Synthesizer Section
The first LO uses four PLLs to phase-lock to the internal 10 MHz standard in the instrument.
See Figure 7-5.
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 First LO
The spectrum analyzer uses a method called lock and roll to sweep the first LO (All 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 Al4 frequency control assembly. It is applied to
either the All YTO main coil or the All YTO FM coil. For LO
spansl2.0
MHz, the YTO
PLL remains locked and the fractional N PLL sweeps while remaining phase locked. The
frequency/span relationships are as follows:
All YTO Spanwidth
Sweep Applied to
20.1 MHz to 3.8107
GHz
All YTO main coil
2.01 MHz to 20.0 MHz
All YTO FM coil
100 Hz to 2 MHz Fractional N phase locked loop
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 phase detector output of the YTO
PLL.
Reference PLL (part of
A15)
The 600 MHz reference PLL provides 600 MHz for the second LO, 300 MHz for the third LO,
and the sampling oscillator reference, and 10 MHz to 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.
YTO PLL (A7, All, part of A14, part of A15)
The YTO PLL produces the instrument first LO (3.0 to 6.81 GHz). The YTO output is
mixed with a harmonic of the sampling oscillator in the sampler (A15A2), and the resulting
frequency is phase-locked to the output of the fractional N PLL.
The
A15UlOO
sampler mixes the LO signal from the A7 SLODA with a harmonic of the
sampling oscillator. The mixing product, the sampler IF, is between 60 and 96 MHz (same
frequency range as the fractional N PLL).
7-46 General Troubleshooting