Agilent Technologies 856290216 service manual Synthesizer Section, Sweeping the First LO

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