Installation guide

Troubleshooting
Block Descriptions for Option UNJ and Option 506
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Frequency Synthesis Block Description for Option UNJ and Option 506
Overview
Frequency synthesis for Option UNJ and Option 506 signal generators is made from portions of the following:
• A18 Reference (Option UNJ or Option 506)
• A15 Sampler (Option UNJ or Option 506)
• A16 Frac-N (Option UNJ or Option 506)
• A21 YTO Driver (Option UNJ or Option 506)
• A13 Output (Option UNJ)
or A13 Output (Option UNB)
• A14 Extended Frequency Output (Option 506)
• A23 Motherboard
The Frequency Synthesis Block consists of the A18 Reference, A15 Sampler, A16 Frac-N, and the
A21 YTO Driver. Frequency synthesis begins on the A18 Reference with a 10 MHz reference Oven Controlled
Xtal Oscillator (OCXO). The OCXO may be phase-locked to an external reference or operate stand-alone. The
10 MHz OCXO phase locks a 1 GHz Surface Transverse Wave (STW) resonator on the A18 Reference. The
frequency accuracy of the 10 MHz reference is translated to the final output frequency accuracy. The 10 MHz
and 1 GHz signals are distributed throughout the signal generator. In this discussion, we will concentrate on
the 10 MHz going to the A16 Frac-N and the 1 GHz going to the A15 Sampler. The 10 MHz is also routed to
the Internal Modulation Source Function Generator located on the A23 Motherboard, and to the rear panel.
The 1 GHz is also routed to the heterodyne circuit on the A13 Output.
Frequency generation is accomplished with a YIG Oscillator (YO) on the A21 YTO Driver. When the
instrument's frequency is changed, a DAC on the A21 YTO Driver is used to coarse tune the YIG.
The 10 MHz reference is part of a Phase Lock Loop (PLL) on the A16 Frac-N. The PLL tunes a Voltage
Controlled Oscillator (VCO), which is then routed to the A15 Sampler.
On the A15 Sampler, the 1 GHz from the A18 Reference is divided down and mixed to generate 750 MHz. It is
then mixed with the VCO from the A16 Frac-N with the resulting IF being one input to a phase comparator.
The other input to the phase comparator is derived from a portion of the YO signal being fed back to the
A15 Sampler. The A15 Sampler down converts the YO signal to a Sampler IF. The Sampler IF is the other
input to the phase comparator. The output of the phase comparator is integrated and routed to the
A21 YTO Driver. The A21 YTO Driver sums the coarse tune DAC with the integrator signal from the
A15 Sampler and uses the signal to fine tune the YO.
A18 Reference
Essential to frequency accuracy and low phase noise are clean and stable reference signals. The reference
signals on the A18 Reference are a 10 MHz OCXO and a 1 GHz STW. To minimize frequency drift, the
10 MHz oscillator has an oven to maintain a constant crystal temperature. If the instrument has been
without electrical power, a "Reference Oven Cold" warning is displayed when it is powered on. The warning
message is controlled by a timer and should go out after five minutes of being connected to ac power. The
10 MHz OCXO may also be phase locked to an external 2, 5, or 10 MHz reference. The 10 MHz reference is
also available at the signal generator’s rear panel and may be used to phase lock other instruments in a
system. The 10 MHz OCXO is routed to the A16 Frac-N and is part of a PLL to tune a VCO.