Keysight NFA Series Noise Figure Analyzers Service Guide 271
L.O. Synthesizer/Reference Troubleshooting
A14 L.O. Synthesizer Theory of Operation
A14 L.O. Synthesizer Theory of Operation
The main oscillator is a low frequency Voltage Controlled Oscillator (VCO) that
has a frequency range from 570 MHz to 1150 MHz. This VCO has low-phase
noise that is multiplied to the desired 1st LO frequency range.
This VCO/Multiplier technology uses built-in filters after each multiplier to
suppress the undesired harmonics. The resultant output has enough power
with a wide enough frequency range to measure RF input signals up to 44 GHz.
Brief Description of the Major Blocks:
—The Field Programmable Gate Array (FPGA) L.O. Controller has the
capability to perform automatic pretune update, unlock detection, reference
signal phase adjustment, and automatic segment sequencing using block
RAM and its segment state machine.
— The fractional-N divider performs frequency division of the feedback VCO
signal.
— The VCO is a low noise, voltage control oscillator multiplied up to the actual
LO frequency. This wide-band oscillator features isolated input ports,
allowing the main tune port and the FM tune port to be formed. It has more
than an octave of tuning range with relatively flat tuning sensitivity.
— The LO Multiplier is a frequency multiplier with switched filters and
amplifiers to produce the required output frequency.
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