0 MHz A11 YTO FM coil
100 Hz to 2 MHz Fractional N phase locked
loop
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