Reference/Synthesis Loop Block Description; A7 Reference; A5 Sampler - Agilent Technologies PSG Series Service Manual

Troubleshooting

Reference/Synthesis Loop Block Description

Reference/Synthesis Loop Block Description
The function of the Reference/Synthesis Loop is to establish the frequency characteristics of the signal
generator. These frequency characteristics are the fundamental frequency range, phase noise, and frequency
accuracy. The Reference/Synthesis loop consists of the A7 Reference, A5 Sampler, A6 Frac-N, A9 YIG Driver,
YIG Oscillator (YO), and A29 20 GHz Doubler. The Reference/Synthesis Loop description covers the
generation of 3.2 to 10 GHz signals and the RF Path description covers frequency generation of frequencies
other than 3.2 GHz to 10 GHz.
The basic functions required to generate an accurate frequency are coarse tuning, fine tuning, and output
frequency monitoring or feedback. Coarse tuning is achieved using the Pretune DACs on the A9 YIG Driver.
Fine tuning is achieved using the A5 Sampler and A6 Frac-N. To help explain the operation of the
Reference/Synthesis loop, the following paragraph describes what happens when a signal generator is set to a
frequency of 5 GHz.
When the user enters the frequency, the A18 CPU sets the A9 YIG Driver Pretune DAC to a value that results
in the YO being coarsely tuned to 5 GHz (within ~ 3 MHz). At the same time, the CPU sets the Sampler VCO
and the A6 Frac-N VCO to the frequencies necessary to generate an exact 5 GHz YO output. The output of the
YO is connected to the A29 20 GHz Doubler where a portion of the signal is coupled off and routed to the
A5 Sampler. On the A5 Sampler, the coarsely tuned 5 GHz is converted to an IF frequency and then filtered.
The A6 Frac-N VCO is divided down on the A5 Sampler to approximately the same frequency as the IF from
the coasely tuned YO signal and is used as the reference signal in the phase comparator. The output of the
phase comparator is proportional to the phase (frequency) error between the two signals. This error voltage is
integrated and summed with the pretune voltage, fine tuning the YO to exactly 5 GHz.

A7 Reference

Essential to frequency accuracy and low phase noise are clean and stable reference signals. In this case, there
are two reference signals required: the 1 GHz signal used by the A5 Sampler and A8 Output, and the 10 MHz
signal used by the A6 Frac-N. The A7 Reference (Standard) uses a 1 GHz STW Oscillator and 10 MHz OCXO.
The low phase noise A7 Reference (Option UNJ) uses a 100 MHz VCXO that is multiplied by 10 to achieve
1 GHz and a high performance 10 MHz OCXO. The A6 Frac-N uses the 10 MHz reference to maintain the
A6 Frac-N's VCO phase coherency.

A5 Sampler

The A5 Sample contains a microwave sampler, used to convert the YO output to lower frequencies for phase
comparison. A portion of the YO's output is coupled to the A5 Sampler to use as the RF input to the sampler.
The VCO on the A5 Sampler is used as the LO input to the sampler. The VCO output is a frequency between
618 to 905 MHz. The LO and RF signals generate an IF signal between 30 and 64 MHz. The IF signal passes
through an 80 MHz low pass filter eliminating all signals above 80 MHz that may pass through the sampler.
The IF signal is an input to a phase comparator. The other phase comparator input is the 10 to 80 MHz signal
that is the result of dividing down or mixing the 500 to 1000 MHz signal from the A6 Frac-N VCO. The phase
comparator's output is integrated and the integrated output summed with the pretune voltage on the
A9 YIG Driver to fine tune the YO output to the desired frequency.
In FM OFF mode, the phase noise level is improved on the A5 Sampler by dividing down the A6 Frac-N VCO
signal to get the desired frequency between 10 and 80 MHz. In FM ON mode and rates above 230 Hz, phase
noise performance is not as good as in FM OFF mode because the A6 Frac-N VCO uses a mixer to get the
desired frequency between 10 and 80 MHz. The mixer circuit is needed to allow for higher FM rates.
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